Pulse-shaped Orthogonal Frequency Division Multiplexing Research Articles

Pulse shaping design for OFDM systems

Spectrally contained OFDM-based waveforms are considered key enablers for a flexible air interface design to support a broad range of services and frequencies as envisaged for 5G mobile systems. By allowing for the flexible configuration of physical layer parameters in response to diverse requirements, these waveforms enable the in-band coexistence of different services. One candidate from this category of waveforms is pulse-shaped OFDM, which follows the idea of subcarrier filtering while fully maintaining the compatibility with CP-OFDM. In this paper, we provide an overview of pulse shaping methods in OFDM systems and propose a new pulse-shaped design method with arbitrary length constraint and good time-frequency localization property. Based on the pulse design, we discuss different receiver realizations and present a criterion for pulse shape evaluation. In addition, the parameterizations of OFDM system to address diverse requirements of the services envisaged for the 5G systems are described. Link and system performance results for selected scenarios show that a proper design of the OFDM numerologies and pulse shapes could substantially improve the performance under time and frequency distortions. Furthermore, pulse-shaped OFDM is able to support asynchronous transmissions and reduce the signal sensitivity to Doppler distortions, rendering it beneficial for various applications from the context of vehicular communications and the Internet-of-things.

EMC Regulations and Spectral Constraints for Multicarrier Modulation in PLC

This paper considers electromagnetic compatibility (EMC) aspects in the context of power line communication (PLC) systems. It offers a complete overview of both narrow band PLC and broad band PLC EMC norms. How to interpret and translate such norms and measurement procedures into typical constraints used by designers of communication systems is discussed. In particular, the constraints to the modulated signal spectrum are considered and the ability of pulse shaped OFDM (PS-OFDM), used in most of the PLC standards as IEEE P1901 and P1901.2, to fulfill them is analyzed. In addition, aiming to improve the spectrum management ability, a novel scheme named pulse shaped cyclic block filtered multitone modulation (PS-CB-FMT) is introduced and compared to PS-OFDM. It is shown that PS-CB-FMT offers better ability to fulfill the norms which translates in higher system capacity.

Pairwise subcarriers weighting for suppressing out-of-band radiation of OFDM

This paper proposes a subcarrier weighting technique to suppress the out-of-band radiation of OFDM signals. By mapping and weighting the same data on an adjacent pair of subcarriers, the spectrum sidelobes are suppressed perfectly through sidelobes mutual cancellation. The optimum weighting factor is derived based on a rectangular pulse-shaped OFDM spectrum model. Compared with existing out-of-band suppression schemes, the proposed scheme not only requires less computational burden but also achieves better spectral roll-off. For example, when the cyclic prefix of a one-eighth OFDM-block length is added, the proposed scheme suppresses the 10-dB radiation at the center frequency between two subbands which are using cognitive radio. Analytical and simulation results also show that the proposed scheme improves the system carrier-to-interference ratio by 10 dB at a normalized frequency offset above 0.1, which leads to the performance improvement in terms of the BER on AWGN channel and multipath fading channel. Copyright © 2011 John Wiley & Sons, Ltd.

Adaptive pulse-shaped OFDM with application to in-home power line communications

We propose an adaptive pulse-shaped orthogonal frequency division multiplexing (OFDM) scheme where the overhead and active sub-channels are adapted to jointly maximize capacity and fulfill the notching mask for coexistence with other devices operating in the same spectrum. The overhead includes the guard interval to partly compensate the channel dispersion, and the roll-off factor used by the shaping window. We show that significant gains are obtained compared to the case of using a fixed guard interval and roll-off factor. To simplify the implementation complexity we also propose to limit the amount of adaptation by deploying a finite set of overhead parameters. Numerical results are reported for typical wideband power line communication indoor channels.

Blind Symbol Timing Estimation for OFDM/OQAM Systems

In this correspondence, we consider the problem of blind symbol timing (ST) estimation for pulse-shaping orthogonal frequency division multiplexing (OFDM) systems based on offset quadrature amplitude modulation (OQAM). In particular, under the assumption of low signal-to-noise ratio, the joint maximum-likelihood (ML) phase offset and ST estimator for additive white Gaussian noise (AWGN) channel is derived. Since the phase estimate is in closed form, the ST estimate requires a one-dimensional maximization procedure with respect to a continuous parameter. Specifically, the ST estimate depends on both the unconjugate and the conjugate correlation function of the transmitted OFDM/OQAM signal and exploits the cyclostationarity of the OFDM/OQAM signal that is related to the bandwidth of the considered pulse-shaping filter. In particular, for the adopted pulse-shaping filter the OFDM/OQAM signal results to be stationary with respect to its unconjugate correlation function and, then, the ML cost function depends only on the conjugate correlation function. The performance of the proposed ML estimator is assessed via computer simulations both in AWGN and multipath channel.

