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The performance of conventional DS-CDMA systems is greatly affected by asynchronous or multipath propagation. We show that, with a certain way of multicode assignment, chip-interleaved DS-CDMA (CIDS-CDMA) can perform better in such a condition thanks to its ability to preserve the synchronous correlations among the spreading codes. Compared to the recently proposed chip-interleaved block-spread CDMA (CIBS-CDMA) that requires a single-user equalizer, the presented scheme can attain better or comparable performance in channel-coded transmission with a rake receiver that has lower complexity

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.

The capacity of conventional direct-sequence code-division multiaccess (DS-CDMA) systems is limited by multiaccess interference (MAI) that arises from correlation among user signals, especially in multipath channels. While inducing an intersymbol-interference (ISI) problem, chip-interleaved DS-CDMA (CIDS-CDMA) can preserve the orthogonality among user signals and effect MAI-free transmission. We propose a multicode CIDS-CDMA (MCIDS-CDMA) technique which can also remove ISI in addition to MAI when a sufficient number of multicodes are assigned. This design facilitates a low complexity receiver structure. In addition, a low complexity ISI canceller can be used to deal with the residual ISI, should the number of codes be insufficient.

Multiple access interference (MAI) is a major limiting factor of DS-CDMA system capacity. Parallel interference cancellation (PIC) is an effective means to mitigate MAI. However, if the initial decisions on which the PIC is based have high error probabilities, then PIC cannot perform well. To improve the performance of PIC, we propose chip-interleaving to provide intra-bit diversity. Then the initial decisions can be more accurate so as to enhance the PIC's performance. We design a chip-interleaving scheme based on some 3GPP WCDMA system features, such as the use of the Q-branch to transmit control bits and the I-branch to transmit data bits in a QPSK-like modulation scheme. Simulation results show that the proposed chip-interleaved WCDMA (CI-WCDMA) transmission can have significant performance advantage compared to simple WCDMA in transmission over fading multipath channels with MAI.

In mobile wireless communication, the signal is usually subject to fading. Gain-combining diversity is a common remedy for fading effects. Practical channel coded direct-sequence code-division-multiple-access (DS-CDMA) uses bit interleaving to effect gain-combining time diversity through the redundancy provided by the channel code. Recently, some have proposed chip-interleaved DS-CDMA (CIDS-CDMA). Simple CIDS-CDMA considers a set of bits at a time. Similarly indexed chips in the bits are grouped together for transmission. Such interleaving has been shown to yield diversity gain in uncoded transmission. This paper considers the performance of CIDS-CDMA with channel coding. For simplicity, synchronous DS-CDMA is considered. In addition to simple CIDS-CDMA, we also consider combined bit-and-chip interleaving (BCIDS-CDMA). We present theoretical analysis and simulation results for both cases under long-code and short-code spreading. It is concluded that the code period is vital for simple CIDS-CDMA but is of no importance for BCIDS-CDMA. Another conclusion is that BCIDS-CDMA yields the best performance for it provides the largest diversity gain.

Successive interference cancellation (SIC) is an effective technique to reduce multiple access interference and thereby increase capacity. While a conventional receiver attains best performance under equal received power for all users, a SIC-based receiver favors unequal powers. We consider ways to assign powers to the received user signals for the benefit of SIC-based multiuser detection. Two power assignment methods, both allocating user signal powers according to their sequence of detection in SIC, are investigated. Through numerical simulation, the methods are shown to greatly improve the bit error rate (BER) performance compared to equal-power assignment. The simulation results also show that, under these methods of power assignment, SIC yields superior performance to parallel interference cancellation (PIC) in high-data-rate transmission.