Erasure decoding in burst-error channels

Burst-error channels have been used to model a large class of modern communication media, and the problem of communicating reliably through such media has received much study [1]-[9]. Existing techniques include two-way communication schemes that involve error detection and retransmission, and schemes that utilize error correcting codes in code interleaving. The error-detection and retransmission scheme is simple, but its applicability has been restricted to limited environments. On the other hand, the concept of code interleaving has proved to be versatile and effective. Code interleaving distributes the error detection and correction burden among the component codes and thus lowers the overall redundancy requirement. However, the memory characteristics of the burst-error channel have not been used. This omission has prompted the investigation presented in this paper to utilize the inherent information embedded in the code interleaving scheme when used with burst-error channels. The concept of erasure decoding is introduced, leading to some useful coding and decoding strategies. Theoretical formulations are devised to predict code performance, and their validity is verified with computer simulations.     

Performance Analysis of Tandem Burst-Error Links with Applications to ISDN

In this paper, tandem links with burst errors are analyzed using the Gilbert model for individual links. The results, apart from being of general interest, are of particular relevance to the Integrated Services Digital Networks (ISDN) for which the International Telegraph and Telephone Consultative Committee (CCITT) is currently developing performance specifications based on hypothetical reference connections (HRX's), i.e., composite channels consisting of several tandem reference circuits. The average error rate, the error gap distribution, its mean, and a tight upper bound on its standard deviation are analytically derived for the composite channel (equivalent to an HRX) withNtandem links (circuits). These statistics are then related to the performance measures proposed for ISDN. Some numerical results are presented to highlight the large differences in the estimated performance with a burst-error model approach and with an independent-error approach (having identical average error probabilities).     

A New Class of Generative Models for Burst-Error Characterization in Digital Wireless Channels

Accurate and efficient generative models are significant for the design and performance evaluation of wireless communication protocols as well as error-control schemes. In this paper, deterministic processes are used to derive a new class of hard and soft generative models for simulation of digital wireless channels with hard and soft decision outputs, respectively. The proposed deterministic-process-based generative models (DPBGMs) are all based on a properly parameterized and sampled deterministic process followed by a threshold detector and two parallel mappers. The target hard and soft error sequences are provided by computer simulations of uncoded enhanced general packet radio service (EGPRS) systems with typical urban and rural area channels. Simulation results indicate that the proposed DPBGMs enable us to approximate very closely all the interested burst-error statistics of the target hard and soft error sequences. The validity of the suggested DPBGMs is further confirmed by the excellent match of the simulated frame-error rates and residual bit-error rates of coded EGPRS systems obtained from the target and generated error sequences     

Reliability and Throughput Analysis of a Concatenated Coding Scheme

In this paper, the performance of a concatenated coding scheme for error control in ARQ systems is analyzed for both randomerror and burst-error channels. In particular, the probability of undetected error and the system throughput are calculated. In this scheme, the inner code is used for both error correction and error detection, and the outer code is used for error detection only. Interleaving/deinterleaving of the outer code is assumed. A retransmission is requested if either the inner code or the outer code detects the Presence of errors. Various coding examples are considered. The results show that concatenated coding can provide extremely high system reliability (i.e., low probability of undetected error) and high system throughput.     

交织编码在遥测图像传输中的应用

介绍了在数据通信中用于抗突发连串错码而常常采用的交织编码技术,并针对遥测图像传输中所使用的差错控制编码———奇偶监督矩阵,提出了一种新的交织编码技术,采用这种交织编码技术,可有效克服普通交织编码时延大的缺点,并使纠错编码在一个奇偶监督矩阵中可以抵抗15位突发错误,使遥测图像传输的抗误码能力大大提高。     

