107 research outputs found
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
Interference Localization for Uplink OFDMA Systems in Presence of CFOs
Multiple carrier frequency offsets (CFOs) present in the uplink of orthogonal
frequency division multiple access (OFDMA) systems adversely affect subcarrier
orthogonality and impose a serious performance loss. In this paper, we propose
the application of time domain receiver windowing to concentrate the leakage
caused by CFOs to a few adjacent subcarriers with almost no additional
computational complexity. This allows us to approximate the interference matrix
with a quasi-banded matrix by neglecting small elements outside a certain band
which enables robust and computationally efficient signal detection. The
proposed CFO compensation technique is applicable to all types of subcarrier
assignment techniques. Simulation results show that the quasi-banded
approximation of the interference matrix is accurate enough to provide almost
the same bit error rate performance as that of the optimal solution. The
excellent performance of our proposed method is also proven through running an
experiment using our FPGA-based system setup.Comment: Accepted in IEEE WCNC 201
Timing and Frequency Synchronization and Channel Estimation in OFDM-based Systems
Orthogonal frequency division multiplexing (OFDM) due to its appealing features, such as robustness against frequency selective fading and simple channel equalization, is
adopted in communications systems such as WLAN, WiMAX and DVB. However, OFDM systems are sensitive to synchronization errors caused by timing and frequency offsets. Besides, the OFDM receiver has to perform channel estimation for coherent detection. The goal of this thesis is to investigate new methods for timing and frequency synchronization and channel estimation in OFDM-based systems.
First, we investigate new methods for preamble-aided coarse timing estimation in OFDM systems. Two novel timing metrics using high order statistics-based correlation and differential normalization functions are proposed. The performance of the new timing metrics is evaluated using different criteria including class-separability, robustness to the carrier frequency offset, and computational complexity. It is shown that the new timing metrics can considerably increase the class-separability due to their more distinct values at correct and wrong timing instants, and thus give a significantly better detection performance than the existing timing metrics do. Furthermore, a new method for coarse estimation of the start of the frame is proposed, which remarkably reduces the probability of inter-symbol interference (ISI). The improved performances of the new schemes in multipath fading channels are shown by the probabilities of false alarm, missed-detection and ISI obtained through computer simulations. Second, a novel pilot-aided algorithm
is proposed for the detection of integer frequency offset (IFO) in OFDM systems. By transforming the IFO into two new integer parameters, the proposed method can largely reduce the number of trial values for the true IFO. The two new integer parameters are detected using two different pilot sequences, a periodic pilot sequence and an aperiodic
pilot sequence. It is shown that the new scheme can significantly reduce the computational complexity while achieving almost the same performance as the previous methods do.
Third, we propose a method for joint timing and frequency synchronization and channel estimation for OFDM systems that operate in doubly selective channels. Basis expansion
modeling (BEM) that captures the time variations of the channel is used to reduce the number of unknown channel parameters. The BEM coefficients along with the timing
and frequency offsets are estimated by using a maximum likelihood (ML) approach. An efficient algorithm is then proposed for reducing the computational complexity of the
joint estimation. The complexity of the new method is assessed in terms of the number of multiplications. The mean square estimation error of the proposed method is evaluated in comparison with previous methods, indicating a remarkable performance improvement by the new method.
Fourth, we present a new scheme for joint estimation of CFO and doubly selective channel in orthogonal frequency division multiplexing systems. In the proposed preamble-aided method, the time-varying channel is represented using BEM. CFO and BEM coefficients
are estimated using the principles of particle and Kalman filtering. The performance of the new method in multipath time-varying channels is investigated in comparison with
previous schemes. The simulation results indicate a remarkable performance improvement in terms of the mean square errors of CFO and channel estimates.
Fifth, a novel algorithm is proposed for timing and frequency synchronization and channel estimation in the uplink of orthogonal frequency division multiple access (OFDMA)
systems by considering high-mobility situations and the generalized subcarrier assignment.
