Electronic Design

How MIMO Works

The 11n standard is based on orthogonal frequency-division multiplexing (OFDM), which is also used in the 802.11a/g standards. It's a proven way to extend range and data rate in multipath environments.

Adding a technology known as multiple-input/multiple-output (MIMO) to OFDM creates a nearly bulletproof multiple antenna/transceiver technology that not only substantially boosts speed but also increases range and link robustness in mulitpath environments.

One form of MIMO involves diversity reception, an ancient radio technique that uses multiple antennas spaced several wavelengths apart. The antenna with the strongest signal is switched to the receiver for optimum reception. While some designers consider diversity to be a form of MIMO, chip makers aren't going in that direction.

In the basic MIMO concept implemented for 11n, the data to be transmitted is scrambled, encoded, and interleaved (see the figure). It's then divided up into parallel data streams, each of which modulates a separate transmitter (TX).

Modulation is OFDM using binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16QAM (quadrature amplitude modulation), or 64QAM, depending on the data rate. Both transmitters operate in the same 20-MHz band. Transmitting two different data streams in the same bandwidth doubles the throughput. Throughput scales linearly with the number of transmitters.

The multiple signals arrive at the receivers at different times in different phases, depending on the different paths they take. Some signals will be direct, others via multiple different paths. Each signal is unique as defined by the characteristics of the path it takes. Such a technique is referred to as spatial multiplexing.

The unique signatures produced by each signal over the multiple paths allow the receivers to sort out the individual signals using special algorithms implemented with DSP techniques. The same signals from different antennas then can be combined to reinforce one another, improving signal-to-noise ratio and, therefore, the reliability and range.

While many systems will stick with two transmitters and receivers, the standard provides for other versions using different numbers of transmitters and receivers. Other possibilities include 2 by 3 (number of transmitters by number of receivers), 3 by 2, 3 by 3, 3 by 4, 4 by 3, and 4 by 4. Beyond the 4-by-4 configuration, very little additional gain is achieved. So far, a 2-by-3 arrangement seems to be the most popular.

Transmitting two or more data streams in the same bandwidth multiplies the data rate by the number of streams used. In addition, MIMO in the 11n standard permits two signal-free adjacent 20-MHz channels to be bonded together into a single 40-MHz+ channel, which can provide even higher data rates.

With this channel bonding and four streams, a maximum potential data rate of 600 Mbits/s is achievable. Though it's not clear who needs that kind of speed, a data rate of over 100 Mbits/s can be sustained easily over a 100-m range in a rather hostile environment.

Perhaps the greater benefit of MIMO is the transmission's increased range and robustness. The spatial multiplexing mitigates the multipath problem experienced by most microwave transmissions. MIMO techniques do permit multiple streams, but that helps improve the signal-to-noise ratio and the reliability significantly over other versions of the standard.

There's no doubt that MIMO will make its major debut in Wi-Fi products. However, it's also expected to show up in future OFDM wireless systems like WiMAX and 4G cell phones.

TAGS: Digital ICs
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