Chip interleaving for performance improvement of coded DS-CDMA systems in Rayleigh fading channels

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.