Rate control for robust video transmission over burst-errorwireless channels

We study the problem of rate control for transmission of video over burst-error wireless channels, i.e., channels such that errors tend to occur in clusters during fading periods. In particular we consider a scenario consisting of packet based transmission with automatic repeat request (ARQ) error control and a back channel. We start by showing how the delay constraints in real time video transmission can be translated into rate constraints at the encoder, where the applicable rate constraints at a given time depend on future channel rates. With the acknowledgments received through the back channel we have an estimate of the current channel state. This information, combined with an a priori model of the channel, allows us to statistically model the future channel rates. Thus the rate constraints at the encoder can be expressed in terms of the expected channel behavior. We can then formalize a rate distortion optimization problem, namely, that of assigning quantizers to each of the video blocks stored in the encoder buffer such that the quality of the received video is maximized. This requires that the rate constraints be included in the optimization, since violating a rate constraint is equivalent to violating a delay constraint and thus results in losing a video block. We formalize two possible approaches. The first one seeks to minimize the distortion for the expected rate constraints given the channel model and current observation. The second approach seeks to allocate bits so as to minimize the expected distortion for the given model. We use both dynamic programming and Lagrangian optimization approaches to solve these problems. Our simulation results demonstrate that both the video distortion at the decoder and packet loss rate can be significantly reduced when incorporating the channel information provided by the feedback channel and the a priori model into the rate control algorithm     

Convolutionally interleaved PSK and DPSK trellis codes forshadowed, fast fading mobile satellite communication channels

Using a model from the literature, the performance of convolutionally interleaved phase-shift-keying (PSK) and differential phase-shift-keying (DPSK) trellis codes for digital speech transmission over shadowed mobile satellite communication channels is determined by computer simulation. First the characteristics of fading channels are examined and analyzed in terms of the probability distributions of amplitude, phase, and burst errors. A statistical method, using a histogram approach, is utilized along with the simulations of fading channels to generate these probability distributions. A test for channel burst error behavior is presented. A periodic convolutional interleaver/deinterleaver to be used with trellis coding to combat slow fading in digital, shadowed mobile satellite channels is designed. This interleaver ha less than half the time delay for the same bit error performance than a block interleaver. The results show that the periodic convolutional interleaver provides considerable improvement in the error and time delay performance of mobile satellite communication channels for up to average shadowing conditions as compared to other techniques     

A method to analyze performance of digital connections

A comprehensive performance analysis method that models, at bit level, the error performance of individual links in an end-to-end connection is presented. The link model accounts for the burst-error behaviour of each individual link. A method to concatenate several individual links and extract a model for the end-to-end connection is given. This resulting end-to-end model can be used to calculate performance measures such as bit error rate and block error rate for any given block size. A procedure to compute the probability distribution of errors within a specific block is also developed. Finally, a method to compute the probability distribution of blocks having a certain error rate over a given period of time is presented. The utility and power of the model are illustrated with the help of an example connection     

STBC结合波束成型的方案在相关MIMO信道中的性能研究

本文用指数相关模型对空间衰落相关的多输入多输出信道进行了建模,并基于此模型研究了A lam outi空时分组码结合波束成型的方案的比特错误概率,为便于比较,也研究了传统A lam outi空时分组码的比特错误概率,通过仿真得到了两种方案在不同相关信道中的误比特率性能。     

A rate adaptation transcoding scheme for real-time video transmission over wireless channels

In this work, we investigate the problem of bit-rate adaptation transcoding for transmitting video over burst-error wireless channels, i.e., channels such that errors tend to occur in clusters during fading periods. In particular, we consider a scenario consisting of packet-based transmission with an Automatic Repeat reQuest (ARQ) error control and a feedback channel. With the acknowledgements received through the feedback channel and a statistical channel model, we have an estimate of the current channel state, and effective channel bandwidth. In this paper, we analyze the buffering implications of inserting a video transcoder at the wireless access point with the variable bit-rate channel as the target. We derive the conditions that both the source encoder and transcoder buffers have to meet for preventing the end decoder buffer from underflowing. Furthermore, we propose a bit-rate adaptation algorithm for VBR transcoders used in the wireless access point. Our experimental results demonstrate that the proposed algorithm can accurately control the bit-rate of the transcoded video stream and reduce the number of frames been skipped without violating the end-to-end delay requirement.     

