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From Wikipedia:

In radio, multiple-input and multiple-output, or MIMO (/ˈmaɪmoʊ, ˈmiːmoʊ/), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation.

Spatial multiplexing or space-division multiplexing (often abbreviated SM, SDM or SMX) is a multiplexing technique in MIMO wireless communication, fibre-optic communication and other communications technologies used to transmit independent channels separated in space.

But consider this radio unit from Mimosa Networks: https://mimosa.co/products/specs/b5c

This is a dual-polarization 4x4:4 MIMO microwave radio unit. It connects to a dual-polarization parabolic antenna via two RF cables.

According to their chat support, it creates 4 separate channels by using 2 separate frequency bands per polarization:

  • Channel 1 - Horizontal_frequency1

  • Channel 2 - Horizontal_frequency2

  • Channel 3 - Vertical_frequency1

  • Channel 4 - Vertical_frequency2

But isn't this frequency-division multiplexing (per polarization)?

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1 Answer 1

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This a daffodils versus yellow flowers problem, all daffodils are yellow flowers, but not all yellow flowers are daffodils.

MIMO means 'Multiple Input, Multiple Output', no more than that.

Having many antennae can allow you to do various things, assuming you have the additional hardware and software to enable them

  • Spatial Diversity
  • Beamforming
  • Antenna Diversity
  • Frequency and Polarisation Diversity

In the particular case of your fixed network vendor, having FP diversity with 4 channels is a robust way to get a four times speedup on a fixed link, which is completely impractical on a link to a mobile. However, 4x4 SDM MIMO can at best get three times speedup and often less, but also works with mobile links. They are used for very different things, in different contexts.

Antenna Diversity has always been possible without 'clever' software, but for that reason it's not often described as MIMO.

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