Capacity, coding and interference cancellation in multiuser multicarrier wireless communications systems

Multicarrier modulation and multiuser systems have generated a great deal of research during the last decade. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation generated with the inverse Discrete Fourier Transform, which has been adopted for standards in wireless and wire-line communications. Multiuser wireless systems using multicarrier modulation suffer from the effects of dispersive fading channels, which create multi-access, inter-symbol, and inter-carrier interference (MAI, ISI, ICI). Nevertheless, channel dispersion also provides diversity, which can be exploited and has the potential to increase robustness against fading. Multiuser multi-carrier systems can be implemented using Orthogonal Frequency Division Multiple Access (OFDMA), a flexible orthogonal multiplexing scheme that can implement time and frequency division multiplexing, and using multicarrier code division multiple access (MC-CDMA). Coding, interference cancellation, and resource sharing schemes to improve the performance of multiuser multicarrier systems on wireless channels were addressed in this dissertation. Performance of multiple access schemes applied to a downlink multiuser wireless system was studied from an information theory perspective and from a more practical perspective. For time, frequency, and code division, implemented using OFDMA and MC-CDMA, the system outage capacity region was calculated for a correlated fading channel. It was found that receiver complexity determines which scheme offers larger capacity regions, and that OFDMA results in a better compromise between complexity and performance than MC-CDMA. From the more practical perspective of bit error rate, the effects of channel coding and interleaving were investigated. Results in terms of coding bounds as well as simulation were obtained, showing that OFDMAbased orthogonal multiple access schemes are more sensitive to the effectiveness of the code to provide diversity than non-orthogonal, MC-CDMA-based schemes. While cellular multiuser schemes suffer mainly from MAI, OFDM-based broadcasting systems suffer from ICI, in particular when operating as a single frequency network (SFN). It was found that for SFN the performance of a conventional OFDM receiver rapidly degrades when transmitters have frequency synchronization errors. Several methods based on linear and decision-feedback ICI cancellation were proposed and evaluated, showing improved robustness against ICI. System function characterization of time-variant dispersive channels is important for understanding their effects on single carrier and multicarrier modulation. Using time-frequency duality it was shown that MC-CDMA and DS-CDMA are strictly dual on dispersive channels. This property was used to derive optimal matched filter structures, and to determine a criterion for the selection of spreading sequences for both DS and MC CDMA. The analysis of multiple antenna systems provided a unified framework for the study of DS-CDMA and MC-CDMA on time and frequency dispersive channels, which can also be used to compare their performance

Adaptive interference cancelation techniques for multicarrier modulated systems

Current wireline systems and wireless broadcasting systems employ multicarrier modulation (MCM). This includes the high-rate digital subscriber line (HDSL), digital audio broadcasting system (DAB) and the digital terrestrial television broadcasting system (dTTb). Multicarrier modulation is also envisioned for high-speed indoor wireless local area networks (WLAN). Additionally, multicarrier code division multiple access (MC-CDMA), a hybrid of orthogonal frequency division multiplexing (OFDM) and CDMA, is proposed for the downlink (base-to-mobile) of a 3rd generation wireless system as part of the IMT-2000 standardization process. The performance of an MC-CDMA system--similar to a direct sequence CDMA (DS-CDMA) system--is limited by the presence of multiple access interference (MAI) . Downlink communications also suffers from MAI as a result of the multipath channel effect, even if it implements orthogonal code multiplexing. Additionally, transmissions aimed at different mobile users may be assigned different powers in order to increase the system capacity, essentially creating a near-far problem for some users. Due to the MC-CDMA signal structure the conventional decorrelator (based on the inverse of the correlation matrix) is dependent on the channel coefficients, suggesting the use of an adaptive multiuser detector, which can track a time-variant channel. The performance of a blind adaptive multiuser detector for MC-CDMA, based on the bootstrap algorithm, is investigated and compared to the performance of the conventional decorrelator. Additionally, the performance is investigated for different channel conditions. First, for a non-faded flat additive white Gaussian noise (AWGN) channel. Second, for a frequency selective channel with and without correlation between the channel coefficients at the different subcarriers. In general, the mobile terminal suffers from limited available resources such as computing power or battery life and, therefore, cannot accommodate the same level of receiver complexity as the base station. For the downlink, however, the received signal structure is less complex due to the assumed synchronized transmission. Moreover, the mobile receiver is merely required to detect the desired user\u27s data stream. To reduce the complexity, detectors are proposed that do not require knowledge of the active users nor their respective codes, but rather use a combined code to represent all the interfering users at once. The performance of the reduced complexity conventional decorrelator is compared to the performance of an adaptive reduced complexity detector using the bootstrap algorithm. The performance of these detectors is also investigated for the aforementioned channel types. For spectral-efficiency, closely spaced subcarriers are used in a multicarrier modulated system. A resulting drawback is a high sensitivity of the performance to a frequency offset. This results from a Doppler shift, due to mobile movement, as well as from a mismatch between the carrier frequencies at the transmitter and receiver. To mitigate this problem an adaptive decorrelator based frequency offset correction scheme is developed for OFDM and its performance is investigated. Additionally, a blind frequency offset estimation and correction structure is proposed based on a stochastic gradient method. The convergence and statistical properties of this estimator are investigated. A blind adaptive joint multiuser detection and frequency offset correction structure for downlink MC-CDMA is developed. This detector is a combination of the structures for multiuser detection for MC-CDMA and frequency offset correction for OFDM. Moreover, the performance of this detector is investigated and compared to a joint detector based on a minimum mean square error (MMSE) criterion

