Determining Optimum Modulation for Inter-Satellite Communications Systems

In the small satellite world, it is becoming increasingly popular to use a constellation of small satellites to do the work previously performed by one large satellite. Implicit in this scenario is the requirement for clusters of small satellites to communicate among themselves via an inter-satellite link (ISL). Traditionally, an ISL has taken the form of a time-division-multiple-access (TDMA) communications system. In a TDMA system, each satellite gets the use of the channel (i.e. gets to transmit) at a particular time and for a particular length of time. Usually this system uses binary-phase-shift keying (BPSK) or some other type of coherent modulation. The choice of BPSK is made because it is more power-efficient than other types of modulation, such as frequency-shift keying (FSK). When using a BPSK system, there are inherent delays that occur when one satellite’s time slot ends and another’s begins. These delays are caused by the time required for the BPSK receivers to coherently lock to the incoming BPSK signal. These delays, which cause a direct reduction in system throughput, have to be factored into the overall system design. It is well known that FSK provides essentially instantaneous communication (i.e. no lock-up time). The penalty for using FSK, however, is the higher transmitter power required compared to BPSK. As TDMA systems become more crowded, meaning more users to be provided time slots within a given time period, the lock-up time becomes a significant portion of the total available time. This corresponds to an overall reduction in system data throughput. At some point, the advantages of BPSK are outweighed by the required lock-up time when the user changes. This paper quantitatively examines the tradeoffs involved in selecting a modulation type when the above parameters are considered. A set of metrics is developed to assist in determining the optimum modulation type and multi-variable charts are presented that show the optimum modulation to use for a given scenario.