Understanding MIMO (Multiple Input, Multiple Output) – LTE Speed & Cell Booster Implications

An early MIMO prototype...

An early MIMO antenna prototype?

For RVers and Cruisers, grasping what MIMO is, how it works, and how it can be used to enhance cellular speeds has the potential to make mobile internet on the road a more productive experience.

For anyone who knows a thing or two about wireless communications, modern LTE cellular radios are borderline miraculous.

Consider the first iPhone - which launched in 2007 with a maximum theoretical cellular speed of around 500kbps using AT&T's 2G EDGE cellular network.

A mere decade later - the latest flagship cellular devices can support max theoretical speeds of 1000mbps.

That's more than a 1000x increase - in just 10 years!

Of course theory rarely equals reality - and the cellular networks need to be substantially upgraded and built out to even come close to being able to deliver speeds like this to real people outside of a lab. And in the real world - you will be sharing this speed with perhaps hundreds of others connected to the same cell tower.

But real LTE speeds over 50Mbps are actually not at all uncommon nowadays, and speeds over 100Mbps are starting to be found.

One of the key technologies making these sorts of speeds possible is known as MIMO (Multiple Input, Multiple Output) - an incredibly clever technique for putting multiple antennas to work to increase both data transmission speed and reliability.

MIMO is fundamental to LTE - but cellular boosters and MIMO have some... challenges... working together.

Read on to get a grasp of what MIMO is, how it works, and how you can use a little bit of MIMO awareness to potentially double your cellular speeds.

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LTE MIMO In A Nutshell

MIMO is one of the core technologies enabling LTE cellular, and almost every LTE mobile device (whether a phone or a hotspot) actually has TWO or more cellular antennas on board to enable the magic of MIMO.

On the other end of the line - LTE cell towers typically have two or four antennas working together in tight synchronization to communicate with you.

With more antennas transmitting a signal, there are more possible echoes and reflections (read the "how it works" section below to understand the magic here) for the receiving device to extract a signal from.

And more reception antennas on the cell tower better enable your carrier to receive a weak remote signal from your handset.

This figure illustrates a 4x2 MIMO deployment:

Many LTE networks are 4x2 MIMO deployments - with 4 antennas on the cell tower and 2 in the mobile device working together. LTE-Advanced supports up to 8x8 deployments, but building more than two antennas into a device the size of a cellphone will prove to be a challenge.

Many LTE networks are 4x2 MIMO deployments - with 4 antennas on the cell tower and 2 in the mobile device working together. The LTE-Advanced standard supports up to 8x8 configurations, and we're starting to see cellular devices with more than 2 antennas. .

Depending on the conditions, the LTE tower and receiver will negotiate one of several possible MIMO modes. These are the important ones to know about:

Get the Book
  • Mode 1 - This is a fallback mode using a single transmit and receive antenna, essentially disabling MIMO.
  • Mode 2 "Transmit Diversity" - This is the default mode for LTE, and it calls for the tower to transmit the same data stream over multiple antennas, just encoded differently. This does not result in any increase in speeds, but the redundancy makes for a more reliable signal, especially in areas where the signal is weak and there are not a lot of "echoes" - such as rural areas without many buildings to reflect a signal off of.
  • Mode 3 "Spatial Multiplexing" - This is the Shannon's Law-busting turbo-mode, where different data streams are transmitted over each antenna, and are then combined at the receiver for a 2x speed boost.

In urban areas with a lot of signal reflections, MIMO's spatial multiplexing mode can almost magically double cellular speeds.

And in rural areas, transmit diversity allows for LTE devices to work wonders with weak signals - often delivering seemingly impossible speeds when the signal strength is less than 100dB, the sort of signal that would have been barely useable on 3G or 4G networks.

In the future LTE cell towers will feature as many as 8 transmit and 8 receive antennas per cell, and LTE receivers will be able to take advantage of 4x or even someday 8x overlapping streams for even more increased speeds and reliability.

It really is pretty darn amazing stuff!

MIMO And Cellular Boosters

A "booster" like this ear trumpet can help you pick up a far away signal, but you are giving up the joys of MIMO along the way.

A "booster" like this ear trumpet can help you pick up a far away signal, but you are giving up the joys of MIMO along the way.

A cellular booster works by taking a signal picked up by an external antenna, amplifying it, and then rebroadcasting it on an interior antenna.

One exterior antenna.

One interior antenna.

And one amplifier.

In a pre-LTE world, this worked wonderfully - almost always improving your connectivity situation.

But with LTE - funneling your cellular signal through a single antenna and amplifier means that all the advantages of MIMO are being thrown out the window, and the two antennas on your phone or hotspot are now picking up essentially the same thing.

When confronted with a boosted signal, LTE modems often have no choice but to fall back to the non-MIMO Mode 1.

Which in moderate signal areas can actually result in substantially decreased speeds. Not what most want with a booster for cellular data purposes.

Additional Member Only Content

If you're a MIA member, please log in to see the rest of this guide - which contains additional information on:

  • More on MIMO And Cellular Boosters
  • Optimizing Cellular Reception in a MIMO World
  • MIMO - Not Just For LTE
  • How It Works: Breaking The Data Transmission Speed Of Light

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