The problem of the efficient use of Hybrid Automatic-Repeat-reQuest (Hybrid-ARQ) in wireless communication has attracted a considerable amount of research. In this thesis, the use and implementation of turbo codes as the Forward Error Correction (FEC) code for Hybrid-ARQ is investigated. The major accomplishments of the research include both the analysis and implementation of turbo Hybrid-ARQ. The thesis begins by obtaining a tractable bound for the performance of turbo codes with M-ary Quadrature-Amplitude-Modulation (M-ary QAM). The research considers the design problem of turbo coded Hybrid-ARQ optimized for AWGN and fading channels. The design problem of turbo Hybrid-ARQ in wideband channels is considered and an optimization strategy is proposed based on Orthogonal-Frequency-Division- Multiplexing (OFDM). The research also presents a novel rate scalable encoder structure that optimal selects a disparate but optimal pair of component codes given the channel conditions. A second part of the thesis considers the implementation of turbo Hybrid-ARQ in Very Large Scale Integration (VLSI ) systems. A design for a single architecture for Type-I and Type-II turbo Hybrid-ARQ is suggested in addition to approaches for improving performance of the Soft-Output-Viterbi-Algorithm(SOVA) decoder core. The research also proposes a SOVA decoder architecture that exploits reliability information to select between the SOVA and bi-directional SOVA.

There are two important techniques, automatic repeat request (ARQ) and forward error control (FEC), in error control systems. ARQ uses redundancy for detecting errors. While an error is detected in a transmitted code word, the receiver requests the transmitter to re-transmit the corrupted code word and the transmitted code word with detected error is discarded. However, the transmitted code word with detected error still contains a lot of information. ARQ schemes require a feedback channel. FEC relies on the controlled use of redundancy in the transmitted code word to detect and correct errors. Whether the decoding of the received code word is successful, no further processing is performed at the receiver. Therefore, FEC requires only a one-way link between the transmitter and receiver. In error control systems, a powerful FEC, turbo coding, was first proposed in 1993. Its performance was investigated by simulation to be close to the Shannon limit. Hybrid ARQ (HARQ) systems after properly combining the ARQ techniques and FEC techniques, can be classified into type-I, type-II and type-III, and its performance can be elevated far more than ARQ systems. HARQ adopts the FEC methods to detect and correct the error patterns caused in the process of transmission. While errors are detected in the demodulated signals at receiver and fail to be corrected, the receiver will activate ARQ mechanism to request the transmitter to retransmit related information to achieve an error-free transmission. In this thesis, a feasible type-II hybrid ARQ algorithm is proposed. Performance of the proposed algorithm is evaluated and analyzed by computer simulations. Compared with the traditional HARQ algorithms, the proposed algorithm can offer better performance in delay time and number of retransmissions with approximate throughput.

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.

The last three decades have seen significant advances in the wireless communication field. As the data rates of wireless systems are increasing, the demand for mobile services also is increasing rapidly. Like other natural resources, radio spectrum suitable for mobile wireless communications is also limited. In order to keep up with this ever increasing demand, there is a requirement of new signal processing algorithms. Diversity is a technique used in wireless systems to combat the effects of fading and thereby improve reliability of data transfer. There are many ways in which algorithms can exploit diversity in wireless channels. Hybrid-automatic repeat request (H-ARQ) schemes and relaying mechanisms are two such diversity extracting techniques. Even though these diversity achieving techniques have been well understood in theory, there are many ways in which one can optimize these techniques for specific application scenarios.In this thesis, we focus on improving the performance of retransmission schemes and relaying systems. In the first part of the thesis, we improve the performance of H-ARQ schemes in the 3GPP- long term evolution (LTE) system by improving the performance of feedback signaling. We employ complex-field coding to extract the inherent frequency diversity available in the resources. Next, we provide a sub-optimal solution to the outage-optimal power allocation problem in incremental redundancy based H-ARQ system, whose performance is practically the same as that of the optimal solution. In the later part of the thesis, we propose a retransmission scheme based on superposition coding (SPC) for the symmetric relaying scenario. We provide packet error probability (PEP) expressions and solutions for the optimal fraction of power allocated for the partners' data. Finally, we study the optimal bits-to-symbol mappings for SPC and its effect on an H-ARQ scheme and the symmetric relaying scenario using SPC.

Dans les standards actuels tels que HSDPA ou LTE, des protocoles de retransmissions (ARQ: Automatic Repeat reQuest) sont utilisés conjointement au codage de canal afin de palier aux erreurs dues à l'absence ou la mauvaise de connaissance de canal à la transmission. On garantit ainsi la fiabilité du lien physique pour les couches OSI supérieures (du moins un taux d'erreur paquet faible). De tels protocoles sont appelés protocoles de retransmission hybrides (HARQ). L'objet de cette thèse est de proposer des outils permettant l'analyse et l'optimisation des systèmes de communication en présences de protocoles HARQ avec une emphase particulière sur les systèmes cognitifs.Dans la première partie, nous étudierons un système point-à-point dans lequel trois différents protocoles HARQ adaptatifs seront considérés. Dans un premier temps, nous considérerons le régime asymptotique (i.e. codes optimaux gaussiens). Nous proposerons, dans ce cas, deux optimisations possibles : la minimisation de la puissance moyenne sous la contrainte de débit moyen et la maximisation du débit moyen sous une contrainte de puissance moyenne. Nous montrerons que les Processus de Décision Markoviens (MDP) sont des outils adaptés aux problèmes d'optimisation considérés.Dans les standards actuels tels que HSDPA ou LTE, des protocoles de retransmissions (ARQ: Automatic Repeat reQuest) sont utilisés conjointement au codage de canal afin de palier aux erreurs dues à l'absence ou la mauvaise de connaissance de canal à la transmission. On garantit ainsi la fiabilité du lien physique pour les couches OSI supérieures (du moins un taux d'erreur paquet faible). De tels protocoles sont appelés protocoles de retransmission hybrides (HARQ). L'objectif de cette thèse est de proposer des outils permettant l'analyse et l'optimisation des systèmes de communication en présences de protocoles HARQ avec une emphase particulière sur les systèmes cognitifs. La radio cognitive est une approche permettant à des utilisateurs non-licenciés de communiquer dans les mêmes bandes de fréquences que des utilisateurs licenciés afin d'augmenter l'efficacité spectrale des réseaux sans fil. Les utilisateurs secondaires doivent néanmoins limiter les interférences générées sur les signaux des utilisateurs primaires. Dans ce contexte, nous étudierons les débits atteignables par un utilisateur secondaire utilisant l'observation du protocole HARQ de l'utilisateur primaire afin de contrôler son interférence. / Automatic Repeat Request protocols (ARQ) are widely implemented in current mobile wireless standards such as HSDPA and LTE. In general, ARQ protocols are combined with channel coding to overcome errors caused by the lack of channel knowledge at the transmitter side. These protocols are called Hybrid ARQ protocols (HARQ). HARQ protocols ensure a good reliability (at least a small packet error rate) of the physical layer for the OSI upper layers. The purpose of this thesis is to provide tools for the analysis and the optimization of HARQ communication systems with an emphasis on cognitive systems. Cognitive Radio (CR) is an approach aiming to increase the spectral efficiency of wireless networks. In a CR context, unlicensed users are allowed to communicate within the same frequency bands and at the same time as licensed users. Secondary users must however limit the amount of interference generated on the primary users signals. In this thesis, we consider a scenario in which the secondary user interferes a primary user employing a HARQ protocol. When the secondary user knows the state of the primary HARQ protocol, we show that a joint power and rate allocation can be performed to limit the interference.

<p>The ongoing wireless communication evolution offers improvements for industrial applications where traditional wireline solutions causes prohibitive problems in terms of cost and feasibility. Many of these new wireless applications are packet oriented and time-critical. The deadline dependent coding (DDC) communication protocol presented here is explicitly intended for wireless real-time applications. The objective of the work described in this thesis is therefore to develop the foundation for an efficient and reliable real-time communication protocol for critical deadline dependent communication over unreliable wireless channels.</p><p>Since the communication is packet oriented, block codes are suitable for error control. Reed-Solomon codes are chosen and incorporated in a concatenated coding scheme using iterative detection with trellis based decoding algorithms. Performance bounds are given for parallel and serially concatenated Reed-Solomon codes using BPSK. The convergence behavior of the iterative decoding process for serially concatenated block codes is examined and two different stopping criteria are employed based on the log-likelihood ratio of the information bits.</p><p>The stopping criteria are also used as a retransmission criterion, incorporating the serially concatenated block codes in a type-I hybrid ARQ (HARQ) protocol. Different packet combining techniques specifically adapted to the concatenated HARQ (CHARQ) scheme are used. The extrinsic information used in the iterative decoding process is saved and used when decoding after a retransmission. This technique can be seen as turbo code combining or concatenated code combining and is shown to improve performance. Saving the extrinsic information may also be seen as a doping criterion yielding faster convergence. As such, the extrinsic information can be used in conjunction with traditional diversity combining schemes. The performance in terms of bit error rate and convergence speed is improved with only negligible additional complexity.</p><p>Consequently, CHARQ based on serially concatenated block codes using iterative detection creates a flexible and reliable scheme capable of meeting specified required realtime constraints.</p>

The use of wireless technology in the process industry is becoming increasingly important to obtain fast deployment at low cost. However, poor channel quality often leads to retransmissions, which are governed by Automatic Repeat Request (ARQ) schemes. While ARQ is a simple and useful tool to alleviate packet errors, it has considerable disadvantages: retransmissions lead to an increase in energy expenditure and latency. The use of Forward Error Correction (FEC) however offers several advantages. We consider a Hybrid-ARQ-Adaptive-FEC scheme (HAF) based on BCH codes and Channel State Information. This scheme is evaluated on AWGN and fading channels. It is shown that HAF offers significantly improved performance both in terms of energy efficiency and latency, as compared to ARQ.

Data intensive applications, requiring reliability and strict delay constraints, have emerged recently and they necessitate a different approach to analyzing system performance. In my work, I establish a framework that relates physical channel parameters to the queueing performance for a single-user wireless system. I then seek to assess the potential benefits of multirate techniques, such as hybrid-ARQ (Automatic Repeat reQuest), in the context of delay-sensitive communications. Present methods of analysis in an information theoretic paradigm define capacity assuming that long codewords can be used to take advantage of the ergodic properties of the fading wireless channel. This definition provides only a limited characterization of the channel in the light of delay constraints. The assumption of independent and identically distributed channel realizations tends to over-estimate the system performance by not considering the inherent time correlation. A finite-state continuous time Markov channel model that I formulate enables me to partition the instantaneous data-rate received at the destination into a finite number of states, representing layers in a hybrid-ARQ scheme. The correlation of channel has been incorporated through level crossing rates as transition rates in the Markov model. The large deviation principle governing the buffer overflow of the Markov model, is very sensitive to channel memory, is tractable, and gives a good estimate of the system performance. Metrics such as effective capacity and probability of buffer overflow, that are obtained through large deviations have been related to the wireless physical layer parameters through the model. Using the above metrics under QoS constraints, I establish the quantitative performance advantage of using hybrid-ARQ over traditional systems. I conduct this inquiry by restricting attention to the case where the expected transmit power is fixed at the transmitter. The results show that hybrid-ARQ helps us in obtaining higher effective capacity, but it is very difficult to support delay sensitive communication over wireless channel in the absence of channel knowledge and dynamic power allocation strategies.

The ongoing wireless communication evolution offers improvements for industrial applications where traditional wireline solutions causes prohibitive problems in terms of cost and feasibility. Many of these new wireless applications are packet oriented and time-critical. The deadline dependent coding (DDC) communication protocol presented here is explicitly intended for wireless real-time applications. The objective of the work described in this thesis is therefore to develop the foundation for an efficient and reliable real-time communication protocol for critical deadline dependent communication over unreliable wireless channels. Since the communication is packet oriented, block codes are suitable for error control. Reed-Solomon codes are chosen and incorporated in a concatenated coding scheme using iterative detection with trellis based decoding algorithms. Performance bounds are given for parallel and serially concatenated Reed-Solomon codes using BPSK. The convergence behavior of the iterative decoding process for serially concatenated block codes is examined and two different stopping criteria are employed based on the log-likelihood ratio of the information bits. The stopping criteria are also used as a retransmission criterion, incorporating the serially concatenated block codes in a type-I hybrid ARQ (HARQ) protocol. Different packet combining techniques specifically adapted to the concatenated HARQ (CHARQ) scheme are used. The extrinsic information used in the iterative decoding process is saved and used when decoding after a retransmission. This technique can be seen as turbo code combining or concatenated code combining and is shown to improve performance. Saving the extrinsic information may also be seen as a doping criterion yielding faster convergence. As such, the extrinsic information can be used in conjunction with traditional diversity combining schemes. The performance in terms of bit error rate and convergence speed is improved with only negligible additional complexity. Consequently, CHARQ based on serially concatenated block codes using iterative detection creates a flexible and reliable scheme capable of meeting specified required realtime constraints.

The HF band of radio frequencies, from 3-30 MHz, is unique in its property that it is refracted by the ionosphere. This property allows long distance radio telecommunications around the world without requiring infrastructure. High frequency (HF) communication has been largely superseded by satellite and cellular technologies for day-to-day communications, due to the tight bandwidth constraints and technical difficulties inherent in using it. However there is still a need for HF communications devices where existing infrastructure is not available, such as in remote or polar locations, or in emergency situations due to natural disasters. This research is aimed at the development of an asymmetric HF communications link, with a battery-powered remote unit that transmits a small amount of data to a mains-powered base station. New technologies are identified and evaluated for use in the link, with the aim of reducing the power requirements of the remote unit. Error correction techniques are investigated. Low-density parity check (LDPC) codes, which are powerful codes used for forward error correction, are suggested for use in the link. Quasi-cyclic LDPC codes allow the low-power transmitter unit to use a computationally simple encoder based on feedback shift registers for generating the LDPC block codes cheaply. Semi-blind LDPC turbo equalisation is a powerful technique that can be used at the base station which utilises the structure of the LDPC code to encode the data stream. This equalises a received signal with a minimal amount of training data required, reducing the duty cycle of the remote unit. Hybrid automatic repeat request (HARQ) techniques are also investigated, which increase the throughput of a link when data repeats are required. A novel HARQ techniquewas created and proven to increase throughput in links with noise. As the proposed system may be deployed in remote locations, or locations where it might be difficult or undesirable to erect a proper HF antenna, a selection of buried antennas are characterised. A design for a remote unit is suggested. This unit was manufactured and used to test the capability of inexpensive, low power hardware to implement the proposed remote unit algorithms.