In direct-sequence code division multiple access (DS/CDMA) multiuser communication systems in multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath channel characterizes the propagation effects including the timing offset and delays, etc. Traditionally, we use the delay-locked loop (DLL) code tracking loop to recover the timing delay. But DLL cannot work well in multipath environment. In this paper, we propose a low-complexity adaptive receiver to suppress ISI/MAI and solve the timing offset problems without using conventional DLL code tracking loop. The proposed receiver employs an adaptive filter whose weights are adapted using a block least-mean square error algorithm with fractional sampling. Simulations confirm the good performance, including learning curves and theoretical analysis of minimum mean-square error, of the proposed receiver. Copyright (C) 2010 John Wiley & Sons, Ltd

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Adaptive antennas, with spatial processing, used in code division multiple access (CDMA) systems can enhance range, reliability, and capacity. Moreover, the adaptive beamforming technologies can remove unwanted noise and interference from the received signal. In this paper, we propose the adaptive direct sequence (DS) CDMA antenna receivers and compare their performances in three existing multirate methodologiesvariable processing gain (VPG), variable chip rate, and multicode systems for multiuser communications in multipath fading channels. Simulation results show that the processing powers of both narrowband and wideband adaptive antennas are not enough. We propose a powerful processing technique named Wiener Code Filter (WCF) to retrieve the shortcomings. Mean square error (MSE) analyses are also given in this paper. Some simulation examples are given to compare the performances of the proposed multirate structures. Copyright (c) 2010 John Wiley & Sons, Ltd

Both intersymbol interference (ISI) and multiple-access interference (MAI) are the important issues in code division multiple access (CDMA) multiuser communication systems. The step size of the traditional least-mean-square (LMS) adaptive receivers must be substantially adjusted to overcome the effect due to different channel responses or different numbers of active users. The normalized-LMS (NLMS) algorithm call automatically manipulate the adjustment to avoid the problem just discussed. This paper proposes the symbol-based interference rejection filter with NLMS algorithm and derives both finite impulse response (FIR)- and infinite impulse response (IIR)-type algorithms. We also derive the optimal step sizes and minimum mean-square errors (MSEs) for both the FIR and IIR symbol-based receivers. The complexity of our proposed FIR receiver is lower than that of the conventional chip-based receivers. Ill addition, it is shown that the bit error rate performance of our proposed symbol-based receivers is superior to conventional one in the simulations. Simulations also show the correctness Of Our theoretical analysis of minimum MSE. Copyright (C) 2008 John Wiley & Sons, Ltd

A flexible time-hopping (TH) multiple-access (MA) ultrawideband (UWB) system employing M-ary orthogonal pulse position modulation (PPM) is implemented and analyzed in this paper. In contrast to the conventional TH method using the fixed partition of TH slots, a new flexible TH scheme that utilizes the whole frame to carry out TH and PPM is proposed. The advantage of the proposed scheme is to increase the modulation level of PPM and improve the capability of MA. Based on the analytical CF technique, we also derive the probability distribution of the MA interference (MAI) and the average symbol error probability of the proposed scheme. Simulation results are provided to demonstrate the effectiveness of the proposed TH system and verify the analytical frameworks in both the additive white Gaussian noise (AWGN) channel and the realistic UWB fading channels. Irrespective of the adopted modulation level of PPM, the proposed TH system outperforms the conventional TH scheme. The tradeoff of the system parameters for the proposed TH system under the constraints of fixed bit rate and signal bandwidth is also examined