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

Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER

Spectral efficiency of CDMA based ad-hoc networks

Spectrum efficiency and energy efficiency are two important attributes driving innovation in wireless communication. Efficient spectrum utilization and sharing with multiple access techniques and using under-utilized spectra by cognitive radios is the current focus due to the scarcity and cost of the available radio spectrum. Energy efficiency to increase operating time of portable handheld devices like smartphones that handle simultaneous voice/video streaming and web browsing and battery powered nodes in a sensor network where battery capacity determines the lifetime of the network is another area attracting researchers. The focus of this thesis is the spectral efficiency of multicarrier code division multiple access (CDMA) in wireless ad-hoc networks. Furthermore, energy efficiency to maximize lifetime of a network are also studied.In a multicarrier CDMA system inter-carrier interference (ICI) due to carrier frequency offset and multiple access interference (MAI) are two major factors that deteriorate the performance. Previous work in this area has been mostly focused on simulation results due to the complexity of the analysis due to the large number of random variables involved. Taking into account accurate statistical models for ICI and MAI that account for the correlation between adjacent subcarriers, this thesis presents new mathematical analysis for the spectral efficiency of multicarrier CDMA communication systems over a frequency selective Rayleigh fading environment. We analyze and compare three multicarrier CDMA schemes which are multicarrier CDMA, multicarrier direct-sequence CDMA and multitone CDMA. We also present simulation results to confirm the validity of our analysis. We also analyze the performance of three simple multiple access techniques or coexistence etiquettes in detail, which are simple to implement and do not require any central control. Accurate interference models are developed and are used to derive accurate expressions for packet error rates in the case of direct sequence CDMA and slotted packet transmission schemes. These results are then used to study the performance of the coexistence etiquettes and compare them with each other. Finally we present a new joint node selection and power allocation strategy that increases lifetime of an ad-hoc network where nodes cooperate to enable diversity in transmission.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Joint Impact of Frequency Synchronization Errors and Intermodulation Distortion on the Performance of Multicarrier DS-CDMA Systems

The performance of multicarrier systems is highly impaired by intercarrier interference (ICI) due to frequency synchronization errors at the receiver and by intermodulation distortion (IMD) introduced by a nonlinear amplifier (NLA) at the transmitter. In this paper, we evaluate the bit-error rate (BER) of multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) downlink systems subject to these impairments in frequency-selective Rayleigh fading channels, assuming quadrature amplitude modulation (QAM). The analytical findings allow to establish the sensitivity of MC-DS-CDMA systems to carrier frequency offset (CFO) and NLA distortions, to identify the maximum CFO that is tolerable at the receiver side in different scenarios, and to find out the optimum value of the NLA output power backoff for a given CFO. Simulation results show that the approximated analysis is quite accurate in several conditions

Performance Analysis of Multicarrier Code Division Multiple Access (MC-CDMA) Systems

A thesis presented to the faculty of the College of Science and Technology at Morehead State University in partial fulfillment of the requirements for the Degree of Master of Science by Pravinkumar Patil on August 11, 2008

Adaptive parallel interference cancellation receivers with diversity combining for multicarrier DS CDMA systems

Master'sMASTER OF ENGINEERIN