Minimum-shift keyed modem implementations for high data rates

Abstract

IGITAL communications links capable of conveying data at hundreds of megabits per second (Mb/s) are becoming increasingly important in various applications. An example is the use of time division multiple access (TDMA) transmission through a satellite relay wherein several data channels are routed through single uplink and downlink paths (antenna beams). Separation of the channels from each other is accomplished through interleaving and deinterleaving in time, with possible rerouting taking place in the satellite. When carrying traffic from large metropolitan areas, the required data rates can exceed 100 Mb/s. One such implementation, currently under study by NASA,' involves data transmission at rates of up to 550 Mb/s. Such applications require the use of modulation schemes that use the available bandwidth efficiently. Furthermore, because power is at a premium on board a satellite, the utmost in power efficiency is desired of the chosen modulation scheme. This implies the use of a constant-envelope modulation scheme since amplifiers, such as traveling wavetube amplifiers, are most efficient when operated near saturation. Three popular constant-envelope modulation schemes for efficient transmission of digital data are quadriphase-shift keying (QPSK), offset (or staggered) quadriphase-shift keying (OQPSK), and minimum-shift keying (MSK). Two excellent recent articles [1,2] in IEEE Communications Magazine have discussed their general properties and attributes as well as those of other modulation schemes with constant (or nearly constant) envelopes. Other recent papers [3-61 have presented analytical results and computer