MaximumPC 2005 11

A

ntennas, whether transmitting or receiv-

ing, are transducers. On the sending

side, they convert alternating current (AC)

signals into a radio frequency (RF). On the

receiving end, they convert that RF signal back

into AC. Between those two points, however,

every wall, building, cloud, and signpost in

the world is waiting to defl ect, refl ect, or block

that signal. Technically, this refl ection is called

“multipath signal propagation.”

The original antenna designs were meant

to span vast open spaces, not concrete jun-

gles. The ghosting you see on your television

is the result of refl ected signals bouncing off

objects and arriving at the receiving antenna

at different times. (Don’t snicker if you have

cable. As time goes by and your cable line

degrades, resistance will build up in the wire

and you’ll see ghosting, too.) Just try to get

radio reception inside a modern building or

even while driving under a bridge. When you

realize that broadcast sources are unable to

penetrate much of the physical world even

with tens of thousands of watts of power at

their disposal, it’s easy to understand why the

Wi-Fi setup in your home or offi ce, working

off only a few thousandths of a watt, can’t

seem to make it through those two plaster-

board walls and down the stairs reliably.

This is where MIMO (multiple input,

multiple output) steps up and says, “I can

do that.” It’s a revolutionary application of

antenna technology that, backed by fi rm-

ware in Wi-Fi equipment, has been opti-

mized to turn the weakness of multipath

signals into a huge bandwidth advantage.

DEFINING MIMO

There are pages upon pages of fi nely tuned

calculations—enough to glaze the eyes of

even the most stalwart geek—describing the

propagation characteristics of the average

antenna under a variety of confi gurations

and conditions. Add in the possibilities of

multiple antennas, as we do with MIMO

schemes, and it’s doubtful you’d be able to

budge the world of associated math even

with a very long lever and an extremely fi rm

place to stand. While making the defi nition

of MIMO hyper accurate, these equations

are best avoided.

Simply put, MIMO uses an array of

antennas (more than one, thus the “mul-

tiple” on both sides of its acronym) when

sending and receiving. At fi rst you might

think this is just another iteration of the

“smart antenna” that’s been around for

decades. Those devices fall into two

general categories: SIMO (single input,

multiple output) and MISO (multiple input,

single output). Basically, SIMO and MISO

systems rely on a predicted level of inter-

ference, for which their antennas are then

tuned. These optimizations compensate

for much of the noise (refl ections, etc.) that

arises between send and receive points.

The more antennas used on either side of

the system, the better the results.

The theory behind SIMO and MISO

technology is an attempt to mitigate the

problems caused by multipath signal propa-

gation. MIMO, on the other hand, although

foundationally based on its predecessors,

goes one giant step further: MIMO technol-

ogy attempts to exploit those propagation

effects to provide increased bandwidth and

signal reliability.

SIFTING THROUGH

THE STATIC

Here’s how it works: The outgoing data

stream is broken down into multiple streams

r&dBREAKING DOWN TECH —PRESENT AND FUTURE

58 MA XIMUMPC NOVEMBER 2005

White Paper: MIMO Technology

Marconi’s idea of sending HOW IT WORKS MIMO: a new approach to Wi-Fi

information from point A to

point B without wires changed

the world, but multipath signal

propagation is radio’s Achilles’

heel. MIMO promises to

transform this weakness

into a Wi-Fi benefi t

BY BILL O’BRIEN

ÑWalls and other

physical objects

between the

transmitting and

receiving antennas

scatter the data

signals, so they

arrive at different

times. A MIMO

router assembles

these scattered

signals into one.

Common household

appliance

Laptop

Walls and plumbing

MIMO antenna array

The three

antennas on this

Linksys router

reveal its MIMO

genes.