SDM: Parallel Spatial Channels
Space Division Multiplexing (SDM) increases capacity by creating several physically separate transmission paths. Each path carries a different signal at the same time, while spatial separation limits mutual interference. Fibers, conductor pairs, cores, or radio beams can provide these paths.
A highway illustrates the principle. When one lane is congested, additional lanes increase the number of vehicles that can move at the same time. Data transmission obtains a similar effect by creating several parallel paths.
Examples of Spatial Channels
- Multi-pair copper cables: A typical Ethernet cable contains four pairs of twisted copper conductors. Each pair forms a physically separate path, while the transmission standard determines how many pairs it uses.
- Fiber optic bundles: A telecommunications cable may contain a bundle of hundreds of individual optical fibers. Each fiber is a distinct spatial channel capable of carrying a large amount of data, while its separation from neighboring fibers limits coupling between channels.
In these cases, the channels are spatially separate along the transmission path. Transmitters and receivers use interfaces matched to the number and type of paths.
SDM in Wireless Communication
In wireless systems, open space is the shared medium. Spatial separation comes from antenna directivity, radio beams, and distance between cells, without separate physical wires.
- Directional antennas and spot beams:A highly directional antenna focuses an electromagnetic wave into a narrow beam. A satellite can use this arrangement to create multiple "spot beams," each covering a different geographical area, such as different states or countries. Each beam forms a spatial channel and can carry different content through the same satellite.
- Cellular network architecture: A cellular network divides a geographical area into smaller regions called cells. Each cell is served by its own . The limited radio-frequency spectrum is managed by reusing the same frequencies in cells separated enough to limit co-channel interference. Spatial separation and frequency reuse let millions of users communicate simultaneously. This planning method is described by the .
MIMO as Spatial Multiplexing
A widely used wireless form of SDM is . Multipath propagation makes signals reflect from objects and reach the receiver along different paths. The receiver distinguishes these paths from the channel response and uses them to separate the streams.
MIMO Transmission Process
- Data splitting: The initial data stream is divided into several layers or sub-streams.
- Multi-antenna transmission: The layers are transmitted simultaneously from different antennas at the transmitter.
- Multipath channel: The signals reach the receiver along different paths, including reflections from walls and buildings. Each receiving antenna obtains a different mixture of all transmitted signals.
- Signal separation: A digital signal processor (DSP) estimates the channel response and applies a detector to separate the mixtures. Separation depends on channel rank, signal-to-noise ratio, and the quality of channel information.
- Data recombination: The separated layers are combined into one output stream.
MIMO is used in Wi-Fi (Wi-Fi 5, 6, and 7) and mobile networks such as LTE and 5G. When channel conditions allow several layers to be distinguished, it sends multiple "spatial streams" at the same time in one frequency band, increasing throughput without assigning wider spectrum.