Adaptive H/sub ∞/-subweight equalizer for DS-CDMA communication system in time-varying multipath channels

Based on Alamouti code, two-branch transmit diversity DS-CDMA systems, namely, space-time block coded DS-CDMA (STBC-DS-CDMA) systems are discussed. Besides the simple maximum-likelihood (SML) detector, three novel detectors are applied to improve the STBC-DS-CDMA systems. Additionally, for random binary spreading code (RBSC), the performance of all systems in spatially uncorrelated time-varying multipath Rayleigh fading channels are derived theoretically. In consequence of the simple expression for the variances of the multipath interference (MPI) and the multiuser interference (MUI), our analytical results are easy to calculate. Numerical outcomes have revealed that significant performance improvement can be achieved even when the systems are operated in highly selective channels.

In this letter, for the first time, we propose to employ the adaptive frequency-domain pre-equalization (FDPE) into an electro-absorption modulator laser (EML)-based optical self-interference cancellation (OSIC) system to suppress the time-varying multipath radio frequency (RF) self-interference (SI) signal, which is implemented for in-band full-duplex (IBFD) communication. Faced with time-varying multipath wireless channel, the OSIC system estimates multipath channel response using a least square estimation algorithm, and adjusts the coefficients of frequency-domain pre-equalizer adaptively to combat time-varying multipath SI. Based on the proposed FDPE-aided OSIC, the IBFD transmission of OFDM RF signals with time-varying multipath channel is demonstrated experimentally, achieving 30-dB OSIC depth over 1.22-GHz broad baseband.

This paper develops a general framework for communication over time-varying multipath fading channels via orthogonal short-time Fourier (STF) basis functions. In general, STF basis functions interfere with each other due to the loss of orthogonality by channel dispersion. It is shown that the channel spread factor, the product of multipath and Doppler spreads, plays a key role in determining system performance. Smaller spread factors result in lower interference. A simple and approximately optimal pulse scale adaptation rule is derived to minimize interference by matching pulse properties to channel characteristics. For sufficiently small channel spread factors, a scale-adapted STF basis serves as a set of approximate eigenfunctions of the channel. However, for relatively larger spread factors, residual interference may cost a large degradation in performance. A highly effective iterative interference cancellation technique, sequential iterative interference cancellation (SIIC), is proposed to mitigate the residual interference between basis functions. Analytical and simulation results demonstrate the excellent performance of the proposed signaling framework.

Multicarrier code-division multiple access (MC-CDMA) combines multicarrier transmission with direct-sequence (DS) spread spectrum techniques. In this study, a novel multiple channel estimator, which consists of several possible channel models and can be self-adjusted to fit different channel uncertain conditions, is proposed for MC-CDMA systems to effectively estimate channel coefficients in the frequency domain over time-varying multipath fading channels. Based on the proposed channel estimation, an enhanced minimum mean-square error (MMSE) equalizer is employed for symbol detection of MC-CDMA systems. Simulation results demonstrate that the proposed multiple channel estimator can track the channel more accurately than the conventional channel estimators. With the consideration of channel estimation error, the enhanced equalizer based on the proposed multiple channel estimator performs much better than those based on the conventional channel estimators in bit error rate (BER).

In this paper, we proposed a frequency-hopped space-time coded orthogonal frequency-division multiplexing (FHST-OFDM) over time-varying multipath channels. Although OFDM is robust against frequency-selective fading channels, it is more vulnerable to the time-varying channels and has a higher peak-to-average power ratio (PAPR) than single-carrier systems. In space-time block coded OFDM (ST-OFDM), channel time variations cause not only the intertransmit-antenna interference (ITAI), but also the inter-carrier interference (ICI). In this paper, based on the analysis of the ITAI and ICI in ST-OFDM systems, the FHST-OFDM transmission scheme is proposed to reduce both ITAI, ICI, and PAPR simultaneously. By combining the Alamouti scheme and the frequency hopping, full data rate and frequency diversity can be achieved by the proposed FHST-OFDM over the time-varying multipath channels. Finally, simulation shows that the proposed FHST-OFDM scheme gets better performance over time varying channels.

Effects of frequency offset compensation error on channel estimation for OFDM system under mobile radio channels

Channel interpolation for LTE uplink systems with high mobility using Slepian sequences

The architecture of an OFDM system is shown as Fig. 1. In the transmitter, the frequency-domain signal of j-th OFDM symbol can be expressed asWhere N is the number of subcarriers and Nsym is the number of OFDM symbols in one transmission frame. The time-domain signal of j-th OFDM symbol can be expressed asWhere F is the N × N discrete Fourier transform (DFT) matrix with e-j(m-1)(n-1)/N to be its (m, n) element, and FH is the Hermitian transpose of F. The time-varying multipath fading channel with CFO and additive white Gaussian noise (AWGN) are considered. The discrete-time impulse response of fading channel with the maximum path delay L is denoted asg(n,l) = [g(n,0) g(n,1) … g(n,L-1)]. (3)Normalize CFO, ε, and AWGN with zero mean and variance sn2 is involved. In the receiver, thetime-domain signal of j-th OFDM symbol removed GI is given byε= +j jy E Gx n( ) , (4)Where(5)(6)Andn = −n n n N T[ (0) (1) ( 1)] .

