e-varying matched filter output. Note that there is a unique sample point in every coded bit time interval for each of the trans- mitting radio signals. These samples correspond to the usual correlator out- puts in the DS/BPSK receiver except that with the use of a single matched filter, correlator outputs for all transmitted radio signals can be obtained at once. The sample sequence for each transmitting radio is then decoded separately. Figure 2.11 illustrates the difference between receiving signals with dif- ferent keys and a common key. For the uncoded case the detector is a sim- ple hard quantizer. Also here we assume known phase and bit time epoch for each signal. The receiving radio terminal described here uses one matched filter fol- lowed by a sampling circuit and individual decoders for each transmitted sig- nal. We assumed that the relative time shifts of each transmitted signal were known at the receiving radio terminal which samples the match filter out- DETECTOR QUANTIZER/ DECODER DETECTOR COMMON QUANTIZER/ MATCHED DECODER FILTER DETECTOR QUANTIZER/ DECODER Figure 2.11. Transmissions with a common key. Multiple Access put at the correct time. For DS/BPSK waveforms it is usually impractical for radio transmitters to accurately time shift their transmissions SO that they are within a prescribed chip time at the receiving radio terminal. A more practical approach is to have each radio transmission introduce a random shift within a time interval and have the receiver system estimate appropri- ate sample times. This requires some sample time estimation scheme at the matched filter output. In addition, this introduces the possibility that two or more transmitting radios will have relative time shifts less than a chip time interval, with the result that their matched filter output sample times will coincide and cause "collisions." The probability of such collisions is deter- mined in a manner analogous to the Aloha random access scheme. It depends on the number of simultaneous transmitting signals and the