Coded transmit diversity in CDMA over Nakagami-m fading channels

With applications such as video conferencing, extensive web browsing and live video streaming, future wireless systems become extremely demanding in terms of high data rates and improved signal quality. In this thesis the performance of a space-time spreading transmit diversity scheme is examined over a frequency-flat Nakagami- m fading channel. The Nakagami- m channel model is considered as it is well known for modeling signal fading conditions ranging from severe to moderate, to light fading or no fading, through its parameter m. We also propose in this thesis a coded transmit diversity scheme which is based on a combination of a convolutional code with a space-time transmit diversity scheme that uses direct-sequence code division multiple access (DS-CDMA) for multiuser access. Our focus will be on the uplink of the communication system. The space-time scheme employs N = 2 and N r antennas at the mobile station (MS) side and at the base station (BS) side respectively. DS-CDMA is used to support many users and a linear decorrelator detector is used to combat the effect of multiuser interference. We study the performance of both the uncoded and coded transmit diversity schemes over slow fading and fast fading channels. In all cases, the investigations start by determining the probability density function (PDF) of the signal to interference and noise ratio at the output of the space-time combiner at the BS receiver side. Using this PDF we derive a closed-form (or an approximation) expression for the bit error rate (BER) of the system under consideration. The accuracy of the PDF and BER expressions are verified when compared to simulation results for different values of the fading figure m and for different combinations of transmit and receive antennas. In the case of the coded space-time transmit diversity scheme, the pairwise error probability and the corresponding BER upper bounds are obtained for fast and slow fading channels. The derived error bounds, when compared to system simulations, are shown to be tight at high signal-to-noise ratios. Furthermore, our analytical results explicitly show the achieved system diversity in terms of the number of transmit and receive antennas and the fading figure m. When the coded space-time scheme is considered, its diversity is shown to be a function of the minimum free distance d free of the convolutional code used. Furthermore we show that the diversity of the different schemes considered is always independent of the system loa

Performance of turbo multi-user detectors in space-time coded DS-CDMA systems

Includes bibliographical references (leaves 118-123).In this thesis we address the problem of improving the uplink capacity and the performance of a DS-CDMA system by combining MUD and turbo decoding. These two are combined following the turbo principle. Depending on the concatenation scheme used, we divide these receivers into the Partitioned Approach (PA) and the Iterative Approach (IA) receivers. To enable the iterative exchange of information, these receivers employ a Parallel Interference Cancellation (PIC) detector as the first receiver stage

Near maximum likelihood multiuser receivers for direct sequence code division multiple access

Wideband wireless access based on direct-sequence code-division multiple access (DS-CDMA) has been adopted for third-generation mobile communications systems. Hence, DS-CDMA downlink communications systems form the platform for the work in this thesis. The principles of the spread spectrum concept and DS-CDMA technology are first outlined, including a description of the system model and the conventional receiver. The two classes of codes used in this system, namely spreading codes and forward error correction codes (including Turbo codes), are discussed. Due to the fact that practical communications channels are non-ideal, the performance of an individual user is interference limited. As a result, the capacity of the system is greatly restricted. Fortunately, multiuser detection is a scheme that can effectively counteract this multiple access interference. However, the optimum multiuser detection scheme is far too computationally intensive for practical use. Hence, the fundamental interest here is to retain the advantages of multiuser detection and simplify its implementation. The objective of the thesis is to investigate the optimum multiuser receiver, regarded on a chip level sampling basis. The aim is to reduce the complexity of the optimum receiver to a practical and implementable level while retaining its good performance. The thesis first reviews various existing multiuser receivers. The chip-based maximum likelihood sequence estimation (CBMLSE) detector is formulated and implemented. However, the number of states in the state-transition trellis is still exponential in the number of users. Complexity cannot be reduced substantially without changing the structure of the trellis. A new detector is proposed which folds up the original state-transition trellis such that the number of states involved is greatly reduced. The performance is close to that of the CBMLSE. The folded trellis detector (FTD) can also be used as a preselection stage for the CBMLSE. The FTD selects with high accuracy the few symbol vectors that are more likely to be transmitted. The CBMLSE is then used to determine the most likely symbol vector out of the small subset of vectors. The performance of this scheme is as good as the CBMLSE. The FTD is also applied in an iterative multiuser receiver that exploits the powerful iterative algorithm of Turbo codes

