Achieving Low-Complexity Maximum-Likelihood Detection for the 3D MIMO Code

The 3D MIMO code is a robust and efficient space-time block code (STBC) for the distributed MIMO broadcasting but suffers from high maximum-likelihood (ML) decoding complexity. In this paper, we first analyze some properties of the 3D MIMO code to show that the 3D MIMO code is fast-decodable. It is proved that the ML decoding performance can be achieved with a complexity of O(M^{4.5}) instead of O(M^8) in quasi static channel with M-ary square QAM modulations. Consequently, we propose a simplified ML decoder exploiting the unique properties of 3D MIMO code. Simulation results show that the proposed simplified ML decoder can achieve much lower processing time latency compared to the classical sphere decoder with Schnorr-Euchner enumeration

Order-Theoretic Methods for Space-Time Coding: Symmetric and Asymmetric Designs

Siirretty Doriast

Distributed space time block coding in asynchronous cooperative relay networks

The design and analysis of various distributed space time block coding schemes for asynchronous cooperative relay networks is considered in this thesis. Rayleigh frequency flat fading channels are assumed to model the links in the networks, and interference suppression techniques together with an orthogonal frequency division multiplexing type transmission approach are employed to mitigate the synchronization errors at the destination node induced by the different delays through the relay nodes. Closed-loop space time block coding is first considered in the context of decode-and-forward (regenerative) networks. In particular, quasi orthogonal and extended orthogonal coding techniques are employed for transmission from four relay nodes and parallel interference cancellation detection is exploited to mitigate synchronization errors. Availability of a direct link between the source and destination nodes is studied, and a new Alamouti space time block coding technique with parallel interference cancellation detection which does not require such a direct link connection and employs two relay nodes is proposed. Outer coding is then added to gain further improvement in end-to-end performance and amplify-and-forward (non regenerative) type networks together with distributed space time coding are considered to reduce relay node complexity. Novel detection schemes are then proposed for decode-and-forward networks with closed-loop extended orthogonal coding which reduce the computational complexity of the parallel interference cancellation. Both sub-optimum and near-optimum detectors are presented for relay nodes with single or dual antennas. End-to-end bit error rate simulations confirm the potential of the approaches and their ability to mitigate synchronization errors. A relay selection approach is also formulated which maximizes spatial diversity gain and attains robustness to timing errors. Finally, a new closed-loop distributed extended orthogonal space time block coding solution for amplify-and-forward type networks which minimizes the number of feedback bits by using a cyclic rotation phase is presented. This approach utilizes an orthogonal frequency division multiplexing type transmission structure with a cyclic prefix to mitigate synchronization errors. End-to-end bit error performance evaluations verify the efficacy of the scheme and its success in overcoming synchronization errors

Codificação distribuída em sistemas com diversidade cooperativa

Doutoramento em Engenharia ElectrotécnicaO presente trabalho propõe-se a divulgar as mais significativas técnicas de esquemas cooperativos, de forma a ultrapassar alguns dos problemas dos sistemas móveis sem fios da próxima geração, estendendo a área de cobertura destes sistemas, assim como a sua capacidade e fiabilidade. O estudo de diversos esquemas cooperativos é efetuado em termos de capacidade e de taxa de erros, fazendo variar o número de relays e de antenas em cada elemento do sistema. Diversos algoritmos com aplicação em sistemas cooperativos são desenvolvidos e propostos ao longo desta tese, como códigos espaço-frequência aplicados de forma distribuída nos relays, para sistemas baseados na tecnologia OFDM e sob diversos cenários próximos da realidade. Os sistemas cooperativos são particularmente úteis em situações em que o caminho direto entre dois terminais não está acessível ou tem uma fraca qualidade de transmissão. Tendo este aspeto em consideração, e pretendendo ter a máxima eficiência espetral e máxima diversidade, um algoritmo com precodificação é também proposto para múltiplos relays, cada um equipado com uma ou duas antenas. A formulação matemática associada aos algoritmos propostos é apresentada, assim como a derivação da probabilidade de erro teórica. O desempenho dos sistemas assistidos por relays usando os algoritmos propostos é comparado em relação a outros esquemas cooperativos equivalentes e a esquemas não-cooperativos, considerando cenários com diferentes qualidades de canal, daí advindo importantes conclusões em relação a estes sistemas.Cooperative schemes are promising solutions for cellular wireless networks aiming to improve system fairness, extend coverage and increase capacity. Measurements of these system performances are made in terms of BER and capacity for different configurations, by varying the number of relays and of antennas equipping each node. In this work we propose and evaluate distributed space-frequency codes applied to cooperative systems in a distributed way, with application in OFDM systems and considering realistic scenarios. Moreover, the use of relays is of significant interest to allow radio access in situations where a direct path is not available or has poor quality. Thus, a data precoded relay-assisted scheme is also proposed for a system cooperating with multiple relays, each equipped with either a single antenna or two-antenna array. Mathematical formulation of the proposed algorithms is derived as well as the pairwise error probability. We further present the performances of the proposed algorithms apllied in relay-assisted schemes, and compare them with equivalent cooperative and non-cooperative schemes, for several channel quality scenarios, through which important conclusions are achieved.FCT/FS

