Blind channel estimator for V-BLAST coded DS-CDMA system in frequency-selective fading environment

Pulse shaping OFDM systems based on offset QAM (OFDM/OQAM) are particularly attractive for wireless high-data-rate applications. Since OFDM/OQAM schemes do not employ a cyclic prefix, equalization has to be performed. In this paper, we introduce an algorithm for blind channel identification in pulse shaping OFDM/OQAM systems. Our approach assumes that the receiver knows the transmitter pulse shaping filter. Exploiting cyclostationarity of the received OFDM signal blind channel identification is accomplished using second-order statistics only. The proposed method exhibits low noise sensitivity, is computationally efficient, and does not require knowledge of the channel order. Finally, we provide simulation results demonstrating the performance of the new algorithm.

Blind identification of discrete-time single-user FIR channels with nonminimum phase is studied here. Exploiting higher order cumulants of output signals of unknown channels, a new closed-form solution to the FIR channel impulse response is derived. The algorithm is simple and fast. It relies only on nullspace decomposition of some cumulant matrices. This method neither involves the difficult task of iterative global minimization of nonunimodal cost functions, nor does it require overparametrization, which poses consistency difficulties. It can be used either as the final channel estimate or as a good initial point in nonlinear cumulant matching techniques. The application of this identification method is broad and not limited to the use of any fixed-order cumulants. Its application in identifying data communication systems shows great potential and promise.

This paper addresses the problem radio mobile channel identification and equalization by blind algorithms. For this, we have proposed two algorithms, which are compared with: cumulants based algorithms, and adaptive filter algorithms (such us the constant modulus algorithm (CMA), Fractional Space CMA (FSCMA) and Sign Kurtosis Maximization Adaptive Algorithm (SKMAA)). However, The simulation results in noisy environment and for different SNR values with QPSK or 4QAM modulation shows that the proposed algorithms gives a good identification and equalization versus to the cumulants and the adaptive filter algorithms, for radio mobile channel.

We present a novel trifunctional multiple-input-multiple-output (MIMO) scheme that intrinsically amalgamates space-time spreading (STS) to achieve a diversity gain and a Vertical Bell Labs layered space-time (V-BLAST) scheme to attain a multiplexing gain in the context of generalized multicarrier direct-sequence code-division multiple access (MC DS-CDMA), as well as beamforming. Furthermore, the proposed system employs both time- and frequency-domain spreading to increase the number of users, which is also combined with a user-grouping technique to reduce the effects of multiuser interference. Further system performance improvements can be attained by serially concatenating our proposed scheme with an outer code that is amalgamated with a unity-rate code for the sake of improving the achievable decoding convergence behavior of the proposed system, which is evaluated with the aid of extrinsic information transfer charts. We also propose a novel logarithmic likelihood ratio (LLR) postprocessing technique to improve the iteratively detected system's performance. Explicitly, the proposed system can attain a second-order spatial diversity gain and a frequency diversity gain of order V, where V is the number of subcarriers. Additionally, the proposed system attains a beamforming gain and a multiplexing gain that is twice that of a single-input-single-output system. Furthermore, after I = 10 decoding iterations and employing an interleaver depth of Dint = 160 000 bits, a time-domain spreading factor of Ne = 4, and V = 4 subcarriers, the overloaded system supporting K = 8 users requires an Eb/N0 that is only about 0.45 dB higher than the single-user system.

