PERFORMANCE OF A SATELLITE MODEM TRANSMITTING CONVOLUTIONALLY AND DIFFERENTIALLY ENCODED 8PSK SIGNALS

The paper studies a digital satellite modem for mobile systems. The mo dem transmits a convolutionally and differentially encoded eight-phase -shift-keyed (CDE8PSK) signal over a satellite link and employs a near -maximum-likelihood decoder at the receiver. The signal can tolerate p hase ambiguities that are multiples of pi/4 radians, which normally re sult from the carrier recovery loop at the receiver. However, the carr ier recovery loop is not considered here. The high-power amplifier (HP A) at the earth-station transmitter may introduce non-linear distortio n to the CDE8PSK signal, and predistortion may or may not be used. Sev eral different earth-stations are assumed to have simultaneous access to a given transponder on the satellite, in an arrangement of frequenc y division multiple access, but the satellite transponder is assumed t o be linear. Thus the CDE8PSK signals that occupy the immediately adja cent frequency bands can introduce adjacent-channel interference (ACI) to the desired CDE8PSK signal. Extensive computer-simulation tests ha ve been carried out to evaluate the effects of non-linear distortion w ith predistortion, in the presence of ACI, on the tolerance of the mod em to additive white Gaussian noise. Four different bandwidths of the transmitted signal are considered, together with three different arran gements of the demodulator. The predistorter operates on the baseband signal at the input to the modulator and assumes a prior knowledge of the HPA characteristics. In all simulation tests, an equivalent baseba nd model of the transmission system is used, and the results are used to determine the preferred modem design.