Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

Performance Analysis and Mitigation Techniques for I/Q-Corrupted OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) has become a widely adopted modulation technique in modern communications systems due to its multipath resilience and low implementation complexity. The direct conversion architecture is a popular candidate for low-cost, low-power, fully integrated transceiver designs. One of the inevitable problems associated with analog signal processing in direct conversion involves the mismatches in the gain and phases of In-phase (I) and Quadrature-phase (Q) branches. Ideally, the I and Q branches of the quadrature mixer will have perfectly matched gains and are orthogonal in phase. Due to imperfect implementation of the electronics, so called I/Q imbalance emerges and creates interference between subcarriers which are symmetrically apart from the central subcarrier. With practical imbalance levels, basic transceivers fail to maintain the sufficient image rejection, which in turn can cause interference with the desired transmission. Such an I/Q distortion degrades the systems performance if left uncompensated. Moreover, the coexistence of I/Q imbalance and other analog RF imperfections with digital baseband and higher layer functionalities such as multiantenna transmission and radio resource management, reduce the probability of successful transmission. Therefore, mitigation of I/Q imbalance is an essential substance in designing and implementing modern communications systems, while meeting required performance targets and quality of service. This thesis considers techniques to compensate and mitigate I/Q imbalance, when combined with channel estimation, multiantenna transmission, transmission power control, adaptive modulation and multiuser scheduling. The awareness of the quantitative relationship between transceiver parameters and system parameters is crucial in designing and dimensioning of modern communications systems. For this purpose, analytical models to evaluate the performance of an I/Q distorted system are considered

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

Channel estimation and parameters acquisition systems employing cooperative diversity

Doutoramento em Engenharia Eletrotécnica e TelecomunicaçõesThis work investigates new channel estimation schemes for the forthcoming and future generation of cellular systems for which cooperative techniques are regarded. The studied cooperative systems are designed to re-transmit the received information to the user terminal via the relay nodes, in order to make use of benefits such as high throughput, fairness in access and extra coverage. The cooperative scenarios rely on OFDM-based systems employing classical and pilot-based channel estimators, which were originally designed to pointto-point links. The analytical studies consider two relaying protocols, namely, the Amplifyand-Forward and the Equalise-and-Forward, both for the downlink case. The relaying channels statistics show that such channels entail specific characteristics that comply to a proper filter and equalisation designs. Therefore, adjustments in the estimation process are needed in order to obtain the relay channel estimates, refine these initial estimates via iterative processing and obtain others system parameters that are required in the equalisation. The system performance is evaluated considering standardised specifications and the International Telecommunication Union multipath channel models.Este trabalho tem por objetivo o estudo de novos esquemas de estimação de canal para sistemas de comunicação móvel das próximas gerações, para os quais técnicas cooperativa são consideradas. Os sistemas cooperativos investigados neste trabalho estão projetados para fazerem uso de terminais adicionais a fim de retransmitir a informação recebida para o utilizador final. Desta forma, pode-se usurfruir de benefícios relacionados às comunicações cooperativas tais como o aumento do rendimento do sistema, fiabilidade e extra cobertura. Os cenários são basedos em sistemas OFDM que empregam estimadores de canal que fazem uso de sinais piloto e que originalmente foram projetados para ligações ponto a ponto. Os estudos analíticos consideram dois protocolos de encaminhamento, nomeadamente, Amplify-and-Forward e Equalise-and-Forward, ambos para o caso downlink. As estatísticas dos canais em estudo mostram que tais canais ocasionam características específicas para as quais o filtro do estimador e a equalisação devem ser apropridamente projetados. Estas características requerem ajustes que são necessários no processo de estimação a fim de estimar os canais, refinar as estimativas iniciais através de processos iterativos e ainda obter outros parâmetros do sistema que são necessários na equalização. O desempenho dos esquemas propostos são avaliados tendo em consideração especificações padronizadas e modelos de canal descritos na International Telecommunication Union

Técnicas com múltiplas antenas distribuídas para sistemas sem fios

Mestrado em Engenharia Electrónica e TelecomunicaçõesTransmissão cooperativa, em que uma fonte e um relay cooperam para enviar uma mensagem para o destino, pode proporcionar diversidade espacial contra o desvanecimento nas comunicações sem fios. O objectivo deste projecto é estudar a performance de um sistema de transmissão cooperativo com dois relays equipados com duas antenas, entre o transmissor e o utilizador. Considera-se que a estação base está equipada com duas antenas e o terminal móvel apenas com uma. O sistema cooperativo foi implementado de acordo com as especificações do LTE e avaliado em diversos cenários de propagação, considerando canais com diferentes Relação Sinal Ruído (SNR). Verificou-se que o desempenho do sistema proposto é melhor, quando comparado com o sistema não cooperativo, na maior parte dos cenários estudados.Cooperative transmission, in which a source and relay cooperate to sent a mensage to destination, can provide spatial diversity against fading in wirless telecomunications. The goal of this project is to study the perfomance of a cooperative tranmition systems with two relays equiped with two antennas, between transmitter and user. It is considered that the base station is equipped with two antennas and the mobile terminal with only one. The cooperative system was implemented according to the specifications of the LTE and evaluated at several propagation scenarios, considering channels with diferents Signal to Noise Ratio (SNR). It was found that the perfomance of the proposed system is better when compared with the non-cooperative ones, in most scenarios considered.CODIV/FP7-ICT-200

