Momin Uppal

Compress-forward coding with BPSK modulation for the half-duplex Gaussian relay channel

This paper studies compress-forward (CF) coding with BPSK modulation for the half-duplex Gaussian relay channel. In CF relaying, Wyner-Ziv coding is applied at the relay to exploit the joint statistics between signals at the relay and the destination. We propose Slepian-Wolf coded nested scalar quantization (SWCNSQ) for practical Wyner-Ziv coding at the relay. We first provide the achievable rate of SWCNSQ based CF relaying as a performance benchmark, and then present a practical code design using low-density parity-check (LDPC) codes for error protection at the source, and nested scalar quantization plus irregular-repeat accumulation (IRA) codes for CF coding at the relay. The degree distributions of the LDPC and IRA codes are optimized using extrinsic information transfer charts and Gaussian approximation. Under discretized density evolution for asymptotically large block lengths, our optimized code design operates 0.11-0.21 dB away from the SWCNSQ limit for CF relaying. Simulations with LDPC/IRA codes of length 2 times 105 bits show a performance gap of 0.27-0.38 dB from the achievable rate.

Compress-Forward Coding With BPSK Modulation for the Half-Duplex Gaussian Relay Channel

This paper studies compress-forward (CF) coding with BPSK modulation for the half-duplex Gaussian relay channel. In CF relaying, Wyner-Ziv coding is applied at the relay to exploit the joint statistics between signals at the relay and the destination. We propose Slepian-Wolf coded nested scalar quantization (SWCNSQ) for practical Wyner-Ziv coding at the relay. We first provide the achievable rate of SWCNSQ based CF relaying as a performance benchmark, and then present a practical code design using low-density parity-check (LDPC) codes for error protection at the source, and nested scalar quantization plus irregular-repeat accumulation (IRA) codes for CF coding at the relay. The degree distributions of the LDPC and IRA codes are optimized using extrinsic information transfer charts and Gaussian approximation. Under discretized density evolution for asymptotically large block lengths, our optimized code design operates 0.11-0.21 dB away from the SWCNSQ limit for CF relaying. Simulations with LDPC/IRA codes of length 2 times 105 bits show a performance gap of 0.27-0.38 dB from the achievable rate.

FPGA based implementation of decoder for array low-density parity-check codes

Low density parity check (LDPC) codes have received much attention for their excellent performance, and the inherent parallelism involved in decoding them. We consider a type of structured binary LDPC codes, known as array LDPC codes, which have low encoding complexity and good performance, for implementation on a Xilinx field programmable gate array (FPGA) device.

Code design for MIMO broadcast channels

Recent information-theoretic results show the optimality of dirty-paper coding (DPC) in achieving the full capacity region of the Gaussian multiple-input multiple-output (MIMO) broadcast channel (BC). This paper presents a DPC based code design for BCs. We consider the case in which there is an individual rate/signal-to-interference-plus-noise ratio (SINR) constraint for each user. For a fixed transmitter power, we choose the linear transmit precoding matrix such that the SINRs at users are uniformly maximized, thus ensuring the best bit-error rate performance. We start with Covers simplest two-user Gaussian BC and present a coding scheme that operates 1.44 dB from the boundary of the capacity region at the rate of one bit per real sample (b/s) for each user. We then extend the coding strategy to a two-user MIMO Gaussian BC with two transmit antennas at the base-station and develop the first limit-approaching code design using nested turbo codes for DPC. At the rate of 1 b/s for each user, our design operates 1.48 dB from the capacity region boundary. We also consider the performance of our scheme over a slow fading BC. For two transmit antennas, simulation results indicate a performance loss of only 1.4 dB, 1.64 dB and 1.99 dB from the theoretical limit in terms of the total transmission power for the two, three and four user case, respectively.

Cooperation in the Low Power Regime for the MAC Using Multiplexed Rateless Codes

In this paper, we consider cooperation in the low power (low SNR) regime for the multiple access channel with the assumption that the transmitters have no channel state information. A relevant performance measure to consider is therefore the outage capacity. We develop cooperation methods based on multiplexed coding in conjunction with rateless codes and find the achievable rates and in particular the minimum energy per bit required to achieve a certain outage probability. We consider two modes of operation: full duplex [code-division multiple access (CDMA)], where nodes can transmit and receive simultaneously on the same frequency band, and half duplex [frequency-division multiple access (FDMA)], where the nodes transmit and listen on different frequency bands. We show that, perhaps surprisingly, there is little loss in performance when using FDMA. Furthermore, our results show that multiplexed rateless codes come within 0.1 dB of the outer bound on capacity. We also develop practical rateless coding methods for FDMA using multiplexed Raptor codes which operate within 0.52 and 1.1 dB of the theoretical limit for the two- and four-user case, respectively.