By using BEM to represent a doubly selective channel, a maximum likelihood (ML) approach is proposed to jointly estimate the timing and frequency offsets of different users
as well as the BEM coefficients of the time-varying channels. A space-alternating generalized expectation-maximization algorithm is then employed to transform the maximization
problem for all users into several simpler maximization problems for each user. The computational complexity of the new timing and frequency offset estimator is analyzed and its performance in comparison with that of existing methods using the mean square error is evaluated .
Finally, two novel approaches for joint CFO and doubly selective channel estimation in the uplink of multiple-input multiple-output orthogonal frequency division multiple
access (MIMO-OFDMA) systems are presented. Considering high-mobility situations, where channels change within an OFDMA symbol interval, and the time varying nature
of CFOs, BEM is employed to represent the time variations of the channel. Two new approaches are then proposed based on Schmidt Kalman filtering (SKF). The first approach
utilizes Schmidt extended Kalman filtering for each user to estimate the CFO and BEM coefficients. The second approach uses Gaussian particle filter along with SKF to
estimate the CFO and BEM coefficients of each user. The Bayesian Cramer Rao bound is derived, and performance of the new schemes are evaluated using mean square error.
It is demonstrated that the new schemes can significantly improve the mean square error performance in comparison with that of the existing methods
Inter-carrier interference suppression in orthogonal frequency division multiple access (OFDMA) systems uplink
Ph.DDOCTOR OF PHILOSOPH
Design and evaluation of OFDM radio interfaces for high mobility communications
[Resumo]
Nas dúas últimas décadas, as modulacións multiportadora emerxeron como una solución de
baixa complexidade para combatir os efectos do multitraxecto en comuniacións sen fíos. Entre
elas, Orthogonal Frequency Division Multiplexing (OFOM) é posiblemente o esquema de
modulación máis estudado, e tamén amplamente adoptado como alicerce de estándares da
industria como WiMAX ou LTE. Sen embargo, OFDM é sensible a canles que varian ca tempo,
unha característica dos escenarios con mobilidade, debido á aparición da interferencia entre
portadoras (ICI).
A implementación de equipamento hardware para o usuario final faise normalmente en
chips dedicados, afnda que entornos de investigación, prefírense solucións máis flexibles. Unha
aproximación popular é a coñecida como Software Defined Radio (SOR), onde os algoritmos de
procesado de sinal se implementan en hardware reconfigurable como Digital Signal Processors
(OSPs) e Field Programmable Gate Arrays (FPGAs).
O obxectivo deste traballo é dobre. Por un lado, definir unha arquitectura para
implementacións de tempo real de capas físicas basadas en OFDM usando como referencia
O estándar WiMAX, probada Dunha plataforma composta por OSPs e FPGAs. Por outra banda,
estudar os efectos da selectividade en tempo no sinal OFDM, definindo métodos de estimación
de canle que teñen en conta a ICI, e evaluándoos tanto en simulación como con medidas
experimentais. Seguíronse dúas aproximacións para caracterizar o comportamento de formas de
onda OFDM baixo condicións de mobilidade, unha basada nun emulador de canle que traballa
en tempo real, e outra en inducir grandes ensanchamentos Doppler no sinal mediante a extensión
da duración do símbolo OFOM.[Resumen]
En las dos últimas décadas, las modulaciones multiportadora han emergido como una
solución de baja complejidad para combatir los efectos del multitrayecto en comunicaciones
iDalámbricas. Entre ellas, Orthogonal Frequency Division Mulriplexing (OFDM) es
posiblemente el esquema de modulación más estudiado, y también ampliamente adoptado
como fundamento de estándares de la industria como WiMAX o LTE. Sin embargo, OFDM es
sensible a canales que varían con el tiempo, una característica de los escenarios coo movilidad,
debido a la aparicióo de la interferencia entre portadoras (ICI).
La implementación de equipamiento hardware para el usuario final se hace normalmente en
chips dedicados, aunque eo entornos de investigación, son preferibles soluciones más Hexibles.
