This invention relates to communication devices and more particularly to communication devices that receive multiple replicas, sent from different transmitters, of the same information.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS standard. This application focuses on WCDMA systems for economy of explanation, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (user equipment or users), respectively, in the downlink (base-to-user equipment) direction. User Equipment (UE) communicates with the system through, for example, respective dedicated physical channels (DPCHs). WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a mobile radio cellular telecommunication system 100, which may be, for example, a CDMA or a WCDMA communication system. Radio network controllers (RNCs) 112, 114 control various radio network functions including for example radio access bearer setup, diversity handover, and the like. More generally, each RNC directs UE calls via the appropriate base station(s) (BSs), which communicate with each other through downlink (i.e., base-to-UE or forward) and uplink (i.e., UE-to-base or reverse) channels. RNC 112 is shown coupled to BSs 116, 118, 120, and RNC 114 is shown coupled to BSs 122, 124, 126. Each BS serves a geographical area that can be divided into one or more cell(s). BS 126 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 126. The BSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, and the like. Both RNCs 112, 114 are connected with external networks such as the public switched telephone network (PSTN), the Internet, and the like through one or more core network nodes like a mobile switching center (not shown) and/or a packet radio service node (not shown). In FIG. 1, UEs 128, 130 are shown communicating with plural base stations: UE 128 communicates with BSs 116, 118, 120, and UE 130 communicates with BSs 120, 122. A control link between RNCs 112, 114 permits diversity communications to/from UE 130 via BSs 120, 122.
At the UE, the modulated carrier signal (Layer 1) is processed to produce an estimate of the original information data stream intended for the receiver. The composite received baseband spread signal is commonly provided to a RAKE processor that includes a number of “fingers”, or de-spreaders, that are each assigned to respective ones of selected components, such as multipath echoes or streams from different base stations, in the received signal. Each finger combines a received component with the scrambling sequence and the appropriate channelization code so as to de-spread a component of the received composite signal. The RAKE processor typically de-spreads both sent information data and pilot or training symbols that are included in the composite signal.
FIG. 2 is a block diagram of a receiver 200, such as a UE in a WCDMA communication system, that receives radio signals through an antenna 202 and down-converts and samples the received signals in a front-end receiver (Fe RX) 204. The output samples are fed from Fe RX 204 to a RAKE combiner and channel estimator 206 that de-spreads the received data including the pilot channel, estimates the impulse response of the radio channel, and de-spreads and combines received echoes of the received data and control symbols. An output of the combiner/estimator 206 is provided to a symbol detector 208 that produces information that is further processed as appropriate for the particular communication system. RAKE combining and channel estimation are well known in the art.
A Multimedia Broadcast/Multicast Service (MBMS) for the frequency division duplex (FDD) aspect of the WCDMA system is being standardized by 3GPP. MBMS is described in 3GPP Technical Specification TS23.246 ver. 6.2.0 Technical Specification Group Services and System Aspects; Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description (Release 6) (April 2003) and Technical Report TR23.846 ver. 6.1.0 Technical Specification Group Services and System Aspects; Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description (Release 6) (December 2002), among other places.
MBMS is intended to offer high-speed and high-quality broadcast, or multicast, transmission to mobile stations (UEs). For example, MBMS can offer the end user a selection of movies to watch. Broadcast and multicast are synonyms for point-to-multipoint communication where data packets are simultaneously transmitted from a single source to multiple destinations. The term “broadcast” refers to the ability to deliver content to all users, whereas the term “multicast” refers to services that are solely delivered to users who have joined a particular multicast group.
