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In this paper, we propose two algorithms for joint power allocation and bit-loading in multicarrier systems using discrete modulations. The objective is to maximize the data rate under the constraint of a suitable Bit Error Rate per subcarrier. The first algorithm is based on the Lagrangian Relaxation of the discrete optimization problem in order to find an initial solution. A discrete solution is found by bit truncation followed by an iterative modulation adjustment. The second algorithm is based on Discrete Coordinate Ascent framework with iterative modulation increment of one selected subcarrier at each iteration. A simple cost function related to the power increment per bit is used for subcarrier selection. A sub-optimal low complexity Discrete Coordinate Ascent algorithm is proposed that overcome the limitations of the Hughes-Hartogs algorithm. The Lagrangian Relaxation algorithm provides a suboptimal solution for non-coded system using M-QAM modulations, whereas the low complexity Discrete Coordinate Ascent algorithm provides a near optimal solution for coded as well as for non-coded system using an arbitrary modulation set. Numerical results show the efficiency of the proposed algorithms in comparison with traditional methods.

Generalized frequency division multiplexing (GFDM) is a prominent candidate to be used by the mobile Fifth Generation (5G) physical layer. Nevertheless, the integration of GFDM with Spatial Multiplexing (SM) MIMO system is essential to fulfill the data rate requirements. SM detection of MIMO-GFDM becomes a more challenging topic because of ICI and ISI due to the non-orthogonal nature of GFDM, along with IAI. In this article, the authors propose a system that combines the Offset-Quadrature Amplitude Modulation (OQAM) with GFDM to mitigate self-induced interference, by using a simple Matched Filter (MF) detector and minimum additional processing at the receiver. Simulation results show a considerable achieved improvement in BER by the proposed OQAM/GFDM compared to QAM/GFDM when using MMSE-based Ordered Successive Interference Cancellation (OSIC) detector. Furthermore, this system is unaffected by the roll-off factor variations of used pulse-shaping filters.

In this paper, we analyze a new cross layer combining scheme of adaptive modulation and coding (AMC) at the physical layer and hybrid automatic repeat request (HARQ) protocol at the data link layer. Our purpose is to improve the performance of the communication system in terms of throughput and errors. We propose a new adaptive HARQ protocol based on the estimated mutual information in addition to signal to noise ratio as channel state information (CSI), which are unavailable at the receiver. Unlike the traditional protocols, the transmitter is aware of the transmission mode only, not the instantaneous CSI, which minimize the load on the feedback channel. The transmission is supposed to be done over block fading channel. Mathematical formulation is improved for the performance criterions, in terms of throughput of the new HARQ, average number of transmissions and the spectral efficiency of the AMC/HARQ. The comparison with the traditional variable rate HARQ shows that our new adaptive HARQ protocol provides an upper bound for the throughput. In addition, the simulation results show that, a small number of transmissions, is enough to reach the maximum spectral efficiency while maintaining the error probabilities under certain acceptable level.

In this paper, we present a theoretical model operating in the frequency domain of the effects of a non-linear power amplifier on the bit error performance of orthogonal frequency-division multiplexing (OFDM) transmission systems. Based on Saleh model (Saleh, 1981), we model the in-band distortion caused by the power amplifier by an equivalent complex gain and additive white Gaussian noise. We characterize analytically the parameters of the proposed model as function of the back-off factor and the underlying parameters of the power amplifier. We demonstrate the accuracy of the proposed model by mean of numerical simulations of the bit error performance over an additive white Gaussian noise channel.

In this paper, we present a simple and effective precoding technique to mitigate inter-symbol interference from multiple transmissions of the same packet in automatic repeat request (ARQ) protocols over slowly time-varying frequency-selective channels. We minimize the power of inter-symbol interference by introducing phase-precoding transmission diversity among subsequent ARQ transmissions. For each packet retransmission, only the phases of the modulated symbols are changed according to a specific pattern. In order to find the best precoding phases assuming that no channel state information is available at the transmitter, we first derive a performance criterion on the precoding phases for a maximum likelihood receiver. Then, we propose a low complexity periodic precoding solution. Simulation results show that the proposed precoding scheme provides a substantial gain in terms of frame error rate performance of the joint detector without significant increase in receiver complexity, leading to enhanced throughput efficiency and better dropping rate in comparison with the non-precoded system.

