Browsing by Author "Han, Yunghsiang S."

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A Low-Complexity Maximum-Likelihood Decoder for Tail-Biting Convolutional Codes
(2018-05-01)
Han, Yunghsiang S.
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Wu, Ting-Yi
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Chen, Po-Ning
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Varshney, Pramod K.
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電機工程學系
Due to the growing interest in applying tail-biting convolutional coding techniques in real-time communication systems, fast decoding of tail-biting convolutional codes has become an important research direction. In this paper, a new maximumlikelihood decoder for tail-biting convolutional codes is proposed. It is named bidirectional priority-first search algorithm (BiPFSA) because priority-first search algorithm has been used both in forward and backward directions during decoding. Simulations involving the antipodal transmission of (2, 1, 6) and (2, 1, 12) tail-biting convolutional codes over additive white Gaussian noise channels shows that BiPFSA not only has the least average decoding complexity among the state-of-the-art decoding algorithms for tail-biting convolutional codes but can also provide a highly stable decoding complexity with respect to growing information length and code constraint length. More strikingly, at high SNR, its average decoding complexity can even approach the ideal benchmark complexity, obtained under a perfect noise-free scenario by any sequential-type decoding. This demonstrates the superiority of BiPFSA in terms of decoding efficiency.
A New Step-by-Step Complete Decoding Algorithm for Binary Cyclic Codes
(2017-01-01)
Han, Yunghsiang S.
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Fu, Shu-Wei
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Chen, Po-Ning
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電信工程研究所
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Institute of Communications Engineering
A complete decoder is one that guarantees to decode a received vector to its nearest codeword. For the design of an efficient complete decoding algorithm, storage demand and decoding complexity are two essential factors. Along this consideration, the Zero-Guards algorithm provides a significant improvement as the number of vectors required to be stored has been made much less than what is required by the conventional standard array decoding. When being applied to certain block codes, however, it may be inefficient when code length is short. At this background, a new step-by-step complete decoding algorithm for cyclic codes is proposed in this work. Based on a newly devised transfer function, our algorithm can achieve a better decoding efficiency than the Zero-Guards algorithm, while keeping a smaller number of vectors for most of the cyclic codes examined.
A Self-Orthogonal Code and Its Maximum-Likelihood Decoder for Combined Channel Estimation and Error Protection
(2008)
Wu, Chia-Lung
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Chen, Po-Ning
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Han, Yunghsiang S.
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Wang, Shih-Wei
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電信工程研究所
Recent researches have confirmed that better system performance can be obtained by jointly considering channel equalization and channel estimation in the code design, when compared with the system with individually optimized devices. However, the existing codes are mostly searched by computers, and hence, exhibit no good structure for efficient decoding. In this paper, a systematic construction for the codes for combined channel estimation and error protection is proposed. Simulations show that it can yield codes of comparable performance to the computer-searched best codes. In addition, the structural codes can now be maximum-likelihoodly decodable in terms of a newly derived recursive metric for use of the priority-first search decoding algorithm. Thus, the decoding complexity reduces considerably when compared with that of the exhaustive decoder. In light of the rule-based construction, the feasible codeword length that was previously limited by the capability of code-search computers(1) can now be further extended, and therefore facilitates their applications.
A systematic bit-wise decomposition of M-ary symbol metric
(2006-10-01)
Chang, Chia-Wei
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Chen, Po-Ning
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Han, Yunghsiang S.
