Effects of Multipath on Telemetry Data Transmission

Abstract

Spurious amplitude and phase modulation effects occur in the received telemetry signal when both the direct waves and the indirect waves enter the receiving antenna. In this paper, a simple multipath model is used which has a single direct wave and a single indirect wave. The indirect wave differs from the direct wave by a time delay and a constant amplitude. With such a simple model, the effects of multipath on telemetry data transmission can be fully examined because of the mathematical simplicity. First an FM system operating above the threshold is considered. The intermodulation distortion and the degradation in the data signal-to-noise ratio due to multipath are both examined. Next we consider two digital systems, namely the noncoherent FSK and the PCM/FM with the discriminator detection. Both systems utilize a split-phase baseband signal. And the error rates depend on the multipath time delay. Comparison is made between the split-phase and the NRZ (non-return-to zero) baseband signals. Available performance improvement from using the predetection diversity combining and the synchronization problem are then discussed. Finally several methods to improve the telemetry data are suggested.