A class of adaptive algorithms for acoustic echo cancellation (AEC) and speech dereverberation have been developed and analyzed in this thesis. The starting point of this work is the affine projection (AP) based non-blind channel identification algorithms for AEC. The proposed. sparseness constrained improved proportionate AP algorithms (SC-IPAPA-I and SC-IPAPA-II) exploit the sparseness of the estimated channel and allocate different effective step sizes accordingly. The performance of these algorithms has been studied in the context of single-channel AEC and tracking capability. The performance of a blind channel identification algorithm with additive noise is studied by providing reasons for the degradation in perforrnance of the rnultichannel least-rnean-square (MCLMS) algorithm in the presence of additive noise. Subsequently, it is shown mathematically, through a cross-correlation based cost function, that minimizing the power of a filtered version of the received signals can suppress the noise effect. The performance of MCLMS and improved lVICLlVIS (LVICLMS) are evaluated using the normalized projection rnisalignment via lVlonte Carlo simulations. It is shown through simulations that the well-known normalized rnultichannel frequency-domain least-mean-square (NMCFLMS) algorithrn, originally developed for acoustic impulse response (AIR) identification in the frequency domain, also suffers from the noise robustness problem. A noise robust blind AIR estima.tion algorithm is then proposed. Inspired by the trivial solution achieved by NMCFLJVIS in the presence of noise, the proposed direct-path NMCFLMS algorithm with power constraint (DP-NMCFLMS-PC) jointly applies direct-path and power constraint to improve its robustness against noise. The DP-NMCLFMS-PC not only addresses the noise robustness issue but also achieves fast convergence compared to NMCFLMS. The adaptive multiple input/output inversion theorem (A-MINT) algorithm has been developed for channel equalization with application to speech dereverberation. In order to increase its convergence rate, the proposed algorithm suppresses any undesired non-zero coefficients in the estimated Kronecker delta function iteratively. This is achieved by applying the sparseness measure of the estimated Kronecker delta function and using it as an additional constraint to A-MINT. Unlike existing channel equalization systems, the proposed auto-relation aided MINT (A-RAM) algorithrn, which achieves good equalization performance, takes into account how the received signals are generated during its adaptation process. The differential output signals from two sub-systems are then utilized in the cost function during equalization. Sirnulation results have shown that the proposed A-RAM algorithm can achieve a higher rate of convergence leading to better dereverberated speech signal cornpared to existing MINT-based equalization algorithms.

The next-generation wireless personal and mobile communication systems are expected to accommodate not only high-quality voice services, but also a broad range of other multirate services. Of the various multiaccess techniques, wide-band code-division multiple access (CDMA) has been regarded as an important part of the third-generation wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multirate services. In this paper, we consider a system with a simplified receiver structure for direct-sequence CDMA (DS/CDMA) wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. We arrive at this system by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. We find that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simple one-finger matched filter is used at the receiver. This system combines the advantages of pre-RAKE diversity and transmit antenna diversity. It is shown that significant system performance and capacity improvements are possible. The numerical results also reveal that this system is not too sensitive to channel estimation errors.

A single-input multiple-output (SIMO) architecture of the frequency-modulated (FM) differential code-shift keying (DCSK) modulation technique is proposed. The new scheme employs orthogonal Walsh functions at the transmitter, with parallel substreams transmitted with a single antenna to help achieve a significant increase of the data rate. Multiple antennas are used at the receiver end to form an SIMO structure so as to obtain a diversity gain. Simulation results demonstrate that at a higher signal-to-noise ratio, the proposed SIMO FM-DCSK architecture has an outstanding bit error rate performance, in contrast to the direct-sequence (DS) vertical Bell Labs layered space-time (VBLAST) scheme that uses a complicated Rake receiver and minimum mean-square error detection, at the same data rate over multipath fading channels. In particular, the new scheme does not require any prior knowledge of the channel states, exact synchronization, and the complex Rake receiver, making the proposed algorithm simpler and yet more efficient than the DS-VBLAST scheme.

Based on the space alternating generalized expectation maximization (SAGE) technique, and the ordered serial interference cancellation (OSIC) algorithm, an improved low-complexity inter-carrier interference (ICI) mitigation scheme, named SAGE-OSIC, is proposed for high mobile orthogonal frequency division multiplexing (OFDM) systems, where the subcarriers are updated in series, according to the order of the decreasing signal to interference plus noise ratio (SINR) or signal to noise ratio (SNR). The computational complexity is investigated, and simulation studies are concluded over multipath fading and rapidly time-varying channels. It is indicated that the proposed SAGE-OSIC scheme significantly outperforms the existing low-complexity ICI mitigation methods based on SAGE, without any increase in the computational complexity, especially for the large Doppler shifts in highly mobile environments. Furthermore, it can also be concluded that the performance of the proposed SAGE-OSIC approaches that of the vertical Bell laboratory layered space-time (VBLAST) scheme, which demands very high computation.