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電信工程研究所
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Institute of Communications Engineering
In this paper, we present a systematic recursive formula for bit-wise decomposition of M-ary symbol metric. The decomposed bit metrics can be applied to improve the performance of a system where the information sequence is binary-coded and interleaved before M-ary modulated. A traditional receiver designed for certain system is to de-map the received M-ary symbol, into its binary isomorphism so as to facilitate the subsequent bit-based manipulation, such as hard-decision decoding. With a bit-wise decomposition of M-ary symbol metric, a soft-decision decoder can be used to achieve a better system performance. The idea behind the systematic formula is to decompose the symbol-based maximum-likelihood (ML) metric by equating a number of specific equations that are drawn from squared-error criterion. It interestingly yields a systematic recursive formula that can be applied to some previous work derived from different standpoint. Simulation results based on IEEE 802.11a/g standard show that at bit-error-rate of 10(-5), the proposed bit-wise decomposed metric can provide 3.0 dB, 3.9 dB and 5.1 dB improvement over the concatenation of binary-demapper, deinterleaver and hard-decision decoder respectively for 16QAM, 64QAM and 256QAM symbols, in which the in-phase and quadrature components in a complex M-2-QAM symbol are independently treated as two real M-PAM symbols. Further empirical study on system imperfection implies that the proposed bit-wise decomposed metric also improves the system robustness against gain mismatch and phase imperfection. In the end, a realization structure that avails the recursive nature of the proposed bit-decomposed metric formula is addressed.
A Systematic Space-Time Code Design and Its Maximum-Likelihood Decoding for Combined Channel Estimation and Error Correction
(2009)
Wu, Chia-Lung
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Skoglund, Mikael
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Chen, Po-Ning
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Han, Yunghsiang S.
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電信工程研究所
Several previous works have confirmed that a joint design that combines channel estimation, channel coding and space-time transmission can improve the system performance over that of a separate design. These conclusions are however in general based on unstructured solutions obtained using computer search. The coding gain of these joint designs is therefore limited by both the computer-searchable "short" code length and the compromise between "suboptimal" performance and "high" complexity of their optimal decoding. At this background, we propose a systematic space-time code construction for joint channel estimation and error correction for a two-transmit-antenna and half-rate system. Also proposed is its maximum-likelihood decoder that follows a priority-first search principle. Our systematic code construction, together with a fairly low-complexity optimal decoder, then allows one to work with longer codes with no sacrifice in performance. For codes of short block length, our simulations illustrate that the codes we propose have comparable performance to the best computer-searched codes. For codes of long block lengths that are almost beyond the searchable range of existing computer systems, our codes are still better than some reference designs based on separate channel estimation and error correction components.
An Efficient Tree Search Algorithm for the Free Distance of Variable-Length Error-Correcting Codes
(2018-03-01)
Huang, Chun
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Wu, Ting-Yi
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Chen, Po-Ning
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Alajaji, Fady
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Han, Yunghsiang S.
We propose an efficient tree search algorithm for determining the free distance of variable-length error-correcting codes (VLECs). A main idea behind the algorithm is to structure all pairs of code word-concatenated sequences as a tree, in which we seek the pair of sequences that determine the free distance. In order to speed up the algorithm, we establish constraints that do not compromise optimality in determining the free distance. Experimental results on VLECs algorithmically constructed for the English alphabet show that our algorithm requires a considerably smaller number of bitwise distance computations and covers a much smaller number of tree nodes than Dijkstra's algorithm operating over the pairwise distance graph while being a hundred times faster in terms of execution time.
Applications of an Exact Formula for the Largest Minimum Distance of Block Codes
(2018-01-01)
Chang, Ling-Hua
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Wang, Carol
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Chen, Po-Ning
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Tan, Vincent Y. F.
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Han, Yunghsiang S.
An exact information spectrum-type formula for the maximum size of finite length block codes subject to a minimum pairwise distance constraint is presented. This formula can be applied to codes for a broad class of distance measures, which only requires having the minimum value between a point and itself. As revealed by the formula, the largest code size is fully characterized by the information spectrum of the distance between two independent and identically distributed (i.i.d.) random codewords drawn from an optimal distribution. Under an arbitrary uniformly bounded distance measure, the asymptotic largest code rate (in the block length n) attainable for a sequence of (n; M; n delta)-codes is given exactly by the maximum large deviation rate function of the normalized distance between two i.i.d. random codewords.
