Phase-locked loop, delay-locked loop, and linear decorrelating detector for asynchronous multirate DS-CDMA system

The performance of phase synchronization and code tracking of a digital phase-locked loop (PLL) and delay-locked loop (DLL), respectively, is investigated in wideband asynchronous multirate DS-CDMA system. Dynamic Partial Correlation (DPC) method is proposed to evaluate the autocorrelation and its power spectrum density (PSD) of the cross-correlated terms in the presence of multirate multiple access interference (MMAI) under additive white gaussian noise (AWGN) and fading channel environments. The steady-state probability density function (PDF) and variance of the phase estimator error and code tracking jitter is evaluated by solving the first-order Fokker-Planck equation. Among many linear multiuser detectors which decouple the multiple access interference from each of the interfering users, one-shot window linear decorrelating detector (LDD) based on a one bit period to reduce the complexity of the LDD has attracted wide attention as an implementation scheme. Therefore, we propose Hybrid Selection Diversity/ Maximal Ratio Combining (Hybrid SD/MRC) one-shot window linear decorrelating detector (LDD) for asynchronous DS-CDMA systems. The selection diversity scheme at the input of the Hybrid SD/MRC LDD is based on choosing the branch with the maximum signal-to-noise ratio (SNR) of all filter outputs. The MR Combining scheme at the output of the Hybrid SD/MRC LDD adopts to maximize the output SNR and thus compensates for the enhanced output noise. The Hybrid SD/MRC one-shot LDD with PLL is introduced to track its phase error and to improve the demodulation performance. The probability density functions of the maximum SNR of the SD combiner, the near-far resistance (NFR) of one-shot LDD by Gaussian approximation, and the maximum SNR of the MR combiner for Hybrid SD/MRC LDD are evaluated, and the bit error probability is obtained from these pdfs. The performance of Hybrid SD/MRC one-shot LDD is assessed in a Rayleigh fading channel

Design and performance of CDMA codes for multiuser communications

Walsh and Gold sequences are fixed power codes and are widely used in multiuser CDMA communications. Their popularity is due to the ease of implementation. Availability of these code sets is limited because of their generating kernels. Emerging radio applications like sensor networks or multiple service types in mobile and peer-to-peer communications networks might benefit from flexibilities in code lengths and possible allocation methodologies provided by large set of code libraries. Walsh codes are linear phase and zero mean with unique number of zero crossings for each sequence within the set. DC sequence is part of the Walsh code set. Although these features are quite beneficial for source coding applications, they are not essential for spread spectrum communications. By relaxing these unnecessary constraints, new sets of orthogonal binary user codes (Walsh-like) for different lengths are obtained with comparable BER performance to standard code sets in all channel conditions. Although fixed power codes are easier to implement, mathematically speaking, varying power codes offer lower inter- and intra-code correlations. With recent advances in RF power amplifier design, it might be possible to implement multiple level orthogonal spread spectrum codes for an efficient direct sequence CDMA system. A number of multiple level integer codes have been generated by brute force search method for different lengths to highlight possible BER performance improvement over binary codes. An analytical design method has been developed for multiple level (variable power) spread spectrum codes using Karhunen-Loeve Transform (KLT) technique. Eigen decomposition technique is used to generate spread spectrum basis functions that are jointly spread in time and frequency domains for a given covariance matrix or power spectral density function. Since this is a closed form solution for orthogonal code set design, many options are possible for different code lengths. Design examples and performance simulations showed that spread spectrum KLT codes outperform or closely match with the standard codes employed in present CDMA systems. Hybrid (Kronecker) codes are generated by taking Kronecker product of two spreading code families in a two-stage orthogonal transmultiplexer structure and are judiciously allocated to users such that their inter-code correlations are minimized. It is shown that, BER performance of hybrid codes with a code selection and allocation algorithm is better than the performance of standard Walsh or Gold code sets for asynchronous CDMA communications. A redundant spreading code technique is proposed utilizing multiple stage orthogonal transmultiplexer structure where each user has its own pre-multiplexer. Each data bit is redundantly spread in the pre-multiplexer stage of a user with odd number of redundancy, and at the receiver, majority logic decision is employed on the detected redundant bits to obtain overall performance improvement. Simulation results showed that redundant spreading method improves BER performance significantly at low SNR channel conditions

Blind Detection, Parameters Estimation and Despreading of DS-CDMA Signals in Multirate Multiuser Cognitive Radio Systems

Open Access : http://www.intechopen.com/books/advances-in-cognitive-radio-systems/blind-detection-parameters-estimation-and-despreading-of-ds-cdma-signals-in-multirate-multiuser-cognInternational audienc

Combined turbo coding and interference rejection for DS-CDMA.

Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2004.This dissertation presents interference cancellation techniques for both the Forward Error Correction (FEC) coded and the uncoded Direct Sequence Code Division Multiple Access (DS-CDMA) systems. Analytical models are also developed for the adaptive and the non-adaptive Parallel Interference Cancellation (PlC) receivers. Results that are obtained from the computer simulations of the PlC receiver types confirm the accuracy of the analytical models that are developed. Results show that the Least Mean Square (LMS) algorithm based adaptive PlC receivers have bit error rate performances that are better than those of the non-adaptive PlC receivers. In the second part of this dissertation, a novel iterative multiuser detector for the Turbo coded DS-CDMA system is developed. The performance of the proposed receiver in the multirate CDMA system is also investigated. The developed receiver is found to have an error rate performance that is very close to the single user limit after a few numbers of iterations. The receiver is also resilient against the near-far effect. A methodology is also presented on the use of the Gaussian approximation method in the convergence analysis of iterative interference cancellation receivers for turbo coded DS-CDMA systems

On the performance and capacity of space-time block coded multicarrier CDMA communication systems

Ph.DDOCTOR OF PHILOSOPH

Advancements of MultiRate Signal processing for Wireless Communication Networks: Current State Of the Art

With the hasty growth of internet contact and voice and information centric communications, many contact technologies have been urbanized to meet the stringent insist of high speed information transmission and viaduct the wide bandwidth gap among ever-increasing high-data-rate core system and bandwidth-hungry end-user complex. To make efficient consumption of the limited bandwidth of obtainable access routes and cope with the difficult channel environment, several standards have been projected for a variety of broadband access scheme over different access situation (twisted pairs, coaxial cables, optical fibers, and unchanging or mobile wireless admittance). These access situations may create dissimilar channel impairments and utter unique sets of signal dispensation algorithms and techniques to combat precise impairments. In the intended and implementation sphere of those systems, many research issues arise. In this paper we present advancements of multi-rate indication processing methodologies that are aggravated by this design trend. The thesis covers the contemporary confirmation of the current literature on intrusion suppression using multi-rate indication in wireless communiquE9; networks

SGD Frequency-Domain Space-Frequency Semiblind Multiuser Receiver with an Adaptive Optimal Mixing Parameter

A novel stochastic gradient descent frequency-domain (FD) space-frequency (SF) semiblind multiuser receiver with an adaptive optimal mixing parameter is proposed to improve performance of FD semiblind multiuser receivers with a fixed mixing parameters and reduces computational complexity of suboptimal FD semiblind multiuser receivers in SFBC downlink MIMO MC-CDMA systems where various numbers of users exist. The receiver exploits an adaptive mixing parameter to mix information ratio between the training-based mode and the blind-based mode. Analytical results prove that the optimal mixing parameter value relies on power and number of active loaded users existing in the system. Computer simulation results show that when the mixing parameter is adapted closely to the optimal mixing parameter value, the performance of the receiver outperforms existing FD SF adaptive step-size (AS) LMS semiblind based with a fixed mixing parameter and conventional FD SF AS-LMS training-based multiuser receivers in the MSE, SER and signal to interference plus noise ratio in both static and dynamic environments

Multiuser detection in CDMA using blind techniques

Thesis (Master)--Izmir Institute of Technology, Electronics and Communication Engineering, Izmir, 2004Includes bibliographical references (leaves: 63-65)Text in English; Abstract: Turkish and Englishxiv, 69 leavesIn code division multiple access (CDMA) systems, blind multiuser detection (MUD) techniques are of great importance, especially for downlinks, since in practice, it may be unrealistic for a mobile user to know the spreading codes of other active users in the channel. Furthermore, blind methods remove the need for training sequences which leads to a gain in the channel bandwidth. Subspace concept in blind MUD is an alternative process to classical and batch blind MUD techniques based on principle component analysis, or independent component analysis (ICA) and ICA-like algorithms, such as joint approximate diagonalization of eigen-matrices (JADE), blind source separation algorithm with reference system, etc. Briefly, the desired signal is searched in the signal subspace instead of the whole space, in this type of detectors. A variation of the subspace-based MUD is reduced-rank MUD in which a smaller subspace of the signal subspace is tracked where the desired signal is contained in. This latter method leads to a performance gain compared to a standard subspace method. In this thesis, blind signal subspace and reduced-rank MUD techniques are investigated, and applied to minimum mean square error (MMSE) detectors with two different iterative subspace tracking algorithms. The performances of these detectors are compared in different scenarios for additive white Gaussian noise and for multipath fading channels as well. With simulation results the superiority of the reduced-rank detector to the signal subspace detector is shown. Additionally, as a new remark for both detectors, it is shown that, using minimum description length criterion in subspace tracking algorithm results in an increase in rank-tracking ability and correspondingly in the final performance. Finally, the performances of these two detectors are compared with MMSE, adaptive MMSE and JADE detectors