Browsing International Telemetering Conference Proceedings, Volume 55 (2019) by Subjects
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Adaptive OFDM for Aeronautical ChannelsPrevious work modeled the cruise phase of an aeronautical channel and showed how the channel varied as a function of height, distance, and speed. What was apparent from that analysis was that the ``cruise" channel was remarkable stable and varied slowly and predictably over time. The steady state channel reflected a 2-ray multipath model which exhibits deep nulls in the spectrum which affects serial tone modems significantly. Further the application of parallel tone modulation improves performance except for that portion of the band which was degraded by the null. This points to the use of Adaptive OFDM (AOFDM) structure wherein tones are only sent in portions of the band which are strong and not areas where the signal is weak. This work develops a method for capturing a profile of the Signal to Distortion Ratio (SDR) for each tone for each frame and over time. It also develops a method for converting the SDR per tone to estimate the optimum QAM modulation scheme for each tone for application in Link Dependent Adaptive Radio (LDAR).
Peak-to-Average Power Ratio (PAPR) Reduction for OFDMThe telemetry community has been challenged in its search for additional spectrum for its aeronautical mission. With a fixed amount of spectrum the challenge becomes focused on increased spectrum efficiency. Today’s best solution for spectrum efficiency is Orthogonal Frequency Division Modulation (OFDM). This approach has proven effective with both cellular LTE as well as IEEE 802.11 wireless LAN systems. OFDM has seen limited use in telemetry systems in part due to issues related to high peak to average ratio of OFDM signals. This paper reviews approaches to resolving these issues and proposes a scheme for peak conditioning of OFDM signals to reduce the peak to average ratio. Results of preliminary experimental work are promising.