Multi-carrier CDMA using convolutional coding and interference cancellation

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN016251 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Space-time diversity for CDMA systems over frequency-selective fading channels

Supporting the expected high data rates required by wireless Internet and high-speed multimedia services is one of the basic requirements in broadband mobile wireless systems. However, the achievable capacity and data rate of wireless communication systems are limited by the time-varying nature of the channel. Efficient techniques for combating the time-varying effects of wireless channels can be achieved by utilizing different forms of diversity. In recent years, transmit diversity based on space-time coding (STC) has received more attention as an effective technique for combating fading. On the other hand, most existing space-time diversity techniques have been developed for flat-fading channels. Given the fact that wireless channels are generally frequency-selective, in this thesis, we aim to investigate the performance of space-time diversity schemes for wideband code-division multiple-access (WCDMA) systems over frequency-selective fading channels. The proposed receiver in this case is a rake-type receiver, which exploits the path diversity inherent to multipath propagation. Then, a decorrelator detector is used to mitigate the multiple access interference (MAI) and the known near-far problem. We derive the bit error rate (BER) expression over frequency-selective fading channels considering both the fast and slow fading cases. Finally, we show that our proposed receiver achieves the full system diversity through simulation and analytical results. Most of the work conducted in this area considers perfect knowledge of the channel at the receiver. Hence, channel identification brings significant challenges to multiple-input multiple-output (MIMO) CDMA systems. In light of this, we propose a channel estimation and data detection scheme based on the superimposed training-based approach. The proposed scheme enhances the performance by eliminating the MAI from both the channel and data estimates by employing two decorrelators; channel and data decorrelators. The performance of the proposed estimation technique is investigated over frequency-selective slow fading channels where we derived a closed-form expression for the BER as a function of the number of users, K , the number resolvable paths, L , and the number of receive antennas, V . Finally, our proposed scheme is shown to be more robust to channel estimation errors. Furthermore, both the analytical and simulation results indicate that the full system diversity is achieved. Considering that training estimation techniques suffer either from low spectral efficiency (i.e., conventional training approach) or from high pilot power consumption (i.e., superimposed training-based approach), in the last part of the thesis, we present an iterative joint detection and estimation (JDE) using the expectation-maximization (EM) algorithm for MIMO CDMA systems over frequency-selective fading channels. We also derive a closed-form expression for the optimized weight coefficients of the EM algorithm, which was shown to provide significant performance enhancement relative to the conventional equal-weight EM-based signal decomposition. Finally, our simulation results illustrate that the proposed receiver achieves near-optimum performance with modest complexity using very few training symbols

Interference Mitigation in Wireless Communications

The primary objective of this thesis is to design advanced interference resilient schemes for asynchronous slow frequency hopping wireless personal area networks (FH-WPAN) and time division multiple access (TDMA) cellular systems in interference dominant environments. We also propose an interference-resilient power allocation method for multiple-input-multiple-output (MIMO) systems. For asynchronous FH-WPANs in the presence of frequent packet collisions, we propose a single antenna interference canceling dual decision feedback (IC-DDF) receiver based on joint maximum likelihood (ML) detection and recursive least squares (RLS) channel estimation. For the system level performance evaluation, we propose a novel geometric method that combines bit error rate (BER) and the spatial distribution of the traffic load of CCI for the computation of packet error rate (PER). We also derived the probabilities of packet collision in multiple asynchronous FH-WPANs with uniform and nonuniform traffic patterns. For the design of TDMA receivers resilient to CCI in frequency selective channels, we propose a soft output joint detection interference rejection combining delayed decision feedback sequence estimation (JD IRC-DDFSE) scheme. In the proposed scheme, IRC suppresses the CCI, while DDFSE equalizes ISI with reduced complexity. Also, the soft outputs are generated from IRC-DDFSE decision metric to improve the performance of iterative or non-iterative type soft-input outer code decoders. For the design of interference resilient power allocation scheme in MIMO systems, we investigate an adaptive power allocation method using subset antenna transmission (SAT) techniques. Motivated by the observation of capacity imbalance among the multiple parallel sub-channels, the SAT method achieves high spectral efficiency by allocating power on a selected transmit antenna subset. For 4 x 4 V-BLAST MIMO systems, the proposed scheme with SAT showed analogous results. Adaptive modulation schemes combined with the proposed method increase the capacity gains. From a feasibility viewpoint, the proposed method is a practical solution to CCI-limited MIMO systems since it does not require the channel state information (CSI) of CCI.Ph.D.Committee Chair: Professor Gordon L. StBe

