# The Convergence of Blockchain, IoT and 6G: Potential, Opportunities,
Challenges and Research Roadmap

Abu Jahid, Mohammed H. Alsharif and Trevor J. Hall

###### Abstract

The world is undergoing a profound transformation with the advent of
intelligent information era. The fundamental domains linked with smart living
such as transportation, entertainment, healthcare, and smart cities are
anticipated to elevate the level of service quality assuring a high-end user
experience. 6G networks envisioned being the game changer in next generation
wireless communication systems that will address the challenges of limited
information speed escalated with the augmentation of billions of data
applications encountered by the current fifth generation (5G) networks. Some
key radical technologies in 6G together with existing 5G candidate schemes
will guarantee the expected quality of experience (QoE) to attain ubiquitous
wireless connectivity for the Internet of Everything (IoE) ranging from the
telecom industry to digital smart industries. Recently, Internet of Things
(IoT) is reforming the incumbent industry into smart infrastructure featured
with 6G data-driven architecture. Blockchain technology (BCT) has gained
significant attention due to undertake the decentralization, transparency,
spectrum resource scarcity, inherent privacy and security, poor
interoperability, confidentiality, and emerging smart applications domains
including Industrial IoT and Industry 4.0. The mismatch between the
requirements of many data intensive disruptive IoT applications and 5G network
capabilities steered the demand of decentralized BCT based 6G architecture.
Inspired by these facts, this paper studies an extensive survey to draw a new
direction of blockchain integration into 6G mobile networks, IoT technologies,
and smart industries focusing the potential merits and challenges in terms of
infrastructure sharing, computational loads, latency, bandwidth overhead,
business model, sustainability goals, and edge intelligence. We highlighted
the convergence of IoT in blockchain to enable intelligent distribution in
future industrial IoT and the technical model of 6G networks to realize the
successful deployment of BCT schemes. This paper pointed out the current
intriguing challenges, canvassed the mitigation techniques, and plausible
future research opportunities that may benefit the pursuit of this vision.

###### Index Terms:

Blockchain, 6G, Internet of Things, Industry 4.0, Industrial IoT, 6G business
model, Privacy and security.

##  I Introduction

Beyond the exchanging of voice, image or video information in the fifth
generation cellular networks, researchers are exploring new dimensions of
interactions including five sense communications, ubiquitous instant
communications, virtual/augmented reality (VR/AR), the internet of skills,
wireless brain-computer interfaces, holographic communications, and pervasive
intelligence, leading a true immersion towards distant environment [1, 2, 3].
Concurrently, the evaluation of smart communications in different sectors such
as remote learning, autonomous driving, healthcare, industry internet, secured
transactions, and smart cities offered by 5G networks inspired experts to
research on 6G. 6G wireless networks are envisioned to the provision the
future data-intensive smart societies with full automation through seamless
integration of wireless networks aspects spreading from ground, air, space to
underwater [4]. With the enormous growth of data traffic and emerging
multimedia applications, 6G is envisaged to experience 607 Exabytes by 2025
and 5016 Exabyte by 2030 per month [5, 6]. The 6G mobile networks are
anticipated to be essentially virtualized, software defined, and cloud-based
systems with the motivate to substantial number of ubiquitously connected
heterogeneous devices including the internet of everything (IoE), to enable an
incredible range of network services [2]. With this paradigm shift towards 6G,
Internet of Things (IoT) plays a significant role by enabling promising
applications connecting the physical devices to the cyberspace of
communication world [7].

The advent of future generation wireless networks supports a new set of real
time and data intensive applications including sustainable business models,
agile management, network orchestration, network slicing, spectrum sharing,
and many other vertical services [3]. However, these differentiated services
tend to augment a number of issues such as data privacy, network reliability,
immutability, security, efficient resource utilization, multiple access
control, virtual network functions, etc. [8]. Mobile network operators (MNOs)
cannot handle bulky cellular infrastructure effectively especially during the
peak traffic demand. This could force cellular operators to raise usage prices
for the end mobile users because of the expensive licensing fees for the
additional bandwidth and the increasing number of infrastructure deployments
and management. Therefore, it is essential for MNOs to alter their existing
centralized business model to a flexible decentralized architecture led by
software defined networking, network virtualization, and intelligent
management [9, 10]. The decentralized micro-MNOs paradigm demand a scalable
and trustworthy solution for enabling quasi real time infrastructure
management in addition to improved financial interactions and service level
agreements (SLAs).

Blockchain technology (BCT) plays as an immutable distribute ledger for
performing secure financial transactions associated with cryptographic
symbols. In other words, BCT establishes trusted secure communications among
unknown entities in untrusted networks. BCT operated as a common, mutual, and
distributed ledger used as a large-scale secure index measuring for all
communications architecture particularly in the cyber security domain. BCT has
gained momentum for distributed applications and is recognized as the key
enabling trusted technologies that received deep attention both in research
and industry communities in the context of IoT and 6G mobile communication.
Blockchain offers a scalable and distributed platform to record data
permanently and validate transactions among different entities without
involving central participants [11]. The integration of blockchain with 6G
wireless networks potentially allows MNOs to monitor and manage resource and
spectrum utilization with some added advantages like improving spectrum
auction, reducing administration expenditures, infrastructure sharing, and so
on. Due to the inherited benefit of transparency, BC can record real-time
resource utilization and significantly improve spectrum efficiency through the
dynamic allocation of spectrum in accordance with the demands [12]. With the
wireless networks paradigm shifting toward decentralized solutions, plenty of
base stations are deployed by MNOs where billions of multimedia devices
communicating with each other. As an indication of this trend, fixed spectrum
allocation and corresponding resource sharing mechanisms are not effective and
scalable as well in the future wireless networks. Implementation of blockchain
with the underlying infrastructure not only provides efficient spectrum
sharing and resource management but also indulge incentive due to service
sharing [13].

Bitcoin and Etherum are the most popular implementations of blockchain
augmenting classes of immutable data lists. A decentralized ledger in
blockchain technology is used for data recording of each completed transaction
among multiple parties on a distributed peer to peer (P2P) networks
efficiently [14, 15]. In addition, BCT provides a trusted, secure, and
immutable platform for numerous entities including individuals and industries
in order to collaborate, digital asset exchanging, and pertaining transactions
with no requirement of controlling by the central authority. Beyond the secure
payments and storing financial data, the blockchain applications are expanding
to other domains, for example, supply chain management, industrial
manufacturing, healthcare, and education sector [16]. BCT 1.0 generally deals
with cryptocurrency (e.g., Bitcoin) and payment, BCT 2.0 handles automated
digital finance, and the most recent BCT 3.0 is associated with digital
society, for instance, Industry 4.0, industrial IoT, and smart cities [17].
The fourth industrial revolution named Industry 4.0 refers to a smart factory
setup equipped with intelligent IoT devices. On the other hand, industrial IoT
(IIoT) is a subset of IoT that has a narrow focus in transition from
conventional industry to smart industry 4.0 ameliorated by the
interconnectivity of our digital society and the eminence of IoT devices.

Albeit IoT empowered applications will smooth human life and facilitates smart
living, but it is a daunting job for 5G networks to support such advanced
applications. With IoT evolving, 6G will overcome the limitations of 5G
capabilities supporting ultra-speed data rate, low latency, a wide range of
coverage, localization, and other massive machine type communications. The
integration of blockchain and IoT technologies handling the limitations with
solutions is studied in the literature [18] but failed to identify the exact
problems to resolving the complexity aspects. Authors [19] investigated data
sensing integrity in an integrated IoT-blockchain platform objective function
to enable effortless access to the nodes in a diverse area. However, execution
time, communication, and computational overhead issues are ignored. The
latency and computational power issues remain unsolved while studying the
effectiveness of BC-enabled intelligent IoT systems with artificial
intelligence [20]. Huang et.al. [21] surveyed an overview of the green
evolution of 6G networks describing architectural transformation and related
potential technologies including blockchain. Authors focused on the blockchain
based ultra-dense cellular IoT architecture and machine learning approach from
the perspectives of research challenges and communication technologies [22].
Reference [23] provided an extensive survey on resource management in cellular
networks and IoT systems leveraging machine learning approaches. Although such
works have laid a solid background and breakthrough technologies on 5G based
IoT, the role of blockchain in 6G enabled massive IoT systems has not been
covered in the previous surveys. To fill this gap, this paper surveyed the
functions of blockchain in 6G and industrial IoT in terms of network
architecture, transparency, advanced network management, key achievement of
secured transactions, etc. with a wider range of opportunities. Table I
summarize the existing literature surveys and reviews in the context of
blockchain enabled wireless networks and IoT systems. All the related papers
presented the potential features of blockchain in the context of internet of
things pointing to different applications. In contrast to other relevant
papers, in this article, we dive deeper in the field of BC enabled industrial
IoT and 6G communications with detailed recent trends, challenges, solutions,
and future scopes.

