Xiaofu Wu

Parallel Weighted Bit-Flipping Decoding

A parallel weighted bit-flipping (PWBF) decoding algorithm for low-density parity-check (LDPC) codes is proposed. Compared to the best known serial weighted bit-flipping decoding, the PWBF decoding converges significantly faster but with little performance penalty. For decoding of finite-geometry LDPC codes, we demonstrate through examples that the proposed PWBF decoding converges in about 5 iterations with performance very close to that of the standard belief-propagation decoding.

Polar lattices: Where Arıkan meets Forney

In this paper, we propose the explicit construction of a new class of lattices based on polar codes, which are provably good for the additive white Gaussian noise (AWGN) channel. We follow the multilevel construction of Forney et al. (i.e., Construction D), where the code on each level is a capacity-achieving polar code for that level. The proposed polar lattices are efficiently decodable by using multistage decoding. Performance bounds are derived to measure the gap to the generalized capacity at given error probability. A design example is presented to demonstrate the performance of polar lattices.

New insights into weighted bit-flipping decoding

A natural relationship between weighted bit-flipping (WBF) decoding and belief-propagation-like (BP-like) decoding is explored. This understanding can help us develop WBF algorithms from BP-like algorithms. For min-sum decoding, one can find that its WBF algorithm is the algorithm proposed by Jiang et al. For BP decoding, we propose a new WBF algorithm and show its performance advantage. The proposed WBF algorithms are parallelized to achieve rapid convergence. Two efficient simulation-based procedures are proposed for the optimization of the associated thresholds.

Joint LDPC and Physical-Layer Network Coding for Asynchronous Bi-Directional Relaying

For practical bi-directional relaying, symbols transmitted by two sources cannot arrive at the relay with perfect symbol and frame alignments and the asynchronous multiple-access channel (MAC) should be seriously considered. In this paper, we consider the Low-Density Parity-Check (LDPC)-coded BPSK signalling over the general asynchronous MAC with both frame and symbol misalignments. For the symbol-asynchronous MAC, we present a formal log-domain generalized sum-product-algorithm (Log-G-SPA) for efficient decoding. When the frame-asynchronism is encountered at the relay, we propose an original approach by employing the cyclic LDPC codes and the simple cyclic-redundancy-check (CRC) coding technique. Simulation results demonstrate the effectiveness of the proposed approach.

Adaptive-Normalized/Offset Min-Sum Algorithm

An adaptive-normalized/offset min-sum (AN-/AO-MS) algorithm for decoding low-density parity-check (LDPC) codes is proposed. Unlike the normalized/offset min-sum (NMS/OMS) algorithm, the normalization/offset factor is adaptively adjusted according to the state of check nodes in each iteration. Simulation results show that the proposed AN-/AO-MS algorithm can perform better than the NMS/OMS algorithm while still preserving the low complexity of the min-sum algorithm.

Iterative carrier phase recovery methods in turbo receivers

This letter considers carrier phase recovery in turbo receivers. Firstly, a novel maximum-likelihood (ML) based iterative carrier phase estimator is proposed for coded linear modulations over static phase channels. The proposed carrier phase estimator, combined with the iterative sum-product decoder, performs very close to, the Cramer-Rao bound. Moreover, for practical dynamic phase channels, an efficient carrier phase recovery method is proposed to perform very close to that of a perfect coherent receiver.

Shortening for irregular QC-LDPC codes

Shortening is a technique to obtain codes of shorter length and lower rate from a given LDPC code by putting infinite reliability on some variable nodes, whose positions are assumed to be available to both encoder and decoder. In this paper, we propose a shortening algorithm suitable for irregular QC-LDPC codes. The efficiency of the proposed algorithm is verified by both theoretical analysis and simulation.

A necessary and sufficient condition for determining the girth of quasi-cyclic LDPC codes

The parity-check matrix of a quasi-cyclic low- density parity-check (QC-LDPC) code can be compactly represented by a polynomial parity-check matrix. By using this compact representation, we derive a necessary and sufficient condition for determining the girth of QC-LDPC codes in a systematic way. The new condition avoids an explicit enumeration of cycles for determining the girth of codes, and thus can be well employed to generate QC-LDPC codes with large girth.

New Gallager Bounds in Block-Fading Channels

In this paper, we propose a new upper bound on the error performance of binary linear codes over block-fading channels by employing Gallagers first- and second-bounding techniques. As the proposed bound is numerically intensive in its general form, we consider two special cases, namely, the spherical bound and the DS2-exponential bound, which are found to be tight in nonergodic and near-ergodic block-fading channels, respectively. The tightness of the proposed bounds is demonstrated for turbo codes. Many existing bounds for quasistatic or fully interleaved fading channels can be viewed as special cases of the proposed Gallager bound

Physical-Layer Authentication for Multi-Carrier Transmission

We propose a novel physical-layer challenge-response authentication scheme for multi-carrier transmission. This new scheme exploits both the reciprocity and randomness of the phase responses over multi-carrier channels, and ensures strong security since the channel-phase response is very sensitive to the distance between the transmitter and receiver. A binary hypothesis testing approach is formulated for authentication, which shows a close connection to the problem of pseudo-noise (PN) code acquisition. The proposed scheme can achieve desirable authentication performance even at the signal-to-noise ratio of 5 dB for Rayleigh fading channels.