Utilizing time-bandwidth space for efficient deep-space communication

We discuss modulation formats that warrant high information efficiency of deep-space optical links under given power constraints. The discussion is framed using the theoretical concept of orthogonal optical modes occupying the available time-bandwidth space. With diminishing average signal power, the challenge is to concentrate the entire optical energy in very few of these modes. In the generic case of pulse position modulation (PPM), where optical modes occupy separate time bins, this results in increasing peak-to-average power ratio requirements for the transmitter light source. Equivalent information efficiency can be attained using frequency shift keying (FSK) employing modes that do not overlap in the spectral degree of freedom and have uniformly distributed instantaneous optical power in the temporal domain. Recently, efficient modulation formats have been proposed that use words composed from the binary phase shift keying (BPSK) alphabet. Such words can be converted after transmission into the PPM format with the help of structured optical receivers. Selected technical aspects of physical implementations of links based on different modulation formats are briefly reviewed.