Analysis of coded OFDM system over frequency-selective fading channels

This thesis considers the analysis of system performance and resource allocation for a coded OFDM system over frequency selective fading channels. Due to the inseparable role taken by channel coding in a coded OFDM system, an information theoretical analysis is carried out and taken as the basis for the system performance and throughput. Based on the results of the information theoretical analysis, the optimal system BER performance of a coded OFDM system is first shown to converge to the outage probability for large OFDM block lengths. Instead of evaluating the outage probability numerically, we provide in this thesis a simple analytical closed form approximation of the outage probability for a coded OFDM system over frequency selective quasi-static fading channels. Simulation results of the turbo-coded OFDM systems further confirm the approximation of the outage probability. By taking the instantaneous channel capacity as the analytical building block, system throughput of a coded OFDM system is then provided. With the aim to compare the performance difference between adaptive and uniform resource allocation strategies, the system throughput of different allocation schemes under various channel conditions is analyzed. First, it is demonstrated that adaptive power allocation over OFDM sub-carriers at the transmitter achieves very little gain in terms of throughput over a uniform power distribution scheme. Theoretical analysis is then provided of the throughput increase of adaptive-rate schemes compared with fixed-rate schemes under various situations. Two practical OFDM systems implementing rate-compatible-punctured-turbo-code-based (RCPT-based) hybrid automatic-repeat-request (Hybrid-ARQ) and redundancy incremental Hybrid-ARQ protocols are also provided to verify the analytical results

Multi-antenna OFDM systems in the presence of phase noise and doubly-selective fading

Orthogonal frequency division multiplexing (OFDM), which has been very attractive for future high rate wireless communications, is very robust to channel multipath fading effect while providing high transmission data rate with high spectral efficiency. Multiple antennas can be combined with OFDM to increase diversity gain and to improve spectral efficiency through spatial multiplexing and space-time coding (STC). This dissertation focuses on performance analysis and detection schemes of multi-antenna OFDM systems in the presence of phase noise and doubly-selective fading where channel is both time-selective and frequency-selective. In space-time coded OFDM (ST-OFDM), channel time variations cause not only intercarrier interference (ICI) among different subcarriers in one OFDM symbol, but also intertransmit-antenna interference (ITAI). We quantify the impact of time-selective fading on the performance of quasi-orthogonal ST-OFDM systems by deriving, via an analytical approach, the expressions of carrier-to-interference ratio (CIR) and signal-to-interference-plus-noise ratio (SINR). We also evaluate the performance of five different detection schemes and show that all these schemes suffer from an irreducible error floor. Multiple-input multiple-output (MIMO) antennas combined with OFDM are very attractive for high-data-rate communications. However, MIMO-OFDM systems are very vulnerable to time-selective fading. We apply frequency-domain correlative coding in MIMO-OFDM systems over doubly-selective fading channels and derive the analytical expression of CIR to demonstrate the effectiveness of correlative coding in mitigating ICI. When applied in fast fading channels, common ST-OFDM receivers usually suffer from an irreducible error floor. We apply frequency-domain correlative coding combined with a modified decision-feedback (DF) detection scheme with low complexity to effectively suppress the error floor of quasi-orthogonal ST-OFDM over fast fading channels. Similar to single-antenna OFDM, MIMO-OFDM suffers from significant performance degradation due to phase noise and time-selective fading. After characterizing the common phase error (CPE) caused by phase noise and ICI caused by phase noise as well as time-selective fading, we derive a minimum mean-squared error (MMSE)- based scheme to mitigate the effect of both phase noise and Doppler frequency shift. We also evaluate and compare the performance of various detection schemes combined with the proposed CPE mitigation scheme. Throughout the dissertation, theoretical performance analysis is always presented along with corroborating simulations.Keywords: OFDM, space-time coding, doubly-selective, MIMO, phase nois

Error Rate Analysis for Coded Multicarrier Systems over Quasi-Static Fading Channels

This paper presents two methods for approximating the performance of coded multicarrier systems operating over frequency-selective, quasi-static fading channels with non-ideal interleaving. The first method is based on approximating the performance of the system over each realization of the channel, and is suitable for obtaining the outage performance of this type of system. The second method is based on knowledge of the correlation matrix of the frequency-domain channel gains and can be used to directly obtain the average performance. Both of the methods are applicable for convolutionally-coded interleaved systems employing Quadrature Amplitude Modulation (QAM). As examples, both methods are used to study the performance of the Multiband Orthogonal Frequency Division Multiplexing (OFDM) proposal for high data-rate Ultra-Wideband (UWB) communication.Comment: 5 pages, 3 figures, 2 tables. Submitted to Globecom 200