Analysis of LMS-adaptive MLSE equalization on multipath fadingchannels

We consider a practical maximum-likelihood sequence estimation (MLSE) equalizer on multipath fading channels in conjunction with an adaptive channel estimator consisting of a least mean square (LMS) estimator and a linear channel predictor, instead of assuming perfect channel estimates. A new LMS estimator model is proposed which can accurately characterize the statistical behavior of the LMS estimator over multipath fading channels. Based on this model, a new upper-bound on block error rate is derived under the consideration of imperfect channel estimates. Computer simulations verify that our analytical results can correctly predict the real system performance and are applicable over a wide range of the step size parameter of the LMS estimator     

Allocating data for broadcasting over wireless channels subject to transmission errors

Broadcasting is an efficient and scalable way of transmitting data over wireless channels to an unlimited number of clients. In this paper the problem of allocating data to multiple channels is studied, assuming flat data scheduling per channel and the presence of unrecoverable channel transmission errors. The objective is that of minimizing the average expected delay experienced by the clients. Two different channel error models are considered: the Bernoulli model and the simplified Gilbert–Elliot one. In the former model, each packet transmission has the same probability to fail and each transmission error is independent from the others. In the latter one, bursts of erroneous or error-free packet transmissions due to wireless fading channels are modeled. Particular cases are detected where optimal solutions can be found in polynomial time. For general cases, simulations show that good sub-optimal solutions can be found on benchmarks whose item popularities follow Zipf distributions.     

Stochastic context-free grammars and hidden Markov models for modeling of bursty channels

In order to design good error-control schemes for bursty channels and also to facilitate performance analysis, it is important to develop accurate and simple statistical channel error models for the channels of interest. We propose two novel generative methods to model the end-to-end error profile of radio channels described by long well-defined error bursts interleaved with long error-free intervals. The first method makes use of the power of stochastic context-free grammars to model palindromes. The second utilizes simple hidden Markov models with specific structures, which are suggested by the ideas presented in the first method. Both methods achieve much better performance than previously proposed approaches without introducing more complexity. Although the complexity of the second method is slightly greater than that of the first, its advantage is that it can be easily applied in decoding implementations specifically tailored to deal with bursty channels.     

Decision-Directed Diversity Combiners--Principles and Simulation Results

Diversity combining methods for medium- and high-capacity digital microwave radio links employing QAM signaling are studied theoretically and by computer simulations. The selective fading diversity channels are represented by a two-path model with complex envelopes. Emphasis is put to find simple receiver structures where combining is controlled by detected symbols to minimize a given error criterion. Two criteria are discussed-minimum mean-square error (MMSE) and minimum projection (MP). The latter criterion is believed to be new and its behavior is analyzed in two extreme cases. It turns out that the MP combiners obey the maximal ratio rule in the absence of dispersion and the minimum distortion rule in the absence of noise. This versatility is achieved with minor circuit complexity. The results have been verified by computer simulations for a 4 PSK 70 Mbit/s system. The resulting diversity signatures are very narrow on the frequency axis, indicating good performance. Moreover, the MP combining tolerates extremely well nonminimum phase fading in one diversity branch, situations where most cophasing schemes have difficulties.     

An analysis of nonlinear direct-sequence correlators

An analysis of the performance of nonlinear correlation reception of direct-sequence signals in single- and multiuser channels is presented. The communications channel is modeled as containing non-Gaussian background noise and, in some cases, multiple-access interference as well. The error-probability behavior is studied asymptotically as the lengths of the spreading codes increase without bound, and conditions on the spreading sequences are obtained that assure asymptotic achievement of single-user performance in a multiuser system. A long-spreading sequence approximation to the average error probability is also derived, and this result is applied to the analysis of smooth-limiting correlation receivers in impulsive channels. Simulation results are also provided to verify the analysis. Average bit-error probabilities are computed by Monte Carlo simulations for linear, hard-limiting, and smooth-limiting correlation receivers in both single- and two-user impulsive channels. The simulation results are compared to the error rates by asymptotic approximations for the smooth limiter and also to those from previous studies on linear and hard-limiting correlators     

