Coherent OFDM Research Articles

Enhanced MMSE Channel Estimation Using Timing Error Statistics for Wireless OFDM Systems

Estimation and tracking of the frequency-selective time-varying channel response is a challenging task for wireless communication systems incorporating coherent OFDM. In pilot-symbol-assisted (PSA) OFDM systems, the minimum mean-square-error (MMSE) estimator provides the optimum performance based on the channel statistics (channel correlation function and SNR). In OFDM systems, FFT-block timing error introduces a linear phase rotation to data modulated on individual subcarriers. An MMSE channel estimator designed only using the wireless channel statistics performs only sub-optimally when subcarrier phase rotations due to block timing errors are present. In this paper, we show that by using the block timing error statistics of the OFDM time-synchronizer the performance of the MMSE channel estimation can be significantly improved. Numerical results show that the bit-error-probability (BEP) performance degradation due to timing errors can be almost completely recovered by the proposed technique.

Tracking of time misalignments for OFOM systems in multipath fading channels

In this paper, new time offset tracking techniques for OFDM are proposed without devoting specific pilot symbols for synchronization. This can be attained since, in coherent OFDM, channel estimation is usually performed employing pilot symbol assisted modulation (PSAM), based on inserting known symbols in the time-frequency grid. We show how this two dimensional embedded signalling can also be used for tracking time-frequency offsets. A two-stage procedure is proposed, focusing this work on the time correction step. The ML estimator has been derived, but though optimal, may not be practical due to its complexity. Hence, a frequency-domain low-complex estimator has been proposed, based on only the samples at pilot positions. Also, the stability of the generated estimate can be improved by averaging over a few number of OFDM symbols. This estimator has been proven to be efficient against time dispersive channels.

Symbol time offset estimation in coherent OFDM systems

This paper presents a symbol time offset estimator for coherent orthogonal frequency division multiplexing (OFDM) systems. The estimator exploits both the redundancy in the cyclic prefix and available pilot symbols used for channel estimation. The estimator is robust against frequency offsets and is suitable for use in dispersive channels. We base the estimator on the maximum-likelihood estimator for the additive white Gaussian noise channel. Simulations for an example system indicate a system performance as close as 0.6 dB to a perfectly synchronized system.

Frequency reuse scheme for cellular OFDM systems

A new frequency reuse scheme for coherent orthogonal frequency division multiplexing (OFDM) cellular systems using high level modulation schemes, whereby both data and pilot parts can have different frequency reuse factors, is described. The capacity increase is investigated by simulations.

Frequency offset correction for coherent OFDM in wireless systems

In this paper, a new frequency offset correction approach for OFDM is proposed to track a wider range of offsets than conventional proposals, without devoting specific pilot symbols for synchronization. This can be attained since, in coherent OFDM, channel estimation is usually performed employing pilot-symbol assisted-modulation (PSAM), based on inserting known symbols in the time-frequency grid. We show how this two-dimensional embedded signalling can also be used for frequency synchronization, with no need of additional synchronization symbols which reduce the data-rate. The ML estimator has been derived, but also, a frequency-domain low-complex estimator has been proposed, based on only the samples at pilot positions. Additionally, a novel decision-guided strategy is presented, working jointly with the channel-estimation signalling and reducing the variance of the estimation considerably. Also, the stability of the scheme is shown to get significantly better by averaging over a few OFDM symbols. This estimator is efficient against AWGN and multipath fading channels. Simulation results demonstrate that the presented frequency offset correction scheme provides the accuracy required in practical applications for OFDM wireless systems.

Bit error rate performances of M‐QAM orthogonal multicarrier modulation in presence of time‐selective multipath fading

AbstractOrthogonal Frequency Division Multiplexing (OFDM) is considered as an effective technique in high speed digital transmission under multipath fading. In this paper we theoretically analyze the behaviour of a coherent pilot‐symbol aided M‐QAM‐OFDM system in presence of time selective multipath channel. In the analyzed scheme, pilot symbols are periodically inserted to monitor the channel characteristics. The fading process experienced by the pilot symbols is used to estimate the fade characteristics at symbol‐rate through interpolation. The estimated channel is used to compensate for the random phase variation caused by the Doppler spread, so that the signal can be coherently demodulated. The bit error rate (BER) performance of this coherent OFDM modulation scheme is derived. The obtained results show that, with a reasonable amount of redundancy (frequency of the pilot symbols) introduced, the system performance approaches that of the ideal coherent demodulation, as long as a suitable channel estimate procedure is adopted.

A coherent OFDM switching system for flexible optical network configuration

A coherent optical switching node based on the combination of a frequency conversion stage with a space switching matrix is proposed for flexible interconnection of future integrated broadband communication networks. The basic system concept is presented together with the architecture of a demonstrator, which allows one to study the functionality available in future networks. The major subsystems and components of the demonstrator are described, and experimental results concerning frequency stabilization, frequency switching, and space switching are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>