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Orthogonal frequency division multiplexing (OFDM) provides a promising modulation technique for underwater acoustic (UWA) communication systems. It is indispensable to obtain channel state information for channel estimation to handle the various channel distortions and interferences. However, the conventional channel estimation methods such as least square (LS), minimum mean square error (MMSE) and back propagation neural network (BPNN) cannot be directly applied to UWA-OFDM systems, since complicated multipath channels may cause a serious decline in performance estimation. To address the issue, two types of channel estimators based on deep neural networks (DNNs) are proposed with a novel training strategy in this paper. The proposed DNN models are trained with the received pilot symbols and the correct channel impulse responses in the training process, and then the estimated channel impulse responses are offered by the proposed DNN models in the working process. The experimental results demonstrate that the proposed methods outperform LS, BPNN algorithms and are comparable to the MMSE algorithm in respect to bit error rate and normalized mean square error. Meanwhile, there is no requirement of prior statistics information about channel autocorrelation matrix and noise variance for our proposals to estimate channels in UWA-OFDM systems, which is superior to the MMSE algorithm. Our proposed DNN models achieve better performance using 16QAM than 32QAM, 64QAM, furthermore, the specified DNN architectures help improve real-time performance by saving runtime and storage resources for online UWA communications.

Nonlinear behavior and power efficiency of the Power Amplifier (PA) contradictorily depend on the input signal amplitude distribution. The transmitted signal in multi-carrier modulation exhibits high Peak-to-Average Power Ratio (PAPR) and large bandwidths, leading to the degradation of the radio link and additional generation out-of-band interferences, which degrade the quality of the transmission. Practical solutions exist, like a power back-off, but with unacceptable efficiency performances of the transmitter. This paper deals with efficiency and linearity improvement using a new PAPR reduction method based on the combination of Discrete Cosine Transform and shaping technique. The main principle is to determine an optimal coding scheme according to a trade-off between coding complexity and performance benefits in the presence of PA nonlinearities. Simulation and experimental results in the context of OFDM signal and using a 20 W–3.7 GHz Radio-Frequency Power Amplifier show an improvement on PAPR reduction of about 3.25 dB. Also, the communication criteria like Bit Error Rate and Error Vector Magnitude are improved by about one decade and a half and $$8\%$$ 8 % , respectively.

In this paper, a diamond shape pilot arrangement for OFDM channel estimation is investigated. Such arrangement will decrease the number of pilots transmitted over the communication channel which in turn will increase data throughput while maintaining acceptable accuracy of the channel estimation, the adaptive antenna array (AAA) is combined with orthogonal frequency division multiplexing (OFDM) to combat the intersymbol interference (ISI) and the directional interferences. In this paper, The optimum beamformer weight set is obtained based on minimum bit error rate (MBER) criteria in diamond-type pilot-assisted in 3GPP long term evolution (LTE) OFDM systems under multipath fading channel. The simulation results show that the quadrature phase shift keying signaling based on MBER technique utilizes the antenna array elements more intelligently than the standard minimum mean square error (MMSE) technique.

In this paper, we consider an orthogonal frequency division multiplexing communication system that adopts frame-by-frame transmission in high-speed railway (HSR) scenario. Due to the increase in demand for the QoS sensitive services, an efficient QoS-aware transmission strategy that improves the system performance is required urgently. Many efforts have been devoted to addressing this problem with the assumption of block fading channel in a frame duration. However, due to the frequent channel quality variation in a frame duration and serious inter channel interference in HSR scenario, the throughput of the QoS sensitive services degrades severely. Hence, a robust cross-layer transmission strategy that combines adaptive modulation (AM) scheme with truncated automatic repeat request protocol is proposed. In this cross-layer formulation, the normalized average throughput is optimized subject to the average power and the packet loss rate (PLR) requirements. First, we derive the closed form average bit error rate that represents the PLR requirement at the physical layer. Second, we obtain the solution of robust AM scheme and power allocation policy in the case of continuous rate. Third, we present the adaptive bits and power allocation scheme in the case of discrete rate, which can be implemented in practice. Finally, the performance of the proposed transmission strategy is evaluated by extensive simulations. Comparing with the constant transmit power AM scheme, the throughput increases by 20%, which demonstrates that the proposed robust cross-layer design is suitable for the HSR communication systems.

