7.7.1 Forms Of Spatial Multiplexing
SDM may be implemented in several distinct physical ways.
- Multibeam systems. Multibeam operation is fundamentally an SDM architecture at the system level. In satellite communications, a single spacecraft may generate many narrow spot beams covering different geographic regions. The beams are spatially separated and may reuse the same frequency bands. This technique provides increased system capacity through frequency reuse, improved link budgets due to antenna gain concentration, and reduced co-channel interference between beams.
- Multiple-Input Multiple-Output (MIMO). In terrestrial wireless systems, SDM is most commonly implemented as MIMO. In rich multipath environments, independently fading paths between antenna elements create spatial degrees of freedom. With appropriate precoding and signal processing, multiple independent data streams can be transmitted, capacity increases without expanding occupied bandwidth, and diversity gain improves reliability. Modern 4G and 5G systems routinely employ 2×2, 4×4, 8×8 MIMO, and massive MIMO with tens or hundreds of antenna elements.
- Phased arrays and beamforming. Phased-array antennas electronically steer narrow beams. By forming multiple simultaneous beams users in different directions may share the same frequency, interference may be suppressed via null steering, and spatial selectivity improves spectral efficiency. Beamforming may be viewed as controlled SDM, where orthogonality is achieved through directional isolation.
- Optical spatial division. In optical fiber systems, SDM is implemented through multicore fibers (multiple independent cores), and few-mode fibers (multiple spatial modes). Here, spatial channels are physically separated within a single fiber cladding, allowing dramatic increases in aggregate capacity.