This Inside Wireless episode introduces MIMO, or, Multiple Input Multiple Output principles. MIMO has been all the rage in recent years and we talk about background why Multiple Input Multiple Output principles were developed - namely the physics phenomena behind sub-optimal RF link performance.
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Physics phenomena influencing RF link performance:
Consider an example of a Single Input Single Output link with a one-channel radio connected to a single polarization antenna transmitting RF signal to the same setup at the receiving side. The received signal is not only the one arriving through the line of sight.
It fluctuates due to all kinds of fading, in other words, random addition of signals arriving at the receiver because of:
Reflections - when signal reflects from objects much larger than the wavelength
Diffraction - from edges of obstructions much larger than the wavelength
Scattering - from objects with the size similar to the signal wavelength
Flat or frequency selective fading - affecting either only certain frequencies of wide band signals or all of them equally.
Or Doppler fading - causing frequency shift of signal when receiver is moving.
All these fading components can severely affect the quality and reliability of a wireless communication system. MIMO is a set of techniques used to diminish fading effects and improve throughput capacity, coverage, and reliability of a wireless link.
This is a simple wireless link capacity equation: C = N . B . log (1 + S/N). Besides the higher bandwidth (B), or increasing the Signal to Noise ratio (S/N), growing the number of channels (N) on either side of a link is also a way to increase the throughput capacity - which is where MIMO comes in.
Increasing the number of antennas on either or both sides of a wireless link creates multiple possible paths for the signal to arrive at the receiver. There are a number of benefits this brings:
1. Array gain - an increase of received signal SNR from combining the signals arriving from different directions. Array gain improves resistance to noise and therefore the coverage and maximum range of a wireless link.
2. Reliability - multiple paths through which the signal can reach the receiver increases the probability of successful data transmission.
3. Link capacity - multiple independent data streams in the same frequency channel enable higher link capacity. The minimum of the M and N in an M x N system tells us the minimum number of reliably operable data streams.
In outdoor wireless internet service provider networks, it is common to leverage two independent data streams on each end of a link separated by antenna polarization. This effectively doubles the link capacity despite one dual polarized antenna being used at the same frequency.
0:00 - Intro
0:27 - SISO link & Fading
1:15 - MIMO Basics
1:46 - MIMO benefits
2:35 - WISP MIMO standard
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