Envelope detection is used in visible light communications and in some radio-frequency receivers, for low-power applications. This framework requires modulations to be unipolar, i.e. using real and positive signals. The aim of this work is to maximize the energy efficiency under the constraint of a spectral efficiency below 1 bit/s/Hz, compatible with low data rate applications. Inspired by single-tone asymmetrically clipped hybrid frequency and phase modulations, this study proposes a general framework for a dual-tone extension. We construct a polynomial approximation of the non-linear clipping part of the model, which permits a fine analysis of the frequency-domain waveforms, including a power spectral density formula, and the proposal of a quasi-optimal receiver with low complexity, implemented for two relevant dual-tone transmitter schemes. Performance simulations show that the proposed transmitter/receiver schemes outperform single-tone FSK-based approaches and other benchmarks from the literature, such as OOK, unipolar PAM and ACO-OFDM modulations, in terms of energy efficiency for a given low spectral efficiency. The best performance is achieved when both tones are selected from the same odd-index frequency grid. Robustness to a frequency-selective channel and to a timing offset synchronization error is evaluated.

Envelope detection, commonly used in visible light communication systems or in some radiofrequency receivers, requires the definition of real and unipolar modulations. With the objective of maximizing the energy efficiency of the modulation for a given spectral efficiency (≤ 1 bit/s/Hz, compatible with low power low data rate applications), a hybrid modulation that merges the elements of non-linear and linear modulations (named as hybrid asymmetrically clipped frequency-shift keying (AC FSK)-based modulation) is investigated. Comparing the performance for different linear (amplitude/phase) modulations, it is shown that the use of phase-shift keying (PSK) with 4 or 8 states for the linear part offers the best spectral efficiency vs energy efficiency trade-off. Moreover, a frequency-domain analysis of the waveforms facilitates the implementation of a low complexity quasi optimal harmonic receiver. Finally, a closed-form expression of power spectral density for AC-FSK-PSK modulation is defined, leading to a relevant definition for signal bandwidth and spectral efficiency.

In this article, we present proof-of-concept of M-ary asymmetrically clipped (AC)-frequency-shift keying (FSK) for energy-efficient/low data rate visible light communications (VLC). M-ary AC-FSK uses the odd frequency waveforms from 2M-ary FSK; which exhibit half-wave symmetry. Owing to this half-wave symmetry, the negative amplitude excursions are clipped without loss of information. Hereby, we investigate the performance of three different receivers for M-ary AC-FSK exhibiting different complexities. These receivers are: (i) an optimal time-domain maximum likelihood (ML) receiver; (ii) 1-tap discrete cosine transform (DCT) receiver; and (iii) a 2-tap harmonic receiver which exploits the spectral properties of AC-FSK waveforms. We shall exemplify that the 2-tap harmonic receiver overcomes the energy efficiency versus complexity trade-off that exists for optimal ML and 1-tap DCT receivers. Simulation results shall confirm that the performance of 2-tap harmonic receiver reaches the performance of the previously proposed unipolar (U)-FSK with the optimal receiver, but with a drastically reduced receiver complexity.

This letter deals with channel estimation in mobile-to-mobile communications, possibly including amplify and forward mobile relays, assuming a two-dimensional scattering environment. A second-order autoregressive [AR(2)] model with the Kalman filter is used to estimate the channel gain of the global multihop channel. The parameters of the AR(2) model can be tuned by using a classic correlation matching criterion. Alternatively, the use of a minimization of the asymptotic variance criterion permits a strong performance improvement, although the corresponding analytical study is still missing in the literature—which is the topic of this letter. The closed-form expressions for the optimal tuning of the AR(2) parameters and mean square error performance have been first established as a function of the second and fourth moments of the Doppler spectrum of the global channel. Because the analytical formulas of these moments were not given in the literature, we derive them by exploiting cumulant property of convolution of density functions, after decomposition of the global channel as a cascade of Jakes Doppler power spectral density channels. Consequently, the provided closed-form formula are helpful for the design of Kalman filters for a given state of the source, destination, and mobile relays [Doppler spread and signal to noise ratio].

