Passive beam forming and spatial diversity in meteor scatter communication systems

Abstract

The method of passive beam formation using a four-element Butler matrix to improve the signal availability of meteor scatter communication systems is investigated. Signal availability, defined as the integrated time that the signal-to-noise ratio (snr) exceeds some sf•r threshold, serves as an important indicator of system performance. Butler matrix signal availability is compared with the performance of a single four-element Yagi reference system using --6.5 hours of data from a 720 km north-south temperate latitude link. The signal availability improvement factor of the Butler matrix is found to range between 1.6-1.8 over the snr threshold range of 20-30 dB in a 300-Hz bandwidth. Experimental values of the Butler matrix signal availability improvement factor are compared with analytical predictions. The experimental values show an expected snr threshold dependency with a dramatic increase at high snr. A theoretical analysis is developed to describe this increase. The signal availability can be further improved by --10-20% in a system employing two four-element Butler matrices with squinted beams so as to illuminate the sky with eight high-gain beams. Space diversity is found to increase the signal availability of a single antenna system by 15%, but the technique has very little advantage in a system already employing passive beam formation.One strategy to overcome the low data throughput focuses on improved error coding. Fixed rate forward error correction (FEC) coding offers an improvement of--2•% in the data throughput over uncoded systems [Miller and Milsrein, 1990]. Further slight improvements can be made by using more complex variable-rate coding schemes [Pursley and Sandber•, 1989], but significant advances are constrained by the limited bandwidth of most M$C systems. An alternative technique is •o employ variable data transmission rates, whereby the data rate profile mimics, in some fashion, the decay profile of the trail snr. Such a scheme takes advantage of the Published in 1996 by the American Geophysical Union. Paper number 95RS03258. relatively high snr at the beginning of the trail [e.g., Weitzen et al., 1984; Davidovici and Kanterakis, 1993]. Shukla et al. [1992] and Cannon et al.[1993] investigated increasing the trail duration, and thereby the data throughput, by using antenna space diversity. This technique takes advantage of the fact that toward the end of a trail, and particularly in the case of long-duration trails, the incoming wavefront is no longer planar because of interference between decorrelated signals. Shukla et al. [1992] found that -40% of all trails with duration >0.75 s showed significant signs of spatial decorrelation over antenna separations of 10A. Further breakdown of trail statistics into underdense, overdense, and not-known categories showed that the underdense and not-known categories did not benefit from increasing the antenna separation above 5A. The overdense trails, however, required a separation of 20A before signal decorrelation was apparent. Cannon et al. [1993] ...

A meteor scatter prediction model and its application to adaptive beam steering

“…A theoretical study by Kilpatrick et al [1991] found that both passive and adaptive systems provide useful increases in throughput. Mawrey and Weitzen [1995] and Akram and Cannon [1996] subsequently confirmed experimentally the advantages cited in favor of the passive method by Kilpatrick et al [1991].…”

“…Full details about the hardware and the digital signal processing (DSP) implementation of the phased array are given by Akram and Cannon [1996]. It is sufficient to note here that the five phase locked channels of in-phase and quadrature components were logged at ---300 samples/s in data blocks of 500 ms.…”

“…Antenna enhancements, equivalent to increased transmitter power, are necessary to realize the desired increase in throughput [e.g., Kilpatrick et al, 1991, Mawrey andWeitzen, 1995;and Akram and Cannon, 1996]. The result of such enhancements is increased gain which, if effectively directed, can be used to increase the number of usable trails.…”

“…beam forming systems. The former use a passive phasing network, such as a Butler matrix [Delaney, 1962] to synthesise high gain beams in fixed directions [e.g., Mawrey and Weitzen, 1995;Akram and Cannon, 1996]. Adaptive systems update the direction of a single high-gain beam in response to the direction of individual trails [Kilpatrick et al, 1991] or according to the empirically determined passage of hotspots over a given link [Desourdis et al, 1994].…”

Relative impact of meteor scatter and other long‐distance high‐latitude propagation modes on VHF communication systems

“…Beyond line of sight (BLOS) meteor burst (MB) communications [e.g., McKinley, 1961;Schanker, 1990;Schilling, 1993] via meteor scatter propagation has been studied since the late 1950s, and through this period there has been a steady improvement in system performance [e.g., Cannon and Reed, 1987;Mawrey and Weitzen, 1995;Akram and Cannon, 1996]. One important approach to system improvement has been the propagation experiment which aims to quantify throughput [e.g., Ostergaard et al, 1990] and verify models [e.g., Cannon, 1986].…”