Chapter 7 / 7.7

7.7 SPATIAL DIVISION MULTIPLEXING

Spatial division multiplexing (SDM) exploits the spatial dimension to transmit multiple independent signal streams simultaneously within the same frequency band and time interval. Whereas FDM separates signals in frequency, TDM separates signals in time, and WDM separates signals by optical wavelength, SDM separates signals in space. The evolution of multiplexing techniques to encompass SDM reflects the progressive exploitation of all available physical degrees of freedom in electromagnetic propagation. SDM is not a replacement for FDM or TDM but rather complements them. Modern systems are multi-dimensional, simultaneously exploiting frequency, time, wavelength, and space.

Consider a transmitter equipped with multiple antennas. If the propagation channel between transmitter and receiver provides sufficiently distinct spatial paths, then independent signals may be transmitted simultaneously and separated at the receiver using spatial processing. Thus, in favorable propagation conditions, capacity increases approximately linearly with the number of antennas—without requiring additional spectrum.

Because antennas can radiate and receive directionally, antenna patterns or propagation paths can be made sufficiently orthogonal in the spatial domain, multiple users or data streams may coexist without requiring additional bandwidth or time slots. Spatial multiplexing differs from spatial diversity. Spatial diversity improves reliability by transmitting redundant copies. Spatial multiplexing increases throughput by transmitting independent streams.

In modern high-capacity systems—particularly cellular networks, satellite systems, and optical fiber links—SDM has become a principal technique for increasing throughput without expanding spectrum allocation.