There are three basic parameters that completely describe the quality and usefulness

of any wireless link: speed, range and reliability. Prior to the development of MIMOOFDM,

the three parameters were interrelated according to strict rules. Speed could

be increased only by sacrificing range and reliability. Range could be extended at

the expense of speed and reliability. And reliability could be improved by reducing

speed and range. MIMO OFDM has redefined the tradeoffs, clearly demonstrating

that it can boost all three parameters simultaneously. While MIMO will ultimately

benefit every major wireless industry including mobile telephone, the wireless LAN

industry is leading the way in exploiting MIMO innovations.

Most radio transmit from a single antenna. The transmitted signal is subject to multipath propagation effects. This includes bouncing off objects, atmospheric layers and distortion as radio signals go through various materials.

Multipath signals arrive at the receiver at different times. If all arriving signals are considered by the receiver some signals will be in phase, producing an increase in signal power. Other signals will arrive out of phase an this will produce reduced signal power. A common strategy is to ignore weaker multipath signals and to use strongest multipath signals. This waste the arriving signal power in the first case and cause distorting issues in the second case.

Beamforming (beam steering)

Employs  one transmitter, two  transmitter antennas, and one receiving antenna to receive the best Multipath signal.

Diversity (receiver combining)

Employs  one transmitter, one transmitter antenna, and two receiving antennas to receive the best Multipath signal.

Physically Resembles MIMO

Employs  one transmitter, one transmitter antenna, and two receiving antennas to receive the best Multipath signal. Form of beam steering and diversity with only one signal sent over a channel. This is  not MIMO.

MIMO (Multiple In - Multiple Out)

MIMO, in contrast, takes advantage of multipath propagation to increase throughput,

range/coverage, and reliability. Rather than combating multipath signals, MIMO puts

multipath signals to work carrying more information. This is accomplished by

sending and receiving more than one data signal in the same radio channel at the

same time. The use of multiple waveforms constitutes a new type of radio

communication—communication using multi-dimensional signals—which is the only

way known to improve all three basic link performance parameters (range, speed

and reliability).

Because MIMO transmits multiple signals across the communications channel (rather

than the conventional system’s single signal), MIMO has the ability to multiply

capacity (which is another word for “speed”). A common measure of wireless

capacity is spectral efficiency—the number of units of information per unit of time

per unit of bandwidth—usually denoted in bits per second per Hertz, or b/s/Hz.

Using conventional radio technology, engineers struggle to increase spectral

efficiency incrementally (i.e. one b/s/Hz at a time). By transmitting multiple signals

containing different information streams over the same frequency channel, MIMO

provides a means of doubling or tripling spectral efficiency.

MIMO can also be thought of as a multi-dimensional wireless communications

system. Conventional radio systems try to squeeze as much information as possible

through a one-dimensional pipe. In order to do that, engineers must adapt their

designs to the noise and other limitations of a one-dimensional channel. MIMO

empowers engineers to work in multiple dimensions, creating opportunities to work

around the limitations of a one-dimensional channel.

Greater spectral efficiency translates into higher data rates, greater range, an

increased number of users, enhanced reliability, or any combination of the preceding

factors. By multiplying spectral efficiency, MIMO opens the door to a variety of new

The first paper describing wireless MIMO’s capacity

multiplying capability was published 100 years later in the 1996 Global

MIMO and “smart antenna” systems may look the same on first examination: Both

employ multiple antennas spaced as far apart as practical. But look under the hood,

Smart antennas enhance conventional, one-dimensional radio systems. The most

beamforming and receive combining are only multipath mitigation techniques, and

That’s not to say beamforming and receive combining aren’t useful. Both have

demonstrated an ability to improve performance incrementally in point-to-point

applications (e.g., outdoor wireless backhaul applications). However, while

beamforming and receive combining are valuable enhancements to conventional

radio systems, MIMO is a paradigm shift, dramatically changing perceptions of and

responses to multipath propagation. While receive combining and beamforming

increase spectral efficiency one or two b/s/Hz at a time; MIMO multiplies the b/s/Hz.

Smart antennas and channel bonding are important and useful technologies but theymust not be confused with MIMO technology. If everything else is equal, MIMO-based products will outperform smart antenna- and channel bonding-based products.

However, the different technologies are not necessarily adversaries: MIMO-based products can make judicious use of smart antennas and channel bonding to offer even more benefits.