Table of Contents
Advertisement
Cellular Overview
Digital Cellular
line, such as that shown in the accompany-
ing illustration, will be produced.
If the varying voltages are sampled at some
rate, the instantaneous voltages can be quan-
tified. Let' s say we want to quantify mea-
surements from values of zero to 255 (the
maximum value a binary byte can hold). The
value of 255 would represent the highest pos-
sible voltage we could expect from voice, and
zero would represent silence. Each discrete
integer between zero and 255 would repre-
sent a particular voltage, typically presented
in binary form.Because of the redundancies
of speech and the inability of the human ear
to detect more than a fraction of the intelli-
gence in speech, it is possible to sample a
small portion of the sound produced by a per-
son speaking, reproduce that sound at either
a later time or another place, then filter the
resulting reproduction to produce a " sound"
that is indistinguishable from the original
source.
In the illustration on page 1 - 19 we saw how
speech could be sampled at some rate. Sup-
pose we take only one of every three samples.
If our sampling rate is fast enough, and if we
can compress the samples, it turns out that
we can interleave several different conver-
sations (communication links) on a single fre-
quency. However, we also have to provide
some mechanism for ensuring that the trans-
mitter and receiver are in synchronization,
and we have to provide for some alternative
to the control and response tones used in con-
ventional AMPS. All of these non-voice sig-
nals are digital and take time from the as-
signed time slot, leaving only a relatively
small amount of time to represent voice. For
this reason the digital receiver has to filter
the audio to closely approximate the original
18
audio.
TDMA (Time Division Multiple Access)
Time Division Multiple Access (TDMA) to-
day provides a times-3 increase in the num-
ber of communication links a channel can
carry (just like NAMPS). Eventually TDMA
is expected to take full advantage of all six
time slots, allowing for six communications
links in the bandwidth of a conventional
AMPS channel.
TDMA, like CDMA, employs a form of phase
shift keying to represent symbols. However,
TDMA also compresses the digitized signal,
making use of predictive algorithms to reduce
the number of symbols actually transmitted.
Digitization and TDMA
Here three conversations, represented by
Figure 12. Digitization and TDMA
A
B
C
© 2000 Motorola, Inc.
TDMA Timeport™ P8190
Time
Advertisement
Table of Contents