The Vertical Bell Laboratories Layered Space-Time (V- BLAST) scheme achieves very high spectral efficiency while maintaining low implementation complexity. However, the error propagation resulting from a wrong cancellation limits the overall system performance greatly. The traditional detection methods proceed assuming that the prior-detected sub-streams are correct at each stage. None of them considers the still existed error propagation. In this paper, we propose a novel soft-information-aided detection algorithm which makes use of the log-likelihood ratio (LLR) information of the prior-detected signals. In this new algorithm, the interference canceling and ordering all take the error propagation effect into consideration, thus making the detection more reliable. We derive the new algorithms for ZF-SIC and MMSE-SIC V-BLAST respectively. Simulation results show that the new proposed algorithms achieve good BER performance compared with the traditional ones.

It has been reported that a smart antenna can improve bit error rate (BER) performance significantly in a mobile communications system (Tanaka, S. et al., IEEE JSAC, vol.18, no.8, p.1495-504, 2000). However, the smart antenna has not been applied and analyzed much for the pseudonoise (PN) code tracking loop, which is one of the important aspects in spread spectrum communications receivers. We present a performance analysis of a first order PN-code tracking loop with a smart antenna under a frequency selective fading environment, and show that the smart antenna can also improve the PN code tracking loop significantly, compared to a single antenna element system. We employ the root mean square (RMS) error and mean time to lose lock (MTLL) of the PN code tracking loop as performance criterion.

The combined use of the multicarrier (MC) technique with the vertical Bell Labs layered space-time (V-BLAST) architecture, i.e. the MC V-BLAST system is well suited to the frequency-selective fading environment. A modified rotation invariance coding scheme and corresponding decoding algorithm without knowing channel state information (CSI) are proposed under this environment. Utilizing the time redundancy, this method has the advantage that the number of receive antennas is less than that of the transmit ones. Simulation results demonstrate the validity of the coding scheme and the performance of the direct decoding algorithm.

In direct-sequence code division multiple access (DS/CDMA) mobile communication systems with time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. It has been shown that iterative equalization and decoding method is an efficient technique to combat ISI. Based on this approach, we propose a turbo equalization-like receiver for multiuser DS/CDMA systems in multipath channels. The proposed MAI-rejection/ISI suppression filter is based on a cost function evaluated at the PN code despreader's and data detector's output. Besides, our scheme does not require a training signal. Simulation results show that the proposed iterative receiver performs better than that of non-iterative scheme, and the BER decreases with the number of iteration increases

Vertical Bell labs layered space-time (V-BLAST) is designed to exploit the capacity advantage of multiple antenna system in rich-scattering environments. In this paper, we propose a low complexity detection algorithm with respect to the maximum-likelihood (ML) criterion. This algorithm deploys sorted QR decomposition to construct a ML trellis. A Viterbi-like algorithm is used to find a near optimal solution in the trellis. The complexity of this algorithm is lower than that of full ML. When small size constellation is used, the proposed algorithm is less complex than MMSE-OSIC. The performance of the proposed algorithm is close to that of full ML.

A simple algorithm is proposed for carrier frequency synchronization after PN code acquisition using FFT (fast Fourier transform) in a direct-sequence code division multiple access mobile satellite communications system. The proposed algorithm shows almost the same performance as the phase differential algorithm under additive white Gaussian noise channel environments. The proposed algorithm, however, largely reduces hardware complexity when a two-dimensional search algorithm is employed for PN code timing acquisition and coarse Doppler shift compensation since the proposed algorithm shares the FFT operator with the two-dimensional search.

In this paper, a new receiver for downlink multicarrier code-division multiple-access (MC-CDMA) system is proposed when the channel between the transmitter and each user is multiple-input multiple-output (MIMO). The structure of the proposed receiver is based on the singular value decomposition (SVD) method. The conventional SVD method is able to eliminate the co-space interference (CSI) in single user scenario, however, due to multi-channel environment, it is not able to eliminate the co-space interferences caused by the transmitted signals to the other users that is called multiuser interference (MUI). By combining the SVD and space frequency spreading code, a receiver called total interference cancellation (TIC) is developed that completely eliminates both the CSI and the MUI. The performance of the TIC suffers from small singular values of the subchannel matrices. To improve the performance, a symbol-chip level MMSE receiver is developed in which joint equalization and beamforming is utilized at chip level when the MMSE criterion is employed at symbol level (after despreading). The performance of the proposed receiver is evaluated and compared under different scenarios by computer simulations. The results show that there is no error floor in the bit error rate (BER) performances of the SVD based receiver under both criteria in the multiuser environment. Compared with the TIC criterion, the receiver designed based on the symbol-chip level MMSE criterion significantly improves the BER performance.