In this paper, we tackle the problem of theoretical evaluation for the multistage parallel interference cancellation (PIC) scheme in a DS-CDMA system with orthogonal modulation and long scrambling codes. The studied system operates on the reverse link in a time-varying multipath Rayleigh fading channel. By applying the Central Limit Theorem to multiple access interference (MAI) and intersymbol interference (ISI), as well as identically distributed chips from a single interferer, the bit error rate (BER) performance of the PIC scheme at any stage can be recursively computed from the signal-to-noise ratio, number of users, the number of paths per user, processing gain of the CDMA system, as well as the average received power of each path. The proposed approximative analysis is validated by the Monte-Carlo simulations and proved to be accurate, and it gives insight into the performance and capacity one can expect from the PIC based receivers under different situations.

Correct time and frequency synchronization is important for the performance of orthogonal frequency-division multiplexing (OFDM) system. While using the partial power training sequences to synchronize in orthogonal frequency-division multiplexing (OFDM) system, the received data signals will be interfered by the training sequences. In this paper, we give a method for suppressing the interference caused by the training sequence, and introduce an optimized formula for the partial interference canceling factor of partial power training sequence. The above technique is evaluated by means of pilot assistant coherent OFDM system under time-varying multipath Rayleigh fading channels, and the linear interpolation channel estimator is employed to exploit the frequency domain correlation. Especially we focus on only one OFDM symbol in a packet transmission, that is, both the OFDM data and the training sequences are included in one OFDM symbol, and there are no other symbols before or after it. Both theoretical and simulated results of the above technique are presented. It has been shown that the better BER performance can be achieved as compared with no separation, and only the one OFDM symbol can accomplish user data communication assignment.

We evaluate the BER performance of the subchannel power controlled OFDM system considering more practical cases including the limitation in word length in subchannel power control tap-weights and the time-varying multipath channel. According to simulation results, the total required number of bits is 5 for the optimal subchannel power control and the main source of the performance degradation of the subchannel power controlled OFDM system is the time-varying multipath channel.

This paper considers the problem of robust channel estimation for direct-sequence code-division multiple-access (DS-CDMA) systems with time-varying multipath fading channels. Due to the lack of accurate knowledge of model dynamics and statistics of system noises, a polynomial channel estimator is developed by virtue of the Krein space theory. The estimator is calculated by performing a J-spectral factorization. In this paper, under some conditions, we obtain a closed-form solution to this J-spectral factorization, which greatly reduces the computational load of the estimator design. The simulation results show that the proposed estimator provides a robust estimation performance against various types of noises and modeling errors.

For direct-sequence code-division multiple-access (DS-CDMA) time-varying channels with additive Gaussian noise, we propose a new receiver. Since channel tracking plays an essential role in practice in equalization of the receiver, a new and viable design approach, a modified interacting multiple-model (MIMM) algorithm, is proposed for tracking of multipath time-varying fading channels. We employ a set of second-order auto-regressive (AR2) multiple models associated with a Makov jump process to cover the range of channel dynamics. This setup leads to an AR2-based MIMM channel tracker which can automatically adjust channel model parameters to track time-varying fading channels for all times once some design parameters are initialized. We employ a Kalman filter as a symbol detector (KBSD). Simulation studies demonstrate that the performance of the AR2-MIMM channel tracker together with a conventional KBSD performs at least as well as that of the ideal AR2-based Kalman channel estimator having the exact knowledge of the Doppler rate of each channel in combination with the KBSD

Intercarrier interference caused by fast time-varying multipath fading channels degrades the system performance of high-mobility orthogonal frequency division multiplexing systems. This study consi...

In this paper, we present a blind code-timing estimator for asynchronous DS (direct-sequence) CDMA (code-division multiple-access) systems. that utilize bandlimited chip waveforms. The received signal is first Fourier-transformed to the frequency domain, followed by a frequency deconvolution to remove the effect of the chip waveform. The code-timing is next derived from the output of a narrowband filter with a sweeping center frequency, which is designed to suppress interference of various sources, including, e.g., MUI (multiuser interference), ISI (inter-symbol interference), CCI (co-channel interference), and narrowband interference. The proposed estimator requires only the spreading code of the desired user. It can be used in frequency-flat or frequency-selective, time-invariant or time-varying fading channels. Moreover, the proposed estimator can be readily implemented using standard adaptive schemes, making it appealing not only for acquisition but also for tracking. Numerical results are presented to illustrate the performance of the proposed scheme.