OPEN COMMUNICATION
THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
■ BY LOUIS E. FRENZEL W5LEF
MIMO 101:
Next Generation Wireless Technology is
Finding Its Way Into Most New Systems
IN CASE YOU HAVEN’T HEARD, MIMO means Multiple Input Multiple Output
which — by itself — still does not explain very much. It is a relatively new
wireless technique that makes use of two or more transmitters, receivers, and
antennas to greatly improve the range, reliability, and data rate of digital
wireless systems. It is being incorporated in the newest versions of wireless
LANs and in the new metropolitan (metro) wireless systems like WiMAX. And
it could play a role in future cell phone systems. You can expect to see it soon
in a wireless system that you use. That’s why you should know more about it.
MIMO SOLVES SOME
REAL PROBLEMS
In modern digital wireless systems, the goal is always to transmit
data faster over a longer distance
with greater reliability. That’s a
tough job. It is particularly difficult at
microwave frequencies where most
digital transmissions take place.
Remember that microwave frequencies are those above 1 GHz. This is
where the wireless local area
networks (WLAN) like Wi-Fi IEEE
802.11 standard-based units work.
The 2. 4 GHz band is the most
common, but 5. 8 GHz is also used.
Data transmissions in cell phones
for Internet access, email, instant
messaging, and the like take place
just below 1 GHz in the 850 to 965
MHz range and in the 1.8 to 2.1
GHz range.
In these frequency ranges, signals
are easily reflected, refracted,
diffused, absorbed, and otherwise
affected by their environment.
Buildings, walls and ceilings, trees,
passing cars, or planes all affect the
signal. What happens is that a transmitted signal goes directly to the
receiver but signals reflected from
other sources also end up at the
receiver. Since the reflected signals
take a longer path to
the receiver, they
occur at a slightly
later time so the
effect is to cancel
some of the directly
received signal. The
multipath signals
cause fading and sig-
nal attenuation. And that effect is not
just an occasional thing but an all-the-time thing. Highly directional
antennas — like Yagis and parabolic
dishes — can help in fixed point-to-point applications, but you can’t put
■ FIGURE 1. The basic
concept of MIMO showing a 2x3 arrangement
of transmitters (TX) and
receivers (RX). The
receiver outputs are
digitized in analog-to-digital converters (ADC)
and then manipulated
by a digital signal
processor (DSP) to
combine multipath
signals. The result is
higher data rates over
longer distances with
greater reliability.
April 2007 89
