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Enhancing the PCM/FM Link - Without the Math

April 01, 2007

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ABSTRACT

Since the 1970s PCM/FM has been the dominant modulation scheme used for RF telemetry. However more stringent spectrum availability as well as increasing data rates means that more advanced transmission methods are required to keep pace with industry demands. ARTM Tier-I and Tier-II are examples of how the PCM/FM link can be enhanced. However these techniques require a significant increase in the complexity of the receiver/detector for optimal recovery.

This paper focuses on a quantitative approach to improving the rate and quality of data using existing PCM/FM links. In particular ACRA CONTROL and BAE SYSTEMS set themselves the goal of revisiting the pre-modulation filter, diversity combiner and bit-sync. By implementing programmable adaptive hardware, it was possible to explore the various tradeoffs offered by modifying pulse shapes and spectral occupancy, inclusion of forward error correction and smart source selection. This papers looks at the improvements achieved at each phase of the evaluation.

KEYWORDS:  PCM/FM, Spectral Efficiency, Forward Error Correction, Best Source Selection.

1 INTRODUCTION

As flight test instrumentation becomes more sophisticated it has become increasingly desirable to have a high quality, high speed telemetry link between the aircraft and the ground monitoring station. PCM/FM has been the primary modulation scheme used for such links for over 30 years, mainly due to its simplicity and reliability. Due to the longevity of PCM/FM, most airborne telemetry facilities have invested significant capital in equipment specifically for use with this standard.

It is only in recent years that new modulation techniques (such as the ARTM Tier-I and Tier-II schemes) have been proposed in a standardized form with the goal of improving the spectral efficiency of these telemetry links. Although these modulation methods do accomplish this goal, optimal implementations require significantly more complex modulators, demodulators and detection schemes than those for PCM/FM as well as typically requiring longer lock times.

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