Abstract-We present the joint maximum likelihood (ML) symbol-time and carrier-frequency offset estimator in orthogonal frequency-division multiplexing (OFDM) systems. Redundant information contained within the cyclic prefix enables this estimation without additional pilots. Simulations show that the frequency estimator may be used in a tracking mode and the time estimator in an acquisition mode.

This thesis focuses on advanced signal processing techniques for multicarrier modulation, in particular, orthogonal frequency division multiplexing (OFDM). OFDM promises a substantial increase in data rate and robustness against the frequency selectivity of multipath channels. For coherent detection, channel estimation is essential for receiver design. In this thesis, we will present a receiver design where the channel estimator exploits the sparse nature of the physical channel. We present the most popular subspace algorithm from the array processing literature, namely root-MUSIC, recent sparse identification algorithms in the form of orthogonal matching pursuit (OMP) and basis pursuit (BP), and a hybrid method called path identification (PI) algorithm which is the main contribution of this thesis. We also compare the performance of these estimators with that of the conventional estimators such as least-squares (LS) estimator and linear minimum-mean-squares estimator (LMMSE).iii

No abstract

A new approach to low-complexity channel estimation in orthogonal-frequency division multiplexing (OFDM) systems is proposed. A lowrank approximation is applied to a linear minimum mean-squared error (LMMSE) estimator that uses the frequency correlation of the channel. By using the singular-value decomposition (SVD) an optimal low-rank estimator is derived, where performance is essentially preservedeven for low computational complexities. A fixed estimator, with nominal values for channel correlation and signalto-noise ratio (SNR), is analysed. Analytical meansquared error (MSE) and symbol-error rates (SER) are presented for a 16-QAM OFDM system.

No abstract

-In this paper a novel data-based frame synchronization method for OFDM-systems is presented. OFDM frames are shown to contain sufficient information to synchronize a system without the use of pilots. The cyclic extension, preceding OFDM frames, is of decisive importance for this method. Based on only the sign bits of the in-phase and the quadrature components of the received OFDM signal, the maximum likelihood solution is derived. This solution basically consists of a correlator, a moving sum and a peak detector. The stability of the generated frameclock is improved significantly by averaging over a few number of frames. Simulations show that this lowcomplex, averaging method can be used to synchronize an OFDM system on twisted pair copper wires and in slowly fading radio channels.

Abstruct-In many areas the time delay of arrival (TDOA) is desired. In the case of narrowband signals we propose a fast and simple method to estimate small time delays. This method is shown to have the same or better accuracy as the cross correlation methods for small delays in the order of fractions of the sample interval. It is based on using the Hilbert transform in correlation between two signals and consists of only one scalar product, which makes it fast. It may also be used in applications with narrowband signals where the measurements are repeatable, such as ultrasonic imaging and nondestructive testing. In ultrasonic applications, due to fluctuations in the insonified media, a small random time shift may be present causing the signals to be misaligned in time. Averaging signals under these conditions will result in a distortion of the signal shape. We propose an averaging method to avoid this and to accomplish a higher SNR without the distortion. Simulations and experiments from ultrasonic applications are presented.

No abstract