In this paper, we propose symbol-based receivers for orthogonal frequency division multiplexing OFDM code-division multiple-access CDMA multiple-input-multiple-output MIMO communications in multipath fading channels. For multiuser and multipath fading environments, both intersymbol interference and multiple-access interference must be considered. We propose narrowband and wideband antennas and Wiener code filter for MIMO OFDM-CDMA systems. The proposed receivers are updated symbol-by-symbol to achieve low computational complexity. Simulation results show that the proposed Wiener code filter can improve the system performance for the proposed adaptive antennas. The wideband antenna can achieve better error-rate performance than that of the narrowband antenna when multipath effect exists. The convergence rate of the recursive least squares antennas is faster than that of the least mean square antennas. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, we propose three adaptive blind algorithms for multiuser multicarrier code division multiple-access systems in multipath fading channels. The proposed adaptive blind receivers are based on the property of the discreteness of the input data symbol and are updated in every symbol interval. We also use the concept that the variance of the output signal approaches to the variance of the desired signal to get the cost function. The three proposed receiver structures are the traditional finite impulse response FIR structure, the despreading DES filter structure and the generalized sidelobe canceller GSC structure. The advantage of the FIR filter is that the length of the filter weights does not have to be the same length as the spreading code. For the DES filter, the combination of the adaptive weight and the despreading code has the simplest structure than the other two proposed receiver structures. The constrained GSC filter is superior to the other two proposed receiver structures in the environments dominated by multiple-access interference. By this constraint, the blind GSC filter can guarantee to converge to the desired solution. Simulation results are given to show the effectiveness and comparison of the proposed adaptive blind receivers. Copyright ©2012 John Wiley & Sons, Ltd.

A N-ary biorthogonal pulse position shape modulation (BPPSM) for hybrid time hopping/ direct sequence (TH/DS) multiple access ultrawideband (UWB) system is proposed and analyzed in this paper. The characteristic function (CHF) of the multiple access interference (MAI) is derived and, accordingly, the average symbol error rate (SER) of the proposed system can be accurately evaluated. Simulation are carried out to verify the analytical results in additive white Gaussian noise (AWGN) channel and to illustrate the system performance in realistic UWB fading channels. As increasing the number of adopting pulses, the proposed TH/DS Nary BPPSM system can provide better SER performance compared with the TH/DS Nary BPPM scheme. Given the constraints of fixed bit rate and signal bandwidth, the impact of adjusting various system parameters on the system performance is also investigated.

Abstract 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 methodologies—variable 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 © 2010 John Wiley & Sons, Ltd.

A hybrid time-hopping/direct-sequence (TH/DS) multiple access ultra-wideband (UWB) system combined with N-ary biorthogonal pulse position modulation (BPPM) is proposed and analyzed. Based on the exact characteristic function (CF) technique, we also derive the probability distribution of the multiple access interference (MAI) and the average symbol error rate (SER) of the proposed scheme. Simulation results are provided to verify the analytical frameworks. The tradeoff of the system parameters for the proposed system under the constraints of fixed bit rate and signal bandwidth is also examined.

A time-hopping (TH) multiple access ultrawideband (UWB) system combined with a large modulation level of M-ary pulse position modulation (PPM) is considered and analyzed. Unlike the conventional TH method that the number of the provided TH slots becomes fewer as the adopted modulation level of M-ary PPM is lager and the data rate is fixed, a feasible TH scheme that utilizes the whole frame duration to carry out TH and PPM is proposed in the paper. The advantages of proposed TH system are to use the large modulation level of PPM, reduce the effect of the multiple access interference (MAI) and increase multiple access capability. The characteristic function (CF) of the MAI for proposed sytem is derived to obtain the probability density function of MAI and average symbol error rate. The simulation results of the proposed TH system are provided to verify the analytical results and show the performance improvement over the conventional TH and DS UWB systems.

