Harvind Samra

Symbol mapping diversity design for multiple packet 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.

Optimal symbol mapping diversity for multiple packet transmissions

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.

The quadratic three-dimensional assignment problem: Exact and approximate solution methods

Abstract This paper reports on algorithm development for solving the quadratic three-dimensional assignment problem (Q3AP). The Q3AP arises, for example, in the implementation of a hybrid ARQ (automatic repeat request) scheme for enriching diversity among multiple packet re-transmissions, by optimizing the mapping of data bits to modulation symbols. Typical practical problem sizes would be 8, 16, 32 and 64. We present an exact solution method based upon a reformulation linearization technique that is one of the best available for solving the quadratic assignment problem (QAP). Our current exact algorithm is useful for Q3AP instances of size 13 or smaller. We also investigate four stochastic local search algorithms that provide optimum or near optimum solutions for large and difficult QAP instances and adapt them for solving the Q3AP. The results of our experiments make it possible to get good solutions to signal mapping problems of size 8 and 16.

New MIMO ARQ protocols and joint detection via sphere decoding

In this paper, we develop some new automatic repeat request (ARQ) protocols for multiple-input multiple-out (MIMO) flat-fading channels which adapt the bit-to-symbol mapping of each ARQ retransmission. We begin by defining a model for distinctly mapped transmissions through flat-fading MIMO channels. We characterize the effect that such a mapping diversity has on an integrated receiver utilizing sphere decoding. Varying the symbol mapping complicates the sphere decoding process, particularly for the enumeration of candidate solutions within the sphere. A technique that enables quick candidate enumeration is presented, utilizing concepts from existing closest point search schemes. The advantage of mapping diversity, in reducing bit error rate and reducing computational complexity, is presented along with simulation examples.

A hybrid ARQ protocol using integrated channel equalization

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.

Constrained capacity of linear precoded ARQ in MIMO wireless systems

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.

Symbol mapping diversity design for packet retransmissions through fading channels

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.

Integrated iterative equalization for ARQ systems

An integrated approach is presented to enhance receiver performance for ARQ communication systems in rapidly-changing environments. This approach only requires a simple modification to the traditional turbo-equalizer by integrating multiple transmissions for joint equalization of multiple systematic codewords, leading to a computationally efficient joint receiver with improved performance.

Symbol mapping diversity in iterative decoding/demodulation of ARQ systems

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).

Sphere decoding for retransmission diversity in MIMO 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.