  Microwave digital communication at interstellar distances applies to starship
and extraterrestrial civilization (SETI and METI) communication. Large
distances demand large transmitted power and/or large antennas, while the
propagation is transparent over a wide bandwidth. Recognizing a fundamental
tradeoff, reduced energy delivered to the receiver at the expense of wide
bandwidth (the opposite of terrestrial objectives) is advantageous. Wide
bandwidth also results in simpler design and implementation, allowing
circumvention of dispersion and scattering arising in the interstellar medium
and motion effects and obviating any related processing. The minimum energy
delivered to the receiver per bit of information is determined by cosmic
microwave background alone. By mapping a single bit onto a carrier burst, the
Morse code invented for the telegraph in 1836 comes closer to this minimum
energy than approaches used in modern terrestrial radio. Rather than the
terrestrial approach of adding phases and amplitudes to increases information
capacity while minimizing bandwidth, adding multiple time-frequency locations
for carrier bursts increases capacity while minimizing energy per information
bit. The resulting location code is extremely simple and yet can approach the
minimum energy as bandwidth is expanded. It is consistent with easy discovery,
since carrier bursts are energetic and straightforward modifications to
post-detection pattern recognition can identify burst patterns. The
interstellar coherence hole captures the time and frequency coherence
constraints, and observations of the environment by transmitter and receiver
constrain the burst parameters and limit the search scope.
