Single-Carrier Block Transmission Asynchronous CDMA with Frequency-Domain Equalization

Single-carrier (SC) block transmission with frequency-domain equalization (FDE) (Z. Wang et al., 2004), (Z. Wang and G.B. Giannakis, 2001) is an efficient scheme that avoids complex time-domain equalization in multipath channels. There are also several systems proposed that apply SC-FDE in code-division multiple access (CDMA). However, unlike the conventional CDMA RAKE receiver, most of the present SC-FDE CDMA systems may be not suitable for asynchronous users. In our previous work (Ming-Xian Chang and Chou-Chang Yang, 2006), a circularly sliding (CS) despreading was proposed at the receiver for preliminary separation of asynchronous users´ signals. By neglecting other users´ interference, an FD minimum mean-square error (MMSE) equalization algorithm was derived in (Ming-Xian Chang and Chou-Chang Yang, 2006). However, only the scenario of two asynchronous users was considered, and no further approach for cancellation of other asynchronous users´ interference was applied. In this paper, we first derive an extended signal model for which spreading, channel effect, CS despreading, and equalization can be represented as a series of circular convolutions. The extended signal model helps construct several equivalent receiver architectures. The complexity analysis of these architectures is also given. The extended signal model also implies an FD implementation of the RAKE receiver. To improve the performance of the signal detection for asynchronous users, we propose an interference cancellation (IC) algorithm under the above SC architectures and FD-MMSE equalization. We compare the performance of FD-MMSE equalization with the RAKE receiver and consider the situations with and without the IC algorithm. The simulation results validate the effectiveness of our algorithm.