As the Internet‐of‐Things (IoT) has expanded, multiple solutions have attempted to address the issues of the Low Power Wide Area (LPWA) networks physical layer. In a previous work, we proposed the Turbo‐FSK, a constant envelope modulation with orthogonal alphabet that allows the receiver to operate at very low levels of power (high sensitivity performance) and very low levels of energy per bitEb. The scheme was demonstrated to approach Shannon’s limit as close as 0.29 dB. However, the scheme lacks flexibility in terms of spectral efficiency (always lower than 10−1 bits/s/Hz), especially compared to the recently standardized Narrow‐Band IoT (NB‐IoT) solution. In this work, we propose an evolution of the initial scheme, so‐called Coplanar Turbo‐FSK (C‐TFSK). In order to increase the spectral efficiency of the system, two new features are introduced: a modulation combining linear and orthogonal properties where only subsets of the alphabet are orthogonal and a puncturing mechanism. Several aspects of the scheme are then studied under asymptotic hypothesis, such as the influence of the linear component of the alphabet and the effects of puncturing. The high flexibility in terms of spectral efficiency, the short distance to Shannon’s limit, and the constant envelope property make the C‐TFSK a serious contender for the physical layer of the IoT.

We consider the joint detection and decoding of m-sequences. The receiver has to decide whether an m-sequence is received and possibly to decode its initial state. To do so, it implements an iterative message-passing decoding algorithm that operates on a parity check matrix, built upon a number of reference parity-check equations satisfied by the m-sequence. This matrix concatenates several elementary parity check matrices which are derived from reference equations. Unlike the conventional decoding case, the detection problem imposes to consider false alarms that may occur when the decoder is only fed with noise. While absorbing sets are known to be responsible for the error floor phenomenon of iterative message-passing decoders, we show that they may have a beneficial effect on the detection performance, in that they may prevent the decoder to produce false alarms. We further compute the number of hybrid cycles of length six and eight in the Tanner graph of the decoder and use the minimization of this number as criterion to derive an algorithm for selecting the reference parity check equations. This algorithm was found to be efficient for minimizing the probability of false alarm and decreases also the probability of wrong detection in the very small SNR region. This has been achieved at the cost of a reduction of the probability of correct detection.

Gold sequences are widely used in communications and positioning systems for synchronization purposes or spread spectrum transmissions. This paper addresses the decoding of the initial state of a Gold sequence. This can be used to detect a harmful interferer closed to a 3G femtocell base station and implement interference mitigation techniques. The decoder implements an iterative message-passing algorithm which is built upon a parity check matrix. Thus, it depends on the coding properties of Gold codes. In this paper, we synthesize the coding properties of Gold codes and use them to compute the number of parity check equations of weight t = 3, 4 or 5. Eventually, the impact of the parity check equations used for decoding is highlighted.

This study deals with channel estimation in tracking mode over a flat Rayleigh fading channel with Jakes’ Doppler spectrum. Many estimation algorithms exploit the time‐domain correlation of the channel by employing a Kalman filter based on a first‐order (or sometimes second‐order) approximation model of the time‐varying channel. However, the nature of the approximation model itself degrades the estimation performance for slow to moderate varying channel scenarios. Furthermore, the Kalman‐based algorithms exhibit a certain complexity. Hence, a different model and approach has been investigated in this study to tackle all of these issues. A novel phase‐locked‐loop‐structured third‐order tracking loop estimator with a low complexity is proposed. The connection between a steady‐state Kalman filter based on a random walk approximation model and the proposed estimator is first established. Then, a suboptimal mean‐squared‐error (MSE) is given in a closed‐form expression as a function of the tracking loop parameters. The parameters that minimise this suboptimal MSE are also given in a closed‐form expression. The asymptotic MSE and bit‐error‐ratio simulation results demonstrate that the proposed estimator outperforms the first‐ and second‐order Kalman‐based filters reported in literature. The robustness of the proposed estimator is also verified by a mismatch simulation.

Interference between macro and femtocells is an important issue for the development of WCDMA femtocell networks. More specifically, the uplink signal of a macro User Equipment may generate an unacceptable level of interference at the femto Base Station. To avoid this situation, interference mitigation techniques could be implemented. All the proposed techniques require the knowledge of the uplink scrambling code index of the interferer. Unfortunately, if the femto BS is in a closed access mode, there are no signalling links with the surrounding macro BSs. The femto BS has to estimate blindly this scrambling code index. An algorithm which performs a blind identification of the uplink scrambling code index of a WCDMA transmission is proposed in this article. This gives the possibility to implement interference cancelation algorithm at the femto BS.

