Classical and Bayesian Linear Data Estimators for Unique Word OFDM

Unique word - orthogonal frequency division multiplexing (UW-OFDM) is a novel OFDM signaling concept, where the guard interval is built of a deterministic sequence - the so-called unique word - instead of the conventional random cyclic prefix. In contrast to previous attempts with deterministic sequences in the guard interval the addressed UW-OFDM signaling approach introduces correlations between the subcarrier symbols, which can be exploited by the receiver in order to improve the bit error ratio performance. In this paper we develop several linear data estimators specifically designed for UW-OFDM, some based on classical and some based on Bayesian estimation theory. Furthermore, we derive complexity optimized versions of these estimators, and we study their individual complex multiplication count in detail. Finally, we evaluate the estimators' performance for the additive white Gaussian noise channel as well as for selected indoor multipath channel scenarios.Comment: Preprint, 13 page

Pilot-based carrier frequency offset estimation in OFDM systems

The ability of OFDM to mitigate channel frequency selectivity is impaired by its vulnerability to carrier frequency offset (CFO). In this paper, a new pilot-based carrier frequency offset estimation scheme for frequency selective slowly-varying channels is proposed. This scheme exploits the inherent redundancy introduced by the OFDM signaling and no additional training symbol is needed. The synchronization process is to minimize a LMS-like cost function, which essentially quantifies the variance of the pilot tones across several OFDM symbols. Moreover, a reduced-complexity estimator is also developed to reduce the implementation complexity. The CFO estimation range can span the whole OFDM signal bandwidth by nonuniformly distributing the embedded pilot tones in the frequency domain. Computer simulation results show that the performance of the proposed estimator can meet the requirement of typical OFDM communication systems.published_or_final_versio

Techniques to Improve the Efficiency of Data Transmission in Cable Networks

The cable television (CATV) networks, since their introduction in the late 1940s, have now become a crucial part of the broadcasting industry. To keep up with growing demands from the subscribers, cable networks nowadays not only provide television programs but also deliver two-way interactive services such as telephone, high-speed Internet and social TV features. A new standard for CATV networks is released every five to six years to satisfy the growing demands from the mass market. From this perspective, this thesis is concerned with three main aspects for the continuing development of cable networks: (i) efficient implementations of backward-compatibility functions from the old standard, (ii) addressing and providing solutions for technically-challenging issues in the current standard and, (iii) looking for prospective features that can be implemented in the future standard. Since 1997, five different versions of the digital CATV standard had been released in North America. A new standard often contains major improvements over the previous one. The latest version of the standard, namely DOCSIS 3.1 (released in late 2013), is packed with state-of-the-art technologies and allows approximately ten times the amount of traffic as compared to the previous standard, DOCSIS 3.0 (released in 2008). Backward-compatibility is a must-have function for cable networks. In particular, to facilitate the system migration from older standards to a newer one, the backward compatible functions in the old standards must remain in the newer-standard products. More importantly, to keep the implementation cost low, the inherited backward compatible functions must be redesigned by taking advantage of the latest technology and algorithms. To improve the backward-compatibility functions, the first contribution of the thesis focuses on redesigning the pulse shaping filter by exploiting infinite impulse response (IIR) filter structures as an alternative to the conventional finite impulse response (FIR) structures. Comprehensive comparisons show that more economical filters with better performance can be obtained by the proposed design algorithm, which considers a hybrid parameterization of the filter's transfer function in combination with a constraint on the pole radius to be less than 1. The second contribution of the thesis is a new fractional timing estimation algorithm based on peak detection by log-domain interpolation. When compared with the commonly-used timing detection method, which is based on parabolic interpolation, the proposed algorithm yields more accurate estimation with a comparable implementation cost. The third contribution of the thesis is a technique to estimate the multipath channel for DOCSIS 3.1 cable networks. DOCSIS 3.1 is markedly different from prior generations of CATV networks in that OFDM/OFDMA is employed to create a spectrally-efficient signal. In order to effectively demodulate such a signal, it is necessary to employ a demodulation circuit which involves estimation and tracking of the multipath channel. The estimation and tracking must be highly accurate because extremely dense constellations such as 4096-QAM and possibly 16384-QAM can be used in DOCSIS 3.1. The conventional OFDM channel estimators available in the literature either do not perform satisfactorily or are not suitable for the DOCSIS 3.1 channel. The novel channel estimation technique proposed in this thesis iteratively searches for parameters of the channel paths. The proposed technique not only substantially enhances the channel estimation accuracy, but also can, at no cost, accurately identify the delay of each echo in the system. The echo delay information is valuable for proactive maintenance of the network. The fourth contribution of this thesis is a novel scheme that allows OFDM transmission without the use of a cyclic prefix (CP). The structure of OFDM in the current DOCSIS 3.1 does not achieve the maximum throughput if the channel has multipath components. The multipath channel causes inter-symbol-interference (ISI), which is commonly mitigated by employing CP. The CP acts as a guard interval that, while successfully protecting the signal from ISI, reduces the transmission throughput. The problem becomes more severe for downstream direction, where the throughput of the entire system is determined by the user with the worst channel. To solve the problem, this thesis proposes major alterations to the current DOCSIS 3.1 OFDM/OFDMA structure. The alterations involve using a pair of Nyquist filters at the transceivers and an efficient time-domain equalizer (TEQ) at the receiver to reduce ISI down to a negligible level without the need of CP. Simulation results demonstrate that, by incorporating the proposed alterations to the DOCSIS 3.1 down-link channel, the system can achieve the maximum throughput over a wide range of multipath channel conditions

