Fang-Biau Ueng

Performance Analyses of OFDM–CDMA Receivers in Multipath Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising scheme for multicarrier (MC) transmission. The combination of OFDM and code-division multiple access (CDMA) which is referred to as OFDM-CDMA, has recently attracted much research interest in wireless communications. In this paper, we evaluate the downlink performance for two forms of adaptive OFDM-CDMA receivers in multipath fading channels: 1) MC-CDMA and 2) MC-direct sequence (DS)-CDMA. We propose theoretical minimum mean square errors (MMSEs) for MC-CDMA and MC-DS-CDMA to compare the performances in different situations. Moreover, one drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC-DS-CDMA systems. We propose adaptive receiver structures with both subcarrier- and code-interval equalizations to improve the performance of MC-DS-CDMA systems with shorter or no CP. Simulation results demonstrate the effectiveness of the proposed receivers and the correctness of the theoretical derivations.

A low-complexity adaptive receiver for DS-CDMA systems in unknown code delay environment

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

Adaptive blind equalization using second- and higher order statistics

This paper presents two classes of adaptive blind algorithms based on second- and higher order statistics. The first class contains fast recursive algorithms whose cost functions involve second and third- or fourth-order cumulants. These algorithms are stochastic gradient-based but have structures similar to the fast transversal filters (FTF) algorithms. The second class is composed of two stages: the first stage uses a gradient adaptive lattice (GAL) while the second stage employs a higher order-cumulant (HOC) based least mean squares (LMS) filter. The computational loads for these algorithms are all linearly proportional to the number of taps used. Furthermore, the second class, as various numerical examples indicate, yields very fast convergence rates and low steady state mean square errors (MSE) and intersymbol interference (ISI). MSE convergence analyses for the proposed algorithms are also provided and compared with simulation results. >

A High-Capacity TH Multiple-Access UWB System With Performance Analysis

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.

Smart antennas for multiuser DS/CDMA communications in multipath fading channels

Smart antennas, with spatial processing, used in code division multiple access (CDMA) multiuser communications can enhance range, reliability and capacity. Moreover, adaptive beamforming techniques can remove unwanted noise and jamming from the received signal. We use a least mean squares algorithm and propose four adaptive antenna receiver structures for a direct-sequence (DS) CDMA multiuser environment with multipath fading channels. Narrowband and wideband adaptive array receiver structures are considered. LMS algorithms are employed in both narrowband and wideband receiver structures to adjust the spreading code coefficients. We call these new schemes Wiener code filters. The weights of the adaptive beamformer and the spreading code are updated every bit interval, so the computational complexity is very low. Simulation examples are given to compare the performances of the proposed receiver structures.

Adaptive receiver for DS/CDMA multiuser communications

In code division multiple access (CDMA) mobile communication systems with time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath effect usually change the characteristics of the spreading codes. Modification of the conventional receiver structure is needed to account for the interference of the multipath fading. This paper proposes an adaptive receiver for such multiuser direct sequence (DS) CDMA systems in multipath fading. We also analyze the mean square error (MSE) performances of the proposed receiver and the conventional receiver. Both the MSE analysis and simulation results show that the performance of the proposed receiver are better than that of the conventional receiver.

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.

Adaptive Receivers for DS/CDMA Multiuser Communication in Multipath Fading Channels

In direct-sequence code division multiple access (DS/ CDMA) multiuser communication systems with multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath effect usually changes the characteristics of the spreading codes. Modification of the conventional receiver structure is needed to account for the interference of the multipath fading. This paper proposes four adaptive receivers for such multiuser DS/CDMA systems in multipath fading channels. We employ least mean square (LMS) and recursive least squares (RLS) algorithms for both finite impulse response (FIR) and infinite impulse response (IIR) receiver structures. Mean square error (MSE) and convergence analysis are also given in this paper. Simulation results show the performance comparisons of the four proposed receivers.

Performances of OFDM-CDMA receivers with MIMO communications

This paper considers the joint design of OFDM and code division multiple access CDMA with MIMO communications. The receivers for both Bell Labs Layered Space Time system and space-time block code are designed in this paper. The performances of the MIMO-OFDM-CDMA receivers are investigated in several typical fading channels. The maximum likelihood, zero forcing, and minimum mean square error receivers for the MIMO-OFDM-CDMA systems are derived, and the performances in the typical fading channels are verified by the simulations. Copyright

Blind adaptive beamforming based on generalized sidelobe canceller

Abstract The generalized sidelobe canceller (GSC) is an efficient implementation of the direction constrained adaptive array. Conventional GSC is designed according to a quiescent weight vector and a blocking matrix. The quiescent weight vector provides the array with specified array response at some direction. The blocking matrix is designed based on a priori knowledge of the direction of arrival (DOA) of the desired signal. In this paper, we propose a new GSC-based adaptive array without a priori knowledge of the DOA of the desired signal. This paper utilizes eigensubspace decomposition and statistically cyclostationary properties of the signals to design the adaptive array. A method for constructing the most efficient blocking matrix for the GSC is developed. Simulation examples illustrate the effectiveness of the proposed method.