In the High Speed
Downlink Packet Access (HSDPA) requirements in the 3GPP standard, this is called Hybird-ARQ.
For example, if we assume our system can only support R 3/4 code rate then a ???good??? punctured
code that translates the R 1/2 encoder into a R 3/4 encoder is shown in Fig. 5.39.
PERFORMANCE IMPROVEMENT TECHNIQUES 253
R = 1/2
Encoder
FIFO
Buffer
C0
C1
C0(n) X C0(n??“2)
C1(n) X C1(n??“1)
Rate Encoder 3/4
FIGURE 5.39 Rate 1/2 encoder punctured to supply a rate 3/4 encoding.
Here the encoder rate can be shown as 3 bits enter the R 1/2 encoder, thus producing 6 output
bits, where 2 of them are not transmitted.
(5.59)
From the above block diagram it is apparent that C0(n 1) and C1(n 2) are not transmitted. In
fact, the place holding operation is also not transmitted. Note one cannot simply pick any combination
of bits to puncture and expect the same overall BER performance. Various studies were performed
to investigate puncturing patterns that resulted in good performance; some are presented in
[24]. Since the transmitter is not transmitting the punctured bits, we are essentially introducing errors
into the data stream at the transmitter; this is from the rate 1/2 perspective.
The receiver simply inserts ???null bits??? into these punctured locations; these null bits are generally
forced to have zero value.
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