Interference between macro and femtocells is an important issue for the development of CDMA2000 femtocell networks. More specifically, the reverse link signal of a macro User Equipment may generate an unacceptable level of interference at the femto Base Station. To avoid this situation, interference mitigation techniques could be implemented. All the proposed techniques require to know the state of the scrambling code of the interferer in the reverse link. Unfortunately, it depends on the code mask of the terminal which is unknown by the femto BS. The femto BS has to estimate blindly the state of the scrambling code. An algorithm which performs a blind identification of the scrambling code of a CDMA2000 reverse link transmission is proposed in this article. This gives the possibility to implement interference cancelation algorithm at the femto BS.

This letter deals with the Kalman filter (KF) based on a third-order integrated random walk model (RW3). The resulting filter, noted as RW3-KF, is well suited to track slow time-varying parameters with strong trend behaviour. We first prove that the RW3-KF in steady-state admits an equivalent structure to the third-order digital phase-locked loops (DPLL). The approximate asymptotic mean-squared-error (MSE) is obtained by solving the Riccati equations, which is given in a closed-form expression as a function of the RW3 model parameter: the state noise variance. Then, the closed-form expression of the optimum state noise variance is derived to minimize the asymptotic MSE. Simulation results are given for the particular case where the parameter to be estimated is a Rayleigh channel coefficient with Jakes' Doppler spectrum.

Underwater wireless communications are subject to multipath propagation and Doppler effects. In the receiver side, it is necessary to estimate both Doppler and channel parameters in order to compensate for their effects in the transmitted signal. In this paper, in the OFDM (Orthogonal Frequency Division Multiplexing) context, we derive a new estimation scheme based on the resolution of two harmonic retrieval problems. Unlike standard methods, data resampling and estimation of residual carrier frequency offset are avoided. The efficiency of the proposed scheme is evaluated by means of simulations.

This paper deals with the case of a high speed mobile receiver operating in an orthogonal-frequency-division-multiplexing (OFDM) communication system. Assuming the knowledge of delay-related information, we propose an iterative algorithm for joint multi-path Rayleigh channel complex gains estimation and data recovery in fast fading environments. Each complex gain time-variation, within one OFDM symbol, is approximated by a polynomial representation. Based on the Jakes process, an auto-regressive (AR) model of the polynomial coefficients dynamics is built, making it possible to employ the Kalman filter estimator for the polynomial coefficients. Hence, the channel matrix is easily computed, and the data symbol is estimated with free inter-sub-carrier-interference (ICI) thanks to the use of a QR-decomposition of the channel matrix. Our claims are supported by theoretical analysis and simulation results, which are obtained considering Jakes' channels with high Doppler spreads.

Phase noise is a topic of theoretical and practical interest in electronic circuits. Although progress has been made in the characterization of its description, there are still considerable gaps in its effects especially on multi-carrier spreading systems. In this paper, we investigate the impact of a local oscillator phase noise on the multi-carrier 2 dimensional (2D) spreading systems based on a combination of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) and known as OFDM-CDMA. The contribution of this paper is multifold. First, we use some properties of random matrix and free probability theory to give a simplified expression of signal to interference and noise ratio (SINR) obtained after equalization and despreading. This expression is independent of the actual value of the spreading codes and depends mainly on the complex amplitudes of estimated channel coefficients. Secondly, we use this expression to derive new weighting functions which are very interesting for the radio frequency (RF) engineers when they design the frequency synthesizer. Therefore, based on these asymptotic results, we adapt a new method to predict the bit error rate (BER) at the output of the channel decoder by using an effective SINR value. We show by simulations the validity of our models and that at a given BER, the required signal to noise ratio (SNR) may easily increase due to the carrier phase noise.

This paper addresses the joint path delay and time-varying complex gain estimation for continuous phase modulation (CPM) over a time-selective slowly varying flat Rayleigh fading channel. We propose an expectation-maximization (EM) algorithm for path delay estimation in a Kalman smoother framework. The time-varying complex gain is modeled by a first order autoregressive (AR) process. Such a modeling yields to the representation of the problem by a dynamic Bayesian system in a state-space form that allows the application of EM algorithm in the context of unobserved data for obtaining an estimate of the path delay. This is used with Kalman smoother for state estimation. We derive analytically a closed-form expression of the modified hybrid Cramer-Rao bound (MHCRB) for path delay and complex gain parameters. Finally, some numerical examples are presented to illustrate the performance of the proposed algorithm compared to the conventional generalized correlation method and to the MHCRB.