This paper investigates the optimal step-size mechanism for a stochastic gradient (SG) based direct sequence (DS-) code division multiple access (CDMA) receiver under the influence of the multiple access interference (MAI) and intersymbol interference (ISI) in multi-path environments by utilizing a simple stochastic model. The convergence of the derived optimal step-size sequence is verified analytically. The learning curve is applied to analyze and compare the speed of convergence and residual mean-squared error (MSE) for the fixed and time-varying step-size sequence methodologies. Simulations show that the receiver utilizes the derived optimal step-size sequence not only has faster convergent speed and smaller MSE, but also is robustly to different multi-path channels.

This paper proposes and compares three multi- rate receiver structures with post-fast Fourier transform (FFT) type and the pre-FFT type for orthogonal frequency division multiplexing code-division multiple access (OFDM-CDMA) communication systems. In multipath fading channel, the transmitted signal suffers from the effects of inter-chip interference (ICI) and multiple access interference (MAI). In order to solve these problems and support multimedia traffic, we propose three multi- rate methodologies- variable processing gain (VPG), variable chip rate (VCR) and multi-code systems to investigate the performance of the OFDM-CDMA communications in multipath fading environments. Simulations confirm the good performance of the proposed receiver structures.

In this paper, we propose and analyze a new noncoherent receiver with PN code tracking for direct sequence code division multiple access (DS-CDMA) communication systems in multipath channels. We employ the decision-feedback differential detection method to detect MDPSK signals. An ";error signal"; is used to update the tap weights and the estimated code delay. Increasing the number of feedback symbols can improve the performance of the proposed noncoherent receiver. For an infinite number of feedback symbols, the optimum weight can be derived analytically, and the performance of the proposed noncoherent receiver approaches to that of the conventional coherent receiver. Simulations show good agreement with the theoretical derivation.

The performance of low-density parity-check (LDPC) codes is investigated for fast frequency hopping / binary frequency shift keying (FFH/BFSK) in partial-band noise jamming channels. We employ different diversity combining schemes to compute the codeword bit decision reliability. In this paper, we consider the maximum-likelihood (ML) diversity combining scheme and two low-complexity suboptimal diversity combining schemes: square-law with adaptive gain control (AGC) combining and square-law with equal gain combining. Simulation results show that, by combining both of the diversity and coding gain, our proposed system can provide excellent performance against partial-band noise jamming.

OFDM is a promising scheme for multicarrier (MC) transmission, since it has the advantage of high spectral efficiency, robustness to frequency selective fading, and feasibility of low-cost transceiver implementation. The combination of OFDM and CDMA, referred to as OFDM-CDMA, recently attracted most research interests in wireless communications. In this paper, adaptive receivers for OFDM-CDMA communication systems are developed based on the least mean square (LMS) algorithm. We evaluate the down-link performance for two forms of OFDM-CDMA schemes which are MC-CDMA and MC-DS-CDMA in multipath fading channels. A 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. Thus we propose adaptive receiver structure that exploits both frequency-domain and time-domain equalization to improve the performances of MC-DS-CDMA systems with shorter or no CP. Simulation results show performance comparisons between MC-CDMA and MC-DS-CDMA systems, which also verify the performance improvement of our proposed receiver for MC-DS-CDMA systems exploiting shorter or no CP.

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.

This paper proposes a direct blind equalization algorithm based on multiple-shift correlation (MSC) property of the received data. Using the adaptive beamforming technique, a partially adaptive channel equalizer (PACE) which allows the number of the adaptive weight; to be less than the number of all the channel parameters is developed. The PACE can achieve fast convergence speed and low implementation complexity.

It is well known that an adaptive filter with a large step size in the transient period and a small one in the convergence period gives small mean squared steady state error while achieving a fast convergence rate. Based upon this idea, we present two variable step size (VSS) blind equalizers. The first one employs an intersymbol interference (ISI) estimator to control the step size, the second algorithm uses a mean squared error (MSE) estimator to adjust the step size. Both accomplish what has been expected.

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.>