 The 64-characer redundancy of this code allows it to correct up to 32 char- acter errors (or, equivalently, 256 bit errors) within each received block of length 255. The code rate over GF(28) is 191/255. If an FH/256-ary FSK chan- Coding Countermeasures nel is available, each RS character can be ideally transmitted as a single tone. Other less desirable implementations are the transmission of each RS char- acter as two consecutive tones over a 16-ary channel, or four tones over a 4-ary channel, or even eight binary tones. In general, a Q-bit RS character can be sent as L tones over an M = 2K-ary channel, where (2.152) Because of complexity considerations, RS decoders in use today typically make hard decisions on each received code symbol, and use this reduced observable space to recreate the transmitted data. Berlekamp has generated a table of 2°-ary RS character error rates Po required for a hard-decision decoder to achieve some selected Pb's for codes with power of 2 redundan- cies up to 64 [21]. If L uses of an M-ary channel are needed to send each RS character, and the channel K-bit symbol error rate is PK, then Po=1-(1-Pr). (2.153) With no diversity, the bit error rate for FH/MFSK signals in worst case partial-band noise and n = 1 band multitone jamming is specified by (2.25) and (2.41), respectively, except that EIN must be reduced by the RS code rate. The bit error rate over the M-ary channel then determines the symbol error rate PK according to the familiar relation (2.154) Using this procedure, we were able to measure the relative performance of various RS codes, without additional levels of coding or diversity, over an FH/MFSK channel in worst case jamming. The results are presented in Table 2.13 in the form of the Eb/N, required to achieve Pb = 10-5 out of the RS decoder. These are exact SNR's based on the tabulated PO's in [21]. For the sake of comparison, the convolutional code performance in Table 2.12 is generally much better, particularly in the multitone case, even though the tabulated values of