Christos Kasparis

Robust time-domain timing and frequency synchronization for OFDM systems

A robust and efficient technique for frame/symbol timing and carrier frequency synchronization in OFDM systems is presented. It uses a preamble consisting of only one training symbol with two identical parts to achieve reliable timing and frequency accuracy in the time-domain, over a wide frequency estimation range which can be up to half of the signal sampling frequency. Also, it has a low complexity which is adaptive to the degree of channel distortion. Computer simulations in the Rayleigh fading ISI channel show that the proposed method achieves superior performance to existing techniques in terms of timing and frequency accuracy. Also, its operation in the time-domain helps to achieve faster synchronization convergence.

Improved preamble-aided timing estimation for OFDM systems

An improved method for estimating the frame/symbol timing offset in preamble-aided OFDM systems is presented. It uses a conventional preamble structure and combines autocorrelation techniques with restricted crosscorrelation to achieve a near-ideal timing performance without significant increase in complexity. Computer simulations show that the method is robust in both AWGN and fading multipath channels, achieving better performance than the existing methods.

A Gradient Based Algorithm for PAPR Reduction of OFDM using Tone Reservation Technique

In this paper, we propose a low complexity gradient based approach for enabling the Tone Reservation (TR) technique to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. The performance of the proposed algorithm is evaluated for different pilot location in the frequency domain, and also in combination with the discrete Fourier transform (DFT) spreading technique proposed in [Heung-Gyoon Ryu, et al., 2007]; in order to further reduce the PAPR. Simulation results show that the new technique achieves significant PAPR reductions, which are further enhanced when it is combined with DFT spreading. The simulation results also show that the performance of the technique is dependent on the pilot positions. In addition, further investigation was performed where the reduction tones are constrained, equal to the average power mask for the data tones, by a simple projection rule in the frequency domain both for the TR scheme and for the combined scheme. Simulation results show that the contiguous pilot arrangement provides better PAPR reduction performance in both cases, when the peak-cancellation signal is constrained in the frequency domain.

Doppler spectrum of the multipath fading channel in mobile satellite systems with directional terminal antennas

A new approach for determining the Doppler spectrum of narrowband multipath channels by assuming a three-dimensional propagation model is proposed. The new derivation methodology is first applied to determine the Doppler spectrum of a Rayleigh channel, when an omni-directional mobile-terminal antenna is assumed. The obtained results are shown to agree with the existing analytical results, which are available for particular assumptions regarding the angular distribution of the received multipath power. The main contribution is the derivation of the Doppler spectrum of the mobile satellite channel, where directive tracking terminal antennas are assumed. The results, which are dependent on the antennas beamwidth and bearing, apply to emerging types of commercial vehicular satellite systems.

A cross-correlation approach for improved timing estimation in OFDM broadcasting systems

A computationally simple cross-correlation based approach for improving timing estimation and frame detection performance in OFDM systems is proposed. The new technique requires only one pilot symbol and this can be used further for frequency-offset estimation, by employing standard available techniques. Simulation results show that the proposed approach performs more robustly relative to state-of-the-art autocorrelation based approaches. Copyright

A bootstrap multi-user detector for CDMA based on Tikhonov regularization

A novel multi-user detection (MUD) technique for CDMA systems is introduced. The new method is well suited for cases in which the code cross correlation matrix is ill-conditioned. In practice, this case coincides with having code lengths approximately equal to the number of users - a desirable condition in terms of bandwidth efficiency. Our approach employs an iterative formulation of a well-known regularization method for linear inverse problems, which is suited to the MUD problem. The technique allows knowledge of the finite set in which the solution belongs to be exploited in a computationally efficient manner in order to improve the quality of the estimate iteratively.

Threshold Detection Analysis for OFDM Timing and Frequency Recovery

Some recently proposed robust methods for time-domain timing and frequency synchronization in OFDM systems incorporate the use of cross-correlation based threshold detection in their timing adjustment and joint time-frequency algorithms. In this paper, we analyze the threshold criterion used and derive its theoretical performance. This translates into a better characterization of these OFDM synchronization algorithms, leading to a closer match between design expectations and practical performance. Computer simulations show a strong agreement between theoretical predictions and numerical results.

Simulation of DVB -S2/RCS Performance in a Vehicular Satellite Environment

Currently there is a clear trend towards vehicular satellite systems, which are designed in high frequency bands (Ku/Ka) and support broadband data services. One such system is designed within the European project MOWGLY for broadband services pro vision within aeronautical, maritime and railroad environments, and which features suitably adapted DVB -S2 and DVB -RCS standards as the basic underlying communications technology. This paper presents simulation results for assessing various aspects of the performance of the DVB -S2/RCS based satellite system in different mobile environments. In particular the effects of different fading mechanisms on the Block Error Rate performance of the system is analyzed through link -level simulations. Additionally , sys te m-level simulation results are provided that apart from complementing the link level simulations include the modeling of propagation impairments such as path loss and rain attenuation that cannot be captured in link level simulations.

Position-based DVB-RCS Spotbeam Handover in Vehicular Geostationary Satellite Networks

High-speed Internet access is already a commodity in developed urban areas. We have also witnessed an increase in the number of airlines, railways, and maritime passengers that expect communication services while outside the coverage of terrestrial networks. Satellite systems are viewed as viable access providers for collective user communities on board aircraft, high-speed trains, and maritime vessels outside the coverage of terrestrial communication infrastructure. This article focuses on minimal mobility adaptations required by the ETSI DVB-RCS specification. First the performance of the existing DVB-RCS burst synchronisation is analysed in mobile vehicular networks. Then, a position-based seamless spotbeam handover mechanism is proposed, and its performance is analysed in terms of handover delay and handover failure rate using a ns-based packet-level simulator of DVB-RCS specification in a multi-spotbeam, multi-satellite geostationary constellation.

Improved resolution DoA estimation through shrunk projections on the signal sub-space

Maximum likelihood (ML) estimation of the Direction of Arrival (DoA) parameters of multiple signals impinging on a sensor array is known to provide best performances among existing techniques, under general signal and system assumptions. However, even the ML estimation performance deteriorates severely in system conditions where the angular separations between signal sources are small and the SNR/sample size are low. In an effort to improve on the ML performance in such challenging conditions, the present communication investigates DoA estimators obtained by performing shrunk (non-orthogonal) projections on the signal sub-space (SS). It is argued that suitable selections of the introduced shrinkage parameters help to limit the chance of outlier estimates occurring, which account for the rapid deterioration of ML at low SNR. Simulation results show that a proposed two-stage estimation approach based on the Shrunk Projections (SP) estimator, offers significant performance gains relative to ML.