Also known as: MIMO, multiple-input multiple-output
MIMO (multiple-input multiple-output) is a radio technique that places several antennas at both the transmitter and the receiver so that a single channel carries more data, or carries it more reliably, than any one antenna pair could.1 Where a conventional link has one path, an M×N MIMO link has M×N paths between the two arrays, and a rich multipath environment turns those paths into independent data pipes rather than a source of fading.
How it works
MIMO exploits the fact that, in a scattering environment, the signal from each transmit antenna arrives at each receive antenna with a distinct amplitude and phase. The set of these coefficients forms a channel matrix H. If H is well-conditioned — meaning its paths are sufficiently independent — the receiver can invert it and recover several data streams that were transmitted simultaneously on the same frequency. This is spatial multiplexing, and the number of separable streams (the rank of H, bounded by min(M,N)) sets how many times capacity is multiplied. Two ideas make it work in practice:
- Spatial multiplexing sends independent bit streams from each antenna and relies on H being invertible. Capacity scales roughly linearly with the number of antenna pairs, at constant bandwidth and power — the headline result of MIMO information theory.
- Spatial diversity sends the same information over multiple antennas (often with a space-time code such as Alamouti). Because the paths fade independently, the odds that all of them are in a deep fade at once are small, so the link stays up. This trades the capacity gain for robustness — closely related to receive antenna diversity.
A third mode, beamforming, uses knowledge of H to weight the antennas so their signals add coherently toward the intended receiver; MIMO and beamforming are often combined in modern systems.
Variants
- SU-MIMO serves one user with multiple streams. MU-MIMO splits the streams among several users at once, using the spatial dimension as a shared resource.
- Massive MIMO puts dozens to hundreds of elements at the base station, sharpening beams and serving many users simultaneously; it is a cornerstone of 5G NR.
- MIMO is almost always paired with OFDM, so the channel matrix is estimated and inverted independently on each flat-fading subcarrier — a “one-tap-per-stream” simplification that keeps the equalizer tractable.
Relevance to SDR
MIMO is standard in Wi-Fi (802.11n and later), LTE, and 5G NR, and it is what lets a crowded band deliver hundreds of megabits per second without extra spectrum. For a software-defined radio hobbyist, MIMO shows up mainly on the analysis side: multi-channel, phase-coherent SDRs (for example a KrakenSDR) use several synchronized receivers to do direction finding and passive radar, which are receive-side cousins of MIMO processing.
GopherTrunk is a single-stream trunking decoder for land-mobile protocols (P25, DMR, NXDN, TETRA), none of which use MIMO on their traffic channels, so GopherTrunk does not implement MIMO. Understanding it still matters for context: it explains why the cellular and Wi-Fi bands a scanner sees are so spectrally dense, and it is the reason a single whip antenna cannot demodulate an LTE data stream that was designed for a multi-antenna handset.