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
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
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 dont transmit at the same time and
destroy each others signal. Since there is no really accurate way of
synchronizing clocks (in the light of propagation delay) a length of time
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