RQ precoding for the cooperative broadcast channel

We consider a broadcast channel with multiple transmit antennas, where receiving terminals (users), with single antenna, can cooperate by exchanging information through a secondary local channel. Similar to zero-forcing dirty paper coding (ZF-DPC) in, we consider a RQ precoding, which is based on the RQ decomposition of the channel matrix and transforms the channel matrix into an upper triangular matrix. Without DPC, the RQ precoding is used in conjunction with successive interference cancellation by the users, which send to each other their respectively detected information symbols. We conduct error probability performance analysis for the RQ precoded cooperative broadcast channel and show how errors on the local link degrade the overall performance. Finally we compare the RQ precoding to standard beamforming precoding where the users do not cooperate and observe: (i) improvements in the overall error rates and throughput, (ii) different diversity gains for the different users, (iii) performance degradation resulting in an error floor due to errors on the local channels. This last feature enables either the application of RQ precoding to a system with different quality of service users or the possibility of scheduling the users in order to improve the overall error rates and throughput.