There are two important techniques, automatic repeat request (ARQ) and forward error control (FEC), in error control systems. ARQ uses redundancy for detecting errors. While an error is detected in a transmitted code word, the receiver requests the transmitter to re-transmit the corrupted code word and the transmitted code word with detected error is discarded. However, the transmitted code word with detected error still contains a lot of information. ARQ schemes require a feedback channel. FEC relies on the controlled use of redundancy in the transmitted code word to detect and correct errors. Whether the decoding of the received code word is successful, no further processing is performed at the receiver. Therefore, FEC requires only a one-way link between the transmitter and receiver. In error control systems, a powerful FEC, turbo coding, was first proposed in 1993. Its performance was investigated by simulation to be close to the Shannon limit. Hybrid ARQ (HARQ) systems after properly combining the ARQ techniques and FEC techniques, can be classified into type-I, type-II and type-III, and its performance can be elevated far more than ARQ systems. HARQ adopts the FEC methods to detect and correct the error patterns caused in the process of transmission. While errors are detected in the demodulated signals at receiver and fail to be corrected, the receiver will activate ARQ mechanism to request the transmitter to retransmit related information to achieve an error-free transmission. In this thesis, a feasible type-II hybrid ARQ algorithm is proposed. Performance of the proposed algorithm is evaluated and analyzed by computer simulations. Compared with the traditional HARQ algorithms, the proposed algorithm can offer better performance in delay time and number of retransmissions with approximate throughput.

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The availability of feedback in communication channels can significantly increase the reliability of transmission while decreasing the encoding and decoding complexity. Most of the applications like cellular telephony, satellite communications and internet involve two-way transmission. Hence, it is important to devise coding schemes which utilize the advantages of feedback. Most of the results in code designs, which make use of feedback, concentrate on noiseless and instantaneous feedback. But in real-time systems, the feedback is usually noisy, and is available at the transmitter after some delay. Hence, it is important that we characterize the gains obtained in this case over that of one-way channels. We consider binary erasure channels to keep the problem tractable. For the erasure channels with noisy feedback, we have designed and analyzed a concatenated coding scheme, which achieves lower probability of error than any forward error correcting code of the same rate. Hence, it is shown that even noisy feedback can be useful in increasing the reliability of the channel. We have designed and analyzed a coding scheme using Low Density Parity Check (LDPC) codes along with selective retransmission strategy, which utilizes the limited (but noiseless), delayed feedback to achieve low frame error rates even with small blocklengths, at rates close to capacity. Furthermore, our scheme provides a way to trade off feedback bandwidth for reliability. The complexity of this scheme is lower than that of a forward error correcting code (FEC) of same blocklength and comparable performance. We have shown that our scheme performs better than the Automatic Repeat Request (ARQ) protocol which makes use of 1 bit feedback to signal retransmissions. For fair comparisons, we have also incorporated the rate loss due to the bits which are fed back in addition to the retransmitted bits. Thus, we have shown that for two-way communications with complexity and delay constraints, it is better to utilize the availability of feedback than to use just FEC.

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Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (M.A. (Sociolinguistics))--University of Pretoria, 2002. / Includes bibliographical references.

Bilinguals are generally adept at segregating their two competing languages and switching between them when contextually appropriate, although it has been shown that switching languages incurs a reaction time delay, or switch cost (Kolers, 1966). These switch costs are modulated by language dominance, with bilinguals evidencing greater delays when switching into their dominant language relative to their non-dominant language (e.g. Meuter & Allport, 1999). While these asymmetrical switch costs have formed the basis for theories of bilingual language separation and selection, the key factor of language context, the degree to which each language is employed in a given paradigm or conversation, has yet to be considered. In addition, previous research and subsequent theories of language selection have focused exclusively on the lexical level, yet given the distinct phonetic categories in a bilingual’s two languages (Caramazza et al., 1973), selection must also occur at the phonetic level. Addressing these gaps in the literature, this dissertation investigates the language switching costs and phonetic production of Spanish-English bilinguals in two experimental paradigms: a cued picture-naming task and an oral production task. In both studies, bilinguals (English-dominant, Spanish-dominant, and balanced bilinguals) produced language switches in varying language contexts, from monolingual to bilingual. Analyses focus on switch costs, error rates, and phonetic production, as a means to further the understanding of the language switching mechanism at the lexical and phonetic levels. Drawing on results from the two experimental paradigms, this dissertation makes several major contributions to the ongoing discussion regarding bilingual language selection. First, this study provides evidence for a gradient nature of the language switching mechanism at the lexical level. Second, it contributes an examination of the effects of language switching at the phonetic level, demonstrating asymmetrical phonetic transfer. And third, parallels are drawn between the underlying effects of language switching and the phonetic realizations produced in connected speech. Implications are considered for theories of bilingual language selection, and a gradient account of the Inhibitory Control Model (Green, 1986) is proposed at both the lexical and phonetic levels. / text

Our sensory experiences are encoded in the patterns of activity of the neurons in our brain. While we know we are capable of sensing and responding to a constantly changing sensory environment, we often study neural activity by repeatedly presenting the same stimulus and analyzing the average neural response. It is not understood how the average neural response represents the dynamic neural activity that produces our perceptions. In this work, we use functional imaging of the rodent primary somatosensory cortex, specifically the whisker representations, and apply classic signal-detection methods to test the predictive power of the average neural response. Stimulus features such as intensity are thought to be perceptually separable from the average representation; however, we show that stimulus intensity cannot be reliably decoded from neural activity from only a single experience. Instead, stimulus intensity was encoded only across many experiences. We observed this probabilistic neural code in multiple classic sensory paradigms including complex temporal stimuli (pairs of whisker deflections) and multi-whisker stimuli. These data suggest a novel framework for the encoding of stimulus features in the presence of high-neural variability. Specifically we suggest that our brains can compensate for unreliability by encoding information redundantly across cortical space. This thesis predicts that a somatosensory stimulus is not encoded identically each time it is experienced; instead, our brains use multiple redundant pathways to create a reliable sensory percept.

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