Introduction to digital modulation schemes

The choice of digital modulation scheme will significantly affect the characteristics, performance and resulting physical realisation of a communication system. There is no universal 'best' choice of scheme, but depending on the physical characteristics of the channel, required levels of performance and target hardware trade-offs, some will prove a better fit than others. Consideration must be given to the required data rate, acceptable level of latency, available bandwidth, anticipated link budget and target hardware cost, size and current consumption. The physical characteristics of the channel, be it hardwired without the associated problems of fading, or a mobile communications system to a F1 racing car with fast changing multipath, will typically significantly affect the choice of optimum system. The objective of this paper is to review the key characteristics and salient features of the main digital modulation schemes used, including consideration of the receiver and transmitter requirements. Simulation is used to compare the performance and tradeoffs of three popular systems, MSK, GMSK and BPSK, including analysis of key parameters such as occupied bandwidth and bit error rate (EER) in the presence of additive white Gaussian noise (AWGN). Finally the digital modulation schemes used in the current and proposed cellular standards are summarised.