CODED FREQUENCY SHIFT KEYED SEQUENCES WITH APPLICATIONS TO LOW DATA RATE COMMUNICATION AND RADAR
STANFORD UNIV CA STANFORD ELECTRONICS LABS
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Coded discrete frequency sequences can provide greatly improved performance over conventional techniques when the fluctuation bandwidth of the communication channel is a significant fraction of the transmission bandwidth. These fluctuations result from medium, equipment, and, in the case of radar, random target variations. The ability to reliably detect these sequences under frequency shift is investigated using a simple algorithm to calculate an approximation of the true ambiguity function. This investigation leads to certain necessary conditions and a set of coupled equations which permit a sequence to be synthesized from the ambiguity function approximation. Error rates for CFSK sequences are calculated and criteria for optimizing the performance in rapidly fluctuating channels given. An algorithm for constructing sets of orthogonal sequences with desirable cross-ambiguity properties is developed and the performance of these sets of orthogonal sequences compared with binary code alphabets. Finally, consideration is given to digital and analog implementations of the special receiver required for the CFSK sequences, error correction coding problems associated with M-ary encoding, and acquisition behavior.