Multi-user MIMO Strategies for a Millimeter Wave Communication System using Hybrid Beam-forming

This paper presents the theoretical capacity performance of different downlink multi-user MIMO (MU-MIMO) beam-forming strategies for a millimeter wave communication system using hybrid beam-forming. Hybrid beam-forming uses both RF and digital beam-forming and is a practical way to achieve the antenna array gains required when the large bandwidths available at millimeter wave frequencies are utilized. Since both RF and digital beam-forming are used, there are various possible beam-forming approaches. We investigate a system which uses code book based RF beam-forming with non-code book based digital beam-forming which utilizes sounding reference symbol (SRS) based uplink measurements for TDD operation. For this system we examine different strategies for selecting the RF beams in conjunction with different digital pre-coding schemes to support MU-MIMO. The performances of the different strategies are simulated under the assumption of perfect channel knowledge at the base station and at the mobile station.