Sequence detection and channel state estimation over finite stateMarkov channels

A useful model for general time-varying channels is a finite state Markov chain. In this paper, maximum likelihood sequence estimation (MLSE) for signals over finite state Markov channels (FSMCs) is studied. Also studied is the maximum a posteriori (MAP) channel state estimation. When coded signals with interleaving are transmitted, the channel estimates can be used to make soft-decision decoding. The error performance of the proposed sequence and channel state estimation schemes are evaluated through computer simulations. The effect of channel modeling error is also discussed     

Experimental Study on 42.7-Gb/s Forward-Error-Correction Performance Under Burst Errors

We describe a novel method to experimentally investigate the performances of forward-error-correction (FEC) codes in the presence of burst errors with various burst length and duty cycle. Using this method, we observe that the coding gains of two commercial 42.7-Gb/s FEC codes are greatly reduced by long and dense error bursts. A correlation between the FEC performance and the FEC burst-error correction length is clearly shown. In addition, the measurement also shows an increase in coding gain in cases with certain short error bursts as compared to the reference case of steady-state white Gaussian noise.     

Coded 64-CAP ADSL in an impulse-noise environment-modeling ofimpulse noise and first simulation results

This paper presents the performance of various coding schemes for the asymmetrical digital subscriber line (ADSL) in an impulse-noise environment. Impulse noise is considered to be one of the most damaging impairments in the ADSL, in which compressed video signals are delivered to residential customers. The impulse noise used in this study was measured and collected in German telephone networks. Based on this measurement and the corresponding statistical modeling, a simulation model for impulse noise is proposed and its properties are outlined. The coding schemes considered here utilize burst-error correcting Reed-Solomon codes and/or random error correcting trellis codes as well as symbol interleaving between the two codes. It has been found through computer simulations that a proper concatenation of the two codes could increase the immunity against impulse noise compared to an uncoded scheme. Specifically, a concatenated code, using a 2-dimensional 8-state trellis code and a 4-error-correcting Reed-Solomon code with an interleaving depth of 18 symbols, was able to eliminate all the errors caused by the impulse noise used in the study. It has also been found that the trellis codes are not very effective against impulse noise, unless they are used in conjunction with Reed-Solomon codes and a proper symbol interleaving. Performance results of other coding configurations using Reed-Solomon codes with different error-correcting capabilities are also presented. In addition, we also show the performance results when simple array codes are used instead of the Reed-Solomon codes     

Source-channel optimized trellis codes for bitonal imagetransmission over AWGN channels

We consider the design of trellis codes for transmission of binary images over additive white Gaussian noise (AWGN) channels. We first model the image as a binary asymmetric Markov source (BAMS) and then design source-channel optimized (SCO) trellis codes for the BAMS and AWGN channel. The SCO codes are shown to be superior to Ungerboeck's codes by approximately 1.1 dB (64-state code, 10^-5 bit error probability), We also show that a simple “mapping conversion” method can be used to improve the performance of Ungerboeck's codes by approximately 0.4 dB (also 64-state code and 10 ^-5 bit error probability). We compare the proposed SCO system with a traditional tandem system consisting of a Huffman code, a convolutional code, an interleaver, and an Ungerboeck trellis code. The SCO system significantly outperforms the tandem system. Finally, using a facsimile image, we compare the image quality of an SCO code, an Ungerboeck code, and the tandem code, The SCO code yields the best reconstructed image quality at 4-5 dB channel SNR     

Channel-quality-based opportunistic scheduling with ARQ in multi-rate wireless networks: modeling and analysis

In this paper, we develop a novel framework for analyzing radio link level performance for opportunistic scheduling with automatic repeat request (ARQ)-based error control in multi-rate wireless networks. The multi-rate transmission is assumed to be achieved through adaptive modulation and coding (AMC) to adjust the transmission rate according to the channel condition. The residual error effect due to each AMC setting is counteracted by means of a limited persistence ARQ protocol. The novelty of the proposed analytical framework lies in the fact that we are able to derive complete statistics (in terms of probability mass function) for both short-term and long-term performance measures such as system throughput, per-flow throughput, inter-success delay under both uncorrelated and correlated wireless channels. These performance measures can also be obtained in case of non-identical channels for different users. Analytical results are validated through simulations and the impacts of channel behavior on the different radio link level performance metrics are investigated.