基于LT码的LBCMP迭代纠错方法

针对删除信道中发生错误的数据包,提出联合信道编码的LBCMP迭代纠错方法,该方法充分利用错误数据包中含有的正确信息,将LT编码包作为冗余纠错包与线性分组码相结合,并采用MP迭代译码方法进行纠错.理论分析及实验结果表明,采用LBCMP迭代方法可以减少为恢复错误数据包所需要的信源编码包数量.     

Error resilient transmission of SPIHT coded images over fadingchannels

The paper introduces a method of transmitting error resilient SPIHT coded images over highly error prone Rayleigh fading channels. First, the source significance of the SPIHT coded output is obtained. Based on the significance of the bits, the channel coding is varied accordingly. Channel coding consists of a mixture of rate compatible punctured convolutional (RCPC) codes and interleaving to combat the burst errors produced by the fading channel. An additional error concealment technique is also introduced into the SPIHT decoder to improve its results in cases where errors cause the corruption of the average luminance level during decoding. Comparison with using RS block codes at a total transmission rate of 1.0 bits/pixel is carried out over fading channels with very high error rates to show the superiority of this method over methods using burst error codes     

Equalization and semi-blind channel estimation for space-time block coded signals over a frequency-selective fading channel

In this paper, we investigate the equalization and channel identification for space-time block coded signals over a frequency-selective multiple-input multiple-output (MIMO) channel. The equalization has been considered by taking into account the cyclostationarity of space-time block coded signals. The minimum mean square error (MMSE) solutions have been derived for the linear and decision feedback (DF) equalizers. The channel estimation is required for the equalization. With known symbols (as pilot symbols), MIMO channels can be estimated. In addition, due to the redundancy induced by space-time block code, it is possible to identify MIMO channels blindly using the subspace method. We consider both blind and semi-blind channel estimation for MIMO channels. It is shown that the semi-blind channel estimate has fewer estimation errors, and it results in less (bit error rate) performance degradation of the MMSE linear and DF equalizers.     

基于多方向边界匹配的视频误码掩盖算法

视频压缩码流对于信道误码十分敏感,可能导致重建图像质量严重下降.误码掩盖技术利用图像在时间和空间上的相关性,可以有效地降低误码对视频图像的影响.文中提出了一种基于多方向边界匹配的时域误码掩盖算法.该算法能更精确地恢复错误宏块的运动矢量,从而获得比传统的时域掩盖算法更好的视频质量.     

Enhanced MMSE channel estimation using timing error statistics for wireless OFDM systems

Estimation and tracking of the frequency-selective time-varying channel response is a challenging task for wireless communication systems incorporating coherent OFDM. In pilot-symbol-assisted (PSA) OFDM systems, the minimum mean-square-error (MMSE) estimator provides the optimum performance based on the channel statistics (channel correlation function and SNR). In OFDM systems, FFT-block timing error introduces a linear phase rotation to data modulated on individual subcarriers. An MMSE channel estimator designed only using the wireless channel statistics performs only sub-optimally when subcarrier phase rotations due to block timing errors are present. In this paper, we show that by using the block timing error statistics of the OFDM time-synchronizer the performance of the MMSE channel estimation can be significantly improved. Numerical results show that the bit-error-probability (BEP) performance degradation due to timing errors can be almost completely recovered by the proposed technique.     

On the statistics of block errors in bursty channels

In the development of encoding algorithms for image, video, and other mixed media transmissions, it is important to note that the channel “seen” by the applications is the physical channel as modified by the error-correcting mechanisms used at the physical level. Therefore, the statistics of the residual error process is relevant to the design of encoding algorithms. In this paper, we study the second- and third-order statistics of the residual error process when block transmissions are performed over a bursty channel. The effect of interleaving is explicitly studied. The conditions under which a Markovian model for the block errors is adequate are identified. Derivations of the parameters of the block error process are then presented in terms of the parameters of the bit/symbol error process. At higher data speeds an effective interleaving strategy is found to require a very large buffer     

On computing undetected error probabilities on the Gilbert channel

Two methods for computing the probability of undetected error on the Gilbert (1960) channel are examined. First, using a method proposed by Kittel (1978), we study some standard cyclic redundancy codes and compare the results with those on the binary symmetric channel. Then we consider a general method of approximate code evaluation, proposed by Elliott, which involves P(m, n), the probability of m errors in a block of length n bits. A nonrecursive technique for computing P(m, n) on the Gilbert channel is described     

Reduced Rank Technique for Joint Channel Estimation and Joint Data Detection in TD-SCDMA Systems