TABLE I: Literature reviews on blockchain for IoT and wireless networks Reference |  Journals/Book Chapters |  Year |  Contributions
---|---|---|---
[24] |  IEEE Access |  2018 |  Development, deployment, challenges of BC based IoT applications
[25] |  IEEE Internet of Things Journal |  2018 |  BC based system performance, computational complexity, communication overhead, security issues for IoT networks
[26] |  IEEE Communications Surveys & Tutorials |  2018 |  Review BC based urban sustainability and use cases
[14] |  IEEE Access |  2019 |  Surveys BC implementation in industrial IoT and industry 4.0 applications
[27] |  IEEE Access |  2019 |  Blockchain-based security services including auth entication, confidentiality, integrity, privacy, and related challenges
[28] |  IEEE Access |  2019 |  Blockchain use cases in automotive industry, opportunities, and threat analysis
[29] |  IEEE Access |  2019 |  BCT in gas and oil industry pointed out efficiency, management framework, and information security
[30] |  IEEE Communications Surveys & Tutorials |  2019 |  Contemplating blockchain in edge computing enabling resource management, self-organization, computation validity, functions integration
[31] |  IEEE Communications Surveys & Tutorials |  2019 |  BCT in smart cities in terms of decentralization, transparency, democracy, trust-free, automation, and security
[32] |  Wireless Networks |  2019 |  BC for privacy and security preserving of e-government system
[33] |  IT Professional |  2019 |  Reviews the security vulnerabilities in the convergence of IoT and blockchain
[34] |  Computer Security Threats |  2019 |  Benefits of blockchain in Industry 4.0 automation
[35] |  Sustainability |  2020 |  Surveys BCT in industry 4.0 by means of privacy, integrity, and multi factor authentications
[36] |  Blockchain Technology for Industry 4.0 |  2020 |  BC for data management in Industry 4.0 applications
[37] |  IEEE Access |  2020 |  Demonstrate the integration of BC with IoT for automated operations based on mobile edge computation
[38] |  Digital Communications and Networks |  2021 |  Discuss the integration of blockchain in 6G in terms of resource management, spectrum sharing, network slicing, device-to-device communication
[39] |  Computer Communications |  2021 |  Review a blockchain based IoT architecture for smart city applications
[40] |  National Science Review |  2021 |  Present cross network sharing of the blockchain radio access network

A limited number of blockchain research projects have been emerged in past few
years on industrial IoT and 6G communications, hence, the industry-oriented
research is still in infancy. Beyond the features of authentication,
accountability, asset tracking, BCT plays a significant role in allowing
participant nodes to exchange information mutually in a secured manner in the
perspective of smart industries. The COVID’19 pandemic has led the world to
adopt for virtual video meetings, entertainment, live video communications
ranging from education, healthcare to business. Experts have anticipated that
the performance demand for real-time and virtual interactions, the escalation
of several data and computation intensive verticals, for instance, digital
transactions using cryptocurrencies, industrial automation, augmented reality,
virtual reality, autonomous driving, and device-to-device communications (D2D)
will increase dramatically after post-pandemic age. A number of significant
challenges are identifiable in the area of 6G communications and massive IoT
implementation to cater envisaged demand surge. The distributed blockchain
ledger technology is the key enablers to address limitations and facilitates
operational standards. To the best knowledge of the authors, no literature
review is conducted in the context of BCT applications in industrial IoT and
6G networks perspectives. The key contributions of this survey paper can be
summarized as follows.

  * •

Demonstrate the potential key features of blockchain technology of future
sixth generation wireless networks and industrial internet of things to shed
light in the research communities.

  * •

Discuss an ecosystem focused BC enabled 6G business model for the sustainable
global society, IIoT, industry 4.0 and beyond applications highlighting
financial stability, technology transfer, ecological balance and
reconstruction, environment conservation, and democratization of government
bodies, and innovation and entrepreneurship.

  * •

Outline the fundamental implications of distributed ledger technologies for a
wide range of 6G ecosystem centered business model transformations of multiple
stakeholders e.g., resource brokers, micro-operators, and edge cloud operators
addressing scalability, sustainability, and replicability consequences.

  * •

Provide extensive surveys on integrity, security, and privacy issues against
cyber threats or unauthorized access. After that, necessary steps depend on
various exhaustive layer-wise security risks and extent efficient solutions
are contemplated in light of adaptive consensus algorithms and cryptographic
mechanisms.

  * •

The challenging issues encountered and the potential mitigation techniques as
well as future megatrends toward BC empowered 6G cellular and industry
applications are extensively pointed out.

The rest of the paper is organized as follows. Section II presents the
fundamentals of blockchain technology, architectures, merits and demerits, a
broad range of application scenarios, challenges encountered, and mitigation
techniques. The benefits of blockchain in 6G including technical model, BC
empowered decentralized network design, and socio-economic sustainable
business model is extensively discussed in Section III. The convergence of
blockchain of IoT systems is presented in section IV. Section V demonstrates
the security and privacy issues. Furthermore, the summary of this survey and
the research challenges, future research directions, and lessons learned are
presented in Section VI. Finally, Section VII concludes the review paper.

##  II Background

###  II-A Overview of Blockchain Technology (BCT)

In recent years, BCT has drawn momentum as an emerging technology in both
academia and industry for distributed applications in 6G and industrial IoT
applications over the traditional centralized system. Blockchain is a
distributed public ledger technology that circulates across all the nodes in a
peer-to-peer (P2P) network in order to record and verify transactions between
participants without involvement of the central trusted party, where a ledger
database maintaining a growing list of information blocks securely employing
public key cryptography [41]. The fundamental features of BCT include
transparency, immutability, security, anonymity, and the least processing
time. With the added merits of economic and legislative features, blockchain
has the potential to disrupt mobile network business enabling dynamic mobile
network operator (MNO) switching by end entities, quality enhancement,
borderless roaming, and so on in 6G networks. In blockchain based approach,
IoT nodes can be classified into two categories, full node and light node,
depending on computing and storage capability. The data collected by
intelligent IoT devices combing with the computing resources from nodes
enables the distributed IoT providing services to all participants in the
entire system.

###  II-B Blockchain Architecture

A new block is identified by a hash value using secure hash cryptographic
algorithm 256 [32] according to the information of its parent block where the
hash value of current block (parent) is linked and stored in the subsequent
block (child). Note that the hash value changes in accordance with the
alteration of block contents and the updated information are spreading
throughout all participants in order to invalidate that block. However, the
nodes have secret keys assigned for digital signature and validation of the
transactions made by the entire mining participants in the network. Thereby,
BCT does not need to involve a trusted third party or any central intermediary
authority in the BC architecture and hence, it minimizes the processing time
and cost while performing P2P transactions.

Figure 1: Illustration of BC ledger with block details.

Each block has only a single parent and child that consists of header
information and transaction data. The header primarily comprised a set of
following information such as a number of transactions incurred, timestamp,
block version, proof of difficulty, and the hash of its parent block, and
concatenated hash values of all transactions in the chain. On the other hand,
a chain contains many connected blocks continuously grows as BC users to
perform transactions. A number of transactions are concatenated into the
blocks employing the Merkle root hash tree. The first block called the genesis
block from where a chain of blocks is originated as shown in Fig. 1 1. A new
block is connected to the blockchain through a hash value depending on the
information of its parent block. A validated block is appended at the end of
BC by the reference implying the parent block. A valid block may contain two
or more children blocks, when two or more nodes are connected to a block
simultaneously generating multiple branches from the same parent. However,
timestamp records current time and Merkle root listed hash values of all
transactions linked together in the entire block. Fig. 2 presents the
formation of Merkle tree a pair of transactions are hashed recursively till to
form one root at the top of the tree.

Figure 2: Illustration of concatenated Merkle tree.

###  II-C Features and Potentials of Blockchain Technology

A distributed ledger has the knowledge of network state information and
legitimates any necessary changes to be tracked. Blockchain has proven a
trusted effective key enabling technology by means of scalability,
interoperability, security, and privacy in data communication. In addition, BC
enables the digitalization of real world assets using cryptocurrency and
exchanges them via smart contracts. This subsection discusses the fundamental
features of BC that make it attractive for 6G wireless networks and IoT
systems.

#### Decentralization

Blockchain uses numerous consensus algorithms to validate transactions and
maintain the ledger in a distributed way without the intervention of central
authority (e.g., government, bank). The blocks can be replicated and shared
throughout the network in order to avoid the failure risk, ensuring
flexibility, and strengthening data persistency.

#### Immutability

Blockchain does not permit the deletion of confirmed transactions since each
block is connected with others via the hash value. Modification of the
information or breaking the whole chain is very limited. This feature solves
the altercation among multiple nodes (e.g., MNOs) related to national law
violations, for example, fraudulent transactions by unwanted users.

#### Security and Privacy

The nature of asymmetric cryptography in BC, a unique cryptographic has is
assigned to each block transaction that guarantees the integrity of the
information. The inherent features of consensus algorithm, immutability, and
anonymous identification ensure the trustworthiness, privacy, and security of
the entire chain without any modification of past transactions.

#### Transparency

All the participant nodes have equal rights to access all the information of
transactions in BC. In other words, any nodes can interact with others while
the confirmed transactions remain unchanged. However, a restriction is not
employed for functioning in the entire system.

#### Trust-less

The participants are not definite to others in the network. The network
entities can converse, assist and team build with others without specialized
any digital identity to carry out transactions among them.

BCT primarily include four elementary components.

#### Ledger

It is a distributed and shared database containing the information of all
performed transactions within the network. Ledger ensures the verification and
validation of every transaction among plenty of entities concurrently. The
information cannot be deleted or modified once the transactions are performed
successfully.

#### Consensus

Consensus protocol validates each action in the network in order to avoid
single miner node dominance in terms of the entire blockchain network
management and manipulating the transaction history. However, the participants
have to reach an agreement using predefined consensus blockchain algorithms.
Proof of Stake (PoS), Delegated PoS, Proof of Work (PoW), Proof of Data (PoD),
Proof of Authenticity (PoA), Proof of Existence (PoE), and practical
Byzantine-Fault-Tolerance (PBFT) [14] are the different consensus mechanisms
contemplated to validate transactions. For instance, PoW is used in Bitcoin
whereas Bitshare and Ethureum employ PoS and DPoS [32]. A new block called
mining is created under PoW protocol, which requires to solve a cryptographic
puzzle for each individual block. The winning miner who solves the puzzle
first and generates new blocks are rewarded using cryptocurrency. The miner
nodes add a new block (mine) in order to solve a complicated mathematical
puzzle that demands significant computational power and thereafter, gets
rewards in bitcoins [32].

