Aigang Feng

Equivalently blind time-domain channel estimation for MC-CDMA system over frequency-selective fading channels in multiuser scenario

Many blind channel estimation methods have been proposed for direct sequence (DS) CDMA systems, so we can certainly use them to estimate the finite impulse response (FIR) channel for the multicarrier code division multiple access (MC-CDMA) system. In this paper, we interpret the MC-CDMA system as an equivalently DS-CDMA system with specific spreading codes. Then, an equivalently blind time-domain channel estimator is derived for the MC-CDMA uplink, which is based on second-order statistics of the received data. Because there is no need of a large amount of sampling data, this method is greatly suitable for the fast time varying wireless channels. By exploiting singular value decomposition (SVD) and the finite alphabet property of transmitted symbols, the time-domain channel impulse response (CIR) for the MC-CDMA can be accurately identified. Computer simulations illustrate both the validity and the overall performance of the proposed estimator.

Blind channel estimation in synchronous MC-CDMA system with consideration of carrier offset compensation

The multicarrier code division multiple access (MC-CDMA) is a flexible and adaptive radio interface to provide dynamical bit rate transmission and a variety of services on radio channels. It can sufficiently utilize the subcarrier diversity and meet the demand of diverse multimedia services; therefore it is broadly considered as an effective approach for future wireless multimedia communications. However, it is very sensitive to carrier drifts, while the orthogonal property between the subcarriers is lost with frequency offset. An accurate channel estimate allows the use of multiuser detection. After reviewing some blind channel estimation algorithms with the signal time domain structure and statistic characteristic, we propose a constant modulus based adaptive blind channel estimation at the base station for MC-CDMA systems in a frequency-selective fading channel environment with the consideration of carrier offset compensation. Compared with the SVD-based method and finite alphabet-based method, it illustrates the effectiveness with numerical simulations.

An vertical layered space-time coding and the symbol detection

Recent results in information theory have demonstrated the enormous potential of wireless communication systems with antenna arrays at both the transmitter and receiver. To exploit this potential, vertical layered space-time codes is attractive for its relative simplicity and robust high capacity. Thus far, vertical layered space-time code assumed that perfect estimates of current channel fading conditions are available at the receiver. In certain situations, however, it may be difficult or costly to estimate the channel accurately. In this paper, by applying the subspace method, we propose a vertical layered space-time code to detect symbols without the knowledge of the channel. From Monte Carlo simulations, we show that performance can approach the detection method needed the knowledge of the channel.

Wideband direction-of-arrival estimation using fast chirplet-based adaptive signal decomposition algorithm

The direction-of-arrival (DOA) is one of important parameters, in addition to time-of-arrival (TOA) and attenuation, to describe most radio propagation channel models. Time-frequency analysis is a powerful technique to deal with time-variant or non-stationary signals. Although M.G. Amin (see Gersham, A.B. and Amin, M.G., IEEE Sig. Proc. Letters, vol.7, no.6, p.152-5, 2000; Belouchrani, A. and Amin, M.G., IEEE Sig. Proc. Letters, vol.6, no.5, p.109-10, 1999) and L. Jin et al. (see IEEE Int. Symp. on Circuits and Systems, p.375-8, 2000) combined it with a coherent signal subspace (CSS) approach, it is not easy to extend from the narrowband to the wideband case. We first overview the fast chirplet-based signal decomposition algorithm, then build the general time-frequency covariance matrix to overlap the narrowband and wideband of CSS. After discussing the difference between the two cases, we propose a wideband DOA estimation algorithm with fast chirplet-based adaptive signal decomposition. The algorithm has no restriction to the array manifold. We give the comparison of our proposed algorithm with the optimal solution of maximum likelihood (ML). We can see that the proposed algorithm has very low complexity. Its robust performance is demonstrated with numerical simulations.

Fast algorithm of adaptive chirplet-based real signal decomposition

Great attention has been focused on the chirplet-based signal decomposition in the area of signal processing, and several approaches were recently developed for complex analytic signals. In order to apply these approaches to real signals, analytic processing needs to be performed. The conventional analytic methods include the Hilbert transform in time-domain and the window function method in frequency-domain. They both have certain drawbacks. After reviewing two typical analytical methods, a novel fast algorithm of adaptive chirplet-based real signal decomposition without analytic processing is presented in this paper. Numerical simulations illustrate the effectiveness of such a novel fast algorithm.

