There was no difference in performance for the R 3/4 code when either 75 bits or 100 bits was used
254 CHAPTER FIVE
R = 1/2
Decoder
FIFO
Buffer
Output Bits
Null Bit
C0(n) C0(n??“2)
C1(n) C1(n??“1)
C0(n) C0(n??“1) C0(n??“2)
C1(n) C1(n??“1) C1(n??“2)
Null Bit Rate Decoder 3/4
FIGURE 5.40 Rate 1/2 decoder operating as a punctured rate 3/4 code.
R = 3/4 and R = 7/8 Punctured Performance
1.E??“07
1.E??“06
1.E??“05
1.E??“04
1.E??“03
1.E??“02
1.E??“01
2 3 4 5 6 7 8 9 10 11
Eb/No (dB)
BER
CD QPSK
R = 3/4, Soft, T = 75 bits
R = 7/8, Soft, T = 100 bits
R = 7/8, Soft, T = 75 bits
FIGURE 5.41 R 3/4 and R 7/8 punctured code performance in AWGN channel.
for the truncation path length value. However, for the R 7/8 code we can see a slight degradation
when we used the 75 bits truncation path length. The measured degradation was approximately 0.2 dB.
Also note as the code rate approaches unity, the BER performance improvement diminishes since less
parity bits are inserted into the transmit data stream. As this occurs the error correction capability
reduces.
It is important to note the R 3/4 and R 7/8 curves were generated by puncturing the same R
1/2 encoder. Similarly, the decoders used the same VA as R 1/2 with the addition of inserting null
bits.
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