11.11 10.36 (Repetition) 14.95 14.21 14.58 15.49 16.76 Optimum Binary 11.11 (7, 1) Convolutional Optimum Binary 10.38 (7,1) Convolutional Optimum 4-ary 10.12 (7,1) Convolutional 10.84 Optimum 8-ary (7, 1/1/2 ) Convolutional 11.55 Orthogonal 25.19 17.42 13.58 11.30 Convolutional 27.03 20.63 18.46 18.05 Dual-K, Rate 2 16.72 14.43 13.40 12.89 12.62 Convolutional 15.75 15.47 16.34 17.76 19.56 Dual-K, Rate 3 14.38 11.86 10.60 Convolutional 13.19 12.68 13.31 14.49 16.04 Non-Coherent Frequency-Hopped Systems PARTIAL- BAND NOISE K=4 Eg/N = 10 dB ORTHOGONAL CONVOLUTIONAL CODE UNCODED DIVERSITY 10 Pwc = .30 (OPTIMUM) P wc = .75 DUAL - 4, RATE 1/2 CONVOLUTIONAL CODE PARTIAL BAND FILL FACTOR, P Figure 2.83. Sensitivity of three coded FH/MFSK systems to variations in partial- band noise fill factor p, for K = 4 and Eb/N1 = 10 dB. Although all three cases exhibit comparable sensitivity to p, differences in Pwc suggest a hybrid coding scheme for defeating partial-band noise jamming (see text for details). link switch to the dual-4, rate 1/2 convolutional code. If the jammer can somehow detect this change and reduce p to .15, or if it simply varies p pseudorandomly, the FH system can react accordingly. If the link can mon- itor p and adjust its code quickly enough, it can maintain a mismatch between p and Pwc most of the time. The net effect of this mismatch is that the effectiveness of the partial-band noise jammer will be significantly reduced, to the extent that the jammer may in fact be better off emitting broadband noise. 2.3.2.2 Reed-Solomon Codes We now consider Reed-Solomon (RS) codes (refer to Section 4.10.3, Part 1) in place of convolutional codes, with FH/MFSK signals in worst case par- tial-band noise and multitone jamming. These are block codes, with input and output characters (symbols) over GF(2°). The output block length of a con- ventional RS code is 2° - 1 Q-bit characters, although extended RS codes can have block lengths of 2° or 2° + 1. As an example, the popular (255, 191) RS code uses Q = 8 bit characters.