Yves Louët

A classification of methods for efficient power amplification of signals

This paper presents a classification of methods that have been proposed to address nonlinear power amplification of highly fluctuating signals in telecommunications. The classification proposed uses a tree-like representation wherein each branch refers to a group of methods that all have a common characteristic. By virtue of this representation, each node corresponds to a test used to discriminate between different methods. From top to bottom, these tests are “What is the target of the method?,” Is the method downward-compatible?,” “Is the bit error rate degraded?,” “Is there a useful data rate loss?,” and “Does the method require changes in the amplification function?” By collating all these requirements, an original classification is proposed that is open enough to allow new methods to be added. It only concerns methods located either only at the transmitter or at both transmitter and receiver. The context of this study generally concerns multicarrier signals (especially orthogonal frequency division modulation) but can be applied to any multiplex of modulated signals.

Peak-to-average power ratio reduction using second order cone programming based tone reservation for terrestrial digital video broadcasting systems

High peak-to-average power ratio (PAPR) is a critical issue in any multicarrier systems using orthogonal frequency division multiplexing, as terrestrial digital video broadcasting (DVB). It can result in low power efficiency and large performance degradation of systems, due to the nonlinearity of high-power amplifier (HPA). A PAPR reduction method based on tone reservation technique with second-order cone programming (SOCP) approach in terrestrial DVB systems is proposed. The authors first demonstrate the superiority of the SOCP optimisation algorithm compared with an iterative gradient-based algorithm, using the current DVB-T parameters: significant PAPR reduction gains can be achieved with only a very small set of subcarriers in the useful bandwidth, making the proposed method more promising in terms of spectral efficiency. Moreover, the proposed solution presents a very good trade-off between PAPR reduction gain and mean transmitted power increase. An overall study, taking into account the limitation of the power level of the dedicated subcarriers and the evaluation of the performances in presence of a nonlinear HPA, is presented. These performances are given in terms of adjacent channel power ratio and bit error rate. The resulting PAPR reduction gain demonstrates that the relevance of the proposed method for the future DVB-T standard is straightforward.

Promising Technique of Parameterization For Reconfigurable Radio, the Common Operators Technique: Fundamentals and Examples

In the field of Software Radio (SWR), parameterization studies have become a very important topic. This is mainly because parameterization will probably decrease the size of the software to be downloaded, and also because it will limit the reconfiguration time. In this paper, parameterization is considered as a digital radio design methodology. Two different techniques, namely common functions and common operators are considered. In this paper, the second view is developed and illustrated by two examples: the well known Fast Fourier Transform (FFT) and the proposed Reconfigurable Linear Feedback Shift Register (R-LFSR), derived from the classical Linear Feedback Shift Register (LFSR) structure.

FPGA implementation of a re-configurable FFT for multi-standard systems in software radio context

This study is focused on the Field Programmable Gate Array (FPGA) implementation of a re-configurable Fast Fourier Transform (FFT) operator able to provide Fourier transforms both over complex infinite field X and Galois finite Field GF. This new re-configurable FFT exploits to a great advantage the possibility to share hardware resources when considering multi-standard scenarios for software radio systems. A re-configurable FFT of length N = 256 has been implemented on FPGA. It achieves a performance-to-cost ratio gain from 24% to 9.4% compared to the basic duplicated solution for which no re-configuration is considered. The proposed technology is strongly connected to further consumer handheld devices such as mobile phones intended to support several standards (digital television, mobile communications, wireless local area network etc) where FFT is involved.

Tone reservation technique based on geometric method for orthogonal frequency division multiplexing peak-to-average power ratio reduction

Orthogonal frequency division multiplexing (OFDM), has recently attracted considerable attention because of its features to combat delay spread or frequency selective fading of wireless or wireline channels. One of the major drawbacks of OFDM is the peak-to-average power ratio (PAPR) which leads to an inefficient use of high power amplifiers (HPA) in communications systems. The authors first briefly review the principle of geometric method (GM) that they previously proposed. This method has a couple of well identified drawbacks as the necessity of adding a radio frequency (RF) filter, the increasing of power amplifier band and the bit error rate degradation if PAPR is largely mitigated. Consequently we propose in this paper a new method to improve GM performance. This method mixes tone reservation (TR) technique and GM called ‘TR-GM’ in order to efficiency reduce the high PAPR of OFDM signals. In addition, the proposed TR-GM technique is compared with TR based on second order cone programming (‘TR-SOCP’) and TR based on the gradient-project (‘TR-GP’) techniques in terms of PAPR reduction, average power variation and complexity. The comparative study has shown that, the proposed TR technique seems to be a good compromise between PAPR reduction, complexity, and average power variation.

