We propose a beamforming technique for the multicell downlink of a multicarrier code division multiple access (MC-CDMA) system with sparse signatures. We propose a distributed beamforming algorithm using the sum-product algorithm. The distributed beamforming algorithm converges very quickly to the centralized beamforming solution, minimizing the delay associated with computation of the transmit vector. The complexity of distributed beamforming depends on the number of base stations (BSs) that are in range of each user, not on the size of the entire network.

The performance of optimum combining (OC) in decode-and forward (DF) relaying in the presence of a Poisson field of interferers is analyzed. An approximation for the outage probability of the OC receiver is derived by considering the temporal correlation of the interference. Despite the randomness of the number and the locations of the interferers, the OC receiver suppresses the interference at the destination node and diversity gains are achieved, provided that the relay nodes are noise limited. For single-antenna relay nodes, the end-to-end performance is vulnerable to interference at the relay nodes which is not canceled, resulting in zero diversity gains.

The performance of optimum combining (OC) in amplify-and-forward (AF) relaying systems in the presence of co-channel interference (CCI) is analyzed using a tight approximation for the signal-to-interference-plus-noise ratio (SINR) at the destination. Two types of relaying protocols are studied, all-relay transmission with OC and best-relay transmission with OC. When CCI is present only at the destination, both relaying protocols achieve diversity gains up to M when interferer powers are scaled with the source and the relay powers, where M is the number of relays. Best-relay transmission with OC maximizes the spectral efficiency as each communication consumes only two time slots.

Spatial point processes are commonly used to model the placement and the number of interferers in modern wireless networks, where the ad hoc deployment of transmitters is common. The homogeneous Poisson point process (PPP) is the most popular spatial point process used to model co-channel interference. Optimum combining (OC) is the diversity combining technique that maximizes the signal-to-interference-plus-noise ratio at the receiver. The performance of OC in cooperative relaying in an interferer field modeled by a homogeneous PPP is analyzed. Both decode-and-forward (DF) and amplify-and-forward (AF) relay protocols are studied. Multirelay transmission and relay selection techniques are considered. Accurate approximations for the outage probability are derived for DF and AF relaying when the destination is able to estimate the noise-plus-interference correlation matrix (NICM) perfectly. An approximation for the outage probability of DF relaying is obtained when the destination only estimates the channel state information of the closest interferer. Relay selection outperforms multirelay transmission in both DF and AF relaying protocols. The interference correlation at the relays significantly degrades the outage performance. Limited estimation of the NICM results in better performance than conventional maximal-ratio combining, although it fails to achieve diversity gains.

The diversity gains of cooperative relay networks are degraded in the presence of co-channel interference (CCI), which is the principal limiting factor in a properly planned cellular network. Optimum combining (OC) can be used to mitigate the adverse effects of CCI, which enables achieving diversity gains when CCI is present. The performance of OC in a channel state information (CSI) assisted amplify-and-forward (AF) relay network is analyzed when the destination node is affected by CCI, with the aid of a tight approximation for the signal-to-interference-plus-noise-ratio (SINR) at the destination node. Closed-form expressions are derived for the outage probability and the moment generating function for the approximated SINR. It is proved that OC results in a diversity gain of M, where M is the number of relay nodes. OC shows significant performance improvements over maximal-ratio combining (MRC), which reaches error floors at low-to-medium power levels.

Cooperative relaying achieves a wide range of advantages including diversity gain, coverage extension and mitigation of shadowing. Nevertheless, the performance advantages of cooperative relaying diminish considerably if co-channel interference is present. Optimum combining (OC) can be used to mitigate the adverse effects of co-channel interference in wireless communications. The performance of optimum combining in a decode-and-forward relay network with N equal-power interferers is analyzed. The probability density function of the output signal-to-interference-plus-noise ratio is obtained and a closed-form expression for the exact outage probability is derived for N ≥M+1 where M is the number of relay nodes. An approximation for the symbol error rate (SER) is presented. The performance results and closed-form expressions for outage probability and for SER suggest that the asymptotic diversity gain of OC is equal to M, which is a significant improvement over maximal-ratio combining, whose asymptotic diversity gain is equal to zero when operating in co-channel interference.