MIMO is the new battle cry in waging the war to achieve higher throughput and robustness in IEEE 802.11n wireless systems. The acronym means ‘multiple input-multiple output’ and refers to the physical layer communications channel being enhanced through the use of multiple transmitters, antennas and receivers. MIMO is the basic architecture upon which the yet-to-be-ratified 802.11n standard is based.

The most common implementation of MIMO technology is what is referred to as a 2 x 2 (two-by-two) architecture in which a MAC layer delivers data to two PHY layers including two transmitters and two antennas on one side of the physical channel and two antennas with two receivers and two demodulating PHY layers on the receive side of the physical channel. Of course, other multiples are possible with the added cost of Tx/Rx pairs and system complexity.

Manufacturers are already to market with MIMO technology, often identified as pre-11n solutions, ahead of even the draft approval, which took place earlier this year. However, the actual ratification of the standard is not expected to occur until early 2007. What this means is market wars have begun with products that are not interoperable. Why not?

IEEE 802.11n is much more than just MIMO technology. So when you hear someone mention MIMO, realize that they may be referring to ‘many things’--many options, communications techniques that use the MIMO architecture. MIMO technology can be, and has been, implemented using one or more of the following channel techniques:

- spatial diversity (aka spatial spreading);

- channel multiplexing; and

- beam forming (focusing).

Spatial diversity is what occurs when using MIMO--multiple antennas physically spaced at some distance, which actually generates multipath signals that are received and recombined to create gain instead of signal cancellation. The effect of spatial diversity is to improve the robustness of the connection to preserve theoretical throughput and extend range. Spatial diversity is mandatory in the 802.11n draft.

Channel multiplexing uses the multiple transmitters, antennas and receivers of MIMO architecture to divide the data into multiple streams that are transmitted simultaneously and recombined in the MAC layer on the receive end. In a 2 x 2 architecture, the throughput is potentially doubled. This greatly improved throughput can be preserved over a greater range by applying spatial diversity.

Beam forming is a smart antenna technique that employs active phasing of the antennas to concentrate the transmitted power toward the receiver(s) and to enhance the hearing ability of the receivers. The narrowness and strength of the beam for both transmit and receive depends on the number of antennas employed. Beam forming increases range and preserves the theoretical throughput but does not increase the theoretical data capacity, as does channel multiplexing.

The actual utilization mix of the above and other variations that we cannot address in this space are the reasons why pre-11n MIMO products from different manufacturers, or groups of manufacturers, are not interoperable, even though each may have modes that are interoperable with 802.11a and g. IEEE 802.11n can be viewed as a new technology that shares the same birth pains of other emerging technologies--consumers are left to sort it out based on a published mix of fact and fiction.

Eventually, alignment will occur as early marketers obsolete their own offerings in favor of the standard. Even so, the new standard will allow for options and variations that can be employed to focus the product for specific market needs.

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