In this paper, a quasi-synchronous code-division multiple-access (QS-CDMA) is investigated for application in the reverse link of a microcellular or indoor mobile communication environment. In a QS-CDMA system, the relative time delay between the signals of different users is normally restricted within a few chips. Generalized orthogonal (GO) codes added with guard chips are employed as the spreading sequences in this paper and the strict timing error restrictions for BPSK and M-QAM modulation schemes are derived based on the correlation properties of GO code set which determines the multiple access interference (MAI). The results reveal that the system employing GO code set with bigger GO zone can tolerate more serious timing error, and higher order modulation schemes require stricter synchronization. Based on the system model presented, the BER performance for BPSK and M-QAM is evaluated by Gaussian Approximation (GA) and Monte Carlo simulation. The system capacity in terms of acquirable total bit rates are also evaluated, revealing that if system synchronization error is limited within the GO zone, M-QAM scheme can be utilized to improve the system capacity.

In this paper, a hierarchical bit mapping (HBM) scheme suitable for M-level quadrature amplitude modulation (M-QAM) packet transmission is presented. Generally, M-QAM, such as 16QAM or 64QAM, consists of multiple bits with differing bit error-rate quality. Because the packet error rate when using M-QAM is highly dependent on the performance of the "weakest" (poor quality) bits, the throughput of M-QAM packets can dramatically deteriorate especially in a multi-path environment. This paper proposes the use of a bit mapping scheme conceptually similar to hierarchical QAM to improve packet transmission throughput. For 16QAM under this scheme for example, as there are four bits in a 16QAM symbol, four independent packets can be simultaneously transmitted on each of the different bits when HBM is used. In doing so, at least two packets can be transmitted with a high probability of success even under poor transmission conditions, and under good transmission conditions, four packets can potentially be successfully transmitted. An interference cancellation (IC) method is then presented for HBM, with simulation results showing that the combination of HBM with IC results in very good performance. It was also determined that the HBM scheme can be used easily and effectively with hybrid ARQ.

This paper presents an adaptive burst-mode M-QAM modem architecture suitable for variable rate broadband wireless packet data networks. The core signal processing functions for the modem are common to all constellations resulting in an efficient hardware architecture for field programmable gate array (FPGA) implementation.