MIMO Introduction

Multiple input and multiple output, or MIMO, is a set of techniques that has gotten a lot of attention in recent years. So what is MIMO? Before we go into details of MIMO principles, we will look at a few basics of RF wave propagation. In this short blog you will learn about:

RF propagation phenomena
Diversity principles of RF links
MIMO benefits

RF propagation phenomena

Consider a simple 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.

Single Input Single Output (SISO) link - each radio has one channel and antennas are single polarized.

The received signal is not only the one arriving through the line of sight. It fluctuates due to all kinds of fading, or, 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 all, or only certain frequencies of wide band signals respectively.
Or Doppler fading - causing frequency shift of signal when receiver is moving.

Examples of fading causes - reflections, diffraction, scattering, frequency selective or flat fading, and Doppler fading.

All these fading components can severely affect the quality and reliability of a wireless communication link. MIMO is a set of techniques used to diminish fading effects and improve throughput capacity, coverage, and reliability of a wireless link.

Diversity principles of RF links

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. MIMO is one of the many techniques that falls under so-called diversity, or, redundancy principles used in RF engineering.

Introducing redundancy in a wireless link provides more reliable, effective, failure-proof operation, and potentially higher throughput of a network by leveraging several propagation channels and physics principles. The key word here is ‘potentially’, because the promise is a statistical improvement rather than an immediate one. Today, there are a few diversity principles in RF communication that are a standard part of the technologies used:

Time - already a mainstream feature of wireless ISP systems - a great example is the GPS synced transmission / reception.
Frequency - one of the oldest principles of operating at different frequency channels - separating users by using different channels within the available spectrum.
Polarization - antennas can have horizontal, vertical, circular, or elliptical polarization. WISP industry common MIMO leverages vertical and horizontal polarizations.
Spatial - leveraging a number of propagation paths, either through beamforming, or using multiple receiver, and / or transceiver channels with directive antennas.

Applying particular techniques does not necessarily mean you will see steady, double, or triple data throughputs. MIMO is not a magic spell, but it can help mitigate the problems experienced in wireless links.

MIMO benefits

Increasing the number of antennas on either or both sides of a wireless link creates multiple possible paths for the signal to arrive at a receiver. There are a number of benefits this brings:

1. Array gain - which is 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.

Using multiple antennas causes the signal to arrive at the receiver from different directions. Combining these improves the SNR.

2. Reliability - multiple paths through which the signal can reach the receiver increases the probability of successful data transmission.

More propagation paths increase the probability of a successful transmission thanks to the use of multiple antennas.

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.

The smaller number of antennas from the Tx and Rx sides dictates what the throughput capacity improvement can be.

In outdoor wireless internet service provider (WISP) 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. Leveraging different frequency channels is enough to provide the signal to noise ratio for successful data transfer.

Dual polarized antennas are a standard in the WISP industry effectively doubling the link capacity since the two channels are polarization separated.

If you are a more visual person or want to share the explanations with someone quickly, check our video version of this blog below or check our Inside Wireless series on all kinds of RF engineering topics HERE.

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