MIMO and 802.11n

MIMO an acronym for multiple input, multiple output, and is a system which is deployed along with OFDM (Orthogonal frequency-division multiplexing) and is also used in the 802.11n standard. This standard offers many advantages over conventional standards such as 802.11g as MIMO is a system which exploits multiple transmitters and antennas to increase the bit rate in a wireless LAN link with no additional power or bandwidth consumption by using a method called Spatial Multiplexing (SM).

Benefits include:

  • Improved signal to noise ratio through increased antenna array gain,

  • Improved link reliability using phase nulling techniques

  • Near and Non LOS performance enhancement

  • Improved ability to ignore other signals which inhabit the same band

  • Increased throughput in good conditions due parallel data channelling.

Here is a simplified summary of how a three channel system would work.

The input data stream is split into independent sub-streams which together occupy less bandwidth than is required to transmit the original stream on a single channel. These sub-streams are applied separately to individual transmitters and antennas on the same frequency, where the receivers at the other end recover each sub-stream and merge them back together.

Due to the presence of various scattering objects eg: buildings, walls, cars, trees, etc. signals experience a multi-path propagation and when it is captured by the receiver antenna these signals will arrive with random phase and amplitude. In conventional 802.11g equipment only the strongest signal is used and the other mulit-path signals are rejected as noise.

However in a MIMO device this is turned to an advantage as the different phases and amplitudes have a specific spatial signature. The receivers can be viewed as a bank of superposed spatial weighting filters where every filter aims at extract one of the multiplexed sub-streams by spatially nulling the remaining ones. This not only allows the added benefit of array gain due to multiple antennas, but also diversity techniques which reduce signal fading.

The disadvantages of the MIMO system is mostly the need for multiple Antennas which are phase isolated; the cost of the equipment compared to existing equipment available and limited open source driver support.

Although MIMO offers better distance connections in terms of reliability in conditions where other standards fail, it cannot offer faster speeds in those conditions. Just like other standards to achieve maximum bandwidth a good signal, Line of Site (LOS) and Fresnel Zone clearance is still required for optimum bandwidth.