Delay-Constrained Input-Queued Switch
(2018-11-01)
Deng, Lei
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Wong, Wing Shing
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Chen, Po-Ning
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Han, Yunghsiang S.
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Hou, Hanxu
In this paper, we study the delay-constrained input-queued switch, where each packet has a deadline and it will expire if it is not delivered before its deadline. Such new scenario is motivated by the proliferation of real-time applications in multimedia communication systems, tactile Internet, networked controlled systems, and cyber-physical systems. The delay-constrained input-queued switch is completely different from the well-understood delay-unconstrained one and thus poses new challenges. We focus on three fundamental problems centering around the performance metric of timely throughput: (i) how to characterize the capacity region? (ii) how to design a feasibility/throughput-optimal scheduling policy? and (iii) how to design a network-utility-maximization scheduling policy? We use three different approaches to solve these three fundamental problems. The first approach is based on Markov Decision Process (MDP) theory, which can solve all three problems. However, it suffers from the curse of dimensionality. The second approach breaks the curse of dimensionality by exploiting the combinatorial features of the problem. It gives a new capacity region characterization with only a polynomial number of linear constraints. The third approach is based on the framework of Lyapunov optimization, where we design a polynomial-time maximum-weight T-disjoint-matching scheduling policy which is proved to he feasibility/throughput-optimal. Our three approaches apply to the frame-synchronized traffic pattern but our MDP-based approach can be extended to more general traffic patterns.
Early-Elimination Modification for Priority-First Search Decoding
(2010-12-01)
Shieh, Shin-Lin
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Chen, Po-Ning
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Han, Yunghsiang S.
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Wu, Ting-Yi
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電機工程學系
In order to release the growing demand for computational complexity with respect to increasing information sequence length in the priority-first search decoding algorithm, a path elimination modification is proposed and also analyzed in this work. Specifically, we propose to directly eliminate all paths whose end nodes are Delta-level prior to the farthest node among those that have been visited thus far by the priority-first search. Following the argument on random coding, we then analyze the path elimination window Delta that results in a larger exponent for additional decoding error caused by path elimination than the exponent of the maximum-likelihood error performance, and hence guarantees exponentially negligible performance degradation. Our analytical results indicate that under additive white Gaussian noise (AWGN) channels, the path elimination window required for exponentially negligible performance degradation is just three times the code constraint length for rate one-half convolutional codes. It can be further reduced to 1.7-fold of the code constraint length when rate one-third convolutional codes are considered instead. Simulation results confirm these analytical window sizes. As a consequence, the priority-first search decoding algorithm can considerably reduce its computation burden and memory consumption by directly eliminating a large number of paths with nearly no performance degradation. This makes the priority-first search decoding algorithm with path elimination suitable for applications that demand low-complexity software implementation with near optimal performance.
Fault-tolerance analysis of a wireless sensor network with distributed classification codes
(2006)
Chen, Po-Ning
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Wang, Tsang-Yi
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Han, Yunghsiang S.
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Varshney, Pramod K.
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Yao, Chien
In this work, we analyze the performance of a wireless sensor network with distributed classification codes, where independence across sensors, including local observations, local classifications and sensor-fusion link noises, is assumed. In terms of large deviations technique, we establish the necessary and sufficient condition under which the minimum Hamming distance fusion error vanishes as the number of sensors tends to infinity. With the necessary and sufficient condition and the upper performance bounds, the relation between the fault-tolerance capability of a distributed classification code and its pair-wise Hamming distances is characterized.
Flip CRC modification for message length detection
(2007-09-01)
Shieh, Shin-Lin
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Chen, Po-Ning
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Han, Yunghsiang S.
