It is well known how the combination of increasing
the transmission bit rate and frequency selectivity of the channel leads to a limitation on the transmitted
bit rate, while keeping implementation complexity low. Here we mean that as the frequency
selective fading (FSF) increases, the amount of intersymbol interference (ISI) present increases
(keeping the symbol time constant), thus producing a BER floor.
With this in mind, iDEN introduced a multichannel (carrier) 16-QAM modulation scheme to combat
the fast FSF channel commonly encountered in practice. This was latter called M16-QAM, for
Motorola 16-QAM.
Some of the physical aspects of the air interface protocol are time slot length 15 msec, creating
a frame length of 90 msec, assuming 6 users per carrier. M16-QAM was chosen to produce 64 kbps
in a 25-kHz channel. There are 4 subcarriers, each carrying 16 kbps of information. Ablock diagram
of the M16-QAM transmitter is shown in Fig. 1.34 with 4 subcarriers.
The data rate on each subcarrier has been reduced by 4, compared to the more conventional single
carrier 16-QAM modulation scheme. Hence assuming the channel conditions (i.e., RMS delay
spread) stay the same, then relative to each subcarrier, the channel behaves more frequency flat, thus
reducing the need of an equalizer (or at least its complexity).
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