Frequency domain interpretation of power ratio metric for cognitive radio systems

Software radio (SWR) is an enabling technology for cognitive radio (CR) systems which promises to (de) modulate any signal, at any frequency. SWR signal therefore is composed of different standards signals, and each standards signal is either multicarrier or multiplex of single carriers. This combination leads to high temporal fluctuations and thus SWR signal inherits high peak to average power ratio (PAPR) or simply high power ratio (PR). Nonlinear analogue components (amplifiers, converters etc.) cause distortions (in and out of band distortion) for high PR signals which result in system performance degradation. Usually PR problem is addressed in time domain, and here frequency domain interpretation of PR which is more appropriate in SWR context is presented. Gaussian equivalence between SWR signal and orthogonal frequency division multiplexing (OFDM) signal is proved first to accentuate high PR issue in SWR as OFDM suffers the same problem. Then frequency domain interpretation of PR metric is discussed which results in a PR upper bound. This PR upper bound depends only upon spectral values of the signal thus associates spectrum with PR. As a result this bound assists in spectrum access for CR systems by providing PR metric information related to any available bandwidth. Thus bandwidth allocation in a spectrum access scenario under PR constraint is simplified.

Blind spectrum sensing using symmetry property of cyclic autocorrelation function: from theory to practice

Spectrum sensing has been identified as the key step of the cognition cycle and the most important function for the establishment of cognitive radio. In this paper, a blind cyclostationary feature detector, which is based on the symmetry property of cyclic autocorrelation function (SP-CAF), is implemented and tested using universal software radio peripheral platform and GNU Radio open-source software development toolkit. Performance of the SP-CAF is compared to the classical energy detector via various tests conducted in real scenarios where both detection algorithms are employed to blindly sense the spectrum for opportunistic access. This study shows that the blind cyclostationary feature detector outperforms the classical energy detector while guaranteeing acceptable complexity and low sensing time. Moreover, different experimental results indicate that the blind sensing detector can achieve high detection probability at a low false alarm probability under real channel conditions and low signal-to-noise ratio.

Application of a joint and iterative MMSE-based estimation of SNR and frequency-selective channel for OFDM systems

This article presents an iterative minimum mean square error- (MMSE-) based method for the joint estimation of signal-to-noise ratio (SNR) and frequency-selective channel in an orthogonal frequency division multiplexing (OFDM) context. We estimate the SNR thanks to the MMSE criterion and the channel frequency response by means of the linear MMSE (LMMSE). As each estimation requires the other one to be performed, the proposed algorithm is iterative. In this article, a realistic case is considered; i.e., the channel covariance matrix used in LMMSE is supposed to be totally unknown at the receiver and must be estimated. We will theoretically prove that the algorithm converges for a relevantly chosen initialization value. Furthermore simulations show that the algorithm quickly converges to a solution that is close to the one in which the covariance matrix is perfectly known. Compared to existing SNR estimation methods, the algorithm improves the trade-off between the number of required pilots and the SNR estimation quality.

PAPR Reduction Scheme with SOCP for MIMO-OFDM Systems

A combination of multiple-input multiple-output (MIMO) with orthogonal frequency division multiplexing (OFDM) has became a promising candidate for high performance 4G broadband wireless communications. However, like OFDM, one main disadvantage of MIMO-OFDM is that the signals transmitted on different antennas may exhibit a prohibitively large peak-to-average power ratio (PAPR). In this paper we extend the formulation of the reduction of the PAPR problem as second order cone programming problem (SOCP) [1] to MIMO- OFDM. The investigated PAPR reduction method is originally based on a particular case of tone reservation (TR) approach which uses the unused carriers of standards. This approach does not degrade the bit-error-rate (BER) or the data rate and no side information (SI) is required. Simulation results are presented for a MIMO-OFDM system with two antennas and 256 carriers as in IEEE 802.16 WiMAX standard.

Adding signal for peak-to-average power reduction and predistortion in an orthogonal frequency division multiplexing context

In this study, the authors look into digital predistortion and its combination with the peak-to-average power ratio (PAPR) reduction technique in the case of orthogonal frequency division multiplexing (OFDM)-based systems. First of all, the authors propose a new predistortion methodology which uses the addition of signals. Two different algorithms are investigated for the predistortion signals generation. The first one is based on the traditional predistortion method, where an estimation of the power amplifiers (PAs) characteristics is needed in the first place. The second algorithm to generate the predistortion signal does not need a priori PA estimation. It is based on an iterative compensation of the error between the amplified signal and the OFDM signal. Then, the authors combine this latter algorithm with the tone reservation PAPR reduction method in order to have a global adding signal expression. Two combination scenarios of PAPR reduction and predistortion obtained by means of adding a signal are proposed. The first one combines both of them in series while the second one is parallel. Performances of the proposed predistortion algorithms and the combination scenarios are compared thanks to simulations based on IEEE802.il a/g standards for a memoryless solid state PA.