Speed-increasing measures

There is generally one target within the different standards: to attain a higher physical layer (PHY) rate and, therefore, offer a higher speed to users. MIMO and spatial streams form an important factor within this context, but, in addition, the channel width is also always checked.

 

Channel width

The simplest way in which to attain this is to increase the channel width. We can double the bandwidth by going from 20 MHz to 40 MHz. This can be compared with an additional lane on a motorway: if there are 4 instead of 2 lanes, double the number of vehicles can use the motorway at the same time.

The disadvantage is that the available spectrum is scarce and, therefore, endless expansions are not possible. This is what we are now noticing with the 802.11ac standard where 80 MHz channels are possible. We have 19 channels in Europe if we use 20 MHz channels. This becomes 9 when we switch to 40 MHz (11n) where 5 remain if they are 80 MHz channels. Wave 2 brings 160 MHz channels of which only 2 are available. At least 3 non-overlapping channels are required to set up a professional network and, if you also want capacity and minimum co-channel interference, at least 4 or 5.

In addition, there is a large section of the spectrum where dynamic frequency selection (DFS) must be used. This means that these channels cannot be deployed reliably for critical networks. This means that the number of available channels at 20 MHz is 4, at 40 MHz is 2 and at 80 MHz is 1.

 

MIMO

MIMO is the acronym for Multiple In, Multiple Out. This means that the radio is split up in multiple radio chains that can all transmit and receive data individually and simultaneously. More data can, therefore, be sent or received simultaneously or the same data can be sent or received several times for a reliable signal.

Mimo

A 3x3 system as described above is, therefore, a system where 3 antennas and, by analogue, radio chains are available to transmit and/or receive data. The objective is to use different paths between the transmitter and receiver and, therefore, to also use multipath in a positive and constructive manner when compared to what happened before.

Typically, there are two ways of using MIMO:

  • Spatial Multiplexing
  • Space Time Block Coding

 

Spatial Multiplexing

The most important reason to use spatial multiplexing is to increase speed. Every path will then carry unique data. This is indicated by the number of “streams”. While the 3x3 indicates how many antennas the system has to receive or transmit data, the figure for the spatial streams will indicate how much of the radio chains will definitely be used to transmit unique data. This is, therefore, not linked to the number of radio chains except for the fact that no more streams can be used than that there are radio chains. A 3x3 system can, therefore, for example use 1 stream (3x3:1), 2 streams (3x3:2) or 3 streams (3x3:3). Every stream will add just as much bandwidth.

While we could compare the channel bandwidth with a motorway where an additional lane was added, the expansion here is within the same channel bandwidth and, therefore, can best be compared with a bridge above the existing road. This will, therefore, offer the same expansion but is more difficult to achieve.

The option is available within the 11n standard to achieve up to 4 spatial streams. 11ac defines 4 with an option of going to 8 streams.

Mimo Channel Width Versus Spatial Streams

Space Time Block Coding

Space-time block coding (STBC) is mainly used to gain on robustness and reliability. Multiple radio chains will be used to send multiple times the same data or the bits of the same data over different antennas. This ensures that the same data is sent through different paths and it will also be received different times on multiple antennas. This must increase the probability that at least one set is received uncorrupted as much as possible.