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We present a unified performance analysis for the conventional bit-interleaved direct-sequence (DS) code-division multiple access (BIDS-CDMA) and the more recently proposed chip-interleaved DS-CDMA (CIDS-CDMA), both with channel coding. Simple CIDS-CDMA treats a set of bits at a time and interleaves their chips together for transmission. But bit interleaving may also be used on top of chip interleaving (thus abbreviated BCIDS-CDMA) to enhance performance. For simplicity, we first tackle flat-faded synchronous transmission, in which we treat both the condition with perfect power control and that where the received signal is subject to Rayleigh fading. We then extend the analysis to asynchronous and multipath channels, with the latter treated only briefly. By approximating the correlation among the spreading codes (rather than the ensuing interference) as Gaussian, we obtain novel and relatively simple results for the various conditions above. In general, BCIDS-CDMA performs best, followed by simple CIDS-CDMA and then by BIDS-CDMA. Within simple CIDS-CDMA and BIDS-CDMA, long-code spreading performs better than short-code spreading. For BCIDS-CDMA with perfect interleaving that fully randomizes the fading coefficients, the performance is not affected by the spreading code period. The above ordering of performance follows the amount of diversity each scheme exploits, where the diversity may come from spectrum spreading, channel coding, and independence in fading of different paths. Simulation results agree well with the theoretical analysis.
Date of Publication: May 2005