In time division-synchronous code division multiple access systems, the channel estimation for multiple subscribers requires the computation of very complicated algorithms through short training sequences. This situation causes mismodeling of the actual channels and introduces significant errors in the detected data of multiple users. This paper presents a novel channel estimation method with low complexity, which relies on reducing the rank order of the total channel matrix H. We exploit the rank deficient of H to reduce the number of parameters that characterizes this matrix. The adopted reduced rank technique is based on singular value decomposition algorithm. Equations for reduced rank-joint channel estimation (JCE) are derived and compared against traditional full rank-joint channel estimators: least square (LS) or Steiner, enhanced LS, and minimum mean square error algorithms. Simulation results of the normalized mean square error for the above mentioned estimators showed the superiority of reduced rank estimators. Multi-user joint data detectors based linear equalizers are used to suppress inter-symbol interference and mitigate intra-cell multiple access interference. The detectors: zero forcing block linear equalizer and minimum mean square error block linear equalizer algorithms are considered in this paper to recover the data. The results of bit error rate simulation have shown that reduced rank-JCE based detectors have an improvement by 5 dB lower than other traditional full rank-JCE based detectors.     

Evaluation of the performance of error-correcting codes on aGilbert channel

Presents a combinatorial analysis to derive a closed-form expression for the number of transmission errors that occur in a block transmitted through a Gilbert channel. This expression simplifies the computations needed to investigate the tradeoffs among the decoding error probability, degree of interleaving, and the error-correction ability of a code. The authors illustrate how a designer may apply the method to determine different combinations of the degree of interleaving and error correction ability to achieve a specified decoding error rate     

空间光通信RS-INTERLEAVING信道编码研究

分析了空间激光通信的两种信道数学模型--自由空间光通信信道和大气激光通信信道,针对信道时变突发的特点,引入信道编码技术和交织技术,交织对发送信号进行扰码以克服大气信道相关性,Reed-Solomon(RS)线性分组码克服信道的突发错误,并进行一定程度的纠错.计算机仿真结果表明,RS编码和交织技术非常适合空间光通信信道,能大大降低系统的误码率.     

Huffman code based error screening and channel code optimization for error concealment in perceptual audio coding (PAC) algorithms

The class of perceptual audio coding (PAC) algorithms yields efficient and high-quality stereo digital audio bitstreams at bit rates from 16 kb/sec to 128 kb/sec (and higher). To avoid "pops and clicks" in the decoded audio signals, channel error detection combined with source error concealment, or source error mitigation, techniques are preferred to pure channel error correction. One method of channel error detection is to use a high-rate block code, for example, a cyclic redundancy check (CRC) code. Several joint source-channel coding issues arise in this framework because PAC contains a fixed-to-variable source coding component in the form of Huffman codes, so that the output audio packets are of varying length. We explore two such issues. First, we develop methods for screening for undetected channel errors in the audio decoder by looking for inconsistencies between the number of bits decoded by the Huffman decoder and the number of bits in the packet as specified by control information in the bitstream. We evaluate this scheme by means of simulations of Bernoulli sources and real audio data encoded by PAC. Considerable reduction in undetected errors is obtained. Second, we consider several configurations for the channel error detection codes, in particular CRC codes. The preferred set of formats employs variable-block length, variable-rate outer codes matched to the individual audio packets, with one or more codewords used per audio packet. To maintain a constant bit rate into the channel, PAC and CRC encoding must be performed jointly, e.g., by incorporating the CRC into the bit allocation loop in the audio coder.     

Near-optimal, single-synchronization-error-correcting code

A synchronization error is said to occur when either a bit which does not belong is detected in a channel between bits which were transmitted, or a bit which was transmitted is never detected at the output. A block code which corrects a single synchronization error per block is presented, and it is shown that this code has, at most, three bits more redundancy than that of an optimal code for this class of errors. The code has the beneficial property that it is possible to separate the information positions from the check positions, and an appropriate method of encoding is shown.     

Reduced Data Rates for Energy-Efficient Reed–Solomon FEC on Fading Channels

In the CDMA2000 1xEV-DO broadcast service, Reed-Solomon (RS) coding is applied at the medium access control (MAC) layer. Long bursts of errors are corrected by an additional block interleaving mechanism, which improves the performance of RS coding by increasing the time diversity. This is achieved by increasing the width of the error control block (ECB) to convert error bursts into sparse patterns. On the assumption that the number of ECBs transmitted over the air channel in a given interval of time remains constant, we propose performance and energy models of RS decoding under varying conditions of Rayleigh fading on the broadcast channel, which allows for varying levels of block interleaving. Evaluation of this model shows that a higher data rate for the MAC payload and more efficient recovery from bursty errors can be achieved by increasing the level of block interleaving, although it increases the energy consumption. We therefore make the ECB as small as we can without incurring significant performance reduction by sacrificing the data rate to an extent that depends on the channel conditions. We suggest a way of choosing an adequate but energy-efficient level of ECB interleaving and, thus, determine the data rate. Our approach is effective for energy-constrained mobile devices running multimedia applications, which benefit from an increase in energy efficiency, and can tolerate a certain level of packet errors and a reduction in data rate.     

