11.4 Radio Link Budget

where BRF is the noise bandwidth of the receiver (see Figure 11.4). The noise bandwidth is approximately equal to the bandwidth of the modulated RF signal, which, on the other hand, depends on the baseband signal and the modulation method used. For example, the bandwidth of an analog baseband signal required for voice is 3 kHz, and that for a TV picture is 5 MHz.

The S /N in the input of the receiver is

The S /N required for a good transmission depends on the application. For example, a good analog TV picture requires S /N over 40 dB for the video signal. For FM radio, S /N over 10 dB is satisfactory. Often in the receiving end one can affect the ratio Gr /TS by properly selecting the antenna and receiver. Equation (11.44) in various forms is often called the linkbudget formula.

Example 11.3

Let us consider a 12-GHz satellite TV link, where a geostationary satellite is broadcasting to Scandinavia. The distance between the transmitter and receiver is 40,000 km, and the satellite is seen at an elevation angle of about 20°. The transmitted power is 200W, and the transmitting satellite antenna is a 1.5-m paraboloid with an aperture efficiency of 0.6. The required availability of the system is 99.9%. What is the minimum ratio Gr /TS and the maximum TR that will result in a good TV reception?

Solution

From long-term statistics it is known that the atmospheric attenuation on the radio path of such a geostationary satellite is during 99.9% of time less than or equal to 3 dB. In order to obtain the required video signal S /N of 40 dB, the received FM signal must have at least S /N = 14 dB. By expressing the variables and constants of (11.44) in decibels, we get the link budget in decibels. Now the system properties can be calculated by adding and subtracting the decibel values. Properties of the system in decibels are given in Table 11.1. When the values from Table 11.1 are substituted into (11.44) and it is taken into account that k = −228.6 dBWK−1 Hz−1, it is obtained that Gr /TS = 2.4 dBK−1. If the receiving antenna is a paraboloidal reflector with a diameter of 0.4m and an aperture efficiency of 0.6, its gain is 31.8 dB. Then TS may be at maximum 29.4 dBK or 870K. The antenna noise

306 Radio Engineering for Wireless Communication and Sensor Applications

Table 11.1

Characteristics of a Satellite TV System

temperature may be assumed to be at maximum 150K, and therefore the receiver noise temperature may be at maximum 720K. At the edges of the satellite antenna beam Gt is smaller than at the center of the beam, and therefore a larger receiving antenna or a more sensitive receiver is needed.

References

[1]Collin, R. E., Foundations for Microwave Engