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Joint beamforming and broadcasting in Massive MIMO

The downlink of a massive MIMO system is considered for the case in which the base station must concurrently serve two categories of terminals: one group to which imperfect instantaneous channel state information (CSI) is available, and one group to which no CSI is available. Motivating applications include broadcasting of public channels and control information in wireless networks. | Joint Beamforming and Broadcasting in Massive MIMO Erik G. Larsson and H. Vincent Poor arXiv 1602.03648v2 cs.IT 14 Mar 2016 Abstract The downlink of a massive MIMO system is considered for the case in which the base station must concurrently serve two categories of terminals one group to which imperfect instantaneous channel state information CSI is available and one group to which no CSI is available. Motivating applications include broadcasting of public channels and control information in wireless networks. A new technique is developed and analyzed joint beamforming and broadcasting JBB by which the base station beamforms to the group of terminals to which CSI is available and broadcasts to the other group of terminals to which no CSI is available. The broadcast information does not interfere with the beamforming as it is placed in the nullspace of the channel matrix collectively seen by the terminals targeted by the beamforming. JBB is compared to orthogonal access OA by which the base station partitions the time-frequency resources into two disjunct parts one for each group of terminals. It is shown that JBB can substantially outperform OA in terms of required total radiated power for given rate targets. I. INTRODUCTION Massive MIMO 1 is a leading technology candidate for 5G wireless access. The main concept is that hundreds of base station antennas act phase-coherently together and serve tens of terminals in the same time-frequency resource. Different base stations however do not cooperate. A fundamental assumption in massive MIMO is that the base station antenna array can acquire instantaneous channel state information CSI to the terminals so that closed-loop beamforming can be applied. This is possible by operating in time-division duplex TDD mode with the base station acquiring CSI from uplink pilots and relying on reciprocity of the propagation channel. In wireless networks the base station will also need to broadcast1 information to terminals to which it