Robust Beamforming Method for SDMA with Interleaved Subarray Hybrid Beamforming

This paper considers millimeter-wave access network systems with space division multiple access using an interleaved subarray hybrid beamforming structure, where antenna elements in each subarray scatter uniformly over the whole antenna array. The structure can provide high system capacity by multiplexing users in the space domain with narrow beams. However, imperfect user direction estimation might cause system capacity degradation due to interference between the beams. In particular, severe interference is experienced in the interleaved subarray configuration because many grating lobes are observed due to the wide element spacing in each subarray. Therefore, we propose a robust beamforming method in order to provide high system capacity even if user direction estimation is inaccurate. In the proposed method, multiple subarrays form beams to a common direction when the peaks of beams directed to the respective user directions are overlapped. We conduct computer simulations to evaluate the proposed method with a non-ideal beam search. The simulation results demonstrate that the proposed method can improve system capacity in the SDMA systems with the interleaved subarray hybrid beamforming structure.