Code-timing synchronization in DS-CDMA systems using space-time diversity

The synchronization of a desired user transmitting a known training sequence in a direct-sequence (DS) asynchronous code-division multiple-access (CDMA) sys-tem is addressed. It is assumed that the receiver consists of an arbitrary antenna array and works in a near-far, frequency-nonselective, slowly fading channel. The estimator that we propose is derived by applying the maximum likelihood (ML) principle to a signal model in which the contribution of all the interfering compo-nents (e.g., multiple-access interference, external interference and noise) is modeled as a Gaussian term with an unknown and arbitrary space-time correlation matrix. The main contribution of this paper is the fact that the estimator makes eÆcient use of the structure of the signals in both the space and time domains. Its perfor-mance is compared with the Cramer-Rao Bound, and with the performance of other methods proposed recently that also employ an antenna array but only exploit the structure of the signals in one of the two domains, while using the other simply as a means of path diversity. It is shown that the use of the temporal and spatial structures is necessary to achieve synchronization in heavily loaded systems or in the presence of directional external interference.Peer ReviewedPostprint (published version

Blind subspace DOA estimation in multipath DS/CDMA channels

In this paper, we consider the problem of blind estimation of the directions of arrival (DOA's) of users' paths in a multipath DS/CDMA channel. Making use of the signal that is sampled at multiple antenna elements and using a subspace based MUSIC-like technique, we show the possibility of DOA estimation using two search methods. The first provides path delays and DOAs simultaneously, and the second provides only DOAs. Knowledge of the chip waveform is used in the first method. It is seen that the two methods exhibit good estimation accuracy, besides being extremely near-far resistant

Space time transceiver design over multipath fading channels

Imperial Users onl

Blind adaptive constrained reduced-rank parameter estimation based on constant modulus design for CDMA interference suppression

This paper proposes a multistage decomposition for blind adaptive parameter estimation in the Krylov subspace with the code-constrained constant modulus (CCM) design criterion. Based on constrained optimization of the constant modulus cost function and utilizing the Lanczos algorithm and Arnoldi-like iterations, a multistage decomposition is developed for blind parameter estimation. A family of computationally efficient blind adaptive reduced-rank stochastic gradient (SG) and recursive least squares (RLS) type algorithms along with an automatic rank selection procedure are also devised and evaluated against existing methods. An analysis of the convergence properties of the method is carried out and convergence conditions for the reduced-rank adaptive algorithms are established. Simulation results consider the application of the proposed techniques to the suppression of multiaccess and intersymbol interference in DS-CDMA systems

