End-to-end interstellar communication system design for power efficiency

ABSTRACT

Interstellar radio communication accounting for known impairments due to radio propagation in the interstellar medium (attenuation, noise, dispersion, and scattering) and motion is studied. Large propagation losses and large transmitted powers motivate us to maximize the power efficiency, defined as the ratio of information rate to average signal power. The fundamental limit on power efficiency is determined. The power efficiency for narrow-bandwidth signals assumed in many current SETI searches has a penalty in power efficiency of four to five orders of magnitude. A set of five power-efficient design principles can asymptotically approach the fundamental limit, and in practice increase the power efficiency by three to four orders of magnitude. The most fundamental is to trade higher bandwidth for lower average power. In addition to improving the power efficiency, average power can be reduced by lowering the information rate. The resulting low-power signals have characteristics diametrically opposite to those currently sought, with wide bandwidth relative to the information rate and sparse distribution of energy in both time and frequency. The design of information-free beacons power-optimized for a given observation time is also undertaken. Such beacons need not have wide bandwidth, but at low powers their energy is sparsely distributed in time. The discovery of both beacons and information-bearing signals is analyzed, and shown to require a substantial number of observations (growing as power is reduced) to achieve a high probability of success. The "false alarms" in current searches are characteristic signatures of possible power-efficient and power-optimized signals. Although existing SETI searches will fail to discover these signals, they can be discovered using common algorithms with straightforward modification to current search methodologies.