ABSTRACT
This research work presents the development of a Modified Link Budget for Low Earth Orbiting (LEO)-Based Land Mobile Satellite Communications System operating at Ku, K and Ka frequency bands by taking into cognizance the effect of additional loss due to Doppler frequency shift. Doppler frequency shift poses the problem of receiving higher or lower frequencies than the original transmitted frequency, this may be as a result of a relative movement between the source of the signal and the object (satellite or receiver) or both. In satellite communication system, this phenomenon constitutes the problem of signal loss.Effect of Doppler shift on satellite link budget is assumed negligible in conventional approach thereby resulting in the design of an unrealistic link budget, particularly in Low earth orbit (LEO) where it is more pronounced. In view of this, a more reliable approach to the estimation of satellite link budget at Ku, K, and Ka bands by the inclusion of Doppler frequency shift effect was investigated and its effect was investigated at different satellite orbits (LEO, MEO and GEO). The results obtained show that at maximum satellite converge angle and central frequencies for Ku, K and Ka bands, the Doppler frequencies for LEO (780 km) are: 325.50 kHz, 423.20 kHz and 726.90 kHz; for MEO (20000 km) we have 88.33 kHz, 114.80 kHz and 197.30 kHz; while GEO (35786 km) stood at 55.26 kHz, 71.84 kHz and 123.40 kHz . Variation of Doppler frequency shift with respect to the latitude (location) of the earth's terminal relative to the satellite motion was also studied. A typical earth terminal location in the range of 0 km - 100 km was selected for the study; from which it was verified that effect of Doppler shift in LEO increased as the distance from the initial location of the user terminal increased. These analyses further confirm that Doppler effect is more pronounced in LEO than in MEO and GEO. Comparative analyses between the conventional and the modified link viii budget at Ku, K and Ka bands was achieved thereof. The results obtained show the Carrier to Noise density ratio for Ku frequency banddropped by 40% (from 25dB without Doppler shift to 15dB with Doppler shift). The Carrier to Noise density ratio for K frequency band dropped by 57% (from 70dB without Doppler shift to 30dB with Doppler shift). The Carrier to Noise density ratio for Ka frequency band dropped by 52% (from 110dB without Doppler shift to 53dB with Doppler shift). This further confirmed that Doppler shift is most pronounced at LEO orbit hence the need to incorporate its effect in link budgeting. This work was validated against the work of Snehasis and Barsha, (2014).The results obtained through comparison show the Carrier to Noise density ratio for Ku frequency banddropped by 58% (from 31dB without Doppler shift to 13dB with Doppler shift). The Carrier to Noise density ratio for K frequency band dropped by 62% (from 54dB without Doppler shift to 20dB with Doppler shift). The Carrier to Noise density ratio for Ka frequency band dropped by 55% (from 85dB without Doppler shift to 38dB with Doppler shift). This clearly shows the need for inclusion of Doppler shift effect in LEO-based link budget.