Millimeter Wave SDMA With Limited Feedback: RF-Only Beamforming Can Outperform Hybrid Beamforming

Millimeter wave (mmWave) beamforming (BF) system can increase the spatial capacity by being installed in hotspot areas where a number of users exist. However, it is hard to obtain the channel state information of a large array system at transmitter. Even with the codebook-based BF scheme, the digital-analog hybrid BF structure may require a two-step feedback procedure. In order to reduce the feedback and feedforward overhead required for joint user scheduling and hybrid BF, we propose a codebook-based radio frequency (RF) only BF scheme with one-step feedback procedure. We show that the achievable sum rate of the RF-only BF scheme double-logarithmically scales with the number of users in mmWave channel model, which coincides with the results for the digital BF schemes in rich scattering channel model. We also show that the RF-only BF scheme can outperform the hybrid BF scheme with limited feedback. In addition, we examine the error performance in extremely sparse channel, and find the number of transmitted beams to achieve the desired rate per user in a small number of users regime.