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Adaptive data transmission scheme for DS/SSMA system in bandlimited Rayleigh fading channel

Two basic problems which the wireless mobile communication system designers are faced with are multipath fading of the radio link and interference from other users. The spread spectrum technique is known to be effective in mitigating multipath fading and interference. We consider a convolutionally coded DS/SSMA system in bandlimited slowly Rayleigh fading channel. We examine the relationship between the code rate and the processing gain (defined as the number of chips per coded bit) in maximizing the throughput in connection with the average signal-to-noise ratio (SNR). It is found that the maximum throughput is attained when the code rate is chosen as low as possible irrespective of the SNR, and the processing gain is increased (decreased) as the SNR decreases (increases). Based on this observation, we propose an adaptive transmission scheme that changes the effective processing gain according to an estimate of the SNR. It is found that a significant improvement can be obtained by using the proposed scheme over the nonadaptive scheme. The effects of the channel state estimation error and the user speed are examined.