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Chapter 2: Polarity and MIMO

Modern radios provide multiple simultaneous data-streams, enabling significant boosts in throughput over older single-stream radios. This is achieved in two ways: through polarity separation, and through Multiple-Input-Multiple-Output (“MIMO”) channels. MIMO offers the ability to use spatial streams on top of polarity streams, sometimes doubling effective capacity.

Modern radios provide multiple simultaneous data-streams, enabling significant boosts in throughput over older single-stream radios. This is achieved in two ways: through polarity separation, and through Multiple-Input-Multiple-Output (“MIMO”) channels.

Most long-range radios offer two polarities: horizontal and vertical, separated by ninety degrees. This is a very literal description: the radio waves are aligned along a vertical axis for one polarity, a horizontal axis for the other. There are triple-polarity radios appearing on the market (rotating polarities by sixty degrees), but these are largely un-tested for longer range links at this time.

Some radios, such as the Ubiquiti Bullet M5 only support a single polarity. Typically, you can select which polarity to use with a switch. This has the advantage that one can re-use a single spectrum in a given area with two radios (since the horizontal and vertical “chains” never see one another), but carries the significant disadvantage of halving your effective maximum throughput/capacity. The majority of modern radios are dual polarity, simultaneously carrying data on both the horizontal and vertical chains. This effectively doubles available throughput – at the expense of requiring that both polarities be used in the frequency range.

MIMO provides another secondary path, on each available polarity. A second “MIMO processor” handles radio packets that take a slightly different route through the air between radios (MIMO can be used to either boost signal or boost performance in this respect), catching signals that bounce around, and taking advantage of “multi-path”. This has the advantage of providing another data stream on each polarity, once again effectively doubling the number of simultaneous streams that can be processed.

When reading radio/antenna data-sheets, you will often see this referred to as “1x1” and “2x2” (and on shorter range radios, “3x3”). “2x2” means “two by two” - that is, two polarities and two MIMO streams.

All of the data-rates included in this document assume that you have a 2x2 link.

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