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

New Full-Diversity Space-Time-Frequency Block Codes with Simplified Decoders for MIMO-OFDM Systems

Multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) is known as a promising solution for wideband wireless communications. This is why it has been considered as a powerful candidate for IEEE 802.11n standard. Numerous space-frequency block codes (SFBCs) and space-time- frequency block codes (STFBCs) have been proposed so far for implementing MIMO-OFDM systems. In this paper, at first we propose new full-diversity STFBCs with high coding gain in time-varying channels; the construct method for this structure is using orthogonal space-time block code for any arbitrary number of transmit antenna and then we propose a decoder with linear complexity for our proposed coding scheme. Simulation results verify that the proposed STFBCs outperform other recently published STFBCs

Exact BER Analysis of OFDM Systems Communicating over Frequency-Selective Fading Channels Subjected to Carrier Frequency Offset

Orthogonal Frequency Division Multiplexing (OFDM) has been employed in numerous wireless standards. However, the performance of OFDM systems is degraded by both the Carrier Frequency Offset (CFO) and the Phase Estimation Error (PER). Hence new exact closed-form expressions are derived for calculating the average BER of OFDM systems in the presence of both CFO and PER in the context of frequency selective Nakagami-m fading channels. Our simulation results verify the accuracy of our exact BER analysis. By contrast, the Gaussian approximation slightly over-estimates the average BER, especially when the normalized CFO is small, the number of OFDM subcarriers is low and when the fading is less severe

Space-Time Trellis and Space-Time Block Coding Versus Adaptive Modulation and Coding Aided OFDM for Wideband Channels

Abstract—The achievable performance of channel coded spacetime trellis (STT) codes and space-time block (STB) codes transmitted over wideband channels is studied in the context of schemes having an effective throughput of 2 bits/symbol (BPS) and 3 BPS. At high implementational complexities, the best performance was typically provided by Alamouti’s unity-rate G2 code in both the 2-BPS and 3-BPS scenarios. However, if a low complexity implementation is sought, the 3-BPS 8PSK space-time trellis code outperfoms the G2 code. The G2 space-time block code is also combined with symbol-by-symbol adaptive orthogonal frequency division multiplex (AOFDM) modems and turbo convolutional channel codecs for enhancing the system’s performance. It was concluded that upon exploiting the diversity effect of the G2 space-time block code, the channel-induced fading effects are mitigated, and therefore, the benefits of adaptive modulation erode. In other words, once the time- and frequency-domain fades of the wideband channel have been counteracted by the diversity-aided G2 code, the benefits of adaptive modulation erode, and hence, it is sufficient to employ fixed-mode modems. Therefore, the low-complexity approach of mitigating the effects of fading can be viewed as employing a single-transmitter, single-receiver-based AOFDM modem. By contrast, it is sufficient to employ fixed-mode OFDM modems when the added complexity of a two-transmitter G2 scheme is affordable

Efficient space-frequency block coded pilot-aided channel estimation method for multiple-input-multiple-output orthogonal frequency division multiplexing systems over mobile frequency-selective fading channels

© 2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.An iterative pilot-aided channel estimation technique for space-frequency block coded (SFBC) multiple-input multiple-output orthogonal frequency division multiplexing systems is proposed. Traditionally, when channel estimation techniques are utilised, the SFBC information signals are decoded one block at a time. In the proposed algorithm, multiple blocks of SFBC information signals are decoded simultaneously. The proposed channel estimation method can thus significantly reduce the amount of time required to decode information signals compared to similar channel estimation methods proposed in the literature. The proposed method is based on the maximum likelihood approach that offers linearity and simplicity of implementation. An expression for the pairwise error probability (PEP) is derived based on the estimated channel. The derived PEP is then used to determine the optimal power allocation for the pilot sequence. The performance of the proposed algorithm is demonstrated in high frequency selective channels, for different number of pilot symbols, using different modulation schemes. The algorithm is also tested under different levels of Doppler shift and for different number of transmit and receive antennas. The results show that the proposed scheme minimises the error margin between slow and high speed receivers compared to similar channel estimation methods in the literature.Peer reviewe