| Inter-beam interference is a problem that afflicts conjugate beamforming systems, which are being considered for massive MIMO downlink systems. To deal with inter-beam interference, we propose allowing users to act as mutual relays over orthogonal side-channels to enable V-BLAST for single-antenna users in a system called cooperative inter-beam nulling and cancellation (CINC). In addition to CINC, we also propose systems with only individual components of V-BLAST are enabled over side-channels. For the decorrelator portion of V-BLAST, we propose cooperative inter-beam nulling (CIN), and for the successive interference cancellation portion of V-BLAST, we propose cooperative inter-beam cancellation (CIC).;The key difference between V-BLAST in a multiple antenna receiver and V-BLAST in single antenna users over side-channels is that side-channel resources are limited in power budget and spectrum usage. Therefore, we study how the proposed strategies perform when the number of mutual relayers is limited by side-channel SNR and available side-channel time-bandwidth. We compute the user's effective SINR for each method in Rayleigh channels, then perform numerical simulations to find the achievable rate given side-channel resources.;From our results, we characterize the improvement by showing the percentage increase in sum-rate that can be achieved by each method for different amounts of time-bandwidth resources. In our simulation scenario with Rayleigh channels, 100 base station antennas, and 8 users, we show that CINC, the full V-BLAST over side-channel system, can achieve 21 % average sum-rate improvement with time-bandwidth limited to 3 times the amount available on the main channel, and 139 % with 12 times the time-bandwidth, where there is full cooperation.;We then use our numerical simulations to outline different regimes of side-channel resources where the amplify-and-forward relayers of CINC should be prioritized and where the decode-and-forward relayers of CINC should be prioritized. We find that the amplify-and-forward relaying should be prioritized in general, except when side-channel SNR is less than 5 dB and when side-channel SNR is greater than 20 dB, but time-bandwidth resources on the side-channel are less than the number of users times the amount available on the main channel, where decode-and-forward relaying should be prioritized. |