Cooperative diversity techniques for high-throughput wireless relay networks

Relay communications has attracted a growing interest in wireless communications with application to various enhanced technologies. This thesis considers a number of issues related to data throughput in various wireless relay network models. Particularly, new implementations of network coding (NC) and space-time coding (STC) techniques are investigated to offer various means of achieving high-throughput relay communications. Firstly, this thesis investigates different practical automatic repeat request (ARQ) retransmission protocols based on NC for two-way wireless relay networks to improve throughput efficiency. Two improved NC-based ARQ schemes are designed based on go-back-N and selective-repeat (SR) protocols. Addressing ARQ issues in multisource multidestination relay networks, a new NC-based ARQ protocol is proposed and two packet-combination algorithms are developed for retransmissions at relay and sources to significantly improve the throughput. In relation to the concept of channel quality indicator (CQI) reporting in two-way relay networks, two new efficient CQI reporting schemes are designed based on NC to improve the system throughput by allowing two terminals to simultaneously estimate the CQI of the distant terminal-relay link without incurring additional overhead. The transmission time for CQI feedback at the relays is reduced by half while the increase in complexity and the loss of performance are shown to be negligible. Furthermore, a low-complexity relay selection scheme is suggested to reduce the relay searching complexity. For the acknowledgment (ACK) process, this thesis proposes a new block ACK scheme based on NC to significantly reduce the ACK overheads and therefore produce an enhanced throughput. The proposed scheme is also shown to improve the reliability of block ACK transmission and reduce the number of data retransmissions for a higher system throughput. Additionally, this thesis presents a new cooperative retransmission scheme based on relay cooperation and NC to considerably reduce the number of retransmission packets and im- prove the reliability of retransmissions for a more power efficient and higher throughput system with non-overlapped retransmissions. Moreover, two relay selection schemes are recommended to determine the optimised number of relays for the retransmission. Finally, with respect to cognitive wireless relay networks (CWRNs), this thesis proposes a new cooperative spectrum sensing (CSS) scheme to improve the spectrum sensing performance and design a new CSS scheme based on NC for three-hop CWRNs to improve system throughput. Furthermore, a new distributed space-time-frequency block code (DSTFBC) is designed for a two- hop nonregenerative CWRN over frequency-selective fading channels. The proposed DSTFBC design achieves higher data rate, spatial diversity gain, and decoupling detection of data blocks at all destination nodes with a low-complexity receiver structure

Spatial diversity in MIMO communication systems with distributed or co-located antennas

The use of multiple antennas in wireless communication systems has gained much attention during the last decade. It was shown that such multiple-input multiple-output (MIMO) systems offer huge advantages over single-antenna systems. Typically, quite restrictive assumptions are made concerning the spacing of the individual antenna elements. On the one hand, it is typically assumed that the antenna elements at transmitter and receiver are co-located, i.e., they belong to some sort of antenna array. On the other hand, it is often assumed that the antenna spacings are sufficiently large, so as to justify the assumption of independent fading. In this thesis, the above assumptions are relaxed. In the first part, it is shown that MIMO systems with distributed antennas and MIMO systems with co-located antennas can be treated in a single, unifying framework. In the second part this fact is utilized, in order to develop appropriate transmit power allocation strategies for co-located and distributed MIMO systems. Finally, the third part focuses on specific synchronization problems that are of interest for distributed MIMO systems

Space time coding in MIMO systems

Multiple-input multiple-output (MIMO) antenna technology is promising for high-speed wireless communications without increasing the transmission band- width. Space time coding (STC) is a scheme that employs multiple antennas to increase transmission rate or to improve transmission quality. STC is used widely in mobile cellular networks, wireless local area networks (WLAN) and wireless metropolitan area networks (WMAN). However, there are still many unsolved or partially solved issues in STC. In this thesis, I propose a new STC design from cyclic design. I then propose a systematic method to design quasi-orthogonal space time block codes (QOSTBC) for an arbitrary number of transmit antennas, and derive the optimal constellation rotation angles to achieve full diversity. I also propose an analytical method to derive the exact error probabilities of orthogonal space time block codes (OSTBC). In order to improve the error performance, I introduce an adaptive power allocation scheme for OSTBC. Combining STC with continuous phase modulation (CPM) is an attractive solution for mobile commu- nications for which power is limited. Thus, I apply OSTBC to binary CPM with modulation index h = 0.5, and develop a simplified receiver for such scheme. Finally, I present a decoding method to reduce the complexity of QOSTBC without degrading its error performance