If you are a wireless systems designer, MIMO is in your future. It will change the design paradigm for virtually all wireless technologies from cell phones to broadband. Wherever higher data rates or more robust connections are important --that is, just about everywhere--MIMO will be the technology of choice.
Multiple input, multiple output, as the name implies, broadcasts and receives using multiple radios and antennas. This technology is not simply a paradigm shift; it inverts conventional wireless design principles: that multipath transmission is undesirable and should be eliminated.
This radical departure delivers a radical result: double the physical layer (PHY) data rate of a conventional one-radio, one-receiver system.
Improved performance is not traded for lower robustness. It is accompanied by an 8-dB gain in receive sensitivity as well.
In a two-by-two MIMO configuration, for example, the first step in preparing the source data stream for transmission is to divide the source into two streams. Each stream is broadcast at the same time and over the same frequency.
In a multicast environment, the receiving antennas hear what could be described as an RF tower of Babel. But engineers using high-performance computing hardware have leveraged the unique characteristics of multicast to unscramble the incoming data and reconstruct the source data stream.
Though MIMO is being used in proprietary point-to-point broadband applications by companies like Orthogon Systems, its first widespread deployment began in 2005 in wireless LAN (IEEE 802.11a/b/g) systems fielded by several systems-level OEMs such as Linksys and Belkin -- and powered by Airgo Networks' MIMO technology. The IEEE 802.11a/g specifications are, of course, mute on MIMO.
But the new system-level products were made to play nice with IEEE 802.11 a/b/g by conforming to 802.11 MAC layer specifications. They don't get the double-data-rate advantage when talking to legacy a/b/g systems, but at least they understand each other.
MIMO gains its advantage primarily in the physical layer. Once the technology was demonstrated in working products, it was quickly understood that optimizing the 802.11 MAC and PHY specifications for MIMO performance and interoperability between MIMO-based systems was the wave of the future. An IEEE 802.11n was convened to start the ball rolling.
Those tuned in to the 802.11n discussions throughout 2005 witnessed an intense and controversial standards process. Early in 2006, an agreement was reached within the IEEE 802.11n task group. The standard started working its way through the remaining IEEE machinery and is expected to be ratified by the middle of 2007.
The term "spatial diversity" is central to MIMO and means only that multiple transmit or receive antennas are used and that they are separated in space. Data is transmitted over multiple antennas through what is referred to as a "MIMO channel" to multiple receive antennas. If the antennas in the transmit array and the antennas in the receive array are spaced far enough apart, multipath signals between them will fluctuate or fade in an independent manner. It is this characteristic that allows the data to be unscrambled at the receive end. Typically, a training sequence is placed in the preamble at the beginning of each packet to discover how the multipath signals are cross-coupling.