# Institute for Systems Research Technical Reports

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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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### Browsing Institute for Systems Research Technical Reports by Author "Alajaji, Fady"

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Item Channel Codes That Exploit the Residual Redundancy in CELP- Encoded Speech(1996) Alajaji, Fady; Phamdo, N.; Fuja, Tom E.; ISRWe consider the problem of reliably transmitting CELP-encoded speech over noisy communication channels. Our objective is to design efficient coding/decoding schemes for the transmission of the CELP line spectral parameters (LSP's) over very noisy channels.We begin by quantifying the amount of ﲲesidual redundancy inherent in the LSP's of Federal Standard 1016 CELP. This is done by modeling the LSP's as first and second-order Markov chains. Two models for LSP generation are proposed; the first model characterizes the intra-frame correlation exhibited by the LSP's, while the second model captures both intra-frame and inter-frame correlation. By comparing the entropy rates of the models thus constructed with the CELP rates, it is shown that as many as one-third of the LSP bits in every frame of speech are redundant.

We next consider methods by which this residual redundancy can be exploited by an appropriately designed channel decoder. Before transmission, the LSP's are encoded with a forward error control (FEC) code; we consider both block (Reed- Solomon) codes and convolutional codes. Soft-decision decoders that exploit the residual redundancy in the LSP's are implemented assuming additive white Gaussian noise (AWGN) and independent Rayleigh fading environments. Simulation results employing binary phaseshift keying (BPSK) indicate coding gains of 2 to 5 dB over soft-decision decoders that do not exploit the residual redundancy.

Item Channel Codes That Exploit the Residual Redundancy in CELP- Encoded Speech(1995) Alajaji, Fady; Phamdo, N.; Fuja, Tom E.; ISRWe consider the problem of reliably transmitting CELP-encoded speech over noisy communication channels. Our objective is to design efficient coding/decoding schemes for the transmission of the CELP line spectral parameters (LSP's) over very noisy channels.We begin by quantifying the amount of ﲲesidual redundancy inherent in the LSP's of Federal Standard 1016 CELP. This is done by modeling the LSP's as first-and second-order Markov chains. Two models for LSP generation are proposed; the first model characterizes the intra-frame correlation exhibited by the LSP's, while the second model captures both intra-frame and inter-frame correlation. By comparing the entropy rates of the models thus constructed with the CELP rates, it is shown that as many as one-third of the LSP bits in every frame of speech are redundant.

We next consider methods by which this residual redundancy can be exploited by an appropriately designed channel decoder. Before transmission, the LSP's are encoded with a forward error control (FEC) code; we consider both (Reed-Solomon) codes and convolutional codes. Soft-decision decoders that exploit the residual redundancy in the LSP's are implemented assuming additive white Gaussian noise (AWGN) and independent Rayleigh fading environments. Simulation results employing binary phase-shifting keying (BPSK) indicate coding gains of 2 to 5 dB over soft-decision decoders that do not exploit the residual redundancy.

Item A Communication Channel Modeled on Contagion(1993) Alajaji, Fady; Fuja, Tom E.; ISRWe introduce a binary additive communication channel with memory. The noise process of the channel is generated according to the contagion model of George Polya; our motivation is the empirical observation of Stapper et. al. that defects in semiconductor memories are well described by distributions derived from Polya's urn scheme. The resulting channel is stationary but not ergodic, and it has many interesting properties.We First derive a maximum likelihood (ML) decoding algorithm for the channel; it turns out that ML decoding is equivalent to decoding a received vector onto either the closest codeword or the codeword that is farthest away, depending on whether an ﲡpparent epidemic has occurred. We next show that the Poly-contagion channel is an ﲡveraged channel in the sense of Ahlswede (and others) and that its capacity is zero. We then demonstrate that the Poly- contagion channel is a counter-example to the adage, ﲭemory cannot decrease capacity ; the capacity of the Poly-contagion channel is actually less than that of the associated memoryless channel. Finally, we consider a finite-memory version of the Poly-contagion model; this channel is (unlike the original) ergodic with a non-zero capacity that increases with increasing memory.