Channel coding for digital HDTV terrestrial broadcasting

In order to transmit the HDTV signal in 6 MHz, the four United States digital HDTV proponents, the DigiCipher, DSC-HDTV, ADTV and ATVA-P systems, are reducing the video data rate of HDTV to 15-17 Mb/s, a compression ratio of approximately 60-70 times. The high compression dictates that channel coding be used to avoid block errors and multiframe error propagation. High efficiency in channel utilization required by the 6-MHz limitation means that the channel must be properly equalized and that the multipath and interfering signals must be severely limited. The channel coding techniques used for error reduction include data interleaving, error detection and replacement, and error correction at different levels of protection for bits and blocks of unequal importance     

Optimal resynchronization marker positioning method using a novel information measure

This paper presents a new resynchronization marker (RM) positioning technique to effectively block the error propagation caused by channel errors. The optimal technique can be defined as the one minimizing the information loss under an error condition. For realization of the optimal technique, the amount of the information lost by errors needs to be measured. Thus, we introduce a novel measure that is successful in reflecting the actual importance of the lost data, which is measured by using error concealment techniques. That is, the amount of information lost by an error at a certain location is numerically specified by the difference between the image recovered by the error concealment and the image normally decoded without any error. Since such a measurement method is appropriate for reflecting the actual damage by errors, it is very useful for determining the RM positions. With the new measure, the optimal RM positioning technique is proposed and verified by experiments.     

A Model for the Performance Evaluation of Packet Transmissions Using Type-II Hybrid ARQ over a Correlated Error Channel

Type-II Hybrid-ARQ (Type-II HARQ) has been shown, under certain circumstances, to increase the efficiency and reduce loss of data transmissions over a wireless channel. However, it is difficult to predict how it will perform when transmission symbol errors are correlated. We present a computationally efficient approach to the performance evaluation of packet transmissions over a wireless link employing Type-II HARQ error mitigation when the physical channel is subject to correlated transmission symbol errors. This provides a tool for static or online optimization of system parameters. We present numerical results for a wide range of channel statistics, illustrating the effect of bit error correlation, bit error rate, and block size on packet latency and loss rate.     

Low-Complexity Block Turbo Equalization for OFDM Systems in Time-Varying Channels

We propose low-complexity block turbo equalizers for orthogonal frequency-division multiplexing (OFDM) systems in time-varying channels. The presented work is based on a soft minimum mean-squared error (MMSE) block linear equalizer (BLE) that exploits the banded structure of the frequency-domain channel matrix, as well as a receiver window that enforces this banded structure. This equalization approach allows us to implement the proposed designs with a complexity that is only linear in the number of subcarriers. Three block turbo equalizers are discussed: two are based on a biased MMSE criterion, while the third is based on the unbiased MMSE criterion. Simulation results show that the proposed iterative MMSE BLE achieves a better bit error rate (BER) performance than a previously proposed iterative MMSE serial linear equalizer (SLE). The proposed equalization algorithms are also tested in the presence of channel estimation errors.      

RS码短码的软判决方法

RS码是差错控制领域中一种重要的线性分组码,而相对于普通的硬判决来说,软判决译码具有纠错能力强的优势。针对RS码(15,11),提出了使用Chase算法实现非二进制码的软判决译码方法,并通过计算机模拟,给出在AWGN信道中的译码结果。     

Effects of correlated fading on frequency-hop communications with Reed-Solomon coding

The transmission of frequency-hop spread-spectrum signals over a frequency-selective fading channel results in correlation between the amplitudes of signals that occupy different frequency slots. This correlation produces dependent errors among symbols transmitted at different frequencies. For a system that employs block coding, this dependence results in dependent errors, even if the code symbols are interleaved over the dwell intervals. Using an appropriate mathematical model for wideband frequency-selective fading channel, we present analytical results on the code-word error probability for a simple coding scheme and simulation results for more complex codes.     

Noise reduction of VQ encoded images through anti-gray coding

Noise reduction of VQ encoded images is achieved through the proposed anti-gray coding (AGC) and noise detection and correction scheme. In AGC, binary indices are assigned to the codevector in such a way that the 1-b neighbors of a code vector are as far apart as possible. To detect the channel errors, we first classify an image into uniform and edge regions. Then we propose a mask to detect the channel errors based on the image classification (uniform or edge region) and the characteristics of AGC. We also mathematically derive a criterion for error detection based on the image classification. Once error indices are detected, the recovered indices can be easily chosen from a “candidate set” by minimizing the gray-level transition across the block boundaries in a VQ encoded image. Simulation results show that the proposed technique provides detection results with smaller than 0.1% probability of error and more than 86.3% probability of detection at a random bit error rate of 0.1%, while the undetected errors are invisible. In addition, the proposed detection and correction techniques improve the image quality (compared with that encoded by AGC) by 3.9 dB