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傳播研究所
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電信工程研究所
Cyclic redundancy check (CRC) bits that are conventionally used for error detection have recently found a new application in universal mobile telecommunications system standard for message length detection of variable-length message communications. It was anticipated that the CRC bits, when they are coworked with the inner convolutional code, can be used to detect the receiver-unaware of the message length-without much degradation in their error detection capability. This is unfortunately not true when the offset or difference between the wrong detected length and the true length is small. Two improvements, i.e., the DoCoMo's reverse CRC method and the flip CRC method, were accordingly proposed. In this paper, we revisited the flip CRC modification by considering the impact of joint decoding of the CRC code and the convolutional code. By generalizing the condition for the selection of the flip polynomials, we found that under error-free transmission, the range of the length offsets, at which the false length probability conditioning on the true message length can be made exactly zero (and hence, is minimized), can be extended from l - 1 to l + m - 1, where f and m are, respectively, the number of the CRC bits and the memory order of the convolutional code. In addition, an upper bound and a lower bound for the overall false length probability with respect to a uniform pick of the true message length over a candidate message length set are derived. It is then confirmed numerically that the two bounds almost coincide for moderate (f + m) value. Simulations show that the false length probability obtained analytically under error-free transmission assumption only mildly degrades for moderate-to-high SNRs. Interestingly, we also found that the system block error rate of the flip CRC method can be well approximated by the performance curve of the adopted convolutional code up to a certain SNR, and approach an error floor determined well by the previously derived false length probability bounds beyond this SNR, thereby facilitating the selection of the system parameters, such as the number of CRC bits and the memory order of the convolutional code.
Local Threshold Design for Target Localization Using Error Correcting Codes in Wireless Sensor Networks in the Presence of Byzantine Attacks
(2017-07-01)
Wei, Chun-Yi
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Chen, Po-Ning
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Han, Yunghsiang S.
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Varshney, Pramod K.
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電信工程研究所
In this paper, we revisit the received signal strength (RSS)-based target localization technique presented in Vempaty et al., where a simple threshold quantizer was employed to quantize the RSS values prior to sending them to the fusion center. It was shown that the probability of misclassification of the distributed classification fusion using error correcting codes scheme vanishes as the number of sensors tends to infinity. This result was obtained based on an intuitive threshold design at the local sensors, and the question of how much a careful design of local thresholds can help improve the overall performance was not addressed. In this paper, we demonstrate the significance of threshold design for accurate and robust target localization in wireless sensor networks, particularly, when the number of sensors is finite. With this objective, we derive an upper bound on the probability of misclassification as a function of RSS thresholds by using the union inequality. The RSS thresholds that algorithmically minimize the derived misclassification error bound are then numerically obtained over a mirror-based homomorphic sensor deployment structure. Simulations over fading wireless links show that the scheme based on newly found optimized RSS thresholds considerably outperforms the previous scheme using the thresholds that are intuitively selected, especially in the presence of Byzantine attacks that severely impact information security.
Maximum-Likelihood Priority-First Search Decodable Codes for Combined Channel Estimation and Error Correction
(2009-09-01)
Wu, Chia-Lung
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Chen, Po-Ning
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Han, Yunghsiang S.
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Kuo, Ming-Hsin
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電信工程研究所
The coding technique that combines channel estimation and error correction has received attention recently, and has been regarded as a promising approach to counter the effects of multipath fading. It has been shown by simulation that a proper code design that jointly considers channel estimation can improve the system performance subject to a fixed code rate as compared to a conventional system which performs channel estimation and error correction separately. Nevertheless, the major obstacle that prevents the practice of such coding technique is that the existing codes are mostly searched by computers, and subsequently exhibit no apparent structure for efficient decoding. Hence, the operation-intensive exhaustive search becomes the only decoding option, and the decoding complexity increases dramatically with codeword length. In this paper, a systematic construction is derived for a class of structured codes that support joint channel estimation and error correction. It is confirmed by simulation that these codes have comparable performance to the best simulated-annealing-based computer-searched codes. Moreover, the systematically constructed codes can now be maximum-likelihoodly decoded with respect to the unknown-channel criterion in terms of a newly derived recursive metric for use by the priority-first search decoding algorithm. Thus, the decoding complexity is significantly reduced as compared with that of an exhaustive decoder.