Una aproximación popular es la conocida como Software Defined Radio (SDR), donde los
algOritmos de procesado de señal se implementan en hardware reconfigurable como Digital
Signa! Processors (DSPs) y Field Programmable Gate AIrays (FPGAs).
El objetivo de este trabajo es doble. Por un lado. definir una arquitectura para
implementaciones de tiempo real de capas ¡lSicas basadas en OFDM usando como referencia
el estándar WiMAX, probada en una plataforma compuesta por DSPs y FPGAs. Por otro
lado, estudiar los efectos de la selectividad en tiempo en la señal OFDM, definiendo métodos
de estimacióo de canal que tengan eo cueota la ICI, y evaluándolos tanto en simulación
como con medidas experimenta1es. Se han seguido dos aproximaciones para caracterizar el
comportamiento de formas de onda OFDM bajo condiciones de mobilidad, una basada en
un emulador de canal que trabaja en tiempo real. y otra en inducir grandes ensanchamientos
Doppler en la señal mediante la extensión de la duración del símbolo OFDM.[Abstract]
In Ihe last two decades, multicarrier modulations have emerged as a low complexity solulion
to combal the effects of Ihe multipalh in wireless communicalions. Among Ihem, Orthogonal
Frequency Division Mulliplexing (OFOM) is possibly Ihe mosl sludied modulation scheme,
and has a1so been widely adopted as Ihe foundation of induslry standards such as WiMAX or
LTE. However, OFOM is sensitive lo time selective channels, which are featured in mobility
scenarlos, due lO Ihe appearance of Inler-Carrier Interference (ICI).
Implemenlation of hardware equipmenl for Ihe end user is usually implemenled in dedicaled
chips, bul in researeh environments, more flexible solutions are preferred. One popular
approach is the so ealled Software Defined Radio (SOR), where the signal processing
a1gorithms are implemented in reconfigurable hardware sueh as Digital Signal Processors
(DSPs) and Field Prograrnmable Gate Arrays (FPGAs).
The aim of Ibis work is two-fold. On the one hand, to define an architeclure for Ihe
implementation of real-time OFOM-based physical layers, using as a reference Ihe WiMAX
standard, and it is tested on a platform composed by DSPs and FPGAs. On the olher hand,
to study Ihe effeets of !he time seleetivity on !he OFOM signal, defining channel estimation
me!hods aware of !he ICI, and ils evaluation bo!h in simulation as well as experimental
measuremenls. Two approaches have been followed to assess the behavior of OFOM waveforms
under mobility conditions, one based on a real-time channel emulator, and the other on inducing
large Doppler spreads in !he signal by extending the duration of Ihe OFDM symbols
Fine Timing and Frequency Synchronization for MIMO-OFDM: An Extreme Learning Approach
Multiple-input multiple-output orthogonal frequency-division multiplexing
(MIMO-OFDM) is a key technology component in the evolution towards cognitive
radio (CR) in next-generation communication in which the accuracy of timing and
frequency synchronization significantly impacts the overall system performance.
In this paper, we propose a novel scheme leveraging extreme learning machine
(ELM) to achieve high-precision synchronization. Specifically, exploiting the
preamble signals with synchronization offsets, two ELMs are incorporated into a
traditional MIMO-OFDM system to estimate both the residual symbol timing offset
(RSTO) and the residual carrier frequency offset (RCFO). The simulation results
show that the performance of the proposed ELM-based synchronization scheme is
superior to the traditional method under both additive white Gaussian noise
(AWGN) and frequency selective fading channels. Furthermore, comparing with the
existing machine learning based techniques, the proposed method shows
outstanding performance without the requirement of perfect channel state
information (CSI) and prohibitive computational complexity. Finally, the
proposed method is robust in terms of the choice of channel parameters (e.g.,
number of paths) and also in terms of "generalization ability" from a machine
learning standpoint.Comment: 13 pages, 12 figures, has been accepted for publication in IEEE
Transactions on Cognitive Communications and Networkin
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