QuickStudy: MIMO

Computerworld - It seems like hardly yesterday (well, five years ago) that fast, inexpensive wireless networks were a far-off dream. But Wi-Fi (wireless service based on the IEEE 802.11 standards) came along like gangbusters in the early 2000s; it worked so well and cost so little that it became an instant success both in home and business environments. Although limited in throughput, compared with modern gigabit wired LANs, even the slowest Wi-Fi networks were still a good match for the newly expanding home broadband market being pushed by cable operators, phone companies and others.

Coupled with the availability of sub-$100 router/access- point/firewall appliances, wireless networking became a simple reality in many homes and offices, where multiple computers and simultaneous computer users became the rule, not the exception. After 802.11b came 802.11a, which was five times faster, and then the now nearly universal 802.11g.

MIMO Magic

The magic of MIMO lies in its ability to take multipath reception, which used to be an unavoidable byproduct of radio communications, and convert it into a distinct advantage that actually multiplies transmission speed and improves throughput.

First, let's look at multipath radio reception. Say you're in a car in downtown Manhattan listening to the radio. You know that your car's antenna is receiving the direct signal from the station's transmitter. But your radio is also receiving additional signals of that same broadcast from many different directions, because buildings, wires, geographical features and other structures in the area between the sender and the receiver can reflect or refract those signals. The end result is that each of these additional signals arrives at your car radio via a different path (hence the term multipath) and also at a slightly different time, so that it's out of phase with the original and will randomly boost or cancel out parts of the signal.

This phase differential introduces noise and distortion that you can hear as the car moves within the city, in the form of signal fading, intermittent reception (also called picket-fencing) and sudden signal dropouts. In digital communications, these factors can cause a reduction in data speed and an increase in the number of errors.

Adding antennas, as some wireless systems do, helps sort out signals, allowing the receiver to pick the antenna getting the strongest signal at any given point. How many antennas? Netgear Inc. in Santa Clara, Calif., has recently offered products using seven internal antennas, which combine to create up to 127 different antenna patterns. This is called diversity reception, and though it's not a true MIMO, it's just the beginning of what can be done with multiple antennas.