In this work, we present some simple techniques for enhancing the diversity provided by retransmissions in OFDM and DMT modulations. The primary techniques of interest are symbol interleaving and mapping diversity for retransmissions. Symbol interleaving involves retransmitting packet symbols through different subcarriers of the same channel, while mapping diversity involves adapting the bit-to-symbol mapping for each retransmission. An analysis of the ability of interleaving to produce lower bit error rates is provided. Also, a discussion of optimally adapting the mappings and their application to OFDM and DMT is presented. Simulation results validate the efficiency of these methods in reducing BER and increasing throughputs.

The paper presents a method for enhancing wireless ARQ system performance by combining packet retransmissions through frequency-selective channels. As a measure of system performance, we derive the total mutual information (or channel capacity) provided by multiple transmissions of a packet through different channels. Retransmissions are uniquely linear precoded and/or truncated in relation to the original transmission. With the objective of maximizing total throughput, optimal linear precoders are developed for zero-padded packet transmissions. These precoders effectively implement power loading with the right singular vectors of the original transmission channel serving as basis vectors. Simulation results validate the increase in the overall mutual information and throughput from optimized precoding.

The paper investigates the constrained capacity of sequential ARQ linear precoding in flat fading MIMO systems. To utilize fully the time diversity provided by ARQ, we derive the optimal linear ARQ precoders in a flat fading MIMO channel with the objective of maximizing the mutual information delivered by multiple transmissions of the same packet. The capacity-maximizing ARQ precoders combine water-pouring power loading and the optimal pairing of singular vectors in the current retransmission with those from previous transmissions. This optimal pairing is a special feature unique to sequential ARQ precoding and plays important role in maximizing the mutual information.

In this letter, a new hybrid automatic repeat request (ARQ) approach is presented to enhance receiver performance for communication systems employing forward error-correction codes in frequency-selective fading environments. This new approach involves a simple modification to the traditional turbo equalizer by combining multiple ARQ transmissions via integrated channel equalization. This modification leads to better computational efficiency, better exploitation of channel diversity, better channel-estimation ability, and improved performance (frame-error rates) when concatenated with an outer code. These improvements are verified through evaluations of extrinsic information transfer charts and ARQ simulations when compared with iterative combining of multiple transmissions.

In this paper, we present a simple, but effective method of enhancing and exploiting diversity from multiple packet transmissions in systems that employ nonbinary linear modulations such as phase-shift keying (PSK) and quadrature amplitude modulation (QAM). This diversity improvement results from redesigning the symbol mapping for each packet transmission. By developing a general framework for evaluating the upper bound of the bit error rate (BER) with multiple transmissions, a criterion to obtain optimal symbol mappings is attained. The optimal adaptation scheme reduces to solutions of the well known quadratic assignment problem (QAP). Symbol mapping adaptation only requires a small increase in receiver complexity but provides very substantial BER gains when applied to additive white Gaussian noise (AWGN) and flat-fading channels.

We define an ARQ protocol for MIMO flat-fading channels which varies the bit-to-symbol mapping per retransmission. We begin by defining a model for distinctly mapped transmissions through MIMO channels, and the effect this mapping diversity has on a sphere decoder receiver. Varying the symbol mapping complicates the sphere decoding process, particularly for the enumeration of candidate solutions within the sphere. A technique that promises quick candidate enumeration is suggested, borrowing concepts from existing closest point search schemes. The value of mapping diversity, in reducing BER and reducing computational complexity, is analyzed.

We present a simple, but effective, method of enhancing and exploiting diversity from multiple packet transmissions in systems that employ nonbinary linear modulations such as PSK and QAM. This diversity improvement results from redesigning the symbol mapping for each packet transmission. Using a general framework for evaluating the upper bound of bit error rate (BER) with multiple transmissions, a criterion to obtain optimal symbol mappings is attained. The optimal adaptation scheme reduces to solving the well known quadratic assignment problem (QAP). Symbol mapping adaptation only requires a small increase in receiver complexity but provides very substantial BER gains when applied to flat-fading channels.

We present a simple, but effective, method of creating and exploiting diversity from packet retransmissions in systems that employ nonbinary modulations such as PSK and QAM. This diversity results from differing the symbol mapping for each packet retransmission. By developing a general framework for evaluating the bit error rate (BER) upper bound with multiple transmissions, a criterion to obtain optimal symbol (re)mappings is attained for memoryless AWGN channels. The optimal adaptation scheme reduces to solutions of the quadratic assignment problem (QAP). Symbol mapping adaptation only requires a small increase in receiver complexity but provides very substantial BER gains.

This paper presents a simple, yet effective method of enhancing the inherent diversity among packet retransmissions in digital communication systems that employ high-order modulations such as M-PSK and M-QAM through frequency-selective channels. Diversity is enhanced by uniquely redefining the bit-to-symbol mapping for each retransmission. Finding the optimal mappings leads to iterative solutions of the quadratic assignment problem. More importantly, jointly equalizing transmission with different mappings only requires a marginal increase in complexity, while providing substantial gains in bit error rates (BER) and frame error rates (FER).

In this paper, a new approach is presented to enhance receiver performance for ARQ communication systems that use systematic codes in rapidly-changing environments. This new approach involves a simple modification to the traditional turbo-equalizer by integrating multiple ARQ transmissions for joint equalization of systematic codewords. This modification provides a computationally efficient joint receiver that improves frame error rates and throughput when compared with a few other simple approaches. To analyze the receiver, simulation results are presented for two types of systematic codes: recursive systematic codes (RSC) and low-density parity check codes (LDPC). Results are also presented for a hybrid ARQ system where subsequent transmissions contain only portions of the original transmission.