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An adaptive MAC protocol is developed and analyzed that offers a "best case" scenario by allowing the MAC to control medium parameters thereby fully exploiting the channel of an ad hoc wireless LAN. This new, opportunistic medium access control protocol is called CATER (Code Adapts To Enhance Reliability) and is based on the proposed MAC standard for wireless local area networks (WLAN)-IEEE 802.11 [IEE96]. As currently proposed, IEEE 802.11 uses a fixed pseudo-noise (PN) code for spreading the information signal, implying a fixed process gain at the receiver. When the channel degrades, IEEE 802.11 offers only retransmissions at the MAC layer to combat a corrupt medium. However, CATER allows communicating stations to reconfigure their transceivers to use a longer PN code after a prescribed number of failed retransmissions. This longer code increases the process gain of the receiver and reduces the error rate. After the two stations are reconfigured, the source station sends the frame in question. Immediately after that frame is acknowledged, the source station may send additional frames during the reconfigured period. Simulation and emulation are used to demonstrate and validate the adaptive protocol's capabilities. Results show that this new protocol offers substantial improvement in system throughput when the channel degrades to a point that reliable transmission of frames is not feasible in a standard IEEE 802.11 WLAN. Specifically, CATER continues to function, permitting up to 14 percent normalized aggregate throughput at times when IEEE 802.11 permits no frames to pass through the WLAN. In addition, throughput experiences only a small decrease due to protocol overhead during periods when stations experience a good channel with few bit errors. Moreover, CATER does not adversely affect the predominate transport layer protocol (i.e., TCP), and provides equitable service to all stations within the network. / Ph. D.

The objective of this research is to develop time-varying signal processing methods for rapidly varying non-stationary signals based on time-varying autoregressive (TVAR) modeling, and to apply such methods to frequency-modulated (FM) interference rejection in direct-sequence spread spectrum (DSSS) communications. For fast varying non-stationary signal processing, such as the task to reject an FM interference that could chirp over the entire DSSS bandwidth in a symbol interval, an explicit description of the variation is necessary to form a time-varying filter. This is realized using the TVAR model, which is an autoregressive model whose coefficients are time-varying with the variation modeled as a linear combination of a set of known functions of time. In DSSS communications, when the strength of an interference - which could be a hostile jammer or overlaid communication signal - possibly exceeds the inherent spread spectrum processing gain, interference rejection is necessary to secure a usable bit-error-rate. The contributions of this research include: a) revealed the advantageous performance of TVAR model based instantaneous frequency estimation (TVAR-IF), which is expected to change the prevailing opinion that regards TVAR-IF as a poor estimator; b) proposed a time-varying Prony method to improve TVAR-IF at low SNR; c) proposed to use TVAR-IF for time-varying FIR notch filter based FM jammer suppression in DSSS communications; d) developed TVAR model based time-varying optimum filters, including the TVAR based Kalman filter (TVAR-KF) and the TVAR based Wiener filter (TVAR-WF); e) developed a TVAR-WF based formulation of FM interference soft-cancellation in DSSS communications; and f) proposed a TVAR based linear prediction error (TVAR-LPE) filter for soft-cancellation of FM interference in DSSS communications. For the interference rejection problem, our TVAR-IF controlled notch filter yields high processing gain close to that using the known IF and much higher than that using the WVD based IF estimate. Furthermore, unlike the IF based notch filter approaches, the proposed soft-cancellation methods utilize the full spectral information captured by the TVAR model. Our soft-cancellation approaches, including TVAR-WF and TVAR-LPE, maintain at least the DSSS system performance expected when no filtering is used, even under estimated conditions. The latter is in contrast to the notch filter based approaches, which may cause deterioration of overall system performance at low jammer-to-signal ratios. / Ph. D.

The Interactive Video and Data System is a multidisciplinary research project involving the creation of a means for people to interact with television and printed media without the addition of expensive hardware as required by most interactive systems available today. The IVDS system consists of a Consumer Control Unit which transmits user requests, a Repeater Unit which receives transmissions from the CCUs, and a Host System which takes appropriate actions for user demands. This thesis follows the evolution of the original CCU prototype as more capabilities are added and hardware platforms are changed, focusing on the addition of a bar code interface to the CCU. / Master of Science