Design and analysis of UW-OFDM signals

AbstractUnique word-orthogonal frequency division multiplexing (UW-OFDM) is a novel signaling concept where the guard interval is implemented as a deterministic sequence, the so-called unique word. The UW is generated by introducing a certain level of redundancy in the frequency domain. Different data estimation strategies and the favourable bit error ratio (BER) performance of UW-OFDM, as well as comparisons to competing concepts have already extensively been discussed in previous papers. This work focuses on the different possibilities on how to generate UW-OFDM signals. The optimality of the two-step over the direct approach in systematic UW-OFDM is proved analytically, we present a heuristic algorithm that allows a fast numerical optimization of the redundant subcarrier positions, and we show that our original intuitive approach of spreading the redundant subcarriers in systematically encoded UW-OFDM by minimizing the mean redundant energy is practically also optimum w.r.t. transceiver based cost functions. Finally, we derive closed form approximations of the statistical symbol distributions on individual subcarriers as well as the redundant energy distribution and compare them with numerically found results

Sincronização de quadro e frequência para OFDM no padrão IEEE 802.15.4g : algoritmos e implementação em hardware

Orientadores: Renato da Rocha Lopes, Eduardo Rodrigues de LimaDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: O objetivo deste trabalho é propor métodos de sincronização de quadro e de frequência de portadora para a camada física MR-OFDM do padrão IEEE 802.15.4g, começando pela pesquisa de algoritmos, passando pelas etapas de modelagem e simulação em alto nível, e finalmente implementando e avaliando os métodos propostos em hardware. A sincronização de quadro é o processo responsável por detectar o início do dado transmitido, ou seja, a primeira amostra válida do sinal de interesse. No caso de sistemas OFDM, onde o sinal transmitido é composto por um ou mais símbolos OFDM (cada símbolo sendo composto por uma quantidade fixa de amostras), o objetivo é detectar a borda ou janelamento de tais símbolos OFDM, ou seja, onde começa e termina cada um deles. A sincronização de frequência, por sua vez, consiste em estimar e compensar o erro de frequência de portadora, causado principalmente pelo descasamento dos osciladores do transmissor e do receptor. Com base em estudos preliminares, selecionamos o algoritmo de Minn para a detecção de quadro. Para a correção de erro de frequência, dividimos o processo em duas etapas, como é geralmente proposto na literatura: primeiro, o erro de frequência fracionário é estimado no domínio do tempo durante a detecção de quadro e compensado via rotação de sinal; após a conversão do domínio do tempo para o domínio da frequência, o erro de frequência inteiro é estimado e compensado utilizando um novo e simples algoritmo que será proposto e detalhado neste trabalho. Os algoritmos propostos foram implementados em hardware e uma plataforma de verificação baseada em FPGA foi criada para avaliar o seu desempenho. Os módulos implementados são parte de um projeto que está sendo desenvolvido no Instituto de Pesquisa Eldorado (Campinas) que tem como objetivo implementar em ASIC um transceptor compatível com o padrão IEEE 802.15.4gAbstract: The objective of this work is proposing methods of frame and frequency synchronization for the MR-OFDM PHY of IEEE 802.15.4g standard, starting with the research of state-of-the-art algorithms, passing through modeling, high-level simulations, and finally implementing and evaluating the proposed methods in hardware. Frame synchronization is the process responsible for detecting the beginning of transmitted data and, in the case of OFDM systems, the border of each OFDM symbol, while frequency synchronization consists of estimating and compensating the Carrier Frequency Offset (CFO) caused mainly by a mismatch between the transmitter and receiver oscillators. Based on the initial studies, we selected Minn¿s algorithm for frame detection. For the CFO correction, we split the process into two steps, as commonly proposed in the literature: first, the Fractional CFO is estimated in the time domain during the frame detection and compensated via signal rotation; after the conversion from time to frequency domain, the Integer CFO is estimated and compensated with a novel and simple algorithm that will be detailed in this work. The proposed algorithms were implemented in hardware and inserted in an FPGA-based verification platform for performance measurement. The implemented modules are part of a project that is under development at Eldorado Research Institute (Campinas) and aims to implement in ASIC a transceiver compliant to the IEEE 802.15.4g standardMestradoTelecomunicações e TelemáticaMestra em Engenharia Elétric