TABLE II: Comparison of different consensus algorithms. Algorithm | PoW | PoS | PoW+PoS | PBFT | DPoS | PoA
---|---|---|---|---|---|---
Transaction/sec | 10 | 10 | 100 | 1000 | 1000 | 10000
Latency | 10-60 min | 5-60 min | 5-50 min | <1 sec | <1 sec | <1 sec
Security | High | Low | Medium | Low | Low | High
Scalability | Low | Low | Low | High | High | High
Throughput | Low | Low | Low | High | High | High
Power consumption | High | Low | Medium | Low | Low | Low
Fairness | Medium | Medium | Medium | Low | Low | Low
Anonymity | High | Medium | Medium | Low | High | Low
Type | Permissionless | Permissioned
Block Creation | Probabilistic | Deterministic

The node possessing more computational and processing power has more chances
to be the winning manner. PoW has a feature that any user can build a block
after proving a specified amount of work. However, PoW is an expansive
consensus process that requires considerable computational resources. In PoS,
a node generates a new block deterministically based on its wealth only to
corroborate new transactions and blocks. Unlike PoS, the PoW consensus
protocol does not depend on the mining process but execute block validation.
Each validator who executes block validation owns a security deposit stake in
the network called a bond. In contrast, DPoS employs real time voting and
reputation scheme to solve consensus problems by creating a board of limited
trusted delegates for validation. Finally, in PBFT approach, all the
participants have known each other and can select the priority of importance
[42]. It is worth mentioning that the blockchain performance highly depends on
a suitable consensus algorithm since MNO experiences thousands of transactions
per second based on user mobility, traffic intensity, and other factors.
However, security, scalability, and throughput are three primary concerns
among the aforementioned algorithms. Considering the dynamic deployment
scenarios, new consensus protocols are expected to be conceived in the
foreseeable future. Table II shows the comparison among various consensus
algorithms adopted in BCT [43].

#### Cryptography

This feature secured all network information using asymmetric encryption and
allowed the authorized entities to decrypt the original data.

#### Smart Contracts

This feature allows software-defined algorithm of distinctive transactions
automatically with self-executing computations in accordance with the
predefined conditions. In other words, a smart contract is an automated
digitalized transaction protocol that executed the contents and terms involved
in a multi-step and distributed way. Moreover, smart contracts can empower the
potential of software defined networks in 6G networks for enhanced network
management and flexible agreements between end users and cellular operators
[9]. There have multiple use cases and applications of smart contracts for
facilitating distributed functions such as voting, auctions, crowdfunding,
micropayments, etc. [44]. All peer nodes in the P2P blockchain network can
inspect the operations of smart contracts cryptographically using computerized
transaction protocol.

Based on the accessing strategy and permission of entities to write BCT
network, BCT can be classified into four types [35].

#### Permission less

Everyone with computing power participates in public BCTs to perform
transactions and join in the mining node and consensus process for adding new
blocks of transactions in the chain. This type of BCT normally used PoS or PoW
consensus algorithms [24].

#### Permissioned

Only the approved users can participate for specific business needs in the
scheme. Approved organizations are likely to have connectivity with existing
applications.

#### Public

Limited trusted entities can access the BCT network and be able to
write/modify the data, where entities may opt for consortium BCTs. The public
access chain is a permissionless proof based security scheme, which assures
network reliability for every user regardless of their identity at entry level
access. The public chains essentially bring significant benefits for adhoc
networks where the restriction of identification is broken for panoptic data
exchange. In addition, public chains can substantially regulate the order of
participants and increase the efficiency of the community [2].

#### Private

Only the specific users have the permission to write over BCT is given to a
central body. Private BCT offers flexibility by increasing control over the
transaction rules, which can be changed by overall consensus. The
private/consortium is permissioned where the entry of the participants is
controlled but not kept in secret. The private chain has a stable community
composition and network experiences a smaller number of security threats from
unknown attacks [2].

Figure 3: Layered structure of Blockchain.

Network layer broadcasts transactions and control messages to the entire
network. Each participant fetches unprocessed transactions for newly mined
blocks that will require scaling to downsize the number of incumbent
transactions. However, the network topology and the broadcast protocol can be
reconfigured to be more efficient and dedicated for the high-speed relay
network. Consensus layer assures three way trade-off among consensus speed,
bandwidth, and security for all participants. Sharding is the potential extent
efficient option that divides the consensus tasks among a set of users in
order to enhance consensus speed. Storage layer is the leger that stores
participants’ state changes resulting from read and write operations. On the
other hand, the view layer stores the current state of all nodes and smart
contracts, connected to the storage layer. The off-chain with cryptographic
proof method curtails the computational burden, and thereby increases system
efficiency [45]. The operation of transactions, authentication with
blockchain, and final execution are performed in the side layer. A details of
BCT layered architecture is demonstrated in Fig. 3.

###  II-D Challenges and Solution Outlines

Apart from the aforementioned features, blockchain based IoT and 6G wireless
systems offer some potential advantages.

#### Greater reliability and scalability

Data transactions are stored in different servers which cannot be altered
unless the consent of all active participants through proper consensus
protocol. The BC enabled system can readily scale up allowing new users and
devices to be connected automatically following the consensus policy.

#### Improved resiliency and auditability

The system is resilient to DoS, DDoS, malware attacks by avoiding single point
failure. All the transactions history can be traced easily after operations
since data remain unchanged in the system.

#### Improved accessibility and verifiability

Information ownership belongs to each individual where they are responsible to
validate a new entity to access their information. Storing information at
diverse locations increases speedy access easily. However, all new data
transactions are verified by all active participant nodes before being added
to the blockchain.

#### Enhanced transparency and data quality

All recorded data and new transactions are validated prior to making the
information authentic with desired quality. All the nodes share the same copy
in the network and new transactions are performed in accordance with the
associated consensus mechanism.

#### Higher efficiency and low cost

New users can access recorded data based on the accessibility privilege. New
records are distributed to all nodes in the chain. Operational cost is
significantly reduced since no middle organization is involved to process
transactions.

Albeit the potential benefits of blockchain in 6G and massive IoT
applications, numerous challenges required to be addressed prior its
widespread deployment. Some implementation challenges and possible solutions
of blockchain are discussed following.

#### Scalability

Because of its distributed nature, cryptocurrency such as Bitcoin requires
large bandwidth, ample storage and computing capacity to pledge the ledger
integrity. Bitcoin protocol deals with a massive number of information being
transferred and broadcasted among the participant nodes. Latency and
scalability issues restrict the performance of blockchain enabled
applications. For instance, Bitcoin has maximum achievable throughput of seven
transactions per second of a 10-minute latency for a confirmed block and four
days bootstrap time. In contrast, Visa transaction promises about 56000
transactions/ sec [46].

#### Double spending and other attacks

A user perform two distinct cryptocurrency transactions from the same amount
of currency, thereby, it breaks the integrity of the distributed ledger.
However, a miner may damage honesty of miner blocks in PoW consensus mechanism
to achieve more reward under selfish mining attacks. On the other hand, a user
generates multiple BC accounts to leak transaction privacy and manipulating
transactions.

Different cryptographic algorithms can be employed to prevent disclosing
users’ private information among participants.

#### Ring Signature

A digital signature is applicable for a group participant where all users
remain anonymous and ensure that the message is endorsed by the members. A
privacy-oriented cryptocurrency called Monero uses ring signature to guarantee
the confidentiality of members’ private information [45].

#### Zero Knowledge Proof

This method permits honest entities to convince other parties of statement
validation with high degree of confidentiality. The downside is the increasing
complexity of transactions approval. Zcash, a privacy protecting
cryptocurrency is the example of zero knowledge arguments in BCT [45].

#### Coin Mixing

This method makes obscure the coin owners’ address to assure their anonymity
through shuffling the information and coin transferring. CoinShuffle is a
decentralized coin mixing scheme, where each node generates an output address
before transmitting the information to other participants. When all the nodes
have their own output address, they broadcasted to the whole network. However,
the computation overload and bandwidth usage are further increased in this
policy.

###  II-E Blockchain in Industry 4.0 and Industrial IoT

The concept of Industry 4.0 is expected to lead distributed, dynamic, and
automated production networks driven by multiple key technological enablers
such as blockchain, industrial IoT, artificial intelligence, edge computing,
big data, human web integration, robotics automation, and open source
software. It is anticipated that the ICT sector plays significant role in
sustainable industry automation to support global socio-economic
sustainability. The interconnected cyber physical systems (CPS) enabled
industrial automation in Industry 4.0; thereby, allowing the transformation of
autonomous and dynamic systems from the conventional industrial production
process and infrastructure [35]. In other words, BCT helps to build
sustainable development by providing safe and secure communication schemes
using key authentication to assure access control management, nonrepudiation,
and authenticity. All the smart entities communicate autonomously with each
other to attain the common goal under such a highly integrated network.
Industry 4.0 is based on the idea of a smart contract in blockchain, which
plays a key role of protecting service level agreement (SLA) specification
between a supplier and consumers from violation by enforcing trustability and
data integrity [47]. SLA service is integrated with a smart contract among
different participants to satisfy SLA specifications; thus, blockchain
technology facilitates SLA services in Industry 4.0 applications with a
greater level of integrity and traceability.

The realization of Industry 4.0 encompasses three paradigms with the different
functioning processes including cloud computing, big data analytics,
logistics, enterprise resource planning, and product development [48]. The
smart entities control the resources and regulate the industrial manufacturing
process till the end. Secondly, the conventional production process is
converted into distributed, flexible, self-organizing, and adaptable
production lines done by the smart machine in a cyber physical system. An
augmented operator is the third paradigm that facilitates the capability and
flexibility of a human operator by means of handling, monitoring, verification
of the production process in the manufacturing industry. Leveraging BCT
provides a cooperative environment for augmenting workers using a human-
machine interface collaborating among entities in the whole manufacturing
process. For instance, the real-time information sharing among entities via
digital communication network of remote edge computing and warehouse data
storage capability in a secured way.

Industrial IoT (IIoT) is a cyber physical system (CPS), where humans,
computers, and machines empowering intelligent industrial functioning using
advanced data analytics for radical business outcomes. IIoT allows the
integration of internet infrastructure, communication protocols, and wireless
sensor networks (WSN) to ease the industrial intelligent automated operations
for cognition, analysis, and management, which enhance efficiency and safety
[14]. IIoT architecture primarily consists of three layers namely physical,
communication and application layer. The physical layer comprises sensors,
manufacturing equipment, actuators, smart nodes, and intelligent data servers.
Numerous access network technologies such as 5G, WSN, machine-to-machine (M2M)
communication, device-to-device (D2D) connectivity are used in the
communication layer enabling services to physical devices in the physical
layers for industrial automation and manufacturing. The physical layer and
communication layer form an industrial CPS to support the production and
development of smart supply chains, smart factories under the application
layer. However, the networking, computation, and controlling infrastructures
of IIoT CPS enable the intelligent operations of the manufacturing process.

