EHF Satellite Communications on the Move: Baseband Considerations

摘要

In this report, baseband signal processing and networking protocol issues associated with the development of an EHF land mobile satellite communications system are investigated. Ignoring the very important problems of antenna pointing and tracking, the primary obstacle to be overcome by the system is signal blockage. At EHF, objects in the propagation path are virtually opaque and cast a dark 'shadow' over the communications terminal, resulting in signal attenuations on the order of 20-30dB or more. The duration of these shadow regions or blockage intervals will depend on a number of factors, including object size and vehicle speed. Recommended solutions to this problem include the following: forward error correction coding at the physical layer to mitigate the effects of relatively short blockage intervals (i.e., milliseconds to seconds), and automatic repeat request schemes at the linklayer to ensure reliability over longer shadow regions (i.e., seconds to lOs of seconds) In addition, dynamic routing protocols can be used at the network layer to provide connectivity to blocked terminals via alternate terrestrial communications paths that include at least one unobstructed terminal. Finally, TCP splitting and protocol conversion offer a great deal of promise for achieving reliable end-to-end transport layer services without sacrificing TCP compatibility.