Codebook-Based SDMA for Coexistence with Fixed Wireless Service

A portion of frequency band for International Mobile Telecommunications (IMT)-Advanced is currently allocated to Fixed Wireless Service (FWS) such as Fixed Service (FS), Fixed Satellite Service (FSS), or Fixed Wireless Access (FWA), which requires frequency sharing between both the systems. SDMA, due to its high throughput nature, is candidate for IMT-Advanced. This paper proposes a systematic design of a precoder codebook for SDMA sharing spectrum with existing FWS. Based on an estimated direction angle of a victim FWS system, an interfering transmitter adaptively constructs a codebook forming a transmit null in the direction angle while satisfying orthogonal beamforming constraint. We derive not only asymptotic throughput scaling laws, but also an upperbound on throughput loss to analyze performance loss of the proposed SDMA relative to the popular SDMA called per-user unitary rate control (PURC). Furthermore, we develop a method of evaluating protection distance in order to analyze the spectrum sharing performance of the proposed approach. The simulation results of protection distance confirm that the proposed SDMA efficiently shares spectrum with FWS systems by reducing protection distance to more than 66%. Although our proposed SDMA always has lower throughput compared to PURC in non-coexistence scenario, it offers an intriguing opportunity to reuse spectrum already allocated to FWS.