##  III Blockchain Empowered 6G Wireless Network Architecture

Blockchain is regarded as an emerging technology to unleash the potentials of
6G, enabling distributed network management (e.g., spectrum, caching,
computation) systems that can be used for resource sharing and management,
data storage, spectrum regulator, and so on. A distributed ledger records and
audits the resource trading transactions in a simplified way. Real-time
spectrum sharing market using decentralized BC ledger will bring higher
spectrum trading efficiencies and nullify spectrum scarcity managing unused
relative frequency bands. Blockchain is considered as a resource-hungry
technique in terms of data transmission and complex computation. Few research
works have been carried out dedicated to BC-enabled spectrum management [49,
50, 51], but ignored the efficiency of blockchain itself ranging from the
framework architecture to the optimization problem.

The spectrum regulator from the government body builds strong-level policies
and constraints on BC enabled smart contracts which are implausible to break.
The regulator body maintains fair spectrum regulation throughout the country,
harmonizing various wireless access technologies, handling transceiver power
and interference, controlling spectrum coordination and trading, and so on. On
the other hand, spectrum owners purchase a license from the spectrum regulator
in particular frequency bands on a nationwide basis. Depending on smart
contracts, spectrum owners can exploit bandwidth or share with other MNOs for
better spectrum utilization. In addition, network infrastructure owners can
share their resources such as macrocells, microcells, and data servers with
other wireless operators relying on smart contracts. This approach can drive
new business models where investment by stakeholders will share with existing
MNOs contemplating the blockchain platform. For instance, a construction
company can build cellular stations for a prospective rental by MNOs. User
equipment (UEs) can dynamically switch among MNOs via service level agreement
(SLA) in the blockchain. However, end users can act like infrastructure owners
optionally by using smartphones for device-device (D2D) communications. End
users can select a particular operator among multiple MNOs based on service
quality, price of package, volumes, and coverage areas. The tokenization of
infrastructure and spectrum is the key aspect of the aforementioned trading
over the blockchain. The spectrum regulatory body defined the allocated money
price of the radio resource token (RRT) based on the user request, area of
deployment, and other economic values [52]. Besides, the infrastructure
resource token (IRT) is recognized as a stable coin in fiat currency where the
static price is subject to negotiation in BC smart contract based on location,
type, energy cost, and so on. Finally, the national cryptocurrency (i.e, USD,
EURO, etc.) is digital equivalent to the fiat currency of the respective
country where the cellular networks are deployed. Telecom operators have the
capability to buy and sell RRT or IRT in exchange of nationwide cryptocurrency
via a smart contract platform [52].

A national consortium blockchain ledger process the mobile network information
to maintain a considerable number of wireless nodes. Whereas a worldwide
public BC ledger contains UE information to ensure seamless connectivity over
the globe. National consortium spectrum ledger embodies information about
system bandwidth, licensing zone, downlink/uplink duplex mode, owner of MNO
and assigned MNOs, licensing fees, frequency allocation, and so on. National
consortium infrastructure ledger deals with each eNode B (eNB) such as
frequency band, positioning, cognitive radio technologies, assigned MNOs,
transmit power, coverage area, signal intensity, and so on. National
consortium mobile operator ledger consists of information about the service
functional area of each MNO, supported SLAs, token balance (RTT/IRT), and so
on. Worldwide UE ledger comprises end user’s information, for example, NC
balance, uplink data transmission, and tracking area. Notably, the authorized
government bodies can access UE information and should not be exposed to the
public. On the other hand, MNO can access only particular data that is
permitted to ensure enhance service quality.

###  III-A Spectrum Management

With the unprecedented growth of high speed data demand of 5G and beyond, the
requirement for more spectrum bands also increases accordingly. A dynamic
spectrum management policy has been proposed in Reference [53], which allows
unlicensed participants to access the licensed spectrum opportunistically
without interfering with the licensed users. The implementation of blockchain
as a trusted database platform has emerged in various spectrum managing
aspects such as spectrum auctions, spectrum sensing, spectrum lease mapping,
data mining outcomes, and idle spectrum details are recorded securely in the
blockchain [54]. It has been also identified that the features of blockchain
bring opportunities to reduce the administrative expenses related with dynamic
spectrum management and able to make significant improvement over the
traditional spectrum management approaches. Moreover, blockchain helps to
overcome the incentive conjoined with dynamic spectrum management and the
security challenges in a decentralized manner. A few other advantages of
spectrum management recording information are presented here.

In contrast to the conventional middleman database, BC enables participants to
have direct control of the recorded data, hence guaranteeing the accuracy of
the information including underutilized spectrum, time, interference
protection requirements. A smart contract in blockchain ensures access
fairness over the carrier sensing multiple access (CSMA) technique where the
access history is not coordinated and recorded. Enhanced spectrum utilization
efficiency of both primary and secondary users adjusting traffic demands
achieved through BC enabled efficient management scheme. The characteristic of
BC empowered dynamic spectrum resource allocation prevents fraud by providing
transparency, restrict unauthorized users from accessing the allocated
spectrum, and pledge the auction payments since all the executed transactions
are validated before recoding in the blockchain database [50].

###  III-B Benefits of BCT in 6G

#### Infrastructure and Resource Management

The resource management functions including spectrum sharing, orchestration,
computing, communication, caching, and decentralization are challenging tasks
in the massive connectivity demands of next-generation telecom infrastructure.
Blockchain technology solves infrastructure sharing challenges between
numerous mobile network operators (MNOs) in multiple ways. Firstly, tracking
the infrastructure usage of each MNO and manage the corresponding payment
using cryptocurrencies. Afterthat, infrastructure resources can be booked in
advance using smart contracts. In addition, additional expenses (i.e.,
internet provider, electricity usage, real state, etc.) can be equally
distributed of infrastructure utilization between MNOs allowing the
integration of third parties in the blockchain [8, 55]. On the other hand,
telecom regulatory bodies cannot track real-time spectrum usage and restrain
unfair behavior under existing technology. Moreover, the mixing of different
types (e.g., licensed, unlicensed) of spectrum band from different network
operators and wireless standard introduce additional complexity. Furthermore,
the complexity of interference handling and real time agreements escalate
incentive issues for dynamic spectrum sharing. The integration of automated
spectrum trading mechanism under blockchain technology with the existing
mobile network infrastructure address the aforementioned problem proving smart
contracts agreement among MNOs [49, 50]. A details illustration of resource
management procedure comprising energy infrastructure, communication and
computing architectures using blockchain is demonstrated in Fig. 4.

Figure 4: Blockchain enabled resource management framework in 6G
communications.

#### Network Slicing and Resource Brokerage

The smart contract orders the network slice orchestration relying on the
service level agreement (SLA) from the broker under the blockchain based
network slice trading. A smart contract keeps the record of the usage of each
resource and the performance of the service provider against the SLA. The
trustworthiness feature enabled by BCT diminishes the collaboration barrier,
thereby, ensure efficient ecosystem and security. Moreover, a BC based
approach is used to automate the payment, billing, leasing between service
providers, and reconciliation in diverse geographies while ignoring the
presence of third party in the service layer. Moreover, BCT based automated
resource brokerage of network slicing provides some additional benefits, for
instance, significant reduction of the operational cost, increase operational
efficiency for each network slice, faster the slicing negotiation process and
agreement, uplift the enforcement of straightforward agreement related to
brokering operations. In addition, blockchain technology also facilitates
cross-carrier payment and money transfer across countries, manage license
spectrum access, easy provision of roaming terms and agreement among MNOs, and
authorization of network elements.

#### Authentication, Access Control, Integrity and Accountability

Typically, a mobile user uses a subscriber identity module (SIM) in existing
mobile networks. Whenever a subscriber needs to change MNOs, one must change
SIM card either physically or electronically. A unique wallet ID is assigned
to all end users for identification among all MNOs that enable operator-
agnostic user quality of experience governed by the competitive mobile network
environment [8]. The dispersed ledger technology employed authority,
inspection, and complex security for content-centric 6G wireless networks
which remain as immutable and transparent for each event that can be utilized
during auditing.

#### International Roaming and Service Level Agreement (SLA)

Blockchain mostly depends on the internet connection availability, does not
rely on the administrative borders of countries. However, the conventional
international transaction method is significantly slow and expensive, while
financial transactions using cryptocurrencies across the globe is a matter of
few seconds with negligible costs. Therefore, with the advent feature of
borderless networking, BC eliminates the international roaming constraint as
there is no complexity in data connection between domestic and overseas MNOs
[56]. It is widely observed that the majority of end users are not entirely
satisfied with the package plan because it may be either costly but redundant
or cheap but limited data availability. Selecting MNO and an appropriate
mobile package plan include a trade-off between price plan, signal coverage,
quality of service, and multiple other aspects. Blockchain can address this
shortcoming by enabling flexible service level agreement smart contracts
between end subscriber and MNO in quasi real time [57].

#### Local Area Wireless Networks

Under the current centralized model, very few cellular operators deploy
nationwide wireless networks, experience complex infrastructure development
and maintenance. Owing to expensive license spectrum fees, the micro size
local MNOs cannot compete in the market in a large scale that leads local
telecom operators unprofitable. Blockchain smart contracts address local
spectrum allocation problem in real time where a small MNO acquire unused
frequency band from large operators over the specified coverage area and time.
In addition, blockchain offers greater level policies and monitoring spectrum
usage by MNOs across the country, to the central regulatory commission. This
feature allows a hybrid spectrum distribution market, where MNOs have the
authority to trade parts of spectrum and the regulatory body may restrict
bandwidth usage in particular areas, for example, military, border defense to
avoid interference.

#### Cloud/Edge Computing

Cloud computing facilitates the offloading of complex computations to distant
servers for the resource-constrained user equipment. Edge computing overcomes
the heavy computation burden on backhaul networks, long latency, and privacy
compared to cloud offloading [49]. Notably, trustworthiness and security are
the primary concerns since sensitive information may involve while offloading
computations. Blockchain can build the desired trust between the edge servers
and smart devices guaranteeing the integrity of the remote resources and
offloaded computations [58].