In this paper, we consider the Bayesian Cramer-Rao bound (BCRB) for the dynamical estimation of multi-path Rayleigh channel complex gains in data-aided (DA) and non-data-aided (NDA) OFDM systems. This bound is derived in an on-line and off-line high Doppler scenarios for time-varying complex gains within one OFDM symbol, assuming the availability of prior information. In NDA context, whereas this true BCRB is hard to evaluate, we present a closed-form expression of a BCRB, i.e., the Modified BCRB (MBCRB). We discuss, based on the theoretical and simulation results, the benefit of using the a priori information and, the past and the future observations for the complex gains estimation.

This paper deals with the on-line estimation of a dynamical carrier phase and a frequency offset in a digital receiver. We consider a Brownian phase evolution with a linear drift in a data aided scenario. The proposed study is relative to the use of an oversampled signal model after matched filtering, leading to a coloured reception noise and a non-stationary power signal. We derive a closed-form expression of the hybrid Cramer-Rao bound (HCRB) for this estimation problem. We use a binary offset carrier (BOC) function as shaping pulse. Our numerical results show the potential gain of using the oversampled signal for estimating the dynamical phase and frequency offset, obtaining better performances than using a classical synchronizer.

In this paper, we present an application of the Extended Kalman Filter for the on-line estimation of a dynamical carrier phase offset. The novel approach implies deriving the filter in an oversampled scenario in a digital receiver. We consider a Brownian phase evolution in a Data Aided scenario. Our numerical results using a BOC shaping pulse show that using the oversampled signal for estimating the phase offset we can obtain better performances than using a classical synchronizer.

In this paper, we present an iterative algorithm for channel complex gains estimation with inter-sub-carrier- interference (ICI) reduction in orthogonal-frequency-division- multiplexing downlink mobile communication systems using comb-type pilot. Assuming delays information, the time-variation of the multipath complex gains within one OFDM symbol are obtained by interpolating the symbol time-average values, which have been previously estimated with LS criteria. After that, the channel matrix is computed and the ICI is reduced by using successive interference suppression (SIS) in data symbol detection. The algorithm's performance is further enhanced by an iterative procedure, performing channel estimation and ICI suppression at each iteration. Mean Square Error theoretical analysis and simulation results show a significant performance improvement for high normalised Doppler spread (especially after the first iteration) in comparison to conventional methods.

This paper presents the impact of a carrier frequency offset on the performance of 2 dimensional spreading OFDM-CDMA systems. This is measured by the degradation of the Signal to Interference plus Noise Ratio (SINR) obtained after despreading and equalization. Using some properties of random matrix and free probability theories, a new expression of the SINR is derived. It is independent of the actual value of the spreading codes while still accounting for the orthogonality between codes. This model is validated by means of Monte-Carlo simulations. It is also exploited to compare the sensitivities of MC-CDMA and MC-DS-CDMA systems to carrier offset in a frequency selective channel. This work is carried out for zero forcing (ZF) and minimum mean square error (MMSE) equalizers.

This article presents the impact of a constant timing error on the performance of a downlink 2 dimensional spreading OFDM-CDMA system. This impact is measured by the Signal to Interference plus Noise Ratio (SINR) degradation after equalization and despreading. Using random matrix theory, an asymptotic evaluation of the SINR is obtained. It is independent of the value of users' spreading code while taking into account their orthogonality. Simulation results are provided to evaluate and discuss the validity of this model.

We consider a direct-sequence code-division multiple-access system operating in the downlink. We propose to improve the performances of a conventional delay tracking algorithm, i.e. the early-late algorithm, by prefiltering the received signal. This concept has been studied by A.N. D'Andrea and M. Luise (see IEEE Trans. Commun., vol.44, p.399-406, 1996) for the clock recovery scheme proposed by F.M. Gardner (see IEEE Trans. Commun., vol.34, p.423-9, 1986). Our main contribution is to extend this concept in a multi-user CDMA context. We treat multiple-access interference and deal with the two "timing scales", i.e. symbol and chip duration, due to the spread spectrum. The analysis shows improved tracking performances in comparison to the standard early-late algorithm.

In the downlink TD/CDMA multiple access scheme through a multipath channel, the use of a short complex code enables a strategy of multi-user detection with great sensibility improvement compared to the traditional matched filter. Nevertheless, the performances are damaged compared to the situation with an ideal channel, more especially as the number of users increases. In this paper, we first use the cross-spectral matrix of the aliased coded channels to recall linear multi-user detector (ZF and MMSE) and express a degradation factor of interference and noise to signal ratio due to a non-ideal channel, with or without fading. The degradation and bit error probability are then computed in the concrete situation of indoor and pedestrian channel environments in order to show the great benefit of one or two supplementary sensors at the reception system.