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.
FAQ
Q:Does each transmitter/antenna use a different channel?
A: No, in fact the MIMO system relies on the frequencies being the same, so it can apply phase nulling techniques to extract the independent data sub-streams. Also allowing the summing of signals from each receiver providing a gain boost using multi-path signals.
Q: Does 802.11n use a wider bandwidth than conventional wireless LAN eg 802.11g causing interference to other wireless LAN equipment?
A: Yes and No, most 802.11n devices currently available, have a choice to use either a 20Mhz or 40Mhz Channel Bandwidth. In 802.11g and b the Channel Bandwidth is 22Mhz. With only 13 channels which closely overlapped the potential for interference between any device is high, consequently it will be up to users.
Q: Can you modify the dipole
Q: Can you modify the dipole of an existing directional antenna for MIMO use?
A: Yes and No
A: The antennas used in 802.11n are identical to that used for 802.11b and g however the way it achieves the high data rates (up to 300Mpbs) is by exploiting a method called Spatial Multiplexing which in simple terms means the antennas are spaced apart.
Consequently, although it would be possible to fit multiple dipole feeds to a single dish, it would most likely result in the loss these high data rates, when used over distance.
Unanswered questions
Unfortunately it is currently unclear what an “off-the-shelf” 802.11n can do outdoors. There are high priced commercial point to point MIMO devices on the market which work over distance at high data rates.
So it will be up to enthusiasts like us to go out and try. Just like was done with the 802.11a, b and g - all now are used over distance (in excess of 5 kms). As its important to remember that many manufactures still only quote 300metres for this equipment.
There are many unanswered questions and maybe some knows? Please post them here.
Q: What is maximum distance that Spatial Multiplexing can be maintained?
Q: What is the optimum spacing of antennas against maximum distance?
Q: Is Spatial Multiplexing effected when used with high gain directional antennas?
Q: Are there any device processor limitations in 802.11n (eg ARK timing)?