Improved decoder metrics for DS-CDMA in practical 3G systems

While 4G mobile networks have been deployed since 2008. In several of the more developed markets, 3G mobile networks are still growing with 3G having the largest market -in terms of number of users- by 2019. 3G networks are based on Direct- Sequence Code-Division Multiple-Access (DS-CDMA). DS-CDMA suffers mainly from the Multiple Access Interference (MAI) and fading. Multi-User Detectors (MUDs) and Error Correcting Codes (ECCs) are the primary means to combat MAI and fading. MUDs, however, suffer from high complexity, including most of sub-optimal algorithms. Hence, most commercial implementations still use conventional single-user matched filter detectors. This thesis proposes improved channel decoder metrics for enhancing uplink performance in 3G systems. The basic idea is to model the MAI as conditionally Gaussian, instead of Gaussian, conditioned on the users’ cross-correlations and/or the channel fading coefficients. The conditioning implies a time-dependent variance that provides enhanced reliability estimates at the decoder inputs. We derive improved log-likelihood ratios (ILLRs) for bit- and chip- asynchronous multipath fading channels. We show that while utilizing knowledge of all users’ code sequences for the ILLR metric is very complicated in chip-asynchronous reception, a simplified expression relying on truncated group delay results in negligible performance loss. We also derive an expression for the error probability using the standard Gaussian approximation for asynchronous channels for the widely used raised cosine pule shaping. Our study framework considers practical 3G systems, with finite interleaving, correlated multipath fading channel models, practical pulse shaping, and system parameters obtained from CDMA2000 standard. Our results show that for the fully practical cellular uplink channel, the performance advantage due to ILLRs is significant and approaches 3 dB

Adaptive DS-CDMA multiuser detection for time variant frequency selective Rayleigh fading channel

The current digital wireless mobile system such as IS-95, which is based on direct sequence Code Division Multiple Access (DS-CDMA) technology, will not be able to meet the growing demands for multimedia service due to low information exchanging rate. Its capacity is also limited by multiple accessed interference (MAI) signals. This work focuses on the development of adaptive algorithms for multiuser detection (MUD) and interference suppression for wideband direct sequence code division multiple access (DS-CDMA) systems over time-variant frequency selective fading channels. In addition, channel acquisition and delay estimation techniques are developed to combat the uncertainty introduced by the wireless propagation channel. This work emphasizes fast and simple techniques that can meet practical needs for high data rate signal detection. Most existing literature is not suitable for the large delay spread in wideband systems due to high computational/ hardware complexity. A de-biasing decorrelator is developed whose computational complexity is greatly reduced without sacrificing performance. An adaptive bootstrap symbolbased signal separator is also proposed for a time-variant channel. These detectors achieve MUD for asynchronous, large delay spread, fading channels without training sequences. To achieve high data rate communication, a finite impulse response (FIR) filter based detector is presented for M-ary QAM modulated signals in a multipath Rayleigh fading channel. It is shown that the proposed detector provides a stable performance for QAM signal detection with unknown fading and phase shift. It is also shown that this detector can be easily extended to the reception of any M-ary quadrature modulated signal. A minimum variance decorrelating (MVD) receiver with adaptive channel estimator is presented in this dissertation. It provides comparable performance to a linear MMSE receiver even in a deep fading environment and can be implemented blindly. Using the MVD receiver as a building-block, an adaptive multistage parallel interference cancellation (PIC) scheme and a successive interference cancellation (SIC) scheme were developed. The total number of stages is kept at a minimum as a result of the accurate estimating of the interfering users at the earliest stages, which reduces the implementation complexity, as well as the processing delay. Jointly with the MVD receiver, a new transmit diversity (TD) scheme, called TD-MVD, is proposed. This scheme improves the performance without increasing the bandwidth. Unlike other TD techniques, this TDMVD scheme has the inherent advantage to overcome asynchronous multipath transmission. It brings flexibility in the design of TD antenna systems without restrict signal coordination among those multiple transmissions, and applicable for both existing and next generation of CDMA systems. A maximum likelihood based delay and channel estimation algorithm with reduced computational complexity is proposed. This algorithm uses a diagonal simplicity technique as well as the asymptotically uncorrelated property of the received signal in the frequency domain. In combination with oversampling, this scheme does not suffer from a singularity problem and the performance quickly approaches the Cramer-Rao lower bound (CRLB) while maintaining a computational complexity that is as low as the order of the signal dimension

Performance evaluation of the third generation TD-SCDMA system.

Firstly, a basic reverse TD-SCDMA system is setup, including a transmitter, a receiver and a channel. Within the receiver model, a specific soft Viterbi decoding algorithm is developed. Secondly, the system synchronization requirements and their impact on its performance under realistic conditions are evaluated. The need for uplink synchronization is established and the sensitivity of the system performance to time misalignment is qualified. To mitigate multipath fading, a RAKE receiver and a minimum mean-square-error (MMSE) receiver are considered. Two RAKE receivers are compared and it is observed that the MMSE-based RAKE receiver always outperforms the cross-correlation based RAKE receiver. An MMSE receiver, including fully or fractionally spaced equalizer is also considered. MMSE receivers do improve the performance for some channels, but not in others. Fractionally spaced equalizers outperform the fully spaced equalizer.Among all the third generation (3G) systems, time-division duplex synchronous code-division multiple access (TD-SCDMA) system is a unique system and has a lot advantages. The performance of the TD-SCDMA system is evaluated using both link level computer simulations and analysis. This dissertation is focused on the performance evaluation and improvement from the receiver model perspective, including Viterbi decoding algorithms, receiver structures, channel estimation algorithms, channel equalization algorithms, and smart antenna techniques.Finally, the performance of the system with an antenna array consisting of a linear array of four equally spaced omni-directional antenna elements, and a direct matrix inversion algorithm (DMI) using the MMSE criterion, was also evaluated