M. Bakhuizen and U. Horn, “Mobile broadcast/multicast in mobile networks,” Ericsson Review, Issue 1 (2005) provides an overview of MBMS. There it is described how, for example in WCDMA, MBMS reuses existing logical and physical channels to the greatest possible extent. In particular, the implementation in WCDMA requires only three new logical channels and one new physical channel. The new logical channels are:                MBMS point-to-multipoint control channel (MCCH), which contains details (e.g., spreading factors) concerning ongoing and upcoming MBMS sessions, and is sent repetitively within a period of 480 ms or 1.28 s;        MBMS point-to-multipoint scheduling channel (MSCH), which provides information (e.g., a program guide) on data scheduled on MTCH; and        MBMS point-to-multipoint traffic channel (MTCH), which carries the actual MBMS application data (e.g., movie content).        
The new physical channel is the MBMS notification indicator channel (MICH) by which the network informs UEs of available MBMS information on MCCH. MCCH, MSCH and MTCH reuse the forward access channel (FACH) transport and secondary common control physical channel (S-CCPCH) in WCDMA. The Radio Link Control (RLC) and Medium Access Control (MAC) layers reuse much of the existing protocol stacks.
Since MBMS is a broadcasting service, the same physical channel may simultaneously be received by multiple UEs (e.g., mobile handsets or other mobile equipment). Consequently MBMS is not subject to power control. With no power control, other ways need to be found to guarantee quality of service. Accordingly, to enhance the quality and bit rate of the MBMS transmission, it has been agreed in 3GPP to use large interleaving depths, that is, large Transmission Time Intervals (TTIs) (each TTI includes one transport block and has a length of three slots), to obtain interleaving gain. It has also been agreed to use multicast on Layer 1, that is, the UE should be able to receive multiple replicas of the same bitstream from different base stations, each of which is a Node B in 3GPP vocabulary. In particular, it is proposed that several clusters should send the same information to get space diversity. As used herein, the term “cluster” means a number of radio links (a number of cells) that are aligned in time. The transmission timing of the cluster determines how to combine the sent data in order to yield an improved reception of the information. Three different combining methods are possible:    1) RAKE combining: when all clusters' transmissions are within a certain period of time, for example 296 chips, of one another, RAKE combining (as is done for DPCH) can be performed.    2) Soft combining: when all clusters' transmissions occur within a time span measured as the duration of a TTI plus the duration of one slot (herein denoted “TTI+one slot”), a soft buffer should be maintained, in which the combiner output symbols are added for each cluster. Soft combining is well known in the art, and is used by some kinds of turbo decoders and by hybrid automatic repeat request (HARQ) in high-speed downlink packet access (HSDPA) which is an evolution of WCDMA communication systems.    3) Selection combining: when a cluster's transmission is time misaligned by more than a TTI+one slot from another cluster's transmission, then full decoding of each cluster's sent transport block is performed, and one of the decoded transport blocks is then selected based on, for example, whether the decoded transport block passes a cyclic redundancy check (CRC). Selective combining is well known in the art.
Reception from at most three clusters is supported. This implies at most three S-CCPCHs to receive MTCH data (no multi-code). One of the clusters contains the cell that controls the MBMS transmission. This cell is referred to as the controlling cell, and the other cells transmitting the MTCH are referred to as the neighboring cells. A FACH or dedicated channel (DCH) can be transmitted at the same time as the MTCH.
The UE is usually limited in the number of despreaders it can allocate for demodulating sent data. When the UE is supposed to receive from multiple clusters, the total number of cells contained in all clusters could be larger than the number of available despreaders in the UE. The UE therefore needs to select the best cells to use. The cells whose MTCHs are to be combined are not signaled to the UE; rather, it is up to the UE to decide which cells to use.
U.S. Patent Application Publication US 2004/0081125 A1 discloses a selection diversity strategy based on a comparison of pilot signals on a common pilot channel (CPICH), preferably the primary-CPICH (P-CPICH). However, the power of a signal transmitted on a CPICH may not be a good indicator of the power of a signal transmitted on the MTCH, which is actually transporting the MBMS application data. One reason for this is, for example, that no power control is used on the S-CCPCH.
Thus, it is desired to provide a mechanism for selecting which cells to use when receiving MBMS application data.