MIMO an acronym for multiple input, multiple output, and is a system which is deployed along with OFDM (Orthogonal frequency-division multiplexing) in the new 802.11n standard. This new standard offers many advantages over conventional standards such as 802.11g Wireless LAN equipment of which we have become familiar.
MIMO 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 using a method called Spatial Multiplexing (SM). The benefits of MIMO over conventional 802.11g equipment is pronounced.
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 due parallel data channelling.
Here is a simplified summary of how it all works
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 merg 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 these deferent phases and amplitudes will 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; the cost of the equipment compared to existing equipment available and limited open source driver support.
However as poeple become more aware of the posibilities the standard it is certain to become popular and with this the price should come down significantly. This I’m sure also will see more manufacturers making code available for the Open Source Community, which will enhancing their competitive advantage over other brands and boarder support by wireless network communities and enthusiasts.