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

Novel multiuser detection and multi-rate schemes for multi-carrier CDMA

A large variety of services is [sic] expected for wireless systems, in particular, high data rate services, such as wireless Internet access. Users with different data rates and quality of service (QoS) requirements must be accommodated. A suitable multiple access scheme is key to enabling wireless systems to support both the high data rate and the integrated multiple data rate transmissions with satisfactory performance and flexibility. A multi-carrier code division multiple access (MC-CDMA) scheme is a promising candidate for emerging broadband wireless systems. MC-CDMA is a hybrid of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA). The most salient feature of MC-CDMA is that the rate of transmission is not limited by the wireless channel\u27s frequency-selective fading effects caused by multipath propagation. In MC-CDMA, each chip of the desired user\u27s spreading code, multiplied by the current data bit, is modulated onto a separate subcarrier. Therefore, each subcarrier has a narrow bandwidth and undergoes frequency-flat fading. Two important issues for an MC-CDMA wireless system, multiuser detection and multi-rate access, are discussed in this dissertation. Several advanced receiver structures capable of suppressing multiuser interference in an uplink MC-CDMA system, operating in a frequency-selective fading channel, are studied in this dissertation. One receiver is based on a so-called multishot structure, in which the interference introduced by the asynchronous reception of different users is successfully suppressed by a receiver based on the minimum mean-square error (MMSE) criterion with a built-in de-biasing feature. Like many other multiuser schemes, this receiver is very sensitive to a delay estimation error. A blind adaptive two-stage decorrelating receiver based on the bootstrap algorithm is developed to combat severe performance degradation due to a delay estimation error. It is observed that in the presence of a delay estimation error the blind adaptive bootstrap receiver is more near-far resistant than the MMSE receiver. Furthermore, a differential bootstrap receiver is proposed to extend the limited operating range of the two-stage bootstrap receiver which suffers from a phase ambiguity problem. Another receiver is based on a partial sampling (PS) demodulation structure, which further reduces the sensitivity to unknown user delays in an uplink scenario. Using this partial sampling structure, it is no longer necessary to synchronize the receiver with the desired user. Following the partial sampling demodulator, a minimum mean-square error combining (MMSEC) detector is applied. The partial sampling MMSEC (PS-MMSEC) receiver is shown to have strong interference suppression and timing acquisition capabilities. The complexity of this receiver can be reduced significantly, with negligible performance loss, by choosing a suitable partial sampling rate and using a structure called reduced complexity PS-MMSEC (RPS-MMSEC). The adaptive implementation of these receivers yields a superior rate of convergence and symbol error rate performance in comparison to a conventional MMSEC receiver with known timing. All the above receiver structures are for a single-rate MC-CDMA. Three novel multi-rate access schemes for multi-rate MC-CDMA, fixed spreading length (FSL), coded FSL (CFSL) and variable spreading length (VSL), have been developed. These multi-rate access schemes enable users to transmit information at different data rates in one MC-CDMA system. Hence, voice, data, image and video can be transmitted seamlessly through a wireless infrastructure. The bit error rate performance of these schemes is investigated for both low-rate and high-rate users

