Wireless Standards · Topic 3 of 5
MIMO and Chains
Learn how MIMO uses several antennas, chains and spatial streams to raise speed and reliability, and how chains are set and read in RouterOS.
- 5 min
- MIMO uses several antennas to send and receive several signals on one channel.
- A chain is one transmit and receive path with its own antenna connection.
- A 2x2 device has two transmit chains and two receive chains.
What it is
MIMO means multiple input, multiple output, and it uses several antennas at each end of the link.
With one antenna a radio sends one signal. With several antennas it can send several signals at once on the same channel, or combine them for a more reliable link.
MIMO arrived with 802.11n, and all the later standards build on it.
How it works
Chains, streams and the techniques that use them all affect the speed and the reliability.
Chains
A chain is one transmit and receive path with its own antenna connection. A device with two chains has two antenna connections.
The notation is transmit chains by receive chains. A 2x2 device has two transmit and two receive chains. A 3x3 device has three of each.
Spatial streams
A spatial stream is an independent flow of data sent at the same time on the same channel. The number of streams cannot be higher than the number of chains.
Each stream carries its own data, so two streams give roughly twice the rate of one. Streams need good conditions, with a strong signal and a rich environment that has many reflections.
The main MIMO techniques
| Technique | What it does |
|---|---|
| Spatial multiplexing | Sends different data on several streams for a higher rate |
| Diversity | Uses several antennas to receive or send the same data, for better reliability |
| Maximal ratio combining | Combines the signals from several receive chains for a stronger result |
| Space time block coding | Sends the same data in a coded form over several chains to improve reliability |
| Beamforming | Adjusts the signals so that they add up at the client |
At the edge of coverage, the signal is weak and streams are hard to separate. The link then falls back to fewer streams, and the extra chains improve reliability instead of speed.
Single user and multi user MIMO
Basic MIMO serves one client at a time. MU-MIMO serves several clients at the same time, using different streams.
802.11ac introduced MU-MIMO for the downlink, from the access point to the clients. 802.11ax extends it to the uplink as well.
Polarisation and chains
Many antennas have two connectors with different polarisations, typically one vertical and one horizontal. Each connector feeds one chain.
This lets a link keep working when the client orientation changes. It also gives two streams that are independent of each other.
Setting chains in RouterOS
On the legacy wireless package the chains are set with transmit and receive chain lists.
/interface wireless set wlan1 tx-chains=0,1 rx-chains=0,1
/interface wireless registration-table print statstx-chainslists the chains that the radio uses to transmit.rx-chainslists the chains that the radio uses to receive.- The registration table shows the signal for each chain and the rate that includes the number of streams.
On the wifi driver, the chains are a setting of the configuration profile.
Reading the results
The registration table can show a signal value for each chain. A similar value on each chain shows that both antennas are fine.
A large difference between the chains points to a problem. The cause can be a loose connector, a damaged cable, a poorly placed antenna or a mismatch in polarisation.
If a 2x2 link runs with only one stream, check the signal on each chain before you change any other setting.
Why use fewer chains
You can turn off a chain on purpose. This is rarely useful, but it can help on a link with a weak or damaged antenna. Normally you should use all the chains of the radio.
Effect on the data rate
The rate scales with the number of streams. A client with one stream cannot use the second chain of the access point for speed, though it can still gain reliability from it.
Many phones and small devices have one or two chains, so they do not reach the highest rates in the specification.
The headline speed of an access point assumes the client has the same number of streams. A one stream client never reaches it.
What to remember for the exam
Seven facts cover this topic.
- MIMO uses several antennas to send and receive several signals on one channel.
- A chain is one transmit and receive path with its own antenna connection.
- A 2x2 device has two transmit chains and two receive chains.
- The number of spatial streams cannot exceed the number of chains.
- More streams give a higher rate, and the rate roughly doubles with two streams.
- MU-MIMO serves several clients at once, and it was added for the downlink in 802.11ac.
tx-chainsandrx-chainsselect the chains on the legacy wireless package.
Common mistakes
These four errors are common.
Thinking more chains always means more speed. Streams need good conditions and a client that can use them.
Ignoring the per chain signal. A big difference points to a fault.
Expecting the headline rate from a one stream client. The client limits the speed.
Confusing MIMO and MU-MIMO. MU-MIMO serves several clients together.
Check yourself
Work through these three questions.
What does a 2x2 notation mean?
The device has two transmit chains and two receive chains. It can use up to two spatial streams.
Why might a link use fewer streams at the edge of coverage?
The signal is weak, so streams are hard to separate. The extra chains then add reliability instead of speed.
What is the difference between MIMO and MU-MIMO?
MIMO uses several streams with one client at a time. MU-MIMO serves several clients at the same time.
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