SATELLITE LINK DESIGN - PDF Free Download

1 SATELLITE LINK DESIGN Networks and Communication Department Dr. Marwah Ahmed

Outlines 2 Introduction Basic Transmission Theory System Noise Temperature and G/T Ratio Design of Downlinks Satellite Communication Link Budget

Introduction 3 In Satellite Communications, three factors influence system design: frequency band, atmospheric propagation effect, and multiple access technique. Table 1. Frequency bands allocated for the satellite.

Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright 2003 John Wiley & Sons. Inc. All rights reserved. Introduction: Types of satellites Figure 10.25 (p. 422) One-way propagation delay for the three orbits: LEO, MEO, and GEO. The one-way delay figures shown above have been calculated assuming the radio signal propagates at the speed of light in a vacuum, i.e., 3 X 10 8 m/s. That is, no account has been taken of any delay due to the refractive index of the atmosphere not being unity. Also, no account has been taken of any processing delay imposed on the signal from any source coding, channel coding, modulation, or access scheme used.

Class Work! 5 From the figure below, explain, how do satellites perform communications? And define the two major elements of satellite communications system!

6 Basic Transmission Theory

Basic Transmission Theory 7 The calculation of the power received by an earth station from a satellite transmitter is fundamental to the understanding of satellite communications. Consider a transmitting source, in free space, radiating a total power P t watts uniformly in all directions as shown in Figure 4.2.

Figure 4.2 (p. 101) Flux density produced by an isotropic source. Satellite Communications, 2/E by Timothy Pratt, Charles Bostian, & Jeremy Allnutt Copyright 2003 John Wiley & Sons. Inc. All rights reserved.

Basic Transmission Theory 9 At a distance R meters from the hypothetical isotropic source transmitting RF power P t watts, flux density crossing the surface of a sphere with radius R is given by: P t F = W m2 4πR2

Basic Transmission Theory 10 All real antennas are directional and radiate more power in some directions than in other. Any real antenna has a gain G θ, as sketched below.

Basic Transmission Theory 11 For a transmitter with output P t watts driving a lossless antenna with gain G t, the flux density in the direction of the antenna boresight at distance R meter is: F = P tg t 4πR 2 The product P t G t is often called the effective isotopically radiated power or EIRP

Basic Transmission Theory 13 If we had an ideal receiving antenna with an aperture area of A m 2, as shown in Figure 4.3, we will collect power P r watts given by: P r = F A watts A practical antenna with a physical aperture area of A r m 2 will not deliver the power transmitted, and some is absorbed by lossy components. This reduction in efficiency is described by using an effective aperture A e where: A e = η A A r And η A is the aperture efficiency of the antenna.

Basic Transmission Theory 14 The power received by a real antenna is P t G t G r P r = 4πR λ 2 This expression is known as the link equation, and it is essential in the calculation of power received in any radio link. The term 4πR λ 2 is known as the path loss, L p. Therefore, we can write the P r as: power recieved = EIRP Receiving antenna gain Path loss watts

Basic Transmission Theory 15 In Communication systems, decibel quantities are commonly used to simplify calculations: P r = EIRP + G r L p dbw In practice, we will need to take account of more complex situation in which we have losses in the atmosphere due to attenuation by oxygen, water vapor, and rain, losses in the antennas at each end of the link. P r = EIRP + G r L p L a L ta L ra dbw where L a is the attenuation in the atmosphere, and L ta, L ra are the losses associated with the transmitting and receiving, respectively.

17 System Noise Temperature and G/T

Noise Temperature 18 Is a useful concept in communications receivers, since it provides a way to determining how much thermal noise is generated by active and passive devices in the receiving system. The noise power is given by: P n = kt s B n where k denotes Boltzman s constant = 1.39 10 23 J/K = -228.6 db W/K/Hz, T s is the system noise temperature of source in kelvin degrees, and B n represents the noise bandwidth in which the noise power is measured, in Hz

Research Work! 19 Calculate the total noise power at the output of the IF amplifier of the receiver in the below figure.

G/T Ratio for Earth Stations 20 The link equation can be rewritten in terms of C N at the earth station: C N = P t G t G r kt s B n λ 4πR 2 = P t G t k B n λ 4πR 2 Gr T s Thus, C N G r Ts, and can be used to specify the quality of a receiving earth station or a satellite receiving system.

21 Design of Downlink

Design of Downlinks The design of any satellite communication is based on meeting a minimum C/N ratio for a specified percentage of time. Any satellite link can be designed with very large antenna to achieve high C/N ratios under all conditions, but cost will be high. The art of good system design is to reach the best compromise of system parameters that meet the specification at the lowest cost. For example if a satellite link is designed with sufficient margin to overcome a 20 db rain fade rather than a 3 db fade, earth station antennas with seven times the diameter are required.

Design of Downlinks All satellite communication links are affected by rain attenuation. In the 6/4 GHz band the effect of rain on the link is small In the 14/11GHz (Ku) band and even more in the 30/20 GHz (Ka) band, rain attenuation becomes important. Rain attenuation is very variable phenomenon, both with time and place.

24 Link Budget

Link Budgets C/N is simplified by the used of link budget A link budget is a tabular method for evaluating the received power and noise power in a radio link. Link budget must be calculated for an individual transponder, and must be repeated for each of the individual links. Link budgets are usually calculated for the worst case, the one in which the link will have the lowest C/N ratio.

Link Budgets Factors which contribute to a worst case scenario include: an earth station located at the edge of the satellite coverage zone where the received signal is typically 3 db lower than the center of the zone. This is because the satellite antenna pattern, maximum path length from the satellite to the earth station, low elevation angle at the earth station giving the highest atmospheric path attenuation in clear air and maximum rain attenuation on the link causing loss of received signal power and increase in receiving system noise temperature.

Link Budgets The calculation of carrier to noise ratio in a satellite link is based on the two equations for received signal power and received noise power Pr = EIRP + Gr - Lp - La Lr Lt dbw A receiving terminal with a system noise temperature TsK and a noise bandwidth Bn Hz has a noise power Pn Pn = KTsB

28 Q & A