crambling the order of channel symbols at the transmitter with interleaving and unscrambling it at the receiver with deinterleaving, an approximately memoryless channel is achieved where the impact of jamming or other channel disturbances is independent from transmitted symbol to symbol. Coding techniques are primarily designed for memoryless channels and, therefore, require interleaving and deinterleaving to be effective. Interleaving is a form of time diversity which requires no knowledge of the channel memory other than its approximate length and is consequently very robust to changes in memory statistics. This is particularly important in a jamming environment where jammer waveforms may change. Since in all practical cases, memory decreases with time separation, if all symbols of a given codeword are transmitted at widely spaced intervals and the inter- vening spaces are filled similarly by symbols of other codewords, the statis- tical dependence between symbols of a given codeword can be effectively eliminated. Appendix 3A. Interleaving and Deinterleaving Figure 3A.1 illustrates an example of a block interleaver with depth I = 5 and interleaver span N = 15. Here the coded symbols are written into the interleaver memory filling it along columns while the transmitted symbols are read out of this memory along rows. Thus, if X1, X2, X3, X4, are the coded symbols entering the interleaver, the inputs to the channel are X1, X16, X31, X46, X61, X2, X17, X32, From this figure it is clear that the minimum separation is at least I = 5 for any two symbols entering the block interleaver within a separation of N - 1 = 14 symbols. In general, with a block interleaver with depth I and span N, there are IN symbols of memory required. For synchronous transmission where one such memory is used to write in the symbols while another memory is used for reading out symbols into the channel, there is a total of 2IN symbols of mem- ory required. At the receiver, the deinterleaver simply performs the inverse operation where symbols a