Data networking via satellite relays remains an important means of linking globally-distributed network terminals for both commercial and governmental applications, especially in remote regions. Modern applications demand both spectrum and power efficiency in the network. The archetype network model in such networks is one with two terminals wishing to exchange data via a single satellite transponder. Relative to traditional time-sharing or frequency-sharing for the bidirectional communication paths, information-theory reveals that spectrum efficiency gains of up to 100% can be obtained for a given set of link power resources. These gains are possible when non-orthogonal transmission methods are adopted, and the decoders exploit side-knowledge on previously-transmitted information.
The project codifies various protocols appropriate to this two-terminal data exchange model, including amplify-forward, as well as protocols that involve satellite decoding/re-encoding. The possible gains depend on link resources as well as the desired bidirectional rate targets. Existing research for this problem presumes perfect synchronization and side-information at both terminals, but practical issues of large round-trip delay, carrier phase/frequency synchronization, and symbol synchronization are important obstacles to achieving the promise of information theory. So the investigators develop realistic synchronization protocol designs that approach the ideal information-theoretic limits. In addition, the project studies a new decode-and-forward relaying protocol based on nested LDPC coding on downlinks that is flexible in terms of rate-asymmetry.
The project studied the efficient exchange of data via satellite between two earth terminals, improving upon the conventional method of time-sharing or frequency-splitting for the bidirectional transmissions. Instead, same-time/same-frequency operation is used, doubling the spectrum efficiency over conventional practice. This promises to greatly lower cost of satellite networking, as satellite time charges are driven by spectrum occupancy. Our research codified the available satellite relaying protocols as to their regions of achievable rates. These protocols include amplify-and-forward and decode-and-forward. While the latter is in some situations better, the flexibility, general all-around efficiency, and transparency of amplify-and-forward operation prompted our focus on detailed analysis of this. We identified achievable rates over the nonlinear satellite channel for both one-way and two-way operation, as a function of link signal-to-noise ratio and backoff of the satellite amplifier. Both conventional matched filter receivers and an oversampled front end were studied. The former, though known to be suboptimal, is simple and provides a baseline for study. Oversampling preserves the total information in the received waveform and allows operation at slightly lower signal-to-noise ratio for a given transmission rate. Subsequently these theoretical limits on data transfer were shown to be nearly achievable by practically-realizable receiver processing methods that mitigate the self-interference associated with our mode of operation. Results have been presented in seven professional conferences, plus two pending journal publications. Three graduate degrees have been produced under the project. Talks are in progress with a satellite networking company to incorporate some of our results into their product line.
