8.6.4 Performance Characteristics

The performance of SDMA systems depends primarily on spatial resolution and channel structure rather than solely on bandwidth or time-slot allocation. In systems based on fixed directional antennas, capacity increases as beamwidth narrows and sidelobe suppression improves, enabling more aggressive spatial reuse. In adaptive beamforming systems, capacity depends on the ability to form independent beams and suppress interference dynamically.

In spatial multiplexing systems, performance is governed by the rank and condition of the channel matrix. Rich multipath environments with low spatial correlation among antenna elements support higher multiplexing gains. Environments with strong line-of-sight components or insufficient antenna spacing may limit achievable spatial separation.

Unlike CDMA, which is fundamentally interference-limited, and FDMA, which is often distortion-limited, SDMA performance is frequently resolution-limited. The number of users or streams that can be supported simultaneously depends on antenna aperture, wavelength, array geometry, and propagation conditions.

Power considerations also differ. In beamforming systems, coherent combination of antenna elements can increase effective gain without increasing total transmitted power. In spatial multiplexing, total transmit power must be divided among multiple streams, and performance depends on both signal-to-noise ratio and spatial channel characteristics.