###  III-C Ecosystem Focused 6G Business Model

Given the recent global economic crisis, the business models in 6G will shift
toward distributed local edge small operators, software, and virtualized
network service providers from monopolized service operators since the
existing business structure has a considerable impact on the sustainable
global society. Unlike 5G networks, the 6G network offers a wide range of
connectivity between real-time dynamic cyber physical world and multiple
upfront KPIs, which will inspire various verticals, technology, product, and
applications developers for the next generation society 5.0. Moreover, the
advent features of 6G tangle with virtualization, intelligence, spectrum
sharing between MNOs, edge operations, and a variety of IoT devices;
altogether services will push forward into smart living and will be an
integral option of smart society [59]. Blockchain empowered decentralized
architectures envisioned as prospective extension efficient solutions for
sustainability, user experience and business, which support multiple verticals
in 6G wireless networks perspectives [60]. Sustainability, replicability, and
scalability are the proposed key enablers for future 6G business platforms by
means of service agility, orchestration, open collaboration, and zero touch
management [60]. Sustainability specifies viability, feasibility,
environmental, and social impact, whereas replicability refers to external
flexibility to comply. In contrast, scalability indicates the potential for
internal growth and flexibility. Furthermore, value creation, capture,
sharing, and delivery are regarded as key parameters for functioning the 6G
business model. Fig. 5 depicts the blockchain enabled framework for 6G
business model.

Figure 5: Blockchain enabled framework for 6G business model [61].

It is commonly believed that the future 6G business arena is embodied by an
automated and seamless market data collection from dynamic business platforms
and humans. 6G will support a user-friendly environment of intelligent big
data analysis for sustainable high-end products and services to address
specialized demands by the consumers under a wide range of geographic
locations. In addition, 6G technology will assist crowdsourcing and advanced
cutting-edge distribution to accelerate fair resource distribution in order to
support MNOs sharing in sustainable business models. Ecosystem focused
sustainable 6G business is key to financial stability, human empowerment
driven by knowledge creation, technology transfer and adaption, innovation and
entrepreneurship, ecological balance and reconstruction, environment
conservation, and democratization of government bodies [61]. The advancement
of sustainable 6G business solutions will empower IoT devices and other
electronic gadgets to operate remotely in a standalone manner and simplify
human lives. Peer-to-peer cooperative models and distributed business models
under BC enabled 6G solutions is an acceptable approach against a monopoly
market that will support a circular economy both in urban and rural areas
[59]. In addition, BC based 6G business models offer cloud storage for over
the top (OTT) companies, cognitive services such as context aware operations,
AI, UAV as a service together with calling and data connectivity features,
envisioned to attract customers and increase their profit. Moreover, digital
twins and the FinTech industry will add a new dimension of business pattern
with 6G in addition to the e-government policy regulations. In the case of
industry 4.0 and beyond applications, BC empowered business models will
provide extreme reliability, privacy-aware secure communications, zero touch
assistance, etc. in prolonged lifetime wireless nodes which will bring new
business evolution using these technologies [59]. As a consequence, the
sustainable BC enabled 6G business model will introduce numerous new forms of
business structures and opportunities to uplift revenue.

##  IV Convergence of Blockchain for Massive IoT

In recent years, IoT system integrates multiple distinctive heterogeneous
sensors and objects in our smart living era facilitating the information
exchange among all nodes. With the rapid expansion of network coverage and
intelligent evolution of smart devices, conventional IoT systems failed to
meet advanced efficacy, interoperability, network management, privacy, and
security vulnerabilities, particularly in the high degree of complex data
handling. The centralized IoT architecture experiences high maintenance cost,
is vulnerable to unwanted attacks and suffers scalability problems [33, 62].
To address these challenges, an autonomous distributed system is more feasible
for trusted users to participate independently. Moreover, multi participants
cooperation ensures the system consistency while a node is crashed by single
point failure. A decentralized IoT scheme also reduces the storage load and
computational complexity while participants synchronize the entire system
copying blockchain leader. The autonomic interaction of IoT devices attained
through smart contracts (when the contract clauses are satisfied) in BC that
allowed cooperation among each other without any middleman. A decentralized
blockchain-IoT (BIoT) architecture is shown in fig. 6 illustrating on featured
applications.

TABLE III: Potential contributions and challenges of blockchain enabled IoT system in 6G Reference |  Contributions |  Limitations |  Year
---|---|---|---
[63] |  Fundamental functions of blockchain in IoT privacy issues |  High computational cost |  2018
[64] |  BC structure identify the connectivity between BC and IoT devices for verification |  Poor security issue |  2018
[18] |  Examined the integration of IoT and blockchain |  Computational complexity high |  2018
[32] |  Proposed a decentralized e-government P2P scheme using BCT including privacy issues |  Suitable consensus algorithms are not focused |  2018
[65] |  BC offers solutions for IoT devices handling in a smart way addressing limitations |  Reliability level not improved |  2019
[19] |  BC integrated IoT platform investigate data sensing integrity in diverse area of applications |  High communication overhead |  2019
[49] |  Intelligent and secure architecture improves the effectiveness using deep reinforcement learning |  Privacy level not improved |  2019
[66] |  Logchian for guaranteeing block integrity |  Security aspect not addressed |  2019
[67] |  BC base edge computing enhance IoT system performance |  Resource optimization not addressed |  2019
[68] |  Centralized architecture lessens the computational overhead |  Computation power high |  2020
[69] |  BC-IoT prevent hacking and information infringement |  Authenticity is not validated for assuring security level |  2020
[70] |  Decentralized and asymmetric cryptographic platform avoid the presence of central authority for communication |  Data confidentiality is not improved |  2020
[71] |  BC refines the concept of cybetwin for secure production |  Privacy issues ignored |  2020
[35] |  Implementation of BCT in Industry 4.0 for cyber threats analysis |  Scalability, confidentiality and latency issues are not presented |  2020
[37] |  Investigate security issues of BC enabled IoT and mobile edge computing |  Computational cost not addressed |  2020

Figure 6: BCT based distributed IoT architecture.

###  IV-A Research Challenges

IoT systems are heterogeneous in nature comprises of distinctive machines,
sensors, robots, vehicles, and so on, which generate different types (e.g.,
structured, non-structured and semi-structured) of massive data. Moreover,
numerous resources such as caching, communication, computing, sensing,
control, and localization need to be considered under resource management
challenges. In addition, huge IoT data are challenging to manage in terms of
transmission, elaboration, and storage under a data management scheme.

#### Interoperability

The information exchanging among IoT devices and the capability of data
communications with physical systems is termed as interoperability, which is
accounted as a key challenging concern for IoT in 6G due to multiple protocols
and cross-layer architectures. Multiple operational platforms such as radio
access operators, caching vendors, edge computing vendors, and so on exist
together to provide a broad range of services to diverse types of IoT devices.
It is impractical to collaborate with multiple service operators, vendors,
industrial sectors, different data centers, and IoT systems under existing
network architecture. However, a unified authorization and authentication are
needed to combine individual systems of different bodies. Furthermore, the
provision of diverse services and valid payment methods to various service
providers and associated vendors is required to be audited and ensured without
the presence of any trusted entity.

#### Privacy and Security

It is essential to prevent the disclosure of huge data generated by massive
IoT devices without users’ consent, which may include confidential and private
information. Owing to the complexity, decentralization, and heterogeneity of
IoT systems, the protection of users’ privacy and security is a challenging
task. Moreover, the recent trend of integrating cloud technologies (e.g.,
fog/edge/cloud) with IoT systems empowering diverse applications which is
vulnerable if clouds are attacked, breaching participants’ privacy [72]. Note
that the current IoT methods for ensuring security, privacy, and information
handling capability in 6G heavily rely on third parties.

#### Standard and Latency

When a greater number of devices are connected in P2P network, the risk of
external attack also increases due to the lack of standard regulation device
applications. Guaranteeing the minimum latency is another key challenge for
high-speed communications for various IoT devices while operating in a large
bandwidth range. Table III summarize the potential contributions and
challenges of blockchain enabled IoT system in 6G.

###  IV-B Possible Solutions

An integration of IoT in blockchain is termed as Blockchain of Things (BoT).
With the inherent emerging properties of BC, i.e., decentralized nature,
distributed consensus, cryptographic encryption, trust-less system, and non-
repudiation pledge, BoT is contemplated as the key enabler of massive IoT
deployment together with the revolution of next generation wireless
communication technologies [21]. In addition, BoT offers some extended
advantages including interoperability, traceability, scalability, reliability,
data quality, and trustworthiness. A powerful decentralized BoT architecture
is constructed with developer tools for security, smart contracts, and
contract execution with a greater level of reliability. BoT can reform the
development of IoT with dependence, open and auditable platform. The
integration of BoT with 6G offer some potential benefits for IoT deployment
and end users, particularly in industrial applications, for example, choosing
multiple services from any organization, choosing anything from anywhere,
choosing any route from any network. These features allow BoT to provide some
extended aspects, such as trust building between 6G enabled IoT applications
and end users, cost reduction due to removal of intermediate node and direct
service, and real-time rapid transaction service [37]. Fig. 7 presents the
possible challenges of blockchain empowered IoT automation with potential
solutions.

Figure 7: Potential challenges and solutions of blockchain-IoT (BoT) scheme.

#### Decentralization

Paradigm shifting towards decentralization eliminating failure to escalation
fault tolerance performance. BoT restricts the autarchy of resources against
corruption while managing data information [73].

#### Autonomy and Interoperability

BoT enables autonomy function for implementing industrial IoT applications by
means of cooperating mobile devices with each other. Blockchain composite
layer on P2P network empower the data communications with physical system and
have the capability to exchange information in the entire IoT system with
uniform access by providing a unified authorization, authentication, and
billing system [74].

#### Traceability and Reliability

The feature of historic timestamp in blockchain empower the detection and
authentication of IoT data (e.g, temporal-spatial information of data block).
The trustworthiness of data quality is termed as the reliability of IoT data
which can be guaranteed through asymmetric encryption mechanisms, hash
inclusions, and digital signature. Each user in the BoT architecture is
allowed for validating the transaction legitimacy with full confidence and
responsibility.

#### Security

Information exchanges between inter-device communications are regarded as
transactions considering new agreements to support security. A manufacturer
allows IoT devices for updating securely incorporating secure code deployment.

