P. Sweeney

Modified sum-product algorithms for decoding low-density parity-check codes

The authors deal with the sum-product algorithm (SPA) based on the hyperbolic tangent (tanh) rule when it is applied for decoding low-density parity-check (LDPC) codes. Motivated by the finding that, because of the large number of multiplications required by the algorithm, an overflow in the decoder may occur, two novel modifications of the tanh function (and its inverse) are proposed. By means of computer simulations, both methods are evaluated using random-based LDPC codes with binary phase shift keying (BPSK) signals transmitted over the additive white Gaussian noise (AWGN) channel. It is shown that the proposed modifications improve the bit error rate (BER) performance up to 1 dB with respect to the conventional SPA. These results have also shown that the error floor is removed at BER lower than 10 -6 . Furthermore, two novel approximations are presented to reduce the computational complexity of the tanh function (and its inverse), based on either a piecewise linear function or a quantisation table. It is shown that the proposed approximations can slightly improve the BER performance (up to 0.13 dB) in the former case, whereas small BER performance degradation is observed (<0.25 dB) in the latter case. In both cases, however, the decoding complexity is reduced significantly

Motion adaptive error resilient encoding for MPEG-4

Optimising delivery of video codecs such as MPEG-4 is vital to ensure that acceptable quality can be offered to 3G network customers. The error prone nature of mobile channels means that the video codec that is employed must be fairly robust. In the past, such research has required alteration to existing standards. Given the imminent implementation of 3G technologies, changing the standards is not a convenient option. This paper presents a technique for altering MPEG-4 encoding parameters to increase the error robustness of the output bitstream. It exploits the fact that frames with a high degree motion are often more sensitive to error than those with low amounts of motion. Using a simple model, the video packet length and number of adaptive intra refresh (AIR) blocks are varied according to the amount of motion in a frame. Simulations using a GPRS channel are presented to confirm the benefits of the proposed scheme.

Performance Evaluation of A Modified Sum-Product Decoding Algorithm for LDPC Codes

We propose a modified hyperbolic tangent (tanh) function that is used in the sum-product algorithm (SPA). This is done in order to approximate the infinite argument of the tanh function, when decoding low-density parity-check (LDPC) codes. As shown by computer simulation results, the proposed modification has a significant impact on the error floor reduction of the code and to the coding gain improvement with respect to the SPA with no modification. Computer simulation results with approximations using both piecewise linear function and coarse quantization are also presented

A new method of improving SOVA turbo decoding for AWGN, Rayleigh and Rician fading channels

The iterative soft output Viterbi algorithm (SOVA) is a sub-optimum algorithm when it is used to decode turbo codes. By normalizing its extrinsic information we get a performance improvement compared to the standard SOVA. In particular, if the extrinsic information is increased in the last decoding iteration, an additional coding gain improvement is noticed. For example, this is 0.25 dB for a frame length of 1000 bits in the additive white Gaussian noise (AWGN) channel as well as in an uncorrelated Rician fading channel at bit error rate (BER) of 10/sup -6/. Also, this normalized SOVA is only about 0.25 dB worse than a turbo decoder using the log-MAP algorithm, both in the AWGN channel and in an uncorrelated Rayleigh fading channel at BER of around 10/sup -6/. Furthermore, with an 8-state component code, a frame length of 500 bits performs 0.125 dB better than a 16-state bidirectional (bi)-SOVA turbo decoder at BER of 10/sup -5/ in the AWGN channel.

Sequential Decoding for a Subcode of Reed Solomon Codes

This paper describes the performance of sequential decoding as applied to a subcode of Reed Solomon (RS) codes. As an efficient soft decision decoding technique sequential decoding using a modified Fano algorithm has been attempted. An encoding technique for generating subcode is designed to enhance the performance and the computational efficiency of the decoder. The performance has been estimated on an AWGN channel. Sequential decoding performance of the subcode of (15,11) RS code could produce a comparable performance as well as computational efficiency to Viterbi decoding of rate 1/2 convolutional code with constraint length 7.

Turbo Coding Performance Evaluation Using an Improved Iterative SOVA Decoder

In this paper two normalization methods are considered to improve the performance of the iterative soft output Viterbi algorithm (SOVA). This is mainly because of the nature of the SOVA, which is a sub-optimum algorithm when it is used to decode turbo codes. In addition to a well-known normalization scheme of the extrinsic information that has been used to improve the Max-Log-MAP turbo decoder, we have found that if the extrinsic information is increased in the last decoding iteration, a coding gain improvement is noticed. For example, this is 0.25 dB at bit error rate (BER) of 6 10 - for an 8-state turbo encoder of 1000 bits frame, with or without puncturing and in the presence of the additive white Gaussian noise (AWGN) channel or an uncorrelated (fully interleaved) Rayleigh/Rician fading channel. In addition, the superior performance of the proposed normalization scheme is verified by observing the turbo decoder convergence when using the extrinsic information transfer (EXIT) charts.

Error statistics of optimal and sub-optimal space-time trellis codes: concatenation requirements

The bit error rate and frame error rate performance of space-time trellis codes are analysed. The analysis shows that for increasing number of states in the trellis codes, the FER performance improves; however, the BER performance does not always do so. By considering error event paths in trellises of different complexities, it is shown that this effect is owed to the increased recovery time from an erroneous decision needed by the trellis code of higher complexity. The results from this study also indicate certain design criteria when space-time trellis codes are concatenated with channel codes.

Low Complexity Soft-Decision Sequential Decoding Using Hybrid Permutation for Reed-Solomon Codes

We present a soft-decision decoding method for Reed-Solomon codes (RS codes) using both cyclic and squaring permutations. These permutations are used to provide a convenient sequence which is predicted to have relatively low complex error pattern with respect to a modified Fano sequential algorithm[1]. In order to preserve bit-level soft-decision values, each sequence of those permutation groups must keep equal weight distribution in symbol and bit level. Trellis construction is based on Wolfs method[2] and a binary systematic parity check matrix of RS codes is used for bit-level decoding[9]. In simulation results, it is shown that a hybrid of those two permutations can be used for low complexity decoding approaching maximum likelihood performance.

Robust Reed Solomon Coded MPSK Modulation

In this paper, construction of partitioned Reed Solomon coded modulation (RSCM) which is robust for the additive white Gaussian noise channel and a Rayleigh fading channel is investigated. By matching configuration of component codes with the channel characteristics, it is shown that this system is robust for the Gaussian and a Rayleigh fading channel.

Definition of Protocols for Secure Anonymous Purchase

Digital watermarking is the art of hiding information in digital contents such as images, audio clips, and videos. We can take advantage of this technique by adding author information as well as buying information for a digital purchase. In this paper we propose a protocol for secure anonymous digital purchase to deter buyers from executing illegal redistribution of digital contents. We propose a protocol to allow merchants to obtain a proof of treachery from dishonest buyers