Reduced-dimension multiuser detection based on grouping users and reducing effective length of spread spectrum code in CDMA systems

The output of the decorrelator matched to the user of interest does not have the signal of interfering users. Does the number of interfering users have an effect on the bit-error-probability (BEP) of the decorrelator? From previous research, we cannot know the impact of the number of users on the BEP. We give the relation between the BEP and the number of users and its mathematical proof is also presented. From the process of the mathematical proof, we can explain some phenomena. Based on the relation of the decorrelator to the number of users, we propose a wavelet-based reduced-dimension approach for joint space-time multiuser detection to reduce its computation load. Compared with the full dimension decorrelator, although the dimension of our algorithm is reduced, the performance does not decrease under the condition of low input SNR and large number of users. Simulations confirm our results.

Blind space-time equalization/decoding with carrier frequency synchronization in multicarrier CDMA systems

Diversity technology is an effective technique to combat channel fading. The multicarrier modulation code division multiple access (MCM-CDMA) technique, or the orthogonal frequency division multiplexing code division multiple access (OFDM-CDMA) scheme, can sufficiently utilize subcarrier diversity; therefore it is broadly considered as an effective approach for future wireless multimedia communications. It is very sensitive to a carrier frequency offset, while the orthogonal property between the subcarriers is lost with carrier drift. A space-time constant modulus algorithm (ST-CMA) is implemented for equalization and decoding of MC-CDMA signals in the frequency domain, with carrier frequency synchronization at the same time. The proposed method has faster convergence rate and less computational complexity, without requiring the periodic transmission of training blocks. Numerical simulations illustrate the effectiveness of this algorithm.

Joint space-multipath-Doppler RAKE receiving in DS-CDMA systems over time-selective fading channels

Diversity technology is an effective technique to combat channel fading. The conventional RAKE receiver improves the performance of a system through temporal diversity combination of delayed replicas of the same desired signal. The diversity idea behind the RAKE receiver is now extending from time domain to frequency domain (Doppler shift) and space domain (antenna array beamforming). As is well known, fast fading results in a Doppler spread, and the system performance degrades under these time-selective channels. The joint multipath-Doppler RAKE proposed by A. M. Sayeed (see Sayeed et al., 1998; Sayeed and Aazhang, B., 1998; Bhashyam, S. et al., 2000), achieves diversity gains on the time-frequency domain. By combining adaptive antennas with RAKE receivers, space-time 2D RAKE simultaneously exploits space-time diversity. We propose a new receive structure in which the above two 2D RAKE are combined together, providing a joint space-multipath-Doppler three dimensional (3D) RAKE. Although the computation complexity increases a little, the 3D RAKE matched filter output provides a maximum SINR estimate of the signal by optimally combining the desired signals from different paths, different antenna elements and with different Doppler frequency. Numerical simulations illustrate the effectiveness of this novel scheme.

Blind space-frequency channel estimator for MC-CDMA systems with antenna arrays in frequency-selective fading environment

The multicarrier code division multiple access (MC-CDMA) technique is known to be appropriate for high-data-rate wireless communications due to its robustness to multipath fading and its capability of handling high-data-rate transmissions with a simple one-tap equalizer. This paper investigates a uniform linear antenna array at the base station for macrocell MC-CDMA systems in a frequency-selective fading channel environment, and develops a blind space-frequency channel estimator for an MC-CDMA system with antenna array. As a by-product, the direction-of-arrivals (DOAs) can also be estimated. By constructing a specific auxiliary matrix including both the space-frequency channel and DOA information, and performing eigen decomposition on it, we can obtain the closed-form solution of space-frequency channels and DOAs for all active users within the macrocell. The overall performance of this novel scheme is demonstrated by extensive computer simulations.

Adaptive diversity combination of space-time block coding in OFDM-CDMA systems with pilot-based channel estimation

We investigate the transmit diversity for orthogonal frequency division multiplexing code division multiple access (OFDM-CDMA). Diversity transmission at the base station is an effective technique to combat channel fading. Space-time (ST) coding has been proposed as coding technique to improve the system performance with additional gains. By combing the spatial transmit diversity with the space-time codes for OFDM systems it shows high performance improvement. But it is important to remark that ST decoding requires multichannel state information at the receiver. Thus the achievable diversity gain comes at the price of proportional increase in the amount of training or computation complexity, which incurs efficiency loss especially in a rapidly varying environment. In an OFDM system, there exist pilots used for frame detection, carrier frequency offset estimation and channel estimation. We proposed a receive scheme, which use the pilot structure to estimate the multichannel state information and use the CMA-based channel tracking in the space-time coded transmit diversity OFDM-CDMA systems. Numerical simulations illustrate the effectiveness of this algorithm.