Multi-user detection for multi-rate DS/CDMA systems

Wireless cellular communication is witnessing a rapid growth in market, technology and range of services. Current and future demands for wireless communication services motivate the need for handling multi-media traffic types. In a multimedia communication system, users with different and even time-varying rates and quality of services (QoS) requirements, such as voice, image and data, must be accommodated. The use of Spread Spectrum modulation with Code Division Multiple Access (CDMA) technology is an attractive approach for economical spectrally efficient and high quality cellular and personal communication services. This dissertation explores the technologies of applying different interference cancellation techniques to multi-rate CDMA systems that serve users with different QoS. Multiple Access Interference (MAI) and multipath propagation are the major issues in wireless communication systems. It is also true for multi-rate CDMA systems. Multi-user detection has been shown to be effective in combating the near-far problem and providing superior performance over conventional detection method. In this dissertation, we combine both linear minimum mean squared error (LMMSE) detector, nonlinear decision feedback detector, with other signal processing techniques, such as array processing and multipath combining, to create effective near-far resistant detectors for multi-rate CDMA systems. Firstly, we propose MMSE receivers for synchronous multi-rate CDMA system and compare the performance with the corresponding multi-rate decorrelating detectors. The multi-rate decorrelating detector is optimally near-far resistant and easy to implement. The proposed linear MMSE multi-rate receiver can be adaptively implemented only with the knowledge of the desired user. Due to the fact that MMSE detector offers best trade-off between the MAI cancellation and noise variance enhancement, it is shown that multi-rate MMSE receiver can offer better performance than the multi-rate decorrelator when the interfering users\u27 Signal to Noise Ratio (SNR) is relatively low comparing to the desired user\u27s SNR. Secondly, the asynchronous multi-rate CDMA system, we propose multi-rate multi-shoot decorrelating detectors and multi-rate multi-shot MMSE detectors. The performance of multi-shot detectors can be improved monotonically with increasing the number of stacked bits, but a great computational complexity is going to be introduced in order to get better performance. A debiasing method is introduced to multi-rate multi-shot linear detectors. Debiasing method optimizes multi-rate detectors based on the multi-rate multi-shot model. Debiasing multi-shot MMSE detector for multi-rate signals can offer performance than the corresponding debiasing multi-shot decorrelating detector. Thirdly, we propose linear space-time receivers for multi-rate CDMA systems. The minimum mean-squared error criteria is used. We perform a comparative study on the multi-rate receiver which uses either multipath (temporal) processing or array (spatial) processing, and the one which uses both array and multipath (space-time) processing. The space-time receiver for the multi-rate CDMA signals give us the potential of improving the capacity of multi-rate systems. The space-time processing combined with multiuser detection have the advantages of combating multipath fading through temporal processing, reducing MAI through MMSE method and provide antenna or diversity gain through spatial processing and increasing the capacity of the multi-rate CDMA systems. Lastly, the group-wise interference cancellation methods are proposed for multi-rate CDMA signals. The non-linear decision feedback detection (DFD) schemes are used in the proposed receivers. The proposed interference cancellation schemes benefit from the nature of the unequal received amplitudes for multi-rate CDMA signals. Users with same data rate are grouped together. Users with the highest data-rate are detected first. Interference between the groups is cancelled in a successive order. The results show that the group-wise MMSE DFD yields better performance than multi-rate linear MMSE detector and multi-rate decorrelating detector, especially for highly loaded CDMA systems

Interference suppression and diversity for CDMA systems

In code-division multiple-access (CDMA) systems, due to non-orthogonality of the spreading codes and multipath channels, the desired signal suffers interference from other users. Signal fading due to multipath propagation is another source of impairment in wireless CDMA systems, often severely impacting performance. In this dissertation, reduced-rank minimum mean square error (MMSE) receiver and reduced-rank minimum variance receiver are investigated to suppress interference; transmit diversity is applied to multicarrier CDMA (MC-CDMA) systems to combat fading; packet combing is studied to provide both interference suppression and diversity for CDMA random access systems. The reduced-rank MMSE receiver that uses a reduced-rank estimated covariance matrix is studied to improve the performance of MMSE receiver in CDMA systems. It is shown that the reduced-rank MMSE receiver has much better performance than the full-rank MMSE receiver when the covariance matrix is estimated by using a finite number of data samples and the desired signal is in a low dimensional subspace. It is also demonstrated that the reduced-rank minimum variance receiver outperforms the full-rank minimum variance receiver. The probability density function of the output SNR of the full-rank and reduced-rank linear MMSE estimators is derived for a general linear signal model under the assumption that the signals and noise are Gaussian distributed. Space-time coding that is originally proposed for narrow band systems is applied to an MC-CDMA system in order to get transmit diversity for such a wideband system. Some techniques to jointly decode the space-time code and suppress interference are developed. The channel estimation using either pilot channels or pilot symbols is studied for MC-CDMA systems with space-time coding. Performance of CDMA random access systems with packet combining in fading channels is analyzed. By combining the current retransmitted packet with all its previous transmitted copies, the receiver obtains a diversity gain plus an increased interference and noise suppression gain. Therefore, the bit error rate dramatically decreases with the number of transmissions increasing, which in turn improves the system throughput and reduces the average delay

Novel self-decorrelation and fractional self-decorrelation pre-processing techniques to enhance the output SINR of single-user-type DS-CDMA detectors in blind space-time RAKE receivers.

