Modeling the Dispersion and Gain of RF Wireless Channels inside Reverberent Enclosures
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Sensor and instrumentation networks operating inside aircraft wings, unmanned air vehicle (UAV) fuselage, small submarine craft and/or automobile engine compartments could be significantly enhanced or enabled by implementing radio communications within these spaces. A wired infrastructure can be cumbersome and expensive to install and maintain inside challenging environments such as aircraft and submarines. Furthermore, wire bundles and assemblies can be heavy, and flight vehicle environments are particularly sensitive to mass distribution. Wireless sensor and instrumentation networks would offer an alternative to their wired counterparts, and could alleviate many of the aforementioned concerns associated with wired assemblies. Furthermore, wireless sensor networks could be quickly deployed in these environments, enabling rapidprototype instrumentation and sensor systems. The nodes of this network are envisioned to be quite small such that the environments they are measuring are not appreciably affected by the presence of the nodes themselves. Perhaps the size of a dime, these sensor nodes would nominally contain all of the sensors, processors, antennas, communications subsystems and energy storage elements necessary to perform the required instrumentation operations in these proposed environments. However, the interior enclosures of aircraft wings, UAV fuselage and small submarines are highly reverberant to propagating electromagnetic waves due to the metallic walls surrounding the space, and energy that is transmitted within the enclosure by a radio device can be expected to remain in the space for long durations in time. This lingering of energy can cause the communications channel dispersion in the enclosure to be considerable, potentially limiting and/or degrading communications. The work described in this dissertation studies the wireless RF communications channel native to these enclosures to better understand the unique characteristics of this channel, referred to as the enclosed space radio channel. For the purposes of this research, the enclosures considered are generally described as being 1) several meters or less per dimension, 2) enclosed by metallic boundaries on many or all sides, and 3) filled with non-uniform objects and/or obstructions. Measurements of the channel dispersion in two representative enclosures are made between 200 MHz and 20.0 GHz, utilizing the metrics of RMS delay spread, mean excess delay, and the 50% and 90% coherence bandwidth. A model for the channel dispersion is formed based on simple descriptions of the enclosure, and it is shown that this model matches the measured data well. A model for the gain of the enclosed space channel is developed and presented, and this model also demonstrates a good match to the gain measured in the channel. The dynamic range of the enclosed space channel gain is measured and modeled, and both theory and the measurements show that the dynamic range is bounded in a high-frequency limit. Both the dispersion model and gain model utilize the enclosure quality factor as a critical parameter, and through the quality factor an important tradeoff between gain and dispersion is seen. Measurements of the enclosures used in this work show average RMS delay spreads of 100-300 ns and 50% coherence bandwidths of 1-5 MHz, indicating that the dispersion in these enclosures is less than the dispersion seen in the outdoor mobile communications channel, but greater than the dispersion seen in the indoor wireless communications channel. Measurements of the gain in these enclosures show average values between -10 and -30 dB—considerably larger than the gain typically seen in the indoor or outdoor channels. Single-carrier communications experiments using real signals of several different modulation formats are made between 3.0 and 18.0 GHz, and the results show that data rates up to 5 Mbps are readily obtainable. The orthogonal frequency division multiplexing (OFDM) multicarrier modulation scheme is evaluated in the enclosed space channel, and results suggest that data rates up to 54 Mbps are possible using the IEEE 802.11a communications standard.