Raghu Mysore Rao

Iterative hard-decision decoding of braided BCH codes for high-speed optical communication

Designing error-correcting codes for optical communication is challenging mainly because of the high data rates (e.g., 100 Gbps) required and the expectation of low latency, low overhead (e.g., 7% redundancy), and large coding gain (e.g., >9dB). Although soft-decision decoding (SDD) of low-density parity-check (LDPC) codes is an active area of research, the mainstay of optical transport systems is still the iterative hard-decision decoding (HDD) of generalized product codes with algebraic syndrome decoding of the component codes. This is because iterative HDD allows many simplifications and SDD of LDPC codes results in much higher implementation complexity. In this paper, we use analysis and simulation to evaluate tightly-braided block codes with BCH component codes for high-speed optical communication. Simulation of the iterative HDD shows that these codes are competitive with the best schemes based on HDD. Finally, we suggest a specific design that is compatible with the G.709 framing structure and exhibits a coding gain of >9.35 dB at 7% redundancy under iterative HDD with a latency of approximately 1 million bits.

Analog Impairments in MIMO-OFDM Systems

MIMO-OFDM is being considered for communication systems where high throughput and spectral efficiency are important factors. Analog impairments like I/Q mismatch and phase noise significantly degrade and limit the performance of communication systems. In this paper, we analyze the impact of I/Q mismatch and phase noise in MIMO-OFDM systems as a function of the number of antennas. We show the improvement in performance that is possible when these impairments are cancelled. We also discuss the impact of correlated and uncorrelated phase noise in MIMO-OFDM systems. Finally, we show the results of I/Q mismatch and phase noise cancellation in wireless measurements performed using a 2times2 MIMO-OFDM testbed

Novel Sort-Free Detector with Modified Real-Valued Decomposition (M-RVD) Ordering in MIMO Systems

K-best MIMO detection technique is the prominent method of simplifying the detection complexity in MIMO systems while maintaining BER performance comparable with the optimum maximum-likelihood (ML) detection technique. However, sorting the candidate nodes in the tree search of the conventional K-best detection can take a significant number of cycles which would reduce the achievable data rate of the detector. In order to reduce this delay, and keep high performance at the same time, we propose using a novel sort-free based MIMO detector which avoids the demanding sorting step. Moreover, this detector utilizes a novel modified real-valued decomposition (M-RVD) ordering that, when compared to the conventional real valued decomposition scheme, can improve the BER performance at no extra computational cost. We show that our proposed detector can outperform the conventional K-best detector with a smaller combination of computation and latency requirements.

Beamforming in MISO Systems: Empirical Results and EVM-Based Analysis

We show that efficient implementation of codebook-based beamforming Multiple Input Single Output (MISO) systems with good performance is feasible in the presence of channel-induced imperfections (due to imperfect channel estimate and feedback delay) and implementation-induced imperfections (due to real-world radio hardware effects). To present our results, we adopt a mixed approach of analytical, simulation, and experimental evaluation. Our analytical and simulation results take into account channel-induced imperfections but do not take into account implementation-induced imperfections (which are difficult to model in a tractable way). Thus, we complement these results with experimental results that do take into account both channel and implementation-induced imperfections. This mixed approach provides a more complete picture of expected performance. As part of our study we develop a framework for Average Error Vector Magnitude Squared (AEVMS)-based analysis of beamforming MISO systems which facilitates comparison of analytical, simulation, and experimental results on the same scale. In addition, AEVMS allows fair comparison of experimental results obtained from different wireless testbeds. We derive novel expressions for the AEVMS of beamforming MISO systems and show how the AEVMS relates to important system characteristics like the diversity gain, coding gain, and error floor.

Design and architecture of spatial multiplexing MIMO decoders for FPGAs

Spatial multiplexing multiple-input-multiple-output (MIMO) communication systems have recently drawn significant attention as a means to achieve tremendous gains in wireless system capacity and link reliability. The optimal hard decision detection for MIMO wireless systems is the maximum likelihood (ML) detector. ML detection is attractive due to its superior performance (in terms of BER). However, direct implementation grows exponentially with the number of antennas and the modulation scheme, making its ASIC or FPGA implementation infeasible for all but low-density modulation schemes using a small number of antennas. Sphere decoding (SD) solves the ML detection problem in a computationally efficient manner. However, even with this complexity reduction, real-time implementation on a DSP processor is generally not feasible and high-performance parallel computing platforms such as FPGAs are increasingly being employed for this class of applications. The sphere detection problem affords many opportunities for algorithm and micro-architecture optimizations and tradeoffs. This paper provides an overview of techniques to simplify and minimize FPGA resource utilization of sphere detectors for high-performance low-latency systems.

Demonstration of highly programmable downlink OFDMA (WiMax) transceivers for SDR systems

In this paper, we present the architecture of a highly configurable multi-input multi-output (MIMO) orthogonal frequency division multiple access (OFDMA) platform. The platform is designed to support experimentation with various communication algorithms, thus allowing an intimate understanding of the performance of complex algorithms under real-life constraints. The hardware used is the wireless open access research platform (WARP) which facilitates rapid prototyping utilizing the FPGA and multi radio interfaces available.

A vector mapping scheme for efficient implementation of beamforming MIMO systems

Quantized feedback in multiple antenna systems has the potential to increase throughput and/or reduce probability of outage. However, current beamforming codebook designs lead to large implementation complexity of the channel quantizer. We propose a vector mapping scheme to construct beamforming codebooks from existing codebooks. We show that beamforming codebooks constructed via the proposed vector mapping have a structure that allows an efficient implementation of the channel quantizer. We show that the proposed vector mapping can also be used to simplify the implementation of the channel quantizer in a WiMAX compliant system, where the codebooks have already been defined. Simulation results validate the proposed codebook construction method.

A High Throughput Beamforming Architecture for MIMO Systems

Quantized feedback in multiple antenna systems has potential to increase throughput or reduce probability of outage. In this paper, we present a method to generate quantization codebooks tailored for efficient implementation of beamforming- based MIMO transmissions. The proposed codebooks can reduce computational requirements significantly, making feasible an efficient architecture for a real-time transmit beamforming and receive combining MIMO orthogonal frequency division multiplexing (OFDM) system. Simulation results are used to validate the proposed codebook construction method and architecture.

FPGA in Wireless Communications Applications

In this chapter, we discuss the architectural challenges associated with spatial multiplexing and diversity gain schemes. We introduce FPGA architectures and report experiment results for the FPGA realization of these systems. We introduce a flexible architecture and implementation of a spatial multiplexing MIMO detector, Flex-sphere, and its FPGA implementation. We also present a hardware architecture for beamforming in a WiMAX system as a way to enhance the diversity and performance in next-generation wireless systems. Finally, we introduce the Rice University Wireless Open Access Research Platform (WARP), an FPGA-centric scalable and programmable research platform, and outline how it is used for studying the effects of MIMO systems in real-world scenarios.