1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to the reporting of Power Headroom (PH) from a User Equipment (UE) in a wireless communication system that supports carrier aggregation.
2. Description of the Related Art
Mobile communication systems were originally designed to provide users with voice communication services while they are on the move. Current mobile communication systems are capable of supporting both voice communication services and data communication services for mobile users.
Standardization for a next generation of mobile communication technology for the 3rd Generation Partnership Project (3GPP) is being conducted for Long Term Evolution (LTE). LTE is a broadband packet-based communication technology that is expected to provide download speeds that improve upon existing data transmission rates by up to 100 Megabytes/second (Mbps). In attempting to achieve such a high data rate, studies have been conducted that use a minimum number of nodes in connection with a simplified network topology, and that place a radio protocol as close as possible to radio channels.
FIG. 1 is a diagram illustrating an LTE wireless communication system. The LTE wireless communication system includes a plurality of Evolved Node Bs (ENBs) 105, 110, 115 and 120, a Mobility Management Entity (MME) 125, and a Serving Gateway (S-GW) 130. ENBs 105, 110, 115 and 120 are coupled to the S-GW 130, enabling a UE 135 to connect to a core network. The ENBs 105, 110, 115 and 120 correspond to Node Bs of a Universal Mobile Telecommunications System (UMTS) and perform more complex functions than those of a legacy Node B. In the LTE system, all user traffic, including real time services such as Voice over Internet Protocol (VoIP), are provided through a shared channel. Each of the ENBs 105, 110, 115 and 120 manage one or more cells, and are responsible for the collection of status information from UEs and for the scheduling of traffic.
In order to support transmission bandwidths of up to 20 megahertz (MHz), LTE employs Orthogonal Frequency Division Multiplexing (OFDM) as its basic modulation scheme. LTE also uses Adaptive Modulation and Coding (AMC) to improve data throughput. AMC varies downlink modulation and coding schemes based on channel conditions for each UE. The S-GW 130 is responsible for managing data bearers and establishes or releases data bearers under the control of the MME 125. The MME 125 is in communication with the S-GW 130 and is responsible for control plane functions.
FIG. 2 is a diagram illustrating a user plane protocol stack for use in the LTE architecture of FIG. 1. A mobile terminal, or UE, 200 has a protocol stack having a Packet Data Convergence Protocol (PDCP) layer 205, a Radio Link Control (RLC) layer 210, a Media Access Control (MAC) layer 215, and a Physical (PHY) layer 220. A base station, or ENB, 201 has a protocol stack having a PDCP layer 240, an RLC layer 235, a MAC layer 230, and a PHY layer 225. The PDCP layers 205 and 240 are responsible for Internet Protocol (IP) header compression/decompression. The RLC layers 210 and 235 pack the PDCP Packet Data Units (PDUs) into a size appropriate for transmission and perform an Automatic Repeat reQuest (ARQ) function. The MAC layers 215 and 230 serve multiple RLC layer entities. These layers are capable of multiplexing the RLC PDUs into a MAC PDU, and demultiplexing the MAC PDU into the RLC PDUs. The PHY layers 220 and 225 perform encoding and modulation on upper layer data for transmission through a radio channel, and perform demodulation and decoding on the OFDM symbol received through the radio channel for delivery to upper layers. A data unit that is input to a protocol entity is referred to as a Service Data Unit (SDU) and a data unit that is output from the protocol entity is referred to as a Protocol Data Unit.
A voice communication service of a wireless communication system requires a relatively small amount of dedicated bandwidth. However, a data communication service must allocate resources in consideration of a data amount and a channel condition so that transmission throughput may increase. Thus, a mobile communication system is provided with a scheduler that manages resource allocation with respect to available resources, channel conditions, an amount of transmission data, etc. Resource scheduling is also required in LTE, and a scheduler that is incorporated into a base station, or ENB, is used to manage radio transmission resources.
In order to meet International Mobile Telephony (IMT)-Advanced requirements that extend beyond those of IMT-2000, further technological advancements have allowed for the evolution of LTE into LTE-Advanced (LTE-A). LTE-A is provided with technological components, such as carrier aggregation, to fulfill the IMT-Advanced requirements. Carrier aggregation aggregates multiple carriers to form a larger bandwidth, thereby allowing a UE to transmit and receive data at higher data rates.
FIG. 3 is a schematic diagram illustrating an LTE-A wireless communication system supporting carrier aggregation. An ENB 305 operates on two different carriers 310 and 315, having center frequencies of f3 and f1, respectively. A conventional wireless communication system allows a UE 330 to communicate with the ENB 305 using only one of carriers 310 and 315. However, the LTE-A system supporting carrier aggregation enables the UE 330 to use both carriers 310 and 315 in order to increase transmission throughput. The maximum data rate between the ENB 305 and the UE 330 increases in proportion to the number of carriers that are aggregated.
Due to the fact that uplink transmissions cause inter-cell interference, it is preferable for a UE to calculate an uplink transmission power using a predetermined function, and to control uplink transmission based on the calculation. The predetermined function may utilize variables such as an allocated transmission resource amount, a Modulation and Coding Scheme (MCS), and a path loss value in calculating a required uplink transmission power. The uplink transmission power is limited to a UE maximum transmission power. When the required uplink transmission power is greater than the UE maximum transmission power, the UE performs the uplink transmission using the UE maximum transmission power. However, use of the maximum transmission power instead of the required transmission power degrades the uplink transmission quality. Thus, it is preferable for the ENB to perform scheduling for UE transmissions such that a required transmission power for the UE transmission will not exceed the UE maximum transmission power.
Some parameters utilized in scheduling at the ENB, such as channel path loss, are not capable of being measured at the ENB. When required, the UE may transmit a Power Headroom Report (PHR) to the ENB to report UE Power Headroom (PH) with respect to path loss. However, conventional uplink transmission power determination procedures are performed with respect to a single downlink carrier and a single uplink carrier. Thus, the conventional procedures are not applicable to the LTE-A system supporting carrier aggregation.