##  V Security and Privacy Aware BC

Since 6G connects diverse types of voluminous and distributed smart devices,
services, and applications, so privacy, security, and reliability are the
paramount requirements of data management for high-end users and industries as
well. Developing a reliable ubiquitous trustworthy BC model into 6G networks
coordinate between end users and network entities to collect and prevent
suspicious misbehavior actions including non-repudiation and indirect
reciprocity. Despite the potential benefits of BC technology, numerous privacy
and security issues against cyber threats or unauthorized access have been key
concerns while convergence with 6G networks and IoT devices. It is essential
to guarantee the integrity, confidentiality, and availability of data services
for each transaction. However, integrity is attained by protecting transaction
data without any modification; preventing information to unauthorized users to
achieved confidentiality; whilst service availability refers data information
is available when needed which is free from denial of service (DoS) attack or
other identical service disruption. A DoS attack targets the services or
system availability through flooding with requests to its legitimate users to
make the service unavailable. In other words, the attacker flooded the server
with massive requests pushing the workstation to commit all computer resources
and forcing the network server to malfunction leading to a crash. On the other
hand, distributed DoS (DDoS) contribute simultaneous flood requests come from
multiple sources in the network [35].

The buffer overflow within the server disables security systems, especially
the firewall and intrusion detection system. In addition, the cloud-based
modern intelligent factories and manufacturing production are highly suffered
from multiple DoS attacks if the cloud server is not designed to protect
against such threat [75]. Moreover, the impacts of DoS attack can immensely
affect Industry 4.0 applications because of using a substantial range of
vulnerable sensors. The fundamental challenge of DoS is the detection and
quantifying of the emerging risks in the network. Reports [35, 76] suggest
that the transition to industrial IoT (IIoT) develops control policies, builds
security awareness and authentication mechanisms including behavioral
assessment to prevent hacking and digital encryption technologies. The data
flow in IIoT systems manifests innumerable points of vulnerability, and
hackers can steal data at any point without suitable protective measures.
Particularly, hackers can access cyber physical protection systems which
collaborating with surrounding physical systems either physically or
logically. With the escalation of collaborative threats and industrial
espionage, many smart industries developing intelligent security techniques
and control policies to guarantee trust and transparency aiming to defend
confidential information [77, 29, 15]. Using the hash function and
cryptography proofs among different linked smart contracts can be regarded as
a promising solution in blockchain technology by preserving the integrity and
detecting any malicious modifications. Moreover, contemplating a Mongo
database in BC can also enforce security algorithm, and offers additional
cryptography proofs against confidentiality threats and attacks [35].

Asymmetric encryption and digital signature are employed in the network to
pledge security, whereas a private key is assigned to a particular user for
validating the transaction, which enables shield for unauthorized access or
adversarial attempts to data modification. To increase the privacy of
transaction data stored in the system, public key cryptography is used
employing incomprehensible hashes of all blocks in the chain. Since the
individual user’s data is stored in various nodes in a decentralized P2P
system which ensures the service availability ignoring the single point
failure. Notably, any valid transactions received from the BC network are
permitted by the witness particular for the malicious nodes to prevent
unauthorized connections [32]. The different sorts of security services and
corresponding necessary steps taken to guarantee adequate security and privacy
of transactions are summarized in Table IV.

TABLE IV: Security issues and common steps [32] Services | Necessary Steps
---|---
Access control | Digital signature and encryption
Authentication | BC address and digital signature
Privacy | Encryption
Integrity | Encryption & digital signature
Availability | Distributed
Trust | Encryption & digital signature, distributed

The security concerns can be the network and communication security,
authentication and identity management, establishment of trust protocol,
distributed cooperation, reliable consensus policy, and transaction privacy
and security in designing distributed 6G networks and industrial IoT. All the
nodes in a distributed IoT system interact autonomously without involving a
central server. As a consequence, such openness may lead to numerous impacts,
for example, launching text spoofing, node capture, eavesdropping can filch
system data partially to compromise the stability issue of massive IoT
systems. Compared to RSA based cryptographic algorithm, the Elliptic Curve
Digital Signature Algorithm (ECDSA) outperforms in network security
particularly for smaller key strings and faster operation [78]. In order to
improve computational efficiency, UE devices will run a lightweight node to
store transactions information instead of entire copy of blockchain which
leads to a reduction of storage burden. However, the elliptic curve
cryptography (ECC) method offers a similar privacy level with Rivest-Shamir-
Adleman (RSA) approach, but ECC requires less number of bits for encryption
and digital framework. For example, 256-bit ECC provides the same security
level as 3072-bit key in RSA [32]. However, a shorter asymmetric key occupies
low CPU space, faster key production, and lower memory usage. ECC algorithm
has been recognized as a promising solution in terms of safety and reliability
in the blockchain context such as Ethereum and Bitcoin. It is worth mentioning
that IoT sensing devices are energy constrained and subject to limited storage
capacity and computational algorithms. Applying ECC lightweight cryptographic
algorithm may fail to meet the demand on limited memory and computational load
under energy constrained aspects. According to [33], ECDSA based algorithm
offers optimum communication performance that realizes to balance between
memory limit and energy consumption.

All the transactions containing content, frequency, destinations are shared in
the blockchain which may lead to potential security and privacy risks that
unwanted entities can infer the real identity of participants. In the
convergence of BC and IoE, adversaries can even track physical IP address of
wireless nodes and IoT devices which may eventually make system crashes and
destabilization through denial-of-service attack. Henceforth, it is essential
to protect sensitive information such as physical location, network address,
device class, and other private information about the nodes especially during
recording the transactions data. A homographic and zero knowledge proof are
potential encryption methods for privacy safety via ensuring the
confidentiality of transaction records in the public blockchain [79].

The ever-increasing number of IoT devices would demand substantial storage and
computing capacity to realize identity authentication. Owing to the high
degree of heterogeneity of smart wireless devices, the typical methods in BC
may not be able to cope with the requirements of identity authentication in
the internet of everything (IoE). To tackle such challenges, a public key
infrastructure combing the smart contract with encryption algorithms
autonomously handles identity management with a greater level of reliability.
However, the reliability can be enhanced by designing device registration
including public key, manufacturer, expiry date, identity validation, firmware
update, and device obsolescence [80]. In addition, synchronization of recorded
data and querying the full nodes in the blockchain can effectively implement
identity management among all P2P nodes and minimizes the cost of building
trust.

An effective collaboration among participants is essential to accomplish
specific functions and services in P2P network, since it is challenging to
maintain reliable trust and cooperation in the BC enable IoT systems where
each node can enter and leave the system at any time, thereby the identity
remains anonymous. The reputation assessment technique is the best way to
enhance trust and cooperation in IoT applications such as crowdsourcing and
adhoc networks. In contrast, both entity and data-centric trust models suffers
privacy leakage, lack of data information, latency, and free riding problems
[81]. Establishing node assessment model by extracting the behavioral data of
other participants recorded in the blockchain and evaluating the transparent
block data. However, the calculation of credibility of each node is done
automatically considering their behavior data and timeliness to measure the
system performance. Therefore, it is important to account for the incentives
and node preferences while choosing the proper behavior type and appropriate
time scale to better regulate the node behavior and develop trust.

TABLE V: Layer based security risks and possible solutions. Layers |  Attacks |  Challenges |  Solutions
---|---|---|---
Physical |  Eavesdropping |  Content modification of a tag |  State detection and data recovery schemes
|  Jamming |  Cause exhaustion and radio interference |
|  Physical damage |  Device connection and recovery |
|  Firmware replacement |  Condition monitoring |
Network |  De-synchronization |  Interruption of existing connection |  Hierarchical consensus algorithm
|  denial of service |  Reduce the activity of devices |  Dynamic committee
|  Byzantine attack |  Consensus protocol |
Transport |  Middleman |  Breaking integrity and confidentiality |  Zero-knowledge proof
|  Message spoofing |  Privacy and security |  Homomorphic encryption method
|  Flooding |  Repeating a new connection request and uses frequent HELLO packets until reaches peak level |  Lightweight authentication
|  Sybil |  Duplicate a single node in multiple locations |
Application |  Identity duplication |  Authorization and authentication |  Customized smart contracts
|  False injection |  Sent untrusted data to server |  Protocol based cryptographic algorithm
|  Malware |  Malicious link can attack web and mail services |  Reputation assessment model
|  Selfish attack |  Coalition and trust building |
|  Brute force |  Attempt to guess password |

Owing to the series of problem in the centralized IoT and 6G wireless systems
such as inferior scalability, single/multi-point attack, and privacy leakage,
blockchain technology offers an immense opportunity for developing secured
decentralized infrastructures. Advanced encryption algorithms, reputation
assessment models, authentication, and consensus protocols are vital
technologies to build the convergence of blockchain, IoT, and 6G networks
addressing a high degree of heterogeneity. Table V summarize the layer based
security risks and possible solutions in the blockchain domain.

##  VI Research Directions and Opportunities

With the rapid development of IoT technology, a significant number of
obstacles are incurred to downsize the adoption of IoT devices across many
applications. The centralized structure of IoT and 6G networks need the owners
to trust the entities to keep data safe. With the potential benefits of
blockchain technology, a distributed ledger neglects the challenges of
centralized architecture while constructing trust, privacy, and security with
guaranteed reliability. Moreover, BCT minimizes the cost by eliminating third
party middlemen and intermediary overhead. Blockchain offers many solutions to
address 6G and IoT challenges but convergence between technologies create new
issues. There are some security and performance issues that exist without
solution. For example, the privacy issue of BCT depends on the method and
usage of the hardware and software implementation. On the other hand, the
increasing number of miners leads to high storage cost, and latency, and
lessens the speed of distribution over the entire network.

