Variable Coded Modulation

The below abstract is excerpted from Thomas A. Sielicki's thesis, "Overcoming CubeSat Downlink Limits with VITAMIN: A New Variable Coded Modulation Protocol". For more detail, see the full thesis. The full text can be found here. There is also a IEEE Aerospace Conference paper available on the topic, which can be found here (IEEE Xplore access required).

Abstract

Many space missions, including low earth orbit CubeSats, communicate in a highly dynamicenvironment because of variations in geometry, weather, and interference. At thesame time, most missions communicate using fixed channel codes, modulations, and symbolrates, resulting in a constant data rate that does not adapt to the dynamic conditions.

When conditions are good, the fixed date rate can be far below the theoretical maximum,called the Shannon limit; when conditions are bad, the fixed data rate may not work atall. To move beyond these fixed communications and achieve higher total data volumefrom emerging high-tech instruments, this thesis investigates the use of error correctingcodes and different modulations. Variable coded modulation (VCM) takes advantage ofthe dynamic link by transmitting more information when the signal-to-noise ratio (SNR)is high.

Likewise, VCM can throttle down the information rate when SNR is low withouthaving to stop all communications. VCM outperforms fixed communications which canonly operate at a fixed information rate as long as a certain signal threshold is met.

This thesis presents a new VCM protocol and tests its performance in both software andhardware simulations. The protocol is geared towards CubeSat downlinks as complexityis focused in the receiver, while the transmission operations are kept simple. This thesisexplores bin-packing as a way to optimize the selection of VCM modes based on expectedSNR levels over time. Working end-to-end simulations were created using MATLAB andLabVIEW, while the hardware simulations were done with software defined radios. Resultsshow that a CubeSat using VCM communications will deliver twice the data throughputof a fixed communications system.