Seminar

Low-energy interstellar radio communication

March 20, 2014

David Messerschmitt

The design of an interstellar digital communication system at radio wavelengths and interstellar distances is considered, with relevance to starships and extraterrestrial civilizations (including SETI and METI). We approach the problem as a traditional end-do-end design based on models of the channel impairments introduced in the interstellar medium (ISM) and making use of the observations of pulsar astronomy. The ISM in conjunction with transmitter/receiver motion determine the coherence time and coherence bandwidth of the channel, and the concept of an interstellar coherence hole (ICH) is introduced. The distances involved imply large transmitted power and/or large antennas, the costs of which can be mitigated by reducing the energy delivered to the receiver. In communications there is a fundamental tradeoff between wider signal bandwidth and lower signal energy per information bit, and our design chooses to minimize energy. This means unconstrained bandwidth, which is also helpful in simplifying the design to offset the lack of coordination. Based on statistical techniques resembling thermodynamics, communication is subject to a fundamental lower limit on delivered energy per bit subject to reliable extraction of information. We have shown that the cosmic microwave background (CMB) is the only fundamental limitation on delivered energy per bit, as the remaining impairments can be circumvented by appropriate signal design (including making use of the ICH) and technology. We demonstrate a simple and transparent signal structure that can approach this fundamental limit as bandwidth expands. In the context of known physics, technology available on Earth can approach the fundamental limit, and no other civilization (no matter how advanced) can do better.

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