Minimum Byzantine Effort for Blinding Distributed Detection in Wireless Sensor Networks
(2020-01-01)
Lin, Hsuan-Yin
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Chen, Po-Ning
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Han, Yunghsiang S.
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Varshney, Pramod K.
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電信工程研究所
In this work, we consider the general problem of distributed detection in the presence of Byzantines using wireless sensor networks. Instead of attempting to mitigate Byzantine attacks as a system designer, we investigate the issue from the perspective of a Byzantine attacker. The probability for each individual sensor to be compromised (compromising probability) required to blind the system operation is adopted as the attack measure. Under the system setting that the fusion center (FC) declares the most likely hypothesis to be true based on the local sensors, a Byzantine attack policy that can blind the FC with the minimum compromising probability for each individual sensor is derived under the assumption that the Byzantine attacker knows the statistics of the local outputs. The closed-form expression for a blind-achieving Byzantine transition probability that is used to alter the statistics of the local outputs of compromised sensors is also established. Our results indicate that the statistics of the local outputs is essential for the minimization of an attacker's effort.
On the Asymptotic Performance of Delay-Constrained Slotted ALOHA
(2018-01-01)
Deng, Lei
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Deng, Jing
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Chen, Po-Ning
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Han, Yunghsiang S.
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電機工程學系
Motivated by the proliferation of real-time applications in multimedia communication systems, tactile Internet, networked controlled systems, and cyber-physical systems, supporting delay-constrained traffic become critical for the communication system. In delay-constrained traffic, each packet has a hard deadline and if it cannot be delivered before its deadline, it becomes useless and will be removed from the system. In this work, we consider a slotted ALOHA system where multiple stations need to deliver delay-constrained traffic to a common receiver by accessing a shared channel. We prove that, under the frame-synchronized traffic pattern, the maximum system timely throughput converges to 1/e = 36.8% as the number of stations goes to infinity, which is the same as the asymptotic maximum system throughput for delay-unconstrained slotted ALOHA system with saturate traffic. While this is not completely surprising, we further investigate the speed of such a maximum system throughput approaching 1/e under borderline traffic.
On the Coding Scheme for Joint Channel Estimation and Error Correction over Block Fading Channels
(2009)
Wu, Chia-Lung
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Chen, Po-Ning
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Han, Yunghsiang S.
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Zheng, Yan-Xiu
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電信工程研究所
In this work, we propose a novel systematic code construction scheme for joint channel estimation and error correction for channels with independently varying fading subblocks. Unlike the existing noncoherent codes that are designed with the help of computer search, a code of desired code length and code rate can be directly generated with our coding scheme. We then compare our codes with the three-times-repetitive (12, 6) code proposed by Xu et al. for use of channel quality indicator (CQI) in uplink control for IEEE 802.16m. Simulations show that our constructed (36, 6) code has comparable performance to Xu's code when channel coefficients changes randomly in every 12 symbols. If the channel taps remain constant in the entire coding block of length 36, our code outperforms Xu's code by 0.7 dB. This indicates that the new constructed code adapts more robustly to the two simulated scenarios. For frequency selective channels of unit memory order, our simulation results suggest that our code that takes in consideration the varying characteristic of channels can achieve better performance at median-to-high signal-to-noise ratio over the computer-searched, union-bound-minimized code of length less than the varying subblock size. A side advantage of our code construction scheme is that its systematic structure makes it maximum-likelihoodly decodable by the priority-first search algorithm. The decoding complexity is therefore significantly decreased in contrast to that of exhaustive decoder for the structureless computer-searched codes.
On the Construction and MAP Decoding of Optimal Variable-Length Error-Correcting Codes
(2011-01-01)
Wu, Ting-Yi
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Chen, Po-Ning
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Alajaji, Fady
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Han, Yunghsiang S.