International audience; Abstract|In OFDM systems, peak-to-average power ratio (PAPR) reduction of the signal is one of the main challenges that need to be overcome in order to use the transmitter in an efficient manner. As one of attractive techniques, interleaving can be used in PAPR reduction for multicarrier signals without spectrum distortion. In this paper, the authors propose to extend the possibilities of interleaving to improve PAPR reduction, to use a new coding of interleaver keys at the transmitter and a robust decoding procedure at the receiver. In order not to degrade the data rate, the use of null subcarriers to transmit side information to the receiver is proposed and evaluated.Simulation results in the context of the WLAN 802.11a standard in the presence of a nonlinear power amplifier model with memory, show a reduction of PAPR of approximately 5.2 dB, and an improvement of bit error rate and error vector magnitude of about 2 decades and 4% respectively, while respecting the spectral mask specication.

International audience; Orthogonal Frequency Division Multiplexing (OFDM) has been shown to be an effective technique in wireless communications. It has been successfully used for Radio Frequency applications. However, reducing its main drawback known as PAPR (Peak-to-Average Power Ratio) is one of the main challenges to use efficiently the High Frequency Power Amplifier at the transmitter. As one of attractive technique, interleaving method can be used in PAPR reduction for multicarrier signals without spectrum distortion, but with additional Side-Information. In this paper, we propose to extend the possibilities of interleaving to improve PAPR reduction and make possible the downward compatibility by transmitting interleaver code via null-subcarriers. Simulation in the context of WLAN 802.11a standard in presence of a nonlinear Power Amplifier (PA) model with memory has been performed. Results show a PAPR reduction of about 5,2 dB and an improvement of about 1 decade in BER (Bit Error Rate) and 4% in EVM (Error Vector Magnitude), while respecting the spectral mask.

International audience; In this paper, we propose a new PAPR reduction method using interleaving technique without data rate loss. The main idea is to set up all possibility of interleaving to get the best reduction of PAPR and to transmit the resulting information via null-subcarriers (NS) available in OFDM (Orthogonal Frequency Division Multiplexing) standards. Thus, we develop a new coding of interleaver key based on mapping symbols at the transmitter and a robust decoding procedure at the receiver. Simulation results in the context of WLAN IEEE 802.11a standard show an improvement of PAPR reduction about 5,2 dB, with the same performance in BER (Bit Error Rate), while respecting the communication criteria (Data Rate and spectrum specification).

Terrestrial or Handheld Digital Video Broadcasting (DVB-T/H) and Terrestrial Digital Multimedia Broadcasting (T-DMB) are two popular broadcasting standards that enable digital television transmissions to handheld receivers. This paper presents a comprehensive performance comparison between the physical layers of DVB-T/H and T-DMB when employed for mobile communications. By exploiting a recently proposed fast simulation model, we assess the BER of the two coded OFDM systems in several realistic scenarios, taking into account Rayleigh and Rice channels, different mobile speeds, inner and outer channel coding, channel estimation, and one or two receive antennas. Our comparison shows that the DVB-T/H physical layer performance highly depends on the delay spread of the channel, whereas T-DMB is less sensitive to the frequency selectivity of the channel. As a result, DVB-T/H yields better performance than T-DMB in typical Rayleigh channels with significant delay spread. On the contrary, at high SNR, T-DMB outperforms DVB-T/H in Rice channels with low delay spread. As a side result, we show the performance improvement of DVB-H produced by MPE-FEC at the data link layer.