#### Privacy and Security

IoT systems and wireless networks are usually suffered from security attacks
like eavesdropping attacks and replay attacks [14]. In addition to these
security attacks, blockchain technology brings their own set of security risks
such as message hijack, smart contract vulnerabilities, etc. [82]. The
proactive security policies for dynamic environments will uplift the control
signaling cost. However, reactive security techniques are not preferable for
6G networks. Hence, cost effective proactive mechanisms need to be
investigated. Privacy leakage is another key concern in transactions and
recorded stored data in the blockchain. A massive wireless device
connectivity, multi-hop data routing, and heterogeneity in 6G networks
manifest the question of data privacy. Numerous adaptive consensus policies
need to be designed to handle a massive number of nodes with guaranteed
throughput and security [83]. Outsourcing a wide range of services at self-
organization and edge to attain automation introduces new security issues,
which urge to be studied furthermore. In addition, vibrant research in the
quantum security context seems to be promising.

#### Energy Efficiency and Cost effectiveness

The current consensus mechanisms are greatly resourced intensive and less
efficient, for instance, PoWs consume a great amount of electricity; thus,
make computationally expensive [84]. Note that more power miners need to be
deployed to run heavy algorithms when the dimension of blockchain size is
larger. Proof of trust and DPoS are examples of energy-efficient consensus
algorithms store only the recent transaction data instead of recording the
entire blockchain data. Nevertheless, resource and energy constrained IoT and
wireless nodes are still overwhelmed due to the massive amount of data demand.
However, the design of more reliable and efficient consensus protocols is an
open issue in order to prevent excessive power consumption and cost overhead.

#### Resource Limitations and Spectrum Allocation

The current blockchain deployments demand stable network connections. Smart
IoT devices and wireless nodes are not always ensured reliable network
connectivity, therefore, blockchain implementation is a challenging task under
this scenario. In addition, large network overhead is another challenge for
incorporation into future 6G networks and IIoT applications [85]. Owing to the
limited resource allocations, a degree of decentralization in current
blockchain implementations 6G and IoT networks are also limited. Due to
resource hungry nature of the blockchain system, optimal resource allocation
needs to be ensured between the wireless networks and blockchain. However, the
profit of cellular operators mostly depends on the volume of offered services
instead of the spectrum band leasing option. A limitation of the
implementation of BCT in the next generation 6G mobile systems is still a big
challenge in the near future.

#### Regulation

Since blockchain operates in a distributed way without the intervention of
third-party central bodies, government regulations, industrial and MNO
standards need to be enforced. With the augmentation of BC platforms, the
necessity of timely enforcement of global standards have increased more [14].

#### Machine Learning (ML) enabled BC

Real time data prediction and analysis of industrial IoT nodes are important
for the successful deployment of BC enabled IIoT applications. Machine
learning algorithms can be integrated for data analysis in the node itself to
avoid the data transit for prediction and analysis. Notably, ML empowered BCT
in industrial IoT applications and 6G cellular architectures is an emerging
technology getting deep attention among industry and research communities
owing to tackling privacy violations and security issues by preventing data
movement.

#### Industry 4.0 applications

Digital transformation is a promising solution to elevate industrial
productivity. Addition of more new technologies could lead to more
susceptibility and increasing cyber-attacks intending to manufacture based
industry 4.0. A very few works conducted recently creating a dataset to handle
cyber security threats in the context of IoT under fog/cloud systems using
machine learning and deep learning to develop an adaptive learning framework
to address the aforementioned problem [86, 87, 88]. However, integration of
BCT based new cutting-edge technology for Industry 4.0 and beyond is a
paramount challenge for successful deployment, which is contemplated as a
double-edged sword.

#### AI enabled blockchain

Typically, AI methods depend on a centralized architecture which may lead to
data tampering and huge communication overhead. Consequently, delaying and
erroneous phenomenon incurred while making a decision by AI [89]. Therefore,
decentralized AI is a compelling need to provide reduction of network traffic
congestion and privacy preservation avoiding data uploading collected at edge
equipment to the central data server. Nonetheless, distributed inference and
training still need communications between a central server and distinctive
multiple agents for data sharing. As a result, the model updates may be
disclosed, and it is difficult to assess the contributions of individual agent
and so lacks motivations for data sharing. Distributed AI-based blockchain is
considered as an emerging solution replacing the centralized server where data
sharing is performed autonomously through smart contracts. To unleash the
potentials of two individual disruptive technologies such as artificial
intelligence (AI) and blockchain, it is a promising opportunity to combine
these megatrends to make mutual enhancement and meet the distributed
requirements intelligently. AI brought some potential benefits to blockchain
such as AI can manage blockchain operations efficiently and AI can fully
utilize the encrypted data. The blockchain operation requires a great amount
of computer computation processing power, which makes the system less
efficient. For instance, a brute force approach (i.e., hashing algorithm) for
mining nodes in Bitcoin attempts characters combination until the verification
of the transaction. Moving away from the brute approach, AI enabled smart
contracts provides better performance and desire level of quality [90]. The
information in a blockchain database held in encrypted state offers security,
but experiences challenges for data usage [91]. AI algorithms address this
challenges with further data security and make full use of the BC database.
Research in physical layer security, adaptive channel coding, NFV, network
slicing, SDN employing BCT is still in the initial stage that requires further
investigations. The role of AI with BC to the 6G communications and IoT
architectures will make society more efficient and super smart through solving
more complex problems that are often changing dynamically.

#### Performance enhancement

Blockchain is predominantly resource-hungry in forms of caching, computation,
transmission, and wireless resources particularly at the edge network for new
block generation, validation, ledger storage, consensus policies, etc. BC
normally performs as an overlaid layer to serve the underlaid layer in a large
scale services. Hence, the performance enhancement of BC enabled systems
should be further investigated.

#### Optimization

IoT based applications are largely in heterogenous forms in terms of various
technical requirements, for example, some services require extremely low
latency, while some other services concern only about privacy. Thus,
blockchain protocols needs to be modified with advanced powerful adaptivity to
provide services in 6G networks and IoT systems as well. Notably the
properties of BC such as delay and scalability issues limit its own
performance. Therefore, a novel consensus mechanism aims to be designed to
make the full use of blockchain optimization for enhancing the performance in
terms of throughput, security, time to finality and so on to meet the up-
trending demands.

#### Business Model, Data storage and analytics

Internet of everything (IoE) is more general form than IoT, which connect
users, data, processes, and things seamlessly in an intelligent manner. With
the augmentation of IoE, massive number of things generate real-time data
streams continuously. Consequently, sufficient and efficient data storages are
required, where blockchain plays an important role for decentralized storage
technologies in different domains. However, for the accurate and efficient
decision processing, effective data analyzing research methods are required
within the distributed BC enabled data storage. With the help of digital
technologies, processes are optimized and automated that will re-invent new
business models in variety of industries. With the proliferation of business
agility and velocity in IoE platforms, it is essential to investigate numerous
possibilities of developing modern business models. Furthermore, the impact of
blockchain based technologies intending of interoperability functions among
multiple businesses needs further research. It is readily agreed that 6G will
greatly support sustainable goals (SDGs), which requires mapping and depth
analysis of evaluating KPIs and social impact. Extensive research works need
to be carried out to bring equality in gender, society, and privacy to support
new use cases.

#### Virtualization of Access Network

Although 6G networks are distributed, still an intelligent support are needed
in terms of visualization and SDN. The virtualization in 6G requires to
compatible with all radio access interfaces including quantum radio, smart
surface, THz communications, which is a big obligation to address. The key
concern, will every network node support different vertical requirements such
as wireless resources, cloud computation, latency, caching and QoS
simultaneously?

#### Edge Computing and Mobility

In general, satellite networks generate a significant delay during vertical
transmission under aerial, terrestrial and space domains. However, high
altitude platforms (HAP) in aerial access networks offer edge caching and data
computing facilities minimizing the computations and delay of satellite
networks providing critical services. Therefore, edge caching demand further
robust research in terms of resource allocation, localization, efficient
computing, power efficiency, etc.

A cell-free communication needs precise synchronization and localization in
heterogeneous networks (HetNets) with beamforming, which requires further
investigation. It is infeasible to ensure high precision beamforming when
dealing with high mobility. Hence, location aware mobility tracking for
beamforming looks promising. An efficient integrated protocol design is
necessary to maintain communication and localization during the mobility among
different access networks.

#### Sustainable 6G Ecosystem

Shareable resources are the key assets that are not restricted to inter-domain
communication but spread in computing and energy sectors as well, while the
communication section also depends on computing resource and energy provision
as illustrated in Fig. 4. Nevertheless, a trusted and secured blockchain based
trading ecosystem considering communication, computing, and energy can built
an efficient sustainable 6G. A distributed energy management is a microgrid
system especially in remote areas, where distributed generation (DG) units,
for example, PV solar and wind firms are the best example of a sustainable
ecosystem approach through distributing electricity to remote localities. A
locally generated distributed energy broke with local providers is a cost-
effective approach of trading energy and power line communications instead of
connected to the central grid [92]. The ecosystem is nurtured while the
computing and communication relay services for DG in exchange of empowering up
the hardware. In terms of the security and performance measurements, a public
chain is more preferable to inter-domain blockchain transactions between
operators like first tier MNOs and the national grid. However, the private
chain can be contemplated for locally oriented resources particularly for IoT
and off-grid nodes in remote regions [38]. To achieve the best results, an
ecosystem may add numerous consensuses on different access chains. However,
the sustainable business ecosystem is not bounded to the scope of
communication, computing, energy, rather it can spread itself to augmented
range via cross-field integration, such as financial, automotive, logistics
chain, manufacturing, and so on. Beyond the implementation of blockchain,
hardware deployment plays a crucial role in the ecosystem to realize portable
mobile device solutions like cars, portable IoT devices, and drones.

#### Future Applications

Autonomous Driving and V2V communications:

Intelligent vehicle to vehicle (V2V) transportation systems is one of the
emerging applications in near future supported technical capabilities under 6G
network services. A BC empowered approach develops trust management among
smart vehicles, but the limitation of contemplating to adhoc network is the
shortcoming. Autonomous driving in 6G offloading from connected autonomous
vehicles (CAVs) to edge servers via space networks is demonstrated in
Reference [93]. The implementation of blockchain can store sensitive
information for CAVs and audit the offloading steps, which guarantees the
security of the system. Autonomous deployment including mobility challenges
such as multi junction road intersection requires further investigation.

