  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 (PU2RC). 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 PU2RC in
non-coexistence scenario, it offers an intriguing opportunity to reuse spectrum
already allocated to FWS.