International audience; In this paper, we address the challenging problem of the OFDM reception in the presence of phase distortions. Phase noise and carrier frequency offset seriously degrade the performances of OFDM systems by destroying orthogonality of the subcarriers. Based on a Markov Chain Monte Carlo sampling mechanization, our approach consists in jointly estimating the phase noise, the frequency offset and the transmitted symbols. The proposed algorithm is implemented in the time domain in order to benefit from the redundancy information induced by the cyclic prefix and from the time correlation of the OFDM signal owing to the presence of virtual and/or pilot subcarriers. The algorithm's efficiency is enhanced by incorporating the Rao-Blackwellization technique as well as various sampling improvement strategies. Simulation results, provided in terms of bit error rate (BER) and mean square error (MSE), clearly illustrate the efficiency and the robustness of the proposed estimator.

In this paper we evaluate the coded and uncoded performance of a 2-antenna terminal relaying schem, for downlink OFDM based systems, using efficient space-frequency block coding protocols. We consider that the relay and the base station are equipped with 2 antennas and the user terminal has a single antenna. Two types of relay protocols are accounted for: Equalize-and-Forward and Decode-and-Forward. Signals expressions are derived and the bit error rates obtained for a pratical and preeminent scenario, with LTE parameters and using convolutional channel coding. The performance of the proposed relay-assited scheme is evaluated under realistic scenarios, considering typical pedestrian scenarios based on LTE specifications. Results are compared to those of other relay-assisted design approaches that have been recently proposed for the case of 1-antenna relay and the non-cooperative systems. For this comparison, we implement three propagation scenarios, with different SNR transmission conditions among links. Simulation results show that the availability of a bi-antenna array at the relay significantly improves the cooperative system performance, but improvements are highly dependent on the scenarios and relay protocol as well as of the cooperative second-hop quality. CODIV - FP7-ICT/2007-215477 AGILE - PTDC/EEA-TEL/71417/2006 FCT

This contribution deals with phase noise estimation from pilot symbols. The phase noise process is approximated by an expansion of discrete cosine transform (DCT) basis functions containing only a few terms. We propose a feedforward algorithm that estimates the DCT coefficients without requiring detailed knowledge about the phase noise statistics. We demonstrate that the resulting (linearized) mean-square phase estimation error consists of two contributions: a contribution from the additive noise, that equals the Cramer-Rao lower bound, and a noise independent contribution, that results front the phase noise modeling error. We investigate the effect of the symbol sequence length, the pilot symbol positions, the number of pilot symbols, and the number of estimated DCT coefficients it the estimation accuracy and on the corresponding bit error rate (PER). We propose a pilot symbol configuration allowing to estimate any number of DCT coefficients not exceeding the number of pilot Symbols, providing a considerable Performance improvement as compared to other pilot symbol configurations. For large block sizes, the DCT-based estimation algorithm substantially outperforms algorithms that estimate only the time-average or the linear trend of the carrier phase. Copyright (C) 2009 J. Bhatti and M. Moeneclaey.

In OFDM transmission schemes, phase-noise from oscillator instabilities in the receiver is a potentially serious problem, especially when bandwidth efficient, high order signal constellations are employed. The paper analyses the two effects of phase-noise: inter-carrier interference (ICI) and a phase error common to all OFDM sub-carriers. Through numerical integration, the ICI power can be evaluated and is shown as a function of the number of OFDM sub-carriers and various parameters of the phase-noise model. Increasing the number of sub-carriers causes an increase in the ICI power, which our analysis indeed shows to become a potential problem, since it can lead to a BER floor. The analysis allows the design of low-cost tuners through specifying the required phase-noise characteristics. A similar technique is applied to calculate the variance of the common phase error. After showing that the common phase error is essentially uncorrelated from symbol to symbol, we propose a simple feed-forward correction technique based on pilot cells, that dramatically reduces the degradation due to phase-noise. This is confirmed by BER simulations of a coded OFDM scheme (proposed for terrestrial transmission of digital television) with 64 QAM.