MIMO — Many antennas at both ends
5G Systems Notes · Concept map · MIMO
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Space — layers, ports, reporting
Where it sits
What it is
What it is. Multiple antennas at both ends. The term covers three different uses that are routinely confused, and they trade against one another.
| Use | Needs | Buys | Best when |
|---|---|---|---|
| Spatial multiplexing | A rich, well-conditioned channel and accurate CSI | Rate — several independent streams at once | Strong signal, plenty of scattering |
| Beamforming | Phase of each path — direction, essentially | Reach — more energy at the device, less elsewhere | Weak signal, or interference to be avoided |
| Diversity | Very little | Reliability — several independent chances | Fast fading, or when CSI is unavailable or stale |
Table 1: The three things multiple antennas can be used for, what each needs to know about the channel, and what it buys. A given transmission does one of them at a time, and the choice is the scheduler's - informed by the rank the device reports. The right-hand column is why rank adaptation exists rather than always using the maximum.
They are the same hardware pointed at different problems, and they compete. Energy spent making one stream stronger is energy not spent making a second stream possible. That trade is exactly what the rank report resolves, and it is why rank is reported separately from quality.
Read on
This concept was first written up in ref-mimo, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order:
Keep going — where this sits on the route
5G Systems Notes · Concept map · MIMO
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