In this paper1, we investigate the use of cyclic frequency diversity (CFD) scheme for single-carrier packet retransmission over frequency-selective channels in the context hybrid automatic repeat request protocols (HARQ). For each retransmission of the same packet, the transmitted signal is cyclically shifted in the frequency-domain in order to compensate for channel fading over a part of the channel bandwidth in the previous transmissions. We consider efficient frequency-domain equalization based on the minimum mean square error (MMSE) criterion. We determine an optimal solution for frequency-shift values in order to maximize system throughput by maximizing the signal to interference-plus-noise ratio (SINR) at the output of the joint MMSE equalizer. The CFD scheme has low complexity and provides a substantial gain in terms of frame error rate, especially for high rate channel coding.

International audience; In data packet communication systems over multipath frequency-selective channels, hybrid automatic repeat request (HARQ) protocols are usually used in order to ensure data reliability. For single-carrier packet transmission in slow fading environment, an identical retransmission of the same packet, due to a decoding failure, does not fully exploit the available time diversity in retransmission-based HARQ protocols. In this paper, we compare two transmit diversity techniques, namely, cyclic frequency- shift diversity and bit-interleaving diversity. Both techniques can be integrated in the HARQ scheme in order to improve the performance of the joint detector. Their performance in terms of pairwise error probability is investigated using maximum likelihood detection and decoding. The impact of the channel memory and the modulation order on the performance gain is emphasized. In practice, we use low complexity linear filter-based equalization which can be efficiently implemented in the frequency domain. The use of iterative equalization and decoding is also considered. The performance gain in terms of frame error rate and data throughput is evaluated by numerical simulations.

In this paper, we investigate the performance of a bit-interleaving diversity (BID) scheme formultiple packet transmissions over multiple-input multiple-output (MIMO) frequency-selective channel in the context of hybrid automatic repeat request protocols. We show under the maximum likelihood decoding rule that a BID scheme outperforms an identically interleaved transmit scheme. Moreover, we identify and characterize three types of diversity resulting from the BID scheme: modulation diversity, multiplexing diversity, and interference decorrelation resulting in an enhanced detection performance. By using a distance spectrum analysis, we evaluate their respective importance according to the used modulation and the transmission scenario. Some interleaving design rules are given in order to enhance further the overall transmission diversity. The receiver is implemented using a turbo-equalizer structure with packet combining based on separate linear equalization followed by joint decoding. Simulation results show the efficiency of the BID scheme for different transmission scenarios.

In this paper, we propose a MIMO approach for packet combining in hybrid automatic repeat request (HARQ) protocols using single-carrier multi-layer transmission over block fading channels. Based on this model, the problem of the optimization of the linear superposition coefficients is briefly addressed.

In this paper, we present a simple and effective packet combining method for a turbo-diversity transmit scheme in hybrid automatic repeat request (HARQ) systems over frequency-selective channels. We first show by using a distance spectrum-based analysis that the turbo-diversity scheme outperforms the identical interleaving transmit scheme in terms of frame error rate (FER) performance under a maximum-likelihood receiver, especially for high order modulations. Then, we propose a low complexity combining method with integrated turbo-equalization based on a separate linear equalization for each HARQ transmission followed by a joint decoding instead of an iterative combining strategy. The proposed method has the advantage to be operational for both HARQ modes: namely, Chase combining (CC) and the incremental redundancy (IR). Numerical simulations show that the proposed method can efficiently exploit the potential of the turbo-diversity scheme.

In this paper 1 , we present a simple and effective precoding technique to mitigate intersymbol interference in communication systems that implement automatic repeat request (ARQ) protocols over slowly time-varying frequency-selective channels. In each ARQ transmission, only the phase of modulated symbols are changed according to a specific periodic pattern in order to pre-equalize the Euclidean distance at the output of the noiseless ISI channel in the minimum square error sense. We first derive a performance criterion on the selection of precoding phases. Then, an optimal periodic pattern is built without ap rioriknowledge of the channel state information by the transmitter, based on orthogonal Hadamard matrices. We show that the proposed precoding scheme provides a substantial gain in terms of frame error rate performance of the joint detector without significant increase of the receiver complexity, leading to both enhanced throughput efficiency and dropping rate in ARQ systems, especially for low to medium SNR range. I. INTRODUCTION In high speed data transmission over frequency-selective channels, the received signal suffers from inter-symbol inter- ference (ISI) resulting from a limited bandwidth of the channel or multipath propagation. Equalization techniques are usually used at the receiver to combat the negative effects of the ISI on the performance of the communication system. To face errors introduced by the channel, forward error correction codes and automatic repeat request (ARQ) protocols (1) are used to ensure data reliability. In Hybrid ARQ (HARQ) protocols, a combination of both methods is used to enhance data throughput.