Non-data-aided carrier-frequency offset estimation for pulse-shaping OFDM/OQAM systems

In this paper we consider the problem of blind carrier-frequency offset (CFO) estimation for pulse-shaping orthogonal frequency-division multiplexing (OFDM) systems based on offset quadrature amplitude modulation (OQAM). In particular, under the assumption of low signal-to-noise ratio, three maximum-likelihood (ML) CFO estimators for non-dispersive channels are derived. Due to their significant computational complexity a more feasible closed-form CFO synchronization algorithm is considered. The performance of the derived blind estimators is assessed via computer simulations and compared with that of a blind CFO estimator previously proposed in the literature, exploiting the second-order cyclostationarity property of the OFDM/OQAM signal.

A discrete model for the efficient analysis of time-varying narrowband communication channels

We derive an efficient numerical algorithm for the analysis of certain classes of Hilbert–Schmidt operators that naturally occur in models of wireless radio and sonar communications channels. We show that many narrowband finite lifelength systems such as wireless radio communications can be well modelled by smooth and compactly supported spreading functions. Further, we exploit this fact to derive a fast algorithm for computing the matrix representation of such operators with respect to well time-frequency localized Gabor bases (such as pulseshaped OFDM bases). Hereby we use a minimum of approximations, simplifications, and assumptions on the channel. Moreover, we use a multivariate setting to allow for applications to, for example, antenna arrays. The derived algorithm and software can be used, for example, for comparing how different system settings and pulse shapes affect the diagonalization properties of an OFDM system acting on a given channel.

Optimal ofdm design for time-frequency dispersive channels

Transmission over wireless channels is subject to time dispersion due to multipath propagation and to frequency dispersion due to the Doppler effect. Standard orthogonal frequency-division multiplexing (OFDM) systems, using a guard-time interval or cyclic prefix, combat intersymbol interference (ISI), but provide no protection against interchannel interference (ICI). This drawback has led to the introduction of pulse-shaping OFDM systems. We first present a general framework for pulse shape design. Our analysis shows that certain pulse shapes proposed in the literature are, in fact, optimal in a well-defined sense. Furthermore, our approach provides a simple way to adapt the pulse shape to varying channel conditions. We then show that (pulse-shaping) OFDM systems based on rectangular time-frequency lattices are not optimal for time- and frequency-dispersive wireless channels. This motivates the introduction of lattice-OFDM (LOFDM) systems which are based on general time-frequency lattices. Using results from sphere packing theory, we show how to design LOFDM systems (lattice and pulse shape) optimally for timeand frequency-dispersive channels in order to minimize the joint ISI/ICI. Our theoretical analysis is confirmed by numerical simulations, showing that LOFDM systems outperform traditional pulse-shaping OFDM systems with respect to robustness against ISI/ICI.

Blind estimation of symbol timing and carrier frequency offset in wireless OFDM systems

Orthogonal frequency-division multiplexing (OFDM) systems are highly sensitive to synchronization errors. We introduce an algorithm for the blind estimation of symbol timing and carrier frequency offset in wireless OFDM systems. The proposed estimator is an extension of the Gini-Giannakis (see IEEE Trans. Commun., vol.46, p.400-411, 1998) estimator for single-carrier systems. It exploits the cyclostationarity of OFDM signals and relies on second-order statistics only. Our method can be applied to pulse shaping OFDM systems with arbitrary time-frequency guard regions, OFDM based on offset quadrature amplitude modulation, and biorthogonal frequency-division multiplexing systems. We furthermore propose the use of different subcarrier transmit powers (subcarrier weighting) and periodic transmitter precoding to achieve a carrier frequency acquisition range of the entire bandwidth of the OFDM signal, and a symbol timing acquisition range of arbitrary length. Finally, we provide simulation results demonstrating the performance of the new estimator.

Three non-pilot based time- and frequency estimators for OFDM

Time-domain maximum-likelihood (ML) estimators of time and frequency offsets are derived for three orthogonal frequency division multiplexing (OFDM) signal models: a pulse-shaped one-shot OFDM signal, a stream of multiple OFDM signals and an OFDM signal in a dispersive channel environment. We then develop structures to simplify their implementation. Simulation results show the relative performance and strengths of each of these three estimators.