A sliding window decorrelating receiver for multiuser DS-CDMA mobile radio networks

This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available

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

Wavelet-based multi-carrier code division multiple access systems

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Multicarrier Frequency Hopping Spread Spectrum Techniques With Quasi-Cyclic Low Density Parity Check Codes Channel Coding

This work presents a new proposed Multicarrier Frequency Hopping Spread Spectrum (MCFH-SS) system employing Quasi-Cyclic Low Density Parity Check (QC-LDPC) codes instead of the conventional LDPC codes. A new technique for constructing the QC-LDPC codes based on row division method is proposed. The new codes offer more flexibility in terms of high girth, multiple code rates and block length. Moreover, a new scheme for channel prediction in MCFH-SS system is proposed. The technique adaptively estimates the channel conditions and eliminates the need for the system to transmit a request message prior to transmitting the packet data. The ready-to-use channel will be occupied with a Pseudonoise (PN) code and use for transmission or else, it will be banned

Packet data communications over coded CDMA with hybrid type-II ARQ

This dissertation presents in-depth investigation of turbo-coded CDNIA systems in packet data communication terminology. It is divided into three parts; (1) CDMA with hybrid FEC/ARQ in deterministic environment, (2) CDMA with hybrid FEC/ARQ in random access environment and (3) an implementation issue on turbo decoding. As a preliminary, the performance of CDMA with hybrid FEC/ARQ is investigated in deterministic environment. It highlights the practically achievable spectral efficiency of CDMA system with turbo codes and the effect of code rates on the performance of systems with MF and LMMSE receivers, respectively. For given ensemble distance spectra of punctured turbo codes, an improved union bound is used to evaluate the error probability of ML turbo decoder with MF receiver and with LMMSE receiver front-end and, then, the corresponding spectral efficiency is computed as a function of system load. In the second part, a generalized analytical framework is first provided to analyze hybrid type-11 ARQ in random access environment. When applying hybrid type-11 ARQ, probability of packet success and packet length is generally different from attempt to attempt. Since the conventional analytical model, customarily employed for ALOHA system with pure or hybrid type-I ARQ, cannot be applied for this case, an expanded analytical model is introduced. It can be regarded as a network of queues and Jackson and Burke\u27s theorems can be applied to simplify the analysis. The second part is further divided into two sub topics, i.e. CDMA slotted ALOHA with hybrid type-11 ARQ using packet combining and CDMA unslotted ALOHA with hybrid type-11 ARQ using code combining. For code combining, the rate compatible punctured turbo (RCPT) codes are examined. In the third part, noticing that the decoding delay is crucial to the fast ARQ, a parallel MAP algorithm is proposed to reduce the computational decoding delay of turbo codes. It utilizes the forward and backward variables computed in the previous iteration to provide boundary distributions for each sub-block MAP decoder. It has at least two advantages over the existing parallel scheme; No performance degradation and No additional computation

Cooperative diversity for the cellular uplink: Sharing strategies, performance analysis, and receiver design

In this thesis, we propose data sharing schemes for the cooperative diversity in a cellular uplink to exploit diversity and enhance throughput performance of the system. Particularly, we consider new two and three-or-more user decode and forward (DF) protocols using space time block codes. We discuss two-user and three-user amplify and forward (AF) protocols and evaluate the performance of the above mentioned data sharing protocols in terms of the bit error rate and the throughput in an asynchronous code division multiple access (CDMA) cellular uplink. We develop a linear receiver for joint space-time decoding and multiuser detection that provides full diversity and near maximum-likelihood performance.;We also focus on a practical situation where inter-user channel is noisy and cooperating users can not successfully estimate other user\u27s data. We further design our system model such that, users decide not to forward anything in case of symbol errors. Channel estimation plays an important role here, since cooperating users make random estimation errors and the base station can not have the knowledge of the errors or the inter-user channels. We consider a training-based approach for channel estimation. We provide an information outage probability analysis for the proposed multi-user sharing schemes. (Abstract shortened by UMI.)