Cheung Shun Keung.Thesis (M.Phil.)--Chinese University of Hong Kong, 2002.Includes bibliographical references (leaves 80-83).Abstracts in English and Chinese.Chapter 1 --- Introduction --- p.1Chapter 1.1 --- The Problem --- p.1Chapter 1.2 --- Overview of CDMA --- p.2Chapter 1.3 --- Problems Encountered in Direct-Sequence (DS)CDMA --- p.3Chapter 1.3.1 --- Multipath Fading Scenario in DS-CDMA Cellular Mo- bile Communication --- p.3Chapter 1.3.2 --- Near-Far Problem --- p.4Chapter 1.4 --- Delimitation and Significance of the Thesis --- p.5Chapter 1.5 --- Summary --- p.7Chapter 1.6 --- Scope of the Thesis --- p.8Chapter 2 --- Literature Review of Blind Space-Time Processing in a wire- less CDMA Receiver --- p.9Chapter 2.1 --- General Background Information --- p.9Chapter 2.1.1 --- Time Model of K-User Chip-Synchronous CDMA --- p.9Chapter 2.1.2 --- Dispersive Channel Modelling --- p.10Chapter 2.1.3 --- Combination of K-user CDMA Time Model with the Slow Frequency-Selective Fading Channel Model to form a completed Chip-Synchronous CDMA Time Model --- p.13Chapter 2.1.4 --- Spatial Channel Model with Antenna Array [9] --- p.15Chapter 2.1.5 --- Joint Space-Time Channel Model in Chip-Synchronous CDMA --- p.19Chapter 2.1.6 --- Challenges to Blind Space-Time Processing in a base- station CDMA Receiver --- p.23Chapter 2.2 --- Literature Review of Single-User-Type Detectors used in Blind Space-Time DS-CDMA RAKE Receivers --- p.25Chapter 2.2.1 --- A Common Problem among the Signal Processing Schemes --- p.28Chapter 3 --- "Novel ""Self-Decorrelation"" Technique" --- p.29Chapter 3.1 --- "Problem with ""Blind"" Space-Time RAKE Processing Using Single- User-Type Detectors" --- p.29Chapter 3.2 --- "Review of Zoltowski & Ramos[10,11,12] Maximum-SINR Single- User-Type CDMA Blind RAKE Receiver Schemes" --- p.31Chapter 3.2.1 --- Space-Time Data Model --- p.31Chapter 3.2.2 --- The Blind Element-Space-Only (ESO) RAKE Receiver with Self-Decorrelation Pre-processing Applied --- p.32Chapter 3.3 --- Physical Meaning of Self-Decorrelation Pre-processing --- p.35Chapter 3.4 --- Simulation Results --- p.38Chapter 4 --- """Fractional Self-Decorrelation"" Pre-processing" --- p.45Chapter 4.1 --- The Blind Maximum-SINR RAKE Receivers in Chen et. al.[l] and Wong et. al.[2] --- p.45Chapter 4.2 --- Fractional Self-Decorrelation Pre-processing --- p.47Chapter 4.3 --- The Blind Element-Space-Only (ESO) RAKE Receiver with Fractional Self-Decorrelation Pre-processing Applied --- p.50Chapter 4.4 --- Physical Meaning of Fractional Self-Decorrelation Pre-processing --- p.54Chapter 4.5 --- Simulation Results --- p.55Chapter 5 --- Complexity Analysis and Schematics of Proposed Techniques --- p.64Chapter 5.1 --- Computational Complexity --- p.64Chapter 5.1.1 --- Self-Decorrelation Applied in Element-Space-Only (ESO) RAKE Receiver --- p.64Chapter 5.1.2 --- Fractional Self-Decorrelation Applied in Element-Space- Only (ESO) RAKE Receiver --- p.67Chapter 5.2 --- Schematics of the Two Proposed Techniques --- p.69Chapter 6 --- Summary and Conclusion --- p.74Chapter 6.1 --- Summary of the Thesis --- p.74Chapter 6.1.1 --- The Self-Decorrelation Pre-processing Technique --- p.75Chapter 6.1.2 --- The Fractional Self-Decorrelation Pre-processing Tech- nique --- p.76Chapter 6.2 --- Conclusion --- p.78Chapter 6.3 --- Future Work --- p.78Bibliography --- p.80Chapter A --- Generalized Eigenvalue Problem --- p.84Chapter A.1 --- Standard Eigenvalue Problem --- p.84Chapter A.2 --- Generalized Eigenvalue Problem --- p.8