Abdel-Nasser Assimi

Packet combining for turbo-diversity in HARQ systems with integrated turbo-equalization

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.

Distance distribution for turbo-equalized systems over static frequency selective channels

In this paper we investigate the Euclidean distance distribution in turbo-equalized systems over static frequency-selective (ISI) channels . We propose a novel approach in evaluating the squared Euclidean distance between transmitted sequences at the output of the ISI channel based on the correlogram of error sequences. By inspecting autocorrelation properties of error sequences, we derive the main statistics of the output Euclidean distance. The proposed method provides a more comprehensive tool to predict system behavior with finite or infinite packet length. We exploit obtained results to evaluate the frame error rate (FER) performance of the system under maximum-likelihood sequence estimation.

Phase-precoding without CSI for packet retransmissions over frequency-selective channels

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.

Diversity techniques for single-carrier packet retransmissions over frequency-selective channels

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.

Periodic Hadamard Phase Precoding for HARQ Systems over Intersymbol Interference Channels

In this paper, we present a simple and effective preceding 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 a priori knowledge 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.

Bit-interleaving diversity for HARQ transmission over MIMO frequency-selective channels

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.

On cyclic frequency diversity for single-carrier packet retransmissions

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.

Reliable Communications Using Multi-layer Transmission

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.

Phase precoding with integrated turbo-equalization for packet retransmissions

In this paper, we present an effective phase precoding diversity scheme to mitigate intersymbol interference from multiple transmissions in hybrid automatic repeat request (HARQ) protocols over slowly time varying frequency-selective channels. For each HARQ transmission, only the phases of modulated symbols are changed according to a periodic pattern. This diversity scheme results in a spectrum thinning of the Euclidean distance at the output of the noiseless ISI channel. Periodic phase precoding only requires a small increase in receiver complexity but provides a substantial gains in terms of frame error rate.