An Introduction to Wireless Technology, Second Edition

5.2.2 Code Division Multiple Access (CDMA)

The DSSS technique gives rise to a novel way of sharing the bandwidth.

Multiple transmitters and receivers are able to use the same frequencies at the

same time

without

interfering with each other. This is a by-product of the DSSS

technique. The receiver correlates its received signal with a known (only to it)

random sequence: all other signals are filtered out.

This is interesting because it is really the same process as FDM. When we

receive an ordinary radio station (channels are separated by FDM), we tune in to

that station. The tuning process involves adjusting a resonant circuit to the

frequency we want to receive. That circuit allows the selected frequency to pass

and rejects all other frequencies. What we are actually doing is selecting a

sinusoidal wave from among many other sinusoidal waves by selective filtering.

If we consider a DSSS signal as a modulated waveform, when there are many

overlapping DSSS signals the filtering process needed to select one of them

from among many is exactly the same thing as FDM frequency selection except

that the waveforms are not sinusoidal in shape. However, the DSSS “chipping

sequences” (pseudo-random number sequences)

must be orthogonal (unrelated)

.

Fortunately there are several good simple ways of generating orthogonal

pseudo-random sequences.

For this to work, a receiving filter is needed that can discriminate a single DSSS

signal from among all the intermixed ones. In principle, you need a filter that

can correlate the complex signal with a known chipping sequence (and reject all

others). There are several available filtering techniques which will do just this.

The usual device used for this filtering process is called a Surface Acoustic

Wave (SAW) filter.

CDMA has a number of very important characteristics:

“Statistical” Allocation of Capacity

Any particular DSSS receiver experiences other DSSS signals as noise.

This means that you can just go on adding channels until the

signal-to-noise ratio gets too great and you start getting bit errors. The

effect is like multiplexing packets on a link. You can have many active

connections and so long as the total (data traffic) stays below the

channel capacity, everything will work well. For example, in a voice

system, only about 35% of the time does a channel actually have sound

(the rest of the time is gaps and listening to speech in the other

direction). If you have a few hundred channels of voice over CDMA what

happens is the average power is the channel limit - so you can handle

many more voice connections than are possible by FDM or TDM

methods.

This also applies to data traffic where the traffic is inherently bursty in

nature. However, it has particular application in voice transmission

because when the system is overcommitted there is no loss in service,

only a degradation in voice quality. Degradation in quality (dropping a

few bits) is a serious problem for data but not for voice.

No Guard Time or Guard Bands

In a TDM system when multiple users share the same channel there

must be a way to ensure that they don′t transmit at the same time and

destroy each other′s signal. Since there is no really accurate way of

synchronizing clocks (in the light of propagation delay) a length of time

102 An Introduction to Wireless Technology