. In addition to modulation, the coded symbols are interleaved and the signal is spread in bandwidth for anti-jam protection. At the receiver, despreading is done first, followed by detection. The detector can have many possible forms including hard decision, soft decision, threshold, possibility of erasures, and for MFSK modulation, an ordered list of the energy detec- tor outputs. As shown in Figure 4.1, channel measurements may provide the decoder with side information. Long-term measurements can include slowly varying parameters of the channel such as propagation conditions. Short-term mea- surements will usually be used to determine if a particular coded symbol experienced jamming during transmission. The part of the system shown inside the dotted lines in Figure 4.1 is called the coding channel. It is the effective channel as seen by the encoder and decoder system. Here a general coded bit error bound will be derived which will serve as a basis for evaluating the performance of all such complex com- munication systems. The key feature of this approach is the decoupling of the coding aspects of the system from the remaining part of the communi- cation system. Specifically, the cutoff rate parameter [4], [5], [6], [7] R0 bits/channel use (4.1) is computed. It represents the practically achievable reliable data rate per coded symbol. This cutoff rate will be a function of the equivalent channel CODING CHANNEL System Model ENCODER INTERLEAVER MODULATOR SPREADER PROBE-LONG-TERM CHANNEL JAMMER STATE Noise INFORMATION Fading Jamming MEASUREMENT - LONG-TERM DECODER DEINTERLEAVER DEMODULATOR DESPREADER Figure 4.1. AJ system overview. General Analysis of Anti-Jam Communication Systems symbol energy-to-jammer noise ratio (4.2) which is shown here to be directly related to the bit energy-to-jammer noise ratio given by (assuming jamming limits performance) (4.3) Here R is the code rate in data bits per channel symbol and the usual jam- ming parameters are PG processing gain (4.4) S = signal power J = jammer power Wss = 