End to end design of interstellar radio communication that is thousand to tens of thousands of times more power efficient

This 237 page report has addressed the end-to-end design of an interstellar communication system taking account of all known impairments from the interstellar medium and induced by the motion of source and observer. Both discovery of the signal and post-discovery communication have been addressed. It has been assumed that the transmitter and receiver designers wish to jointly minimize the cost, and in that context we have considered tradeoffs between resources devoted in the transmitter vs. the receiver. The resources considered in minimizing cost are principally energy consumption for transmitting the radio signal and energy consumption for signal processing in a discovery search.

Note – project Icarus had worked out an even more energy efficient communication system if the sender and receiver are at gravitation lensing points of two stars. One tenth of a milliwatt is enough to have perfect communication between the Sun and Alpha Cen through two 12-meter FOCAL spacecraft antennas. A similar bridge between the Sun and a Sun-like star inside M31, using the gravitational lenses of both. We’re working here with a distance of 2.5 million light years, but a transmitted power of about 107 watts would do the trick

Thus study is applicable to communication with interstellar spacecraft (sometimes called ”starships”) in the future, although it will probably be quite some time before these spacecraft wander far enough to invoke many of the impairments considered here. In the present and nearer term this study if relevant to communication with other civilizations. In the case of communication with other civilizations, we of course will be designing either a transmitter, or a receiver, which in either case has to be interoperable with a similar design by the other civilization. In either case, it is very helpful to consider the end-to-end design and resource tradeoffs, as we have attempted here. In our view, a major shortcoming of existing SETI observation programs (which requires the design of a receiver for discovery) is the lack of sufficient attention to the issues faced by the transmitter in overcoming the large distances and the impairments introduced in interstellar space.

One profound conclusion of this study is that if we assume that the transmitter seeks to minimize its energy consumption, which is related to average transmit power, then the communication design becomes relatively simple. The fundamental limit on power efficiency in interstellar communication has been determined, that limit applying not only to us but to any civilization no matter
how advanced. Drawing upon the early literature in power-efficient communication design, five simple but compelling design principles have been identified. Following these principles has been shown to permit designs that approach the fundamental limit. Although the same fundamental limit does not apply to the discovery process, we have defined an alternative resource-constrained
design approach that minimizes processing resources for a given power level, or power level for a given processing resource. Again, application of a subset of the five principles leads to a design that can achieve dramatic reductions in the transmitter’s energy consumption relative to the type of Cyclops beacons that have been a common target of SETI observation programs, at the expense of a more modest increase in the receiver’s energy consumption through an increased observation time in each location.

The power efficiency for narrow-bandwidth interstellar radio communication 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 (thousand to tens of thousands of times more power efficient interstellar communication) 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.

Five principles of power-efficient design
* Use energy bundles
* Avoid channel impairments
* Convey information by location rather than amplitude
* Make energy bundles sparse
* Combat scintillation with time-diversity combining

Required average receive power

The state of the art in receiver design on Earth can achieve a receiver internal noise temperature of 5 degrees kelvin. This would be a representative value that a transmitter would have to assume in order to communicate reliably with us. If we were to construct our own transmitter, it might be reasonable to assume a lower value. Added to this is cosmic background radiation at 3 degrees kelvin, applicable to more advanced civilizations as well as ourselves, giving a total noise temperature of T = 8 degrees kelvin. This is the most optimistic case, since it does not take into account star noise. Based on an assumption of 8 degrees kelvin, the fundamental limit for received energy per bit is

7.66 X 10^-23 joules .

1 bit per second communication would be 7.66 X 10^-23 watts

Required average transmit power

To transmit 500 light years at the fundamental limit at 5 GHz frequency
4632 watts

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