International Telemetering Conference Proceedings / October 28-31, 1996 / Town and Country Hotel and Convention Center, San Diego, California / Testing avionics and military equipment often requires extensive facilities and numerous operators working in concert. In many cases these facilities are mobile and can be set up at remote locations. In almost all situations the equipment is loud and makes communication between the operators difficult if not impossible. Furthermore, many sites must transmit, receive, relay, and record telemetry signals. To facilitate communication, most telemetry and test sites incorporate some form of intercom system. While intercom systems themselves are a not a new concept and are available in many forms, finding one that meets the requirements of the test community (at a reasonable cost) can be a significant challenge. Specifically, the test director must often communicate with several manned stations, aircraft, remote sites, and/or simultaneously record all or some of the audio traffic. Furthermore, it is often necessary to conference all or some of the channels (so that all those involved can fully follow the progress of the test). The needs can be so specialized that they often demand a very expensive "custom" solution. This paper describes the philosophy and design of a multi-channel intercom system specifically intended to support the needs of the telemetry and test community. It discusses in detail how to use state-of-the-art field programmable gate arrays, relatively inexpensive computers and digital signal processors, and some other new technologies to design a fully digital, completely non-blocking intercom system. The system described is radically different from conventional designs but is much more cost effective (thanks to recent developments in programmable logic, microprocessor performance, and serial/digital technologies). This paper presents, as an example, the conception and design of an actual system purchased by the US government.

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This report describes the investigation which has led to the development of a novel technique for the adaptive adjustment of the receiver in a digital data-transmission system, where the latter operates with additive noise and severe intersymbol interference in the received signal. This technique is suitable both for the adjustment of a conventional non-linear (decision-feedback) equaliser, and also for the adjustment of a linear feedforward transversal filter that is employed ahead of a near-maximum-likelihood detector. In the latter case, the technique provides, in addition, an estimate of the sampled impulse-response of the channel and filter, to give the information on the received signal needed by the detector. The adaptive system requires an estimate of the sampled impulse-response of the channel and it involves finding the roots (zeros) of the z-transform of the sampled impulse-response that lie outside the unit circle. The aim of the investigation has been to develop a root-finding algorithm which would be used to find some or all. of the roots, with moduli greater than unity, of a high-degree (>20) complex polynomial. The first part of the report describes the study of an on-line system, where the receiver operates on a continuous stream of received samples subject to both intersymbol interference and noise, and 'it presents the results of a large number. of tests using computer simulation which show the root-finding capability and vulnerability of the system to white Gaussian noise. .The results of the tests point to the potential advantages of an off-line system, which is much simpler to implement and'requires less computation. The modified technique operates solely and directly on the estimate of sampled impulse-response of the channel, which must,again be,provided at the receiver. The report gives the results of computer-simulation tests measuring the speed and accuracy of two different arrangements of the system, for different channels, and it also studies the effects of double roots and roots just outside the unit circle. The off-line system is suitable for a 16-point QAM system operating at 9600 bit/s over the British public switched telephone network, and with slight modification, it can also be used for a 64-point QAM system operating at 19200 bit/s.

Over the last decade, we have witnessed the rapid evolution of Multiple-Input Multiple-Output (MIMO) systems which promise to break the frontiers of conventional architectures and deliver high throughput by employing more than one element at the transmitter (Tx) and receiver (Rx) in order to exploit the spatial domain. This is achieved by transmitting simultaneous data streams from different elements which impinge on the Rx with ideally unique spatial signatures as a result of the propagation paths’ interactions with the surrounding environment. This thesis is oriented to the statistical characterisation and modelling of MIMO systems and particularly of indoor and short-range channels which lend themselves a plethora of modern applications, such as wireless local networks (WLANs), peer-to-peer and vehicular communications. The contributions of the thesis are detailed below. Firstly, an indoor channel model is proposed which decorrelates the full spatial correlation matrix of a 5.2 GHzmeasuredMIMO channel and thereafter assigns the Nakagami-m distribution on the resulting uncorrelated eigenmodes. The choice of the flexible Nakagami-m density was found to better fit the measured data compared to the commonly used Rayleigh and Ricean distributions. In fact, the proposed scheme captures the spatial variations of the measured channel reasonably well and systematically outperforms two known analytical models in terms of information theory and link-level performance. The second contribution introduces an array processing scheme, namely the three-dimensional (3D) frequency domain Space Alternating Generalised Expectation Maximisation (FD-SAGE) algorithm for jointly extracting the dominant paths’ parameters. The scheme exhibits a satisfactory robustness in a synthetic environment even for closely separated sources and is applicable to any array geometry as long as its manifold is known. The algorithm is further applied to the same set of raw data so that different global spatial parameters of interest are determined; these are the multipath clustering, azimuth spreads and inter-dependency of the spatial domains. The third contribution covers the case of short-range communications which have nowadays emerged as a hot topic in the area of wireless networks. The main focus is on dual-branch MIMO Ricean systems for which a design methodology to achieve maximum capacities in the presence of Line-of-Sight (LoS) components is proposed. Moreover, a statistical eigenanalysis of these configurations is performed and novel closed-formulae for the marginal eigenvalue and condition number statistics are derived. These formulae are further used to develop an adaptive detector (AD) whose aim is to reduce the feasibility cost and complexity of Maximum Likelihood (ML)-based MIMO receivers. Finally, a tractable novel upper bound on the ergodic capacity of the above mentioned MIMO systems is presented which relies on a fundamental power constraint. The bound is sufficiently tight and applicable for arbitrary rank of the mean channel matrix, Signal-to-Noise ratio (SNR) and takes the effects of spatial correlation at both ends into account. More importantly, it includes previously reported capacity bounds as special cases.

Hybrid automatic repeat request transmission (hybrid ARQ) schemes aim to provide system reliability for transmissions over noisy channels while still maintaining a reasonably high throughput efficiency by combining retransmissions of automatic repeat requests with forward error correction (FEC) coding methods. In type-II hybrid ARQ schemes, the additional parity information required by channel codes to achieve forward error correction is provided only when errors have been detected. Hence, the available bits are partitioned into segments, some of which are sent to the receiver immediately, others are held back and only transmitted upon the detection of errors. This scheme raises two questions. Firstly, how should the available bits be ordered for optimal partitioning into consecutive segments? Secondly, how large should the individual segments be? This thesis aims to provide an answer to both of these questions for the transmission of convolutional and Turbo Codes over additive white Gaussian noise (AWGN), inter-symbol interference (ISI) and Rayleigh channels. Firstly, the ordering of bits is investigated by simulating the transmission of packets split into segments with a size of 1 bit and finding the critical number of bits, i.e. the number of bits where the output of the decoder is error-free. This approach provides a maximum, practical performance limit over a range of signal-to-noise levels. With these practical performance limits, the attention is turned to the size of the individual segments, since packets of 1 bit cause an intolerable overhead and delay. An adaptive, hybrid ARQ system is investigated, in which the transmitter uses the number of bits sent to the receiver and the receiver decoding results to adjust the size of the first, initial, packet and subsequent segments to the conditions of a stationary channel.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 139-152). Available also in a digital version from Dissertation Abstracts.

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