Practical rateless cooperation in multiple access channels using multiplexed Raptor codes

In this paper we develop practical rateless coded cooperation strategies for a two-user multiple access channel. At the heart of our practical strategies lie Raptor codes concatenated with a space-time code, and an iterative decoding procedure to jointly recover the two users messages at the base station. Since cooperation in multiple access channels is known to be particularly beneficial when considering the outage probability at low SNRs, or equivalently at low transmission rates, we simulate our strategies at a transmission rate of 0.25 bits/sample. Experiments indicate that our schemes perform very close to the theoretical limit, with the performance gap being less than 0.55 dB.

An FPGA Implementation of Dirty Paper Precoder

Dirty paper code (DPC) can be used in a number of communication network applications; broadcast channels, multiuser interference channels and ISI channels to name a few. We study various implementation bottlenecks and issues with implementing a DPC pre-coder based on nested trellis technique. The aim is to achieve a practical hardware realization of the precoder for wireless LAN/DSL applications. We describe the architectural development process and realization of the precoder on a Xilinx Virtex 2V8000 FPGA. To the best of our knowledge this is the first reported DPC pre-coder hardware implementation.

A rateless coded protocol for half-duplex wireless relay channels

We propose a rateless coded protocol for a half-duplex wireless relay channel where all links experience independent quasi-static Rayleigh fading and the instantaneous channel realizations are unavailable at the transmitters. We assume that the network does not have a stringent delay constraint-thus the source and/or the relay continue transmitting until the destination acknowledges successful decoding. We identify rateless coded relaying as the natural choice, where each transmission from the source and/or the relay adds incremental redundancy to help the destination recover the original message. Our proposed protocol utilizes, in conjunction with rateless coding, a combination of the two popular relay cooperation schemes, namely decode-forward and compress-forward. Assuming very limited feedback from the destination, we derive the theoretical performance bounds specifically with binary phase-shift keying. We then implement the rateless coded relaying protocol using Raptor codes. The degree profiles for the Raptor codes are designed to maximize the average throughput-with the design formulated as a convex optimization problem. Using discretized density evolution for asymptotically large block lengths, the optimized codes lose approximately 5% in performance from the theoretical limit, whereas with practical finite block lengths, the performance loss is approximately 9%.

Code Designs for MIMO Broadcast Channels

Recent information-theoretical results show the optimality of dirty-paper coding (DPC) in achieving the capacity of the Gaussian multiple-input multiple-output (MIMO) broadcast channel (BC). This paper presents the first practical limit approaching DPC-based design for the MIMO BC. We start with Covers simplest two-user Gaussian BC and present a code design that operates 1.44 dB away from the capacity region boundary at a transmission rate of 1.0 bit per sample (b/s). Then we consider the non-degraded two-user MIMO fading BC with two transmit antennas. For this setup, the performance loss of our code design is 3.7 dB and 2.45 dB from the sum-rate capacity when the transmission rate for each user is 1.0 b/s and 2.0 b/s, respectively

A Dirty-Paper Coding Scheme for the Cognitive Radio Channel

We implement a dirty-paper coded framework for the cognitive radio channel. We assume that the cognitive user has non-causal knowledge about the primary users transmissions. Thus the secondary receiver can employ dirty-paper coding to counter the effect of any interference from the primary user. In addition, we consider a situation where the introduction of the cognitive user should not affect the performance of the primary system -- nor should the primary system have to change its encoding/decoding process. For the primary user we use a low-density parity-check code and a 4-ary pulse amplitude modulation format. For the cognitive user, we propose a dirty-paper coding scheme which employs trellis-coded quantization as the source code and an irregular repeat-accumulate code as the channel code. At a transmission rate of 1.0 bits/sample, the designed dirty-paper coding scheme operates within 1.23 dB of the theoretical limit.