Optimal cross layer design for CDMA-SFBC wireless systems

The demand for high speed reliable wireless services has been rapidly growing. Wireless networks have limited resources while wireless channels suffer from fading, interference and time variations. Furthermore, wireless applications have diverse end to end quality of service (QoS) requirements. The aforementioned challenges require the design of spectrally efficient transmission systems coupled with the collaboration of the different OSI layers i.e. cross layer design. To this end, we propose a code division multiple access (CDMA)-space frequency block coded (SFBC) systems for both uplink and downlink transmissions. The proposed systems exploit code, frequency and spatial diversities to improve reception. Furthermore, we derive closed form expressions for the average bit error rate of the proposed systems. In this thesis, we also propose a cross layer resource allocation algorithm for star CDMA-SFBC wireless networks. The proposed resource allocation algorithm assigns base transceiver stations (BTS), antenna arrays and frequency bands to users based on their locations such that their pair wise channel cross correlation is minimized while each user is assigned channels with maximum coherence time. The cooperation between the medium access control (MAC) and physical layers as applied by the optimized resource allocation algorithm improves the bit error rate of the users and the spectral efficiency of the network. A joint cross layer routing and resource allocation algorithm for multi radio CDMA-SFBC wireless mesh networks is also proposed in this thesis. The proposed cross layer algorithm assigns frequency bands to links to minimize the interference and channel estimation errors experienced by those links. Channel estimation errors are minimized by selecting channels with maximum coherence time. On top, the optimization algorithm routes network traffic such that the average end to end packet delay is minimized while avoiding links with high interference and short coherence time. The cooperation between physical, MAC and network layers as applied by the optimization algorithm provides noticeable improvements in average end to end packet delay and success rat

D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies

This document provides the most recent updates on the technical contributions and research challenges focused in WP3. Each Technology Component (TeC) has been evaluated under possible uniform assessment framework of WP3 which is based on the simulation guidelines of WP6. The performance assessment is supported by the simulation results which are in their mature and stable state. An update on the Most Promising Technology Approaches (MPTAs) and their associated TeCs is the main focus of this document. Based on the input of all the TeCs in WP3, a consolidated view of WP3 on the role of multinode/multi-antenna transmission technologies in 5G systems has also been provided. This consolidated view is further supported in this document by the presentation of the impact of MPTAs on METIS scenarios and the addressed METIS goals.Aziz, D.; Baracca, P.; De Carvalho, E.; Fantini, R.; Rajatheva, N.; Popovski, P.; Sørensen, JH.... (2015). D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies. http://hdl.handle.net/10251/7675

Implementation of multi carrier-code division multiple access-frequency division multiple access with beyond 4G specifications

Hybrid code division multiple access techniques present the open door for the future of code division multiple access and wireless communications. Multicarrier CDMA is the most popular type of hybrid CDMA because of its robustness against multipath fading channels and flexible multiple access capability. MC-CDMA is a predictable technique for future high data rate wireless communication systems according to these appealed properties. The main drawback of MC-CDMA is the power level in uplink, i.e. the ratio of peak power to the average power is high and leads to high instantaneous power which is required in transmission of mobile station. However, there are many researchers working towards reducing the level of the transmitted power. This research presents new method of peak to average power ratio (PAPR) reduction. The proposed method is making use of the characteristics of uplink for current 4th Generation (single carrier frequency division multiple access) which has low PAPR into current MC-CDMA system to reproduce a new MC-CDMA system (MC-CDMA-FDMA) with low PAPR and keep all the characteristics of the basic MC-CDMA system. MC-CDMA-FDMA reduced the level of power from 10 dB to 2 dB in case of 64 FFT size and Walsh Hadamard code is used in spreading block. In addition bit error rate has been reduced from 96x10-5 bps to 82x10-5 bps comparing to SC-FDMA bit error rate. The proposed system also has high flexibility to deal with modern communication systems with minimum required hardware at the base station through optimization of FFT size. The simulation results show that MC-CDMA-FDMA system will be a good candidate for beyond 4th Generation for mobile communication

Técnicas de diversidade cooperativa para o sistema mobile WIMAX

Mestrado em Engenharia Electrónica e TelecomunicõesAs comunicações cooperativas são uma das áreas de pesquisa em maior crescimento e é provável que sejam uma tecnologia essencial para o uso eficiente do espectro rádio nos próximos anos. A ideia chave na cooperação do utilizador é a da partilha de recursos entre múltiplos nós da rede. Esta dissertação insere-se na área das comunicações sem fios e tem como principal objectivo estudar, implementar e avaliar o desempenho de esquemas de diversidade cooperativa projectados para o sistema WiMAX Móvel. É feito o estudo de técnicas de retransmissão através de terminais acessórios, constituídos por antenas simples ou agregados de antenas. Foram igualmente estudados dois protocolos de retransmissão dos sinais cooperativos: Equalize and Forward e Decode and Forward. Foram comprovadas melhorias no uso de diversidade cooperativa, em especial em sistemas equipados com elementos retransmissores de antenas múltiplas, em comparação com o desempenho de sistemas MIMO não cooperativos. Estas técnicas permitem aumentar a cobertura e contornar os problemas relacionados com a má qualidade de canal entre emissor e receptor, melhorando o seu desempenho, especificamente ao nível da taxa de erros de transmissão.Cooperative communications is one of the emerging research areas and is likely to become an essential technology for an efficient use of the radio spectrum throughout the next years. The key concept in the users cooperation is the resource share between multiple network nodes. This dissertation is included in the wireless communication area and its main objective is to study, implement and evaluate the performance of cooperative diversity schemes for the Mobile WiMAX system. Relaying techniques over terminals equipped with single and multiple antennas were implemented. Two cooperative diversity schemes, Decode and Forward and Amplify and Forward, were also developed. The use of cooperative diversity proved to be beneficial, especially to systems equipped with multiple antenna relays, in comparison to non cooperative MIMO systems. The developed techniques provide wider coverage areas and ensure the mitigation of problems related to the emitter/receiver poor channel quality, improving specifically the bit error rate performance