UAV and Drone:

In recent years, unmanned area vehicles (UAVs) and drones are popularly
deployed for civilian usage, diverging from conventional military
applications. The incorporation of blockchain in drone industry helps to
identity management, air traffic management efficiently with a greater level
of accuracy and security [94]. Integration of blockchain to UAV networks are
the potential areas for future exploration particularly in detection of
compromised UAVs under surveillance application. If a compromised UAVs sending
false information (detecting an intruder) will not be able to join in the
blockchain with the help of smart contracts and suitable consensus algorithms.

Remote Learning:

A blockchain enabled platform could record educational databases in a
transparent and independent way, thereby, educational authority easily
accesses secure records and transcripts. Besides, it also facilitates secure
collaboration among universities and other institutions. The inherent
characteristics of blockchain allow academic bodies to check educational
dishonesty [14]. The addition of BC in educational sectors also prevents
fraudulent activities and tampering via locating transaction records and
verifying the key.

Power Sectors:

With the surge of smart IoT devices ranging from smartphones to smart meters
to smart electric vehicles exploiting variable power demands, the electricity
grid is becoming more complex to handle with mushrooming distinctive types of
power generation. Enabling smart contracts among different devices and
components in the smart grid infrastructure provides optimized grid operations
and accelerates global energy transformation with the least transaction costs
[95]. Blockchain induces consumers to moves toward prosumers (who consumes and
produce simultaneously) facilitating them to save electricity cost by selling
abundant power to the grid with autonomous safe recording information through
smart contracts [96].

Agriculture Field:

The agriculture sectors are largely depending on several external factors such
as climate, water supply availability, reap quality, complex supply chains,
etc. This complexity can be addressed transparently using blockchain
technology that could ease the path from the agricultural farm to the
supermarket, giving buyers a sense of control and security [97]. In addition,
BCT permits small business farms with low assets, can easily access a similar
set of data likewise rich farms which making the trading market more
transparent.

Retail and E-commerce:

E-commerce based IoT deployment is the emerging business sector which demands
the transactions to be autonomous, secure, and lightweight [98]. The necessity
of blockchain in retail industry can be illustrated in several steps of
product tracking. A detail of production including time, place, quantity from
the manufacturer is listed in the blockchain. Then, information of shipped
products is properly logged onto the BC database during transit and data is
then updated at the warehouse end with logistic details. Thereafter,
wholesaler and retailer dealers update the received product information in the
BC. Finally, consumers make anonymous feedback on the products that are
recorded in the blockchain. This set of complete procedure brings a good
reputation for companies and downsize the overall computational overhead [99].
BCT essentially eliminate some difficulties experienced in the e-commerce and
retail business by providing trust, values, and incentive to the consumers.
Such business sectors demand more robust research in automated and secured
transactions in blockchain platform.

Supply Chains:

BCT can considerably reduce time delays and human slip-ups and help to verify
the product authentication by tracing their origin. Data accessibility and
immutability aspects of blockchain increase the reliability, efficiency, and
transparency of the whole supply chain industry [100]. Everyday traders,
merchants, shippers, and other logistic support personnel handle plenty of
choices while shipping a massive number of products across worldwide, each
progression of the journey requires extensive paperwork and communication
management. In spite of technological advancement, logistics management still
needs improvement for keeping and tracking all the records transparently.
Blockchain solves these complexity and fragmented tasks by tracking resource,
recording exchange, developing effective product management framework for
dealing with all archives involved with the logistics procedure [101].

Healthcare:

In the healthcare industry, the data for critical patients can be shared
effectively using blockchain, which can eliminate errors in patient care and
potentially enhance healthcare services [102]. Interoperability feature
offered by blockchain provides the stakeholders (e.g., healthcare providers,
doctors) to access the patient’s health data with proper authentication in a
secured manner. Moreover, BC driven IoT system accelerate the diseases
management and monitoring facilities, for example, smart pills taking,
tracking vital signs, condition investigation remotely, side effects alarm,
and providing feedback using smart wearable devices, and quality control
[103]. The fundamental challenge of maintaining patients’ records is that
leaving the health-related information scattered across different healthcare
providers. Electronic health records cannot ensure to manage lifelong medical
records, accessible across various health service providers. The lack of data
management and coordination among medical service providers create additional
obstacles for accessing the records. Blockchain helps in providing a secured
data sharing structure where any entity can gather all the information of
respective patients including their personal details, medicare, medical
history, etc.

###  VI-A Summary and Lessons Learned

The integration of blockchain technology (BCT) into IoT and 6G networks have
clearly demonstrated a great potential. The fundamental features of BCT
include transparency, immutability, security, anonymity, and the least
processing time. BCT does not need to involve a trusted third party or any
central intermediary authority in the BC architecture and hence, it minimizes
the processing time and cost while performing P2P transactions. Shifting the
blockchain paradigm toward a distributed and digitized economy allows smart
living with a greater level of integrity in the future. The use of blockchain
technology in next generation ultra high-speed 6G networks and massive IoT
systems attain the expected level of services and ultra-high performances
steering to a positive feedback loop. BCT reduces manmade errors where users’
identity and device are authenticated prior to obtaining access to the
network. Besides, each participant has direct control of its own information,
and all participated nodes are authenticated, thereby, BC technology enhanced
public trust. Blockchain enabled 6G will support a user-friendly environment
of intelligent big data analysis for sustainable high-end products and
services to address specialized demands by the consumers. A shared BC ledger
marketplace can be employed in the asset transactions among multiple peers for
network resource sharing, roaming, neutral hosting, network slices, micro-
operators, local licensing, software defined networking (SDN), and network
function virtualization (NFV) based networking and specialized services. In
the case of industry 4.0 and industrial IoT applications, BC empowered
sustainable business models will provide extreme reliability, privacy-aware
secure communications, zero touch assistance with the advent of green
networking, zero carbon technologies. Additionally, blockchain empowered
decentralized architectures offers excellent solution in 6G business models in
terms of service agility, orchestration, open collaboration, and zero touch
management. Moreover, BC based 6G business models offer cloud storage for over
the top (OTT) companies, cognitive services such as context aware operations,
AI, UAV as a service together with calling and data connectivity features,
envisioned to attract customers and increase their profit. Furthermore,
blockchain enabled distributed IoT systems address the challenges of
conventional IoT architectures by means of advanced efficacy,
interoperability, network management, privacy, and security vulnerabilities,
particularly in the high degree of complex data handling.

Despite the numerous potential advantages of BC enabled 6G communications and
IoT system, the considered architecture is suffering distinct types of
shortcomings for implementation. Prior joining to the blockchain, a new block
is validated and agreed upon by the active participants in a transparent way.
All the participant node requires the witness of prior data for the validation
purpose. Moreover, the immutability feature of blockchain restricts the
modification and deletion of confirmed transactions. BCT supports distributed
applications avoiding single point failure and develop augmentation of trust.
As a consequence, blockchain is broadly regarded as a trusted technology in
the future generation, but it suffers privacy issues. The appropriate
consensus algorithms with asymmetric encryption overcome the challenges of
security and privacy aspects. Smart contracts in blockchain allow software
defined contracts among users as transactions and hence, the flexible smart
contracts introduce multiple distinctive new attacks in the network. However,
different cryptographic algorithms such as ring architecture, zero knowledge
proof, coin mixing can be used for privacy and security. The reliability of
transactions can be achieved using asymmetric encryption mechanisms, hash
inclusions and digital signatures. The data communications among multiple
operational platforms are paramount concerns in BC enabled cross layer 6G and
IoT architectures. Ensuring the least level of latency is another key
challenge for BC based high speed system implementation. The communication
overhead and computational complexity introduced by blockchain may impose a
great challenge for the deployment of billions number of smart devices and
corresponding huge transactions experienced by 6G networks and IoT systems.In
addition, the energy consumption of IoT devices, least computational power,
low storage capacity, lack of policies for data security, and compatibility
with 6G are some others research challenges IoT enabled 6G devices. Majority
studies related to BC based 6G networks and massive IoT systems are not
experimentally demonstrated, provide theoretical analysis with some simulation
works. BoT and BC-6G system cover a wide area ranging from individual,
industry to worldwide business models. Multiple studies have discussed
numerous solutions to resolve distinctive technical issues, however, many
aspects remain unexplored.

##  VII Conclusions

With the rapid pace of a massive number of IoT devices, a number of obstacles
incur to downsize the empowering IoT across various applications. IoT device
owners and telecom operators must trust the intermediary node under the
centralized architectures. Blockchain technology is an emergent distributed
guarantor in terms of a cost-effective solution, nonrepudiation, convergence
between embedded technologies, and exploiting traceability to verify the
unauthorized action affecting the sustainability charter. This survey paper
explored the recent research works directed on the applications of BC in 6G
networks and multiple potentials in industrial IoT areas from the perspective
of different commercial implementations. We provide an overview of the
blockchain features, potential, drivers, requirements for massive IoT enabled
by 6G addressing numerous challenges such as integrity of personal data
protection, data secrecy, scalability of block contents, adoption cost,
privacy of other entities, and governmental regulations. Then a discussion on
core technical elements of blockchain including accessing strategies,
extensive comparison of consensus algorithms to fit the desired applications
are carried out. A sustainable ecosystem focused business model driven by
blockchain empowered 6G networks is thoroughly analyzed to address the
contemporary global economic crisis. In addition to support in economic
performance, a smart contract regard as a core component of BCT that curtail
the negative external aspects during manufacturing, enhance the quality of
life, promote employability, uplift societal and environmental performances
under a wide range of industrial applications. The use of blockchain in IoT
industries and 6G use case applications are expected to grow and benefit of
smart living and industry sectors mitigating security threats and provide
integrity solutions. Blockchain deployment is not without limitations, a lot
of challenges lay ahead, and hence, necessary care must be taken to deal with
a number of BC challenges to maintain desired QoS, performance level, and
adequate security. We listed the research challenges and future directions,
outlined numerous possible solutions to address the challenges. It is
envisioning that AI, ML, intelligent surfaces, cybertwin, cell-free
architecture will likely become promising candidates in future blockchain
implementations. This survey paper provides valuable resource for clear
understanding the contemporary research contribution of blockchain technology
in different implications and is anticipated to persuade further effort for
the imminent deployment of next generation wireless networks and industries in
the forthcoming years.

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