Institute for Systems Research Technical Reports
Permanent URI for this collectionhttp://hdl.handle.net/1903/4376
This archive contains a collection of reports generated by the faculty and students of the Institute for Systems Research (ISR), a permanent, interdisciplinary research unit in the A. James Clark School of Engineering at the University of Maryland. ISR-based projects are conducted through partnerships with industry and government, bringing together faculty and students from multiple academic departments and colleges across the university.
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Item Detection of Binary Sources Over Discrete Channels with Additive Markov Noise(1994) Alajaji, Fady; Phamdo, N.; Farvardin, Nariman; Fuja, Tom E.; ISRWe consider the problem of directly transmitting a binary source with an inherent redundancy over a binary channel with additive stationary ergodic Markov noise. Out objective is to design an optimum receiver which fully utilizes the source redundancy in order to combat the channel noise.We investigate the problem of detecting a binary iid non-uniform source transmitted across the Markov channel. Two maximum a posteriori (MAP) formulations are considered: a sequence MAP detection and an instantaneous MAP detection. The two MAP detection problems are implemented using a modified version of the Viterbi decoding algorithm and a recursive algorithm. Necessary and sufficient conditions under which the sequence MAP detector becomes useless as well as simulation results are presented. A comparison between the performance of the proposed system with that of a (substantially more complex) traditional tandem source-channel coding scheme exhibits a better performance for the proposed scheme at relatively high channel bit error rates.
The same detection problem is then analyzed for the case of a binary symmetric Markov source. Analytical and simulation results show the existence of a "mismatch" between the source and the channel. This mismatch is reduced by the use of a rate-one convolutional encoder. Finally, the detection problem is generalized for the case of a binary non-symmetric Markov source.
Item Speech Coding over Noisy Channels(1994) Farvardin, Nariman; ISRThis chapter contains a discussion of quantization over noisy channels. The effects of channel noise on the performance of vector quantizers are discussed and algorithms for the design of noisy-channel vector quantizers are presented. It is argued that in certain practical situations where delay and complexity place hard limits on system parameters, a combined source-channel coding approach might be preferable to the more traditional tandem source-channel coding. Examples of full-searched, multi- stage and finite-state vector quantization designed for a noisy channel are provided for coding of speech line spectrum pair parameters.Item Entropy-Constrained Trellis Coded Quantization: Implementation and Adaptation(1993) Lee, Cheng-Chieh; Farvardin, Nariman; ISREntropy-constrained trellis coded quantization (ECTCQ) of memoryless sources is known to be an efficient source coding technique in the rate-distortion sense. We develop an ECTCQ scheme that employs a symmetric reproduction codebook. The symmetry of the reproduction codebook, while essentially costs no performance loss, is exploited to reduce the memory requirement in entropy coding the ECTCQ output. In practice, a buffer of finite, and preferably small, size is needed to interface the variable-length codewords to the fixed-rate channel. An adaptive ECTCQ (A-ECTCQ) scheme, which uses a buffer-state feedback to control the quantizer characteristics to avoid buffer overflow/underflow, is studied in this work. The choice of encoding delay is an important issue in A- ECTCQ, as too long a delay will adversely impact the performance of the feedback control. We propose a pathwise-adaptive ECTCQ (PA-ECTCQ) that solves the encoding delay problem. Simulation results indicate that, while the buffer overflow/underflow problems of the PA- ECTCQ can be practically eliminated, the overall quantization distortion is increased only negligibly over theoretical performance predictions. Our experiments also suggests that PA- ECTCQ is robust with respect to source mismatch.Item Quantization Over Discrete Noisy Channels Using Rate-One Convolutional Codes(1993) Phamdo, N.; Farvardin, Nariman; ISRWe consider high-rate scalar quantization of a memoryless source for transmission over a binary symmetric channel. It is assumed that, due to its suboptimality, the quantizer's output is redundant. Our aim is to make use of this redundancy to combat channel noise. A rate-one convolutional code is introduced to convert this natural redundancy into a usable form. at the receiver, a maximum a posteriori decoder is employed. An upper bound on the average distortion of the proposed system is derived. An approximation of this bound is computable and we search for that convolutional code which minimizes the approximate upper bound. simulation results for a generalized Gaussian source with parameter a = 0.5 at rate 4 bits/sample and channel crossover provability 0.005 show improvement of 11.9 dB in signal-to-noise ratio over the Lloyd-Max quantizer and 4.6 dB over Farvardin and Vaishampayan's channel-optimized scalar quantizer.Item Trellis-Based Scalar-Vector Quantizer for Memoryless Sources(1992) Laroia, Rajiv; Farvardin, Nariman; ISRThis paper describes a structured vector quantization approach for stationary memoryless sources that combines the scalar-vector quantizer (SVQ) ideas (Laroia and Farvardin) with trellis coded quantization (Marcellin and Fischer). The resulting quantizer is called the trellis-based scalar-vector quantizer (TB-SVQ). The SVQ structure allows the TB-SVQ to realize a large boundary gain while the underlying trellis code enables it to achieve a significant portion of the total granular gain. For large block- lengths and powerful (possibly complex) trellis codes the TB-SVQ can, in principle, achieve the rate-distortion bound. As indicated by the results obtained here, even for reasonable block-lengths and relatively simple trellis codes, the TB-SVQ outperforms all other reasonable complexity fixed-rate quantizers.Item Low Bit-Rate Image Coding Using a Three-Component Image Model(1992) Ran, X.; Farvardin, Nariman; ISRIn this paper the use of a perceptually-motivated image model in the context of image compression is investigated. The model consists of a so-called primary component which contains the strong edge information of the image, a smooth component which represents the background slow-intensity variations and a texture component which contains the textures. The primary component, which is known to be perceptually important, is encoded