COOPERATIVE MIMO-BEAMFORMING FOR MULTIUSER RELAY NETWORKS

INTRODUCTION
Cooperative relaying systems have received considerable attention in the last decade, see  and the references therein. The basic idea of cooperative relaying is to introduce intermediate nodes (relays) that forward the received data from the source to the destination. Cooperative relaying brings a large number of advantages to wireless communication systems. For example, it provides spatial diversity since the relay terminals form a distributed antenna array. This diversity can be further exploited by applying distributed space-time coding. Another advantage of cooperative relaying is the increase in the range of communication which can be further extended via beamforming. Cooperative relaying can also be used to provide spatial multiplexing in multiuser communication scenarios where multiple signal sources are targeting one or more destination nodes. A multiuser relaying scheme called multiuser zero forcing relaying was proposed. This relaying technique uses beamforming to eliminate the interference between different source and destination pairs. Zero forcing relaying requires full knowledge of the channels from the sources to the relays and from the relays to the destination nodes. 

 
This channel information can be obtained using orthogonal pilot sequences broadcasted from the source and destination nodes to all the relay terminals. The channel estimates are then transmitted to a processing center that computes the beamforming coef��cients and feeds them back to the relay terminals. It was shown that zero forcing relaying can greatly increase the average sum rate of the system (measured in bits/channel use) compared to the single source, single relay, single destination case. However, zero forcing beamforming is known to be suboptimal when the signal-to-noise ratio (SNR) of the sources is relatively low as it results in increased noise power at the destination nodes. Also, the zero forcing algorithm proposed was derived under the assumption that each source signal is targeting a distinct destination node, and hence, it cannot handle the more complicated multiplexing case where multiple sources are targeting the same destination node. Finally, this algorithm is only applicable to relaying schemes with single antenna relays and cannot exploit the performance gains due to multiple antennas at the relay terminals.

 
In this project, we develop a multiuser beamforming algorithm for relay networks with multiple antennas at the relays. Our algorithm is derived under the same assumptions as those i.e., the channels between the relay terminals and different source and destination nodes are known with enough accuracy. We design the beamforming matrix such that both the noise received at each destination node and the interference caused by the sources not targeting this node are minimized. We also impose linear constraints that preserve each source signal at its targeted destination. The resulting optimization problem is shown to be convex and is formulated as a second order cone program (SOCP) that can be effciently solved with polynomial complexity using interior point methods.We provide numerical simulations showing the superior performance of our beamforming technique compared to zero forcing beamforming in terms of the received signal-to-interference-plus-noise ratio (SINR) and symbol error rate (SER) when each relay is equipped with a single antenna. Simulation results also indicate that the use of multiple antennas at the relay terminals can significantly improve the system performance compared to single antenna relaying. This can be attributed to the additional degrees of freedom available for beamforming due to the block diagonal structure of the stacked beamforming matrix. 

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