Simulation of multistage detector for spread-spectrum applications
Bais, Viraj Kumar
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The conventional single-user correlation receiver is not optimized for use with a multiple-access channel shared by users transmitting asynchronously independent data streams by modulating a set of assigned signal waveforms with low mutual cross-correlations. Furthermore, the performance is severely degraded as the number of users in a radio-network with fixed bandwidth grows or as the relative powers of the interfering signals become large, as in the "near-far" problem. The optimum multiuser detector based on a dynamic programming algorithm has substantially better performance, but has a variable decoding delay and is computationally intensive due to the NP-hardness of the problem of computing the most likely sequence of symbols from the sufficient statistics and signal cross-correlations. Interest has grown in suboptimal demodulation schemes which exhibit a low order of complexity to ensure their practical implementation, but not exhibiting the impairment of the conventional single-user detector. The multistage detection strategy for coherent demodulation in asynchronous code-division multiple-access systems is such a suboptimal detection scheme based on successive estimation and removal of the multiple-access interference, and requires a computational complexity per symbol which is linear in the number of users, in contrast to the exponential complexity of the optimum demodulator. This project presents simulation results for the performance of a multistage detector for varying number of interfering users, and different combinations of desired user versus interfering users' signal energies. It is shown that the performance approaches that of communIcations as the interfering signals become stronger. The near-far problem is therefore alleviated. Results also indicate that one stage in the receiver is required to reject interference from users at each of the various power levels. This receiver could be modified for use in practical CDMA systems by designing a multistage receiver which forms successive estimates of the channel state in addition to the interference.
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