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 Feedback Does Not Increase the Capacity of Discrete Channels with Additive Noise(1993) Alajaji, Fady; ISRWe consider discrete channels with stationary additive noise. We show that output feedback does not increase the capacity of such channels. This is shown for both ergodic and non-ergodic additive stationary channels.Item New Results on the Analysis of Discrete Communication Channels with Memory(1994) Alajaji, Fady; Fuja, T.E.; ISRThe reliable transmission of information bearing signals over a communication channel constitutes a fundamental problem in communication theory. An important objective in analyzing this problem is to understand and investigate its ﲩnformation theoretic aspects - i.e., to determine the fundamental limits to how efficiently one can encode information and still be able to recover it with negligible loss. In this work we address this problem for the case where the communication channel is assumed to have memory - i.e., the effect of noise lingers over many transmitted symbols. Our motivation is founded on the fact that most real-world communication channels have memory.We begin by proposing and analyzing a contagion communication channel. A contagion channel is a system in which noise propagates in a way similar to the spread of an infectious disease through a population; each ﲵnfavorable event (i.e., an error) increases the probability of future unfavorable events. A contagion-based model offers an interesting and less complex alternative to other models of channels with memory like the Gilbert-Elliott burst channel. We call the model set forth the Polya-contagion channel - discrete binary communication channel with additive errors modeled according to the famous urn scheme of George Polya for the spread of contagion.

We nest consider discrete channels with arbitrary (not necessarily stationary ergodic) additive noise. Note that such channels need not be memoryless; in general, they have memory. We show that output feedback does not increase the capacity of such channels. The same result is also shown for a larger class of channels to which additive channels belong, the class of discrete symmetric channels with memory. These channels have the property that their inf-information rate is maximized for equally likely iid input processes.

Finally, we impose average cost constraints on the input of the additive channels, rendering them non-symmetric. We demonstrate that in the case where the additive noise is a binary stationary mixing Markov process, output feedback can increase the capacity-cost function of these channels.

Item The Performance of Focused Error Control Codes(1990) Alajaji, Fady; Fuja, Tom E.; ISRConsider an additive noise channel with inputs and outputs in the field GF (q ) where q > 2; every time a symbol is transmitted over such a channel, there are q - 1 different errors that can occur, corresponding to the q - 1 non-zero elements that the channel can add to the transmitted symbol. In many data communication/ storage systems, there are some errors that occur much more frequently than others; however, traditional error correcting codes- designed with respect to the Hamming metric - treat each of these q - 1 errors the same. Fuja and Heegard have designed a class of codes, called focused error control codes, that offer different levels of protection against "common" and "uncommon" errors; the idea is to define the level of protection in a way based not only on the number of errors, but the kind as well. In this paper, the performance of these codes is analyzed with respect to idealized "skewed" channels as well as realistic non-binary modulation schemes. It is shown that focused codes, used in conjunction with PSK and QAM signaling, can provide more than 1.0 dB of additional coding gain when compared with Reed- Solomon codes for small blocklengths.Item Quantization of Memoryless and Gauss-Markov Sources Over Binary Markov Channels(1994) Phamdo, N.; Alajaji, Fady; Farvardin, Nariman; ISRJoint source-channel coding for stationary memoryless and Gauss- Markov sources and binary Markov channels is considered. The channel is an additive-noise channel where the noise process is an M-th order Markov chain. Two joint source-channel coding schemes are considered. The first is a channel-optimized vector quantizer - optimized for both source and channel. The second scheme consists of a scalar quantizer and a maximum a posteriori detector. In this scheme, it is assumed that the scalar quantizer output has residual redundancy that can be exploited by the maximum a posteriori detector to combat the correlated channel noise. These two schemes are then compared against two schemes which use channel interleaving. Numerical results show that the proposed schemes outperform the interleaving schemes. For very noisy channels with high noise correlation, gains of 4 to 5 dB in signal-to-noise ratio are possible.Item Strong Converse, Feedback Channel Capacity and Hypothesis Testing(1994) Chen, Po-Ning; Alajaji, Fady; ISRIn light of recent results by Verdu ad Han on channel capacity, we examine three problems: the strong converse condition to the channel coding theorem, the capacity of arbitrary channels with feedback and the Neyman-Pearson hypothesis testing type-II error exponent. It is first remarked that the strong converse condition holds if and only is the sequence of normalized channel information densities converges in probability to a constant. Examples illustrating this condition are also provided. A general formula for the capacity of arbitrary channels with output feedback is then obtained. Finally, a general expression for the Neyman-Pearson type-II exponent based on arbitrary observations subject to a constant bound on the type-I error probability is derived.