A Vectorization Of Synchronization Algorithms For Ofdm Systems

Orthogonal Frequency Division Multiplexing (OFDM) or Multi-carrier Modulation (MCM) is a digital modulation technique that supports high-rate data with sufficient robustness to radio channel impairments (especially multi-path propagation). Due to that, it is emerging as the modulation technique used for the new generation of wireless communication systems (IEEE802.11a and DVB-T). However, one of the arguments against OFDM is that it is highly sensitive to synchronization errors. This raises up the need for optimum synchronization algorithms for OFDM applications such as IEEE802.11a and DVB-T. In this thesis, several synchronization algorithms are presented. Those algorithms deal mainly with two important synchronization problems: Timing and frequency errors and their consequences. We focus on the implementation aspects of synchronization algorithms and propose optimizations which lead to well performing and robust fixed point implementations. In addition, complexity and cost needed for such a project are analyzed leading to a model for classifying different algorithms depending on cost, time-to-market, and performance.

In this thesis, we considered the problem of frame synchronization and channel estimation in Orthogonal Frequency Division Multiplexing (OFDM) systems. Since framing error may cause severe ISI and may disturb the orthogonality of the subcarriers, frame synchronization must be accomplished at the OFDM receiver. Furthermore, the effects of channel must be compensated to obtain the symbols accurately. We investigated several frame synchronization algorithms including a maximum likelihood (ML) synchronizer which relies on the periodicity induced in the OFDM structure, and a robust synchronizer which uses a special training symbol. These algorithms are evaluated in AWGN and Rayleigh fading multipath channels and performances are compared in terms of percentage of ISI-free synchronization, mean squared error and symbol error rate. The IEEE 802.11a framework is used to compare these algorithms with the standard system which utilizes training symbols dedicated for synchronization. It is shown that an adjustment for the frame start estimates must be done to avoid the effects of the channel delay spread. It is also pointed that ideal synchronization is not necessary unless symbol boundaries are detected inside an ISI-free region and the error aroused in ISI-free synchronization can be compensated by applying channel estimation and equalization regarding the same symbol boundaries.

HKUST Call Number: Thesis ECED 2008 ZhangL.

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The first book to provide a detailed discussion of the application of wavelets in wireless communications, this is an invaluable source of information for graduate students, researchers, and telecommunications engineers, managers and strategists. It overviews applications, explains how to design new wavelets and compares wavelet technology with existing OFDM technology. • Addresses the applications and challenges of wavelet technology for a range of wireless communication domains • Aids in the understanding of Wavelet Packet Modulation and compares it with OFDM • Includes tutorials on convex optimisation, spectral factorisation and the design of wavelets • Explains design methods for new wavelet technologies for wireless communications, addressing many challenges, such as peak-to-average power ratio reduction, interference mitigation, reduction of sensitivity to time, frequency and phase offsets, and efficient usage of wireless resources • Describes the application of wavelet radio in spectrum sensing of cognitive radio systems.

Orthogonal frequency-division multiplexing (OFDM) has been adopted by many broadband wireless communication systems for the simplicity of the receiver technique to support high data rates and user mobility. However, studies also show that the advantage of OFDM over the single-carrier modulation schemes could be substantially compromised by timing or frequency estimation errors at the receiver. In this thesis we investigate the synchronization problem for practical OFDM systems using a system model generalized from the IEEE 802.11 and IEEE 802.16 standards. For preamble based synchronization schemes, which are most common in the downlink of wireless communication systems, we propose a novel timing acquisition algorithm which minimizes false alarm probability and indirectly improves correct detection probability. We then introduce a universal fractional carrier frequency offset (CFO) estimator that outperforms conventional methods at low signal to noise ratio with lower complexity. More accurate timing and frequency estimates can be obtained by our proposed frequency-domain algorithms incorporating channel knowledge. We derive four joint frequency, timing, and channel estimators with different approximations, and then propose a hybrid integer CFO estimation scheme to provide flexible performance and complexity tradeoffs. When the exact channel delay profile is unknown at the receiver, we present a successive timing estimation algorithm to solve the timing ambiguity. Both analytical and simulation results are presented to confirm the performance of the proposed methods in various realistic channel conditions. ...