Abstract
Mobile Satellite Systems (MSS), providing world-wide voice and data communications are planned to come into service to complement the existing terrestrial cellular systems. As the demand for these services and the requirement for higher data rates and quality of service increases, the effects of the wideband mobile satellite propagation channel become more critical with respect to the overall system link budget. Propagation measurements have mainly concentrated on the characterisation of the narrowband satellite channel, in order to fully characterise the mobile satellite channel, wideband measurements are required. A purpose-designed wideband channel sounder of the swept time delay crosscorrelation type was produced. The Sounder was designed specifically to operate simultaneously in two frequency bands, at L-Band (1540-1560 MHz) and S-Band (2315-2335 MHz). The main characteristics of the Sounder were a minimum delay resolution of 100ns, a dynamic range of 30dB and a modulation spectrum of bandwidth around 20MHz. Particular features incorporated were a range of code sequences (127, 255, 511 and 1023), in order to maximise the delay range and the ability to measure the absolute delay using an additional reference correlator in the receiver which mimics that in the transmitter. Both I and Q outputs are available from each channel and the reference correlator output. The wideband propagation measurement campaign applicable to mobile satellite communication systems at L and S- bands was implemented in the Urban, Suburban, Open/Highway, Heavily and Lightly Wooded environments covering an elevation angle range of 15° to 80°, around the Guildford area and at the University of Surrey during the Summer of 1995. A helicopter was used as a platform to simulate satellite transmissions and flew orthogonal to the measurement vehicle and in a hash (#) pattern. The former producing worst case azimuth results for attenuation and shadowing, but allowed for the stable emulation of the satellite propagation channel with a high degree of control over parameters such as speed and elevation. Omni-RHCP antennas were used on the helicopter and road vehicle, also LHCP hand-held antennas were used. Data analyses indicate that the satellite propagation channel is entirely different to the conventional terrestrial mobile channel. The main reason for the difference is the dominance of the LOS signal. The particular environment and elevation angle have an effect on the various wideband parameters, due to the geometrical relationships of the various scatterers in the local vicinity within the environment and the elevation angle of the propagating signal. The results showed that the satellite channel usually has short multipath echo delays with Average Delays less than 1 usec and RMS Delay Spreads less than 0.25 μsec. The echo power levels were typically 10-20 dB below the LOS signal and reduce with increasing delay. For short periods, (<1 second), far echoes with delays around 2 μsec were detected, with power levels typically around 25-30dB below the LOS signal. A wideband channel model is proposed applicable to mobile satellite systems at L and S-Bands. The model is of the Tapped Delay-line type, comprising a transversal filter structure with equidistant time shifted taps. The model parameters have been derived from the measurement campaign of this study, and defined by six discrete taps, where each tap is described by its complex time-varying amplitude including its statistical distribution and corresponding Doppler spectrum.