8.6 SPATIAL-DIVISION MULTIPLE ACCESS (SDMA)

Frequency-division, time-division, and code-division techniques separate users by partitioning spectral, temporal, or signal-space resources. Spatial-division multiple access instead exploits a fourth fundamental degree of freedom: space. Because electromagnetic waves propagate through physical space with direction and geometry, signals arriving from different directions or occupying different spatial modes can, in principle, be separated even when they share identical time and frequency resources.

Spatial separation may be achieved passively through antenna directivity or actively through adaptive signal processing. In its simplest form, SDMA allows multiple users to transmit simultaneously in the same frequency band provided that they are sufficiently separated in angle or spatial signature. In more advanced forms, multiple independent data streams may be transmitted and received over the same time–frequency resources by exploiting the structure of the propagation channel itself.

The fundamental idea of SDMA is that electromagnetic propagation is not uniform in all directions. Antennas radiate and receive energy with direction-dependent gain patterns. If two transmitters are located in sufficiently different directions relative to a receiver, and if the receiver employs a directional antenna or array, it can preferentially receive energy from one direction while attenuating energy from another.

This directional selectivity creates a form of spatial orthogonality. Two users may transmit simultaneously on the same frequency and at the same time, yet remain separable because their signals occupy different angular regions or distinct spatial modes of the channel.

At a more general level, the received signal at an antenna array can be represented as a weighted superposition of contributions from multiple spatial paths. By applying appropriate weighting to the array elements, the receiver can enhance signals arriving from one spatial direction while suppressing others. In this way, spatial filtering serves a role analogous to frequency filtering in FDMA or time gating in TDMA.

Spatial separation can therefore be understood as partitioning the channel according to direction or spatial mode rather than frequency, time, or code. The degree of achievable separation depends on antenna aperture, array configuration, wavelength, and the propagation environment. Narrower beams or larger arrays provide greater spatial resolution and improved user isolation.

Unlike FDMA and TDMA, where orthogonality is defined explicitly in frequency or time, spatial orthogonality is often approximate and environment-dependent. Reflections, scattering, and multipath can reduce directional isolation, while carefully designed arrays and signal processing can enhance it.

SDMA thus introduces geometry as an additional resource dimension. The following sections examine how this principle is realized in practice, beginning with large-scale spatial reuse and sectorization, and progressing to adaptive beamforming and spatial multiplexing.