separately by encoding the intensity and geometric information of the strong edge brim contours. Two alternatives for coding the smooth and texture components are studied: Entropy-coded adaptive DCT and entropy-coded subband coding. It is shown via extensive simulations that the proposed schemes, which can be thought of as a hybrid of waveform coding and featurebased coding techniques, result in both subjective and objective performance improvements over several other image coding schemes and, in particular, over the JPEG continuous-tone image compression standard. These improvements are especially noticeable at low bit rates. Furthermore, it is shown that a perceptual tuning based on the contrast-sensitivity of the human visual system can be used in the DCT-based scheme, which in conjunction with the 3- component model, leads to additional subjective performance improvements.Item Finite-State Vector Quantization for Noisy Channels(1992) Hussain, Yunus; Farvardin, Nariman; ISRUnder noiseless channel conditions and for sources with memory, finite-state vector quantizers (FSVQs) exhibit improvements over memoryless vector quantizers. It is shown, however, that in the presence of channel noise, the performance of the FSVQ degrades significantly. This suggests that for noisy channels, the FSVQ design problem needs to be reformulated by taking into account the channel noise. Using some of the developments in joint source-channel trellis coding, we describe two different methods leading to two types of noisy channel FSVQs. We show by means of simulations on the Gauss-Markov source and speech LSP parameters and for a binary symmetric channel that both schemes are fairly robust to channel noise. For the Gauss-Markov source, the proposed noisy channel FSVQs perform at least as well as or better than the channel-optimized VQ, while for speech LSP parameters, they lead to saving of 1.5-4 bits/frame over the channel-optimized VQ depending on the level of noise in the channel.Item A Perceptually Motivated Three-Component Image Model(1992) Ran, X.; Farvardin, Nariman; ISRIn this paper, results of phychovisual studies of the human visual system are discussed and interpreted in a mathematical framework. The formation of the perception is described by appropriate minimization problems and the edge information is found to be of primary importance in visual perception. Having introduced the concept of edge strength, it is demonstrated that strong edges are of higher perceptual importance than weaker edges (textures). We have also found that smooth areas of an image influence our perception together with the edge information, and that this influence can be mathematically described via a minimization problem. Based on this study, we have proposed to decompose the image into three components: (i) primary, (ii) smooth and (iii) texture, which contain, respectively, the strong edges, the background and the textures. An algorithm is developed to generate the three-component image model and an example is provided in which the resulting three components demonstrate the specific properties as expected. finally, it is shown that the primary component provides a superior representation of the strong edge information as compared with the Laplacian-Gaussian Operator scheme which is a popular edge extraction method.Item Optimal Detection of Discrete Markov Sources Over Discrete Memoryless Channels - Applications to Combined Sources-Channel Coding(1992) Phamdo, N.; Farvardin, Nariman; ISRWe consider the problem of detecting a discrete Markov source which is transmitted across a discrete memoryless channel. The detection is based upon the maximum a posteriori (MAP) criterion which yields the minimum probability of error for a given observation. Two formulations of this problem are considered: (i) a sequence MAP detection in which the objective is to determine the most probable transmitted sequence given the observed sequence and (ii) an instantaneous MAP detection which is to determine the most probable transmitted symbol at time n given all the observations prior to and including time n. The solution to the first problem results in a "Viterbi-like" implementation of the MAP detector (with large delay) while the later problem results in a recursive (with no delay). For the special case of the binary symmetric Markov source and binary symmetric channel, simulation results are presented and an analysis of these two systems yields explicit critical channel bit error rates above which the MAP detectors become useful.Applications of the MAP detection problem in a combined source-channel coding system are considered. Here it is assumed that the source is highly correlated and that the source encoder (in our case, a vector quantizer (VQ) fails to remove all of the source redundancy. The remaining redundancy at the output of the source encoder is referred to as the "residual" redundancy. It is shown, through simulation, that the residual redundancy can be used by the MAP detectors to combat channel errors. For small block sizes, the proposed system beats Farvardin and Vaishampayan's channel- optimized VQ by wide margins. Finally, it is shown that the instantaneous MAP detector can be combined with the VQ decoder to form a minimum mean-squared error decoder. Simulation results are also given for this case.
Item On SVQ Shaping of Multidimensional Constellations - High-Rate Large-Dimensional Constellations(1992) Laroia, Rajiv; Farvardin, Nariman; Tretter, S.; ISRAn optimal shaping scheme for multidimensional constellations, motivated by some ideas from a fixed-rate structured vector quantizer (SVQ), was recently proposed by Laroia. It was shown that optimal shaping could be performed subject to a constraint on the CER2 or PAR2 by expressing the (optimally shaped) constellation as the codebook of an SVQ and using the SVQ encoding/decoding algorithms to index the constellation points. Further, compatibility with trellis coded modulation was demonstrated. The complexity of the proposed scheme was reasonable but dependent on the data transmission rate. In this paper, we use recent results due to Calderbank and Ozarow to show that complexity of this scheme can be reduced and made independent of the data rate with essentially no effect on the shaping gain. Also, we modify the SVQ encoding/decoding algorithms to reduce the implementation complexity even further. It is shown that SVQ shaping can achieve a shaping gain of about 1.20 dB with a PAR2 of 3.75 at a very reasonable complexity (about 15 multiply-adds/baud and a memory requirement of 1.5 kbytes). Further, a shaping gain of 1 dB results in a PAR2 of less than 3. This is considerable less than a PAR2 of 3.75 for Forney's trellis shaping scheme that gives about 1 dB shaping gain.