A direct sequence spread spectrum (DSSS) wireless modem system based on frequency shift keying (FSK) is introduced and studied. The modem is designed to support data rates from as low as 10 kbps to as high as 160 kbps, which results in a variable modulation index for the FSK modulation. For the conventional frequency modulation system the modulation index is kept small to achieve the best performance as well as the most efficient bandwidth usage. Using DSSS, the transmitted signal occupies a much wider bandwidth than the information signal. Therefore, bandwidth efficiency is not an issue for this system. However, a large modulation index may degrade the system performance. For the two post detectors considered, a single sample detector and an integrate-and-dump detector, the paper demonstrates that the performance degradation from a large modulation index is significant for the single sample detector, but very small for the integrate-and-dump detector.
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