Multi-user detection for multi-carrier communication systems

Doctor of PhilosophyDepartment of Electrical and Computer EngineeringBalasubramaniam NatarajanWireless broadband communications is a rapidly growing industry. New enabling technologies such as multi-carrier code division multiple access (MC-CDMA) are shaping the future of wireless systems. However, research efforts in improving MC-CDMA receiver performance have received limited attention and there is a need for innovative receiver designs for next generation MC-CDMA. In this thesis, we propose novel multi-user detection (MUD) schemes to enhance the performance of both synchronous and asynchronous MC-CDMA. First, we adapt the ant colony optimization (ACO) approach to solve the optimal MUD problem in MC-CDMA systems. Our simulations indicate that the ACO based MUD converges to the optimal BER performance in relatively few iterations providing more that 95% savings in computational complexity. Second, we propose a new MUD structure specifically for asynchronous MC-CDMA. Previously proposed MUDs for asynchronous MC-CDMA perform the detection for one user (desired user) at a time, mandating multiple runs of the algorithm to detect all users' symbols. In this thesis, for the first time we present a MUD structure that detects all users' symbols simultaneously in one run by extending the receiver's integration window to capture the energy scattered in two consecutive symbol durations. We derive the optimal, decorrelator and minimum mean square error (MMSE) MUD for the extended window case. Our simulations demonstrate that the proposed MUD structures not only perform similar to a MUD that detects one user at a time, but its computational complexity is significantly lower. Finally, we extend the MUD ideas to multicarrier implementation of single carrier systems. Specifically, we employ the novel MUD structure as a multi-symbol detection scheme in CI-CDMA and illustrate the resulting performance gain via simulations

Frequency synchronization in multiuser OFDM-IDMA systems.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2013.Various multiuser schemes have been proposed to efficiently utilize the available bandwidth while ensuring an acceptable service delivery and flexibility. The multicarrier CDMA became an attractive solution to the major challenges confronting the wireless communication system. However, the scheme is plagued with multiple access interference (MAI), which causes conspicuous performance deterioration at the receiver. A low-complexity multiuser scheme called the Interleave Division Multiple Access (IDMA) was proposed recently as a capable solution to the drawback in the multicarrier CDMA scheme. A combined scheme of OFDM-IDMA was later introduced to enhance the performance of the earlier proposed IDMA scheme. The multicarrier IDMA scheme therefore combats inter-symbol interference (ISI) and MAI effectively over multipath with low complexity while ensuring a better cellular performance, high diversity order, and spectral efficiency. Major studies on the OFDM-IDMA scheme emphasis only on the implementation of the scheme in a perfect scenario, where there are no synchronization errors in the system. Like other multicarrier schemes, the OFDM-IDMA scheme however suffers from carrier frequency offset (CFO) errors, which is inherent in the OFDM technique. This research work therefore examines, and analyzes the effect of synchronization errors on the performance of the new OFDM-based hybrid scheme called the OFDM-IDMA. The design of the OFDM-IDMA system developed is such that the cyclic prefix duration of the OFDM component is longer than the maximum channel delay spread of the multipath channel model used. This effectively eliminates ISI as well as timing offsets in the system. Since much work has not been done hitherto to address the deteriorating effect of synchronization errors on the OFDM-IDMA system, this research work therefore focuses on the more challenging issue of carrier frequency synchronization at the uplink. A linear MMSE-based synchronization algorithm is proposed and implemented. The proposed algorithm is a non-data aided method that focuses on the mitigation of the ICI induced by the residual CFOs due to concurrent users in the multicarrier system. However, to obtain a better and improved system performance, the Kernel Least Mean Square (KLMS) algorithm and the normalized KLMS are proposed, implemented, and effectively adapted to combat the degrading influence of carrier frequency offset errors on the OFDM-IDMA scheme. The KLMS synchronization algorithm, which involves the execution of the conventional Least Mean Square (LMS) algorithm in the kernel space, utilizes the modulated input signal in the implementation of the kernel function, thereby enhancing the efficacy of the algorithm and the overall output of the multicarrier system. The algorithms are applied in a Rayleigh fading multipath channel with varying mobile speed to verify their effectiveness and to clearly demonstrate their influence on the performance of the system in a practical scenario. Also, the implemented algorithms are compared to ascertain which of these algorithms offers a better and more efficient system performance. Computer simulations of the bit error performance of the algorithms are presented to verify their respective influence on the overall output of the multicarrier system. Simulation results of the algorithms in both slow fading and fast fading multipath scenarios are documented as well

Proceedings of the Second International Mobile Satellite Conference (IMSC 1990)

Presented here are the proceedings of the Second International Mobile Satellite Conference (IMSC), held June 17-20, 1990 in Ottawa, Canada. Topics covered include future mobile satellite communications concepts, aeronautical applications, modulation and coding, propagation and experimental systems, mobile terminal equipment, network architecture and control, regulatory and policy considerations, vehicle antennas, and speech compression