A time spread diversity technique with integrated coding and interference cancellation

Conventional signal diversity techniques increase system complexity and/or result in an increase in the required signal bandwidth. A novel Time Spread (TS) diversity technique has been proposed in [1] aimed at improving the performance of digital communication systems in flat-fading channels without increasing their complexity or bandwidth usage. With TS, time diversity is obtained by transmitting a modulated spreading sequence, spanning over a temporal period longer than the channel coherence time Tc, for each information symbol. Unfortunately, this TS technique exhibits a Bit Error Rate (BER) performance floor at higher SNR values that renders the original technique less attractive. This dissertation is primarily concerned with the nullification of this BER performance floor. Sec¬ondly, the ability of the TS technique to transform a flat-fading channel into an Additive White Gaus¬sian Noise (AWGN) channel is investigated and exploited to enhance the performance of coding techniques designed for AWGN channels, when used in flat-fading channels. A new method is described by which TS sequences can be temporally expanded, thereby increas¬ing their obtainable time diversity gains. This method also reduces the computational complexity of a TS system, while retaining the signal diversity properties of a longer non-expanded sequence. The BER floor in TS systems is caused by the distortion of the Aperiodic Auto-Correlation (AAC) properties of overlapping spreading sequences in fading channels, resulting in Inter Code(Sequence) Interference (ICI) between spreading sequences. A Pilot Symbol Aided Modulation (PSAM) tech¬nique is adapted for the TS system to provide accurate channel estimates required by the Inter Code(Sequence) Interference Cancellation (lCIC) module. A hybrid ICIC technique, which corrects the fading TS symbol amplitudes during periods of above average instantaneous SNR levels, is shown to be the most effective. This ICIC technique enables the TS technique to provide gains similar to that of conventional third order diversity techniques at average Eb/ No ratios above 10 dB. Finally, the transparency and ability of the TS technique to transform fading channels into Gaus¬sian channels are exploited to allow the integration of conventional convolutional codes with the TS system. The coded TS system achieves substantial gains when operating in a Rayleigh flat-fading channel when a soft decision Viterbi decoder is used in the TS receiver. A strategy by which Turbo Code (TC) techniques can be integrated with the TS technique is discussed as a concluding notion to illustrate the flexibility of the TS technique. Future research areas are identified based on the findings of this dissertation. These include the investigation of more effective adaptive ICIC schemes and the possibility of using Code Division Multiple Access (CDMA) communication techniques over a narrow band channel by employing TS- and Multi-User (MU)-detection methods, combined with existing ICIC techniques. The cryptographic value of the TS technique also provides ground for future research.Dissertation (MEng (Electronic Engineering))--University of Pretoria, 2006.Electrical, Electronic and Computer Engineeringunrestricte

Proceedings of the Fifth International Mobile Satellite Conference 1997

Satellite-based mobile communications systems provide voice and data communications to users over a vast geographic area. The users may communicate via mobile or hand-held terminals, which may also provide access to terrestrial communications services. While previous International Mobile Satellite Conferences have concentrated on technical advances and the increasing worldwide commercial activities, this conference focuses on the next generation of mobile satellite services. The approximately 80 papers included here cover sessions in the following areas: networking and protocols; code division multiple access technologies; demand, economics and technology issues; current and planned systems; propagation; terminal technology; modulation and coding advances; spacecraft technology; advanced systems; and applications and experiments