J.M. Paez-Borrallo

DFT-based channel estimation in 2D-pilot-symbol-aided OFDM wireless systems

An efficient channel estimation algorithm for OFDM in wireless applications has been proposed. Two-dimensional pilot-symbol assisted modulation (2D-PSAM) is employed in coherent OFDM for channel estimation and it is based on inserting known symbols spreaded throughout the 2D time-frequency grid. These scattered symbols are employed to perform an estimation of the channels frequency response. At a first stage, if certain requirements are fulfilled, channel estimation in frequency dimension can be carried out with an efficient DFT-based algorithm, which provides an accurate estimation at time positions where pilot symbols are included. In the second step, linear interpolation is performed in the time direction. This scheme has been proven to be very successful in multicarrier HF communications systems.

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.

Peak power reduction for OFDM systems with orthogonal pilot sequences

In this paper, a novel peak-to-average power reduction approach for orthogonal frequency division multiplexing (OFDM) has been addressed. Two-dimensional pilot-symbol assisted modulation (2D-PSAM) is employed in coherent OFDM for channel estimation, and it is based on inserting known symbols spread throughout the 2D time-frequency grid. These pilot symbols are employed to simultaneously perform distortionless peak power reduction with a suboptimum technique named orthogonal pilot sequences (OPS), which reduces additional system complexity and side information compared to optimum pilot values. This proposal attains a further step over other previous works, since this set of sequences allows blind detection at the receiver without prior knowledge of any side information.

Efficient pilot patterns for channel estimation in OFDM systems over HF channels

In this paper, the distribution of pilot symbols in the time-frequency lattice of a multicarrier system for HF communications has been analysed. The pilot density of an OFDM modem based on the HFDL standard can be reduced, so the data rate can be about 3600 bit/s. An hexagonal pilot pattern has been proposed and compared with rectangular geometries. Hexagonal distribution of the pilots is optimum in terms of sampling efficiency and it provides better performance in terms of BER.

Joint channel estimation and peak-to-average power reduction in coherent OFDM: a novel approach

A new peak-to-average power reduction approach for OFDM has been addressed. Two-dimensional pilot-symbol assisted modulation (2D-PSAM) is employed in coherent OFDM for channel estimation and it is based on inserting known symbols spreaded out throughout the 2D time-frequency grid. We show that these scattered symbols can also be employed to perform distortionless peak power reduction, with a negligible loss in data-rate. To reduce additional system complexity and side information, several suboptimal strategies are proposed. A comparison between the two schemes with different approaches to the joint design of pilot symbols for both channel estimation and peak power reduction, has also been carried out.

Orthogonal pilot sequences for peak-to-average power reduction in OFDM

A new peak-to-average power reduction approach for OFDM has been addressed. Two-dimensional pilot-symbol assisted modulation (2D-PSAM) is employed in coherent OFDM for channel estimation and it is based on inserting known symbols spread throughout the 2D time-frequency grid. We show that these scattered symbols can also be employed to perform distortionless peak power reduction. To reduce additional system complexity and side information, a finite set of orthogonal pilot sequences is proposed, and also blind detection can be performed without prior knowledge of redundancy at the receiver.

Pre-RAKE and multiuser detection techniques in UTRA-TDD systems

This paper is concerned with the application of pre-RAKE processing in the up link of a DS-CDMA system for a wireless Local Loop (WLL) scenario with time division duplex (TDD) access. It is well known that RAKE processing allows optimum time diversity gain in a multipath channel. An alternative approach, known as pre-RAKE processing, exploits the channel reciprocity in TDD systems in the up and down links to perform the optimum combination at the transmitter with the consequent receiver simplification. Typically, the pre-RAKE works at the base station, transmitting all users information with their particular pre-distortion. In this paper we explore another possibility, that is, each user performs its own pre-distortion, in order to simplify the synchronization and multiuser detection at the base station. Several simulations show its feasibility in comparison with standard multiuser detection techniques.

Tracking of time-frequency misalignments in 2D-pilot-symbol-aided coherent OFDM systems

Pilot-symbol assisted-modulation (PSAM) is employed in coherent OFDM for channel estimation and it is based on inserting known symbols in the time-frequency grid. We show how this two-dimensional embedded signalling can be used for time and frequency offset tracking. A two-stage procedure is proposed, focusing in this work on the time correction step. The ML estimator has been derived, but though optimal, it may not be practical due to its complexity. Hence, a frequency-domain low-complexity 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 is efficient against time dispersive channels.

Analysis of a new frequency synchronization scheme in OFDM systems

This paper deals with the analysis of a new method which allows the unambiguous frequency offset acquisition in N-OFDM systems exploiting the knowledge of only one symbol. This new approach provides an important improvement in those systems where fast and accurate time-frequency synchronization is a critical point. In our proposal, the limit of the acquisition range for the carrier frequency offset can be extended to ±N/2 the subcarrier spacing rather than ±1/2 the subcarrier spacing as previous methods. A common formulation of related methods and the calculus of the probability of detection of our present work are also provided. Finally, several computer simulations in particular scenarios with both deterministic and random frequency offsets support our proposal.

Spread Spectrum OFDM Modem for HF Voice-Band Link over Fading Channels

This paper deals with the application of spread spectrum techniques to MultiCarrier HF modems. OFDM modems are a special option for the transmission over the high dispersive HF medium in front of single carrier modems. Our present contribution is now focused on the application of Spread Spectrum techniques to improve its fading robustness. The transmission bandwidth is the standard voice band channel in the HF band where the spread spectrum characteristic means that one symbol is transmitted over N times the spectrum in the classical N-OFDM scheme. The motivation of this idea is the observation of the OFDM modems behaviour over fading channels with deep nulls showing a quite different performance between carriers: those in the vicinity of deep nulls provide a much higher Bit Error Rate (BER) than others far away of the nulls. Our proposal intends to spread different symbols over all the subcarriers simultaneously in order to homogenize performances. This approach, performed by orthogonal codes, shows an evident link with MC-CDMA techniques. In addition, Multi User Detection (MUD) strategies as Interference Cancellation (IC) with coherent detection are also applied in order to improve its performance.