In fact, with the above block diagram
of 4 subcarriers, a max delay spread of approximately 10 sec can be accommodated without the
need of a complex equalizer.
Now let??™s turn our attention to the receiver. Each subchannel has sync words and pilot symbols
inserted into the 16-QAM symbol stream. Assuming that each subchannel will encounter a flat
frequency fading channel, then the pilot symbols can be used to estimate the channel to be used
for coherent detection. The channel estimate between pilot groups can be estimated through interpolation
techniques.
There exist a few options on the receiver side. First, we can simply demodulate each subcarrier
independently and combine all subchannels to create the aggregate symbol rate to be passed to either
the vocoder or multimedia device. Second, we can jointly demodulate all subchannels through the use
of a variety of array signal processing techniques available.
26 CHAPTER ONE
Serial-to-
Parallel
Convertor
16-QAM
Symbols + Pilot
Insertion LPF X
. . .
Symbols + Pilot
Insertion
LPF X
ejw1t
ejw4t +
FIGURE 1.34 M16-QAM transmitter block diagram using 4 subcarriers.
The wireless link bit rate is 64 kbps, where the frequency channel has been split up into 6 time
slots, where each time slot is a radio link.
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