when the decoder decodes all L signals simultaneously. To illustrate the optimum receiver take the simplest case of L = 2 radios each transmitting binary symbols where M = 2. Let fo and f1 represent the two possible transmitted frequencies by the two radios. Ignoring hopping and dehopping, the set of possible frequencies and the corresponding hard quantized chip energy detector outputs are: Inputs Frequency Detector Outputs Radio 1 Radio 2 Output for fo Output for f1 If we take the conventional approach illustrated in Figure 2.26, the sec- ond transmitting radio is regarded as channel interference to the link between the first transmitting radio and the receiving radio. To this link the Spread-Spectrum Multiple Access with FH/MFSK Waveforms (1,0) (1,1) (O, 1) Figure 2.27. Binary erasure channel. interference channel is as shown in Figure 2.27, where we assume all binary symbols are equally likely to occur. Each time the second radio sends a fre- quency different from the first radio, which occurs with probability .5, the receiver detector hard quantized output for the two detectors is (1, 1). Essentially, there is no information about what frequency the first radio transmitted when this occurs and we might as well have "erased" the out- put. This is the binary erasure channel which has channel capacity [103] CBEC = . 5 bits/symbol. = (2.45) This is the maximum data rate possible between the first transmitting radio and the receiving radio. By symmetry this is also the channel between the second transmitting radio and the receiving radio when the first transmit- ting radio's signal is regarded as interference. Since the channel capacity of the interference channel in Figure 2.27 is 1/2, codes of this rate, in principle, can achieve any arbitrarily small bit error probability. Suppose the first radio uses such codes and the receiver decodes with no errors. Then the receiver knows exactly the first radio's transmitted coded symbols and can use this to determine exactly the symbols transmit- ted by the second radio. 