Here parity bits of information are inserted into the transmit
data stream. The receiver makes use of these parity bits to better detect the data bits within a particular
block or group of bits. A conventional-coded system block diagram is shown in Fig. 5.1 [1].
The raw information is introduced to the FEC encoder at an uncoded rate of Ru bits/sec, this corresponds
to the uncoded data rate. Since the encoder adds parity bits to the data bits, the encoded
output data rate is given by Rc bits/sec, with Rc Ru. The coded bits enter the modulator to perform
the complex envelop domain transformation and create symbols, Rs symbols/sec.
5.1.1 Interleaving/De-interleaving
As shown in Chap. 3, multipath fading can be characterized by the following parameters: Doppler
spread, frequency selectivity, average fade time duration (AFTD), and Level Crossing Rate (LCR).
Let us, for the moment, assume that errors only occur when the channel is in a deep fade. In this case
the errors would be bunched together in groups, exhibiting a bursty behavior. If the length of the burst
of errors is greater than the error correction capability of the FEC decoder, then performance improvements
will be greatly diminished [2]. A method to combat this behavior is to use interleaving.
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