Transmission modelling of ENCAP-OQPSK radio links

Offset quaternary PSK-type (OQPSK) digital modulation schemes are strongly recommended for radio transmission within land mobile and/or satellite communication systems requiring good power/bandwidth tradeoffs and low-cost, power-efficient implementations. The ENCAP-OQPSK design method (encoded-amplitude-and-phase) previously proposed by the authors was shown to provide a wide class of OQPSK-type schemes of practical interest; a flexible, digital transmitter implementation can be directly based on the encoding rules of this design method. The paper deals with the transmission modelling of radio links where ENCAP-OQPSK modulators are employed. In spite of the apparent complexity of the encoding rules, it was possible to define an appropriate signal model, both accurate and user-friendly. In fact, the nonlinearly modulated ENCAP-OQPSK signals can always be exactly represented as a sum of linear OQPSK-type components. Similar representations, also exact, can be adopted for any signal obtained by filtering and/or nonlinear amplification of the signals. This modelling approach simplifies spectral computations and performance evaluations, and is quite accurate, in general, when the sum of the two main linear components is used to approximate the transmitted signals