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My oh MIMO

Using multiple transmitters and receivers, new technology offers major boost to wireless LANs.

By Craig Mathias, Network World
August 30, 2004 12:09 AM ET

Network World - Current wireless LAN technologies have their share of technical challenges, including limited bandwidth and transmit power, interference, signal fading and multipath (interfering echoes and reflections). Fortunately, there's a new approach to radio design and implementation that promises a powerful remedy. It's called MIMO - multiple input, multiple output - and we at the Farpoint Group believe MIMO is the single most important technology in wireless today.

MIMO promises improvements in WLAN throughput, range and reliability that will broaden the usefulness of wireless for applications that demand ever-greater performance.

We expect the next evolution of the IEEE 802.11 physical-layer standard - 802.11n, with more than 100M bit/sec single-channel performance - will be based on MIMO technology. And we even expect MIMO to be applied to cellular and other wireless systems in the future. But, for now, let's just consider how MIMO will play in the WLAN world.

Isn't that spatial

Basically, while 802.11a/b/g technologies use single transmitting and dual receiving antennas, MIMO uses multiple transmitting antennas, multiple receiving antennas and a lot of signal processing on both ends to create a complex, 3-D radio-frequency transmission.

What's being inputted and outputted in this case is the radio channel itself. Normally, we think only of frequency and time as the key elements here - the core issue in modulating a signal is the number of bits per Hertz, per second.

MIMO adds a third "spatial" dimension, and in fact depends upon multipath, previously an impediment, for it to work properly. If that sounds counterintuitive, think of MIMO as analogous to 3-D computer graphics - a much richer form of communication than 2-D, with a lot more information content. That's the core idea. Note that single-input, multiple output and multiple-input, single-output approaches are also possible - but the best results are obtained with a true MIMO implementation on both ends of a connection.

Better than bonding

MIMO is so important to the future of WLANs because it's the most effective approach to improving performance. Another option is to simply use more bandwidth, like the proprietary channel-bonding techniques used in 108M bit/sec 802.11a and 802.11g implementations. But channel bonding isn't as spectrally efficient as MIMO and not a good long-term choice because data loads and rates surely will increase over time.

We also could use a more complex modulation scheme and simply attempt to cram more bits into the frequency and time already available. But we likely would end up with modulated signals that are so complex that the damage normally incurred via radio propagation would render the received signal unusable.

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