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電子工程學系及電子研究所
In this paper, we present a novel algorithm that guarantees of finding a variable-length error-correcting code (VLEC) with minimal average codeword length for a fixed free distance d(free). We also propose a low complexity maximum a posterior (MAP) decoding algorithm for our codes under the premise that the receiver knows the number of codewords being transmitted. The resulting VLEC provides significant gains over other codes from the literature. When compared with separate source-channel tandem codes with identical d(free), such as a tandem code consisting of a Huffman source code concatenated with a (2, 1, 4) tail-biting convolutional channel code, our system has only a 0.3 dB performance loss at a bit error rate of 10(-5) while requiring significantly less decoding complexity.
On the design of soft-decision fusion rule for coding approach in wireless sensor networks
(2006)
Wang, Tsang-Yi
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Chen, Po-Ning
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Han, Yunghsiang S.
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Wang, Yung-Ti
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電信工程研究所
In this work, two soft-decision fusion rules, which are respectively named the maximum a priori (MAP) and the suboptimal minimum Euclidean distance (MED) fusion rules, are designed based on a given employed sensor code and associated local classification. Their performance comparison with the distributed classification fusion using soft-decision decoding (DCSD) proposed in an earlier work is also performed. Simulations show that when the number of faulty sensors is small, the MAP fusion rule remains the best at either low sensor observation signal-to-noise ratios (OSNRs) or low communication channel signal-to-noise ratios (CSNRs), and yet, the DCSD fusion rule gives the best performance at middle to high OSNRs and high CSNRs. However, when the number of faulty sensor nodes grows large, the least complex MED fusion rule outperforms the MAP fusion rule at high OSNRs and high CSNRs.
On the Design of Variable-Length Error-Correcting Codes
(2013-09-01)
Wu, Ting-Yi
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Chen, Po-Ning
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Alajaji, Fady
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Han, Yunghsiang S.
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電機資訊學士班
A joint source-channel coding problem that combines the efficient compression of discrete memoryless sources with their reliable communication over memoryless channels via binary prefix-free variable-length error-correcting codes (VLECs) is considered. Under a fixed free distance constraint, a priority-first search algorithm is devised for finding an optimal VLEC with minimal average codeword length. Two variations of the priority-first-search-based code construction algorithm are also provided. The first one improves the resilience of the developed codes against channel noise by additionally considering a performance parameter B-dfree without sacrificing optimality in average codeword length. In the second variation, to accommodate a large free distance constraint as well as a large source alphabet such as the 26-symbol English data source, the VLEC construction algorithm is modified with the objective of significantly reducing its search complexity while still yielding near-optimal codes. A low-complexity sequence maximum a posteriori (MAP) decoder for all VLECs (including our constructed optimal code) is then proposed under the premise that the receiver knows the number of codewords being transmitted. Simulations show that the realized optimal and suboptimal VLECs compare favorably with existing codes in the literature in terms of coding efficiency, search complexity and error rate performance.
On the Maximum Size of Block Codes Subject to a Distance Criterion
(2019-06-01)
Chang, Ling-Hua
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Chen, Po-Ning
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Tan, Vincent Y. F.
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Wang, Carol
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Han, Yunghsiang S.
We establish a general formula for the maximum size of finite length block codes with minimum pairwise distance no less than d. The achievability argument involves an iterative construction of a set of radius-d balls, each centered at a codeword. We demonstrate that the number of such balls that cover the entire code space cannot exceed this maximum size. Our approach can be applied to codes i) with elements over arbitrary code alphabets, and ii) under a broad class of distance measures. Our formula indicates that the maximum code size can be fully characterized by the cumulative distribution function of the distance measure evaluated at two independent and identically distributed random codewords. When the two random codewords assume a uniform distribution over the entire code alphabet, our formula recovers and thus naturally generalizes the Gilbert-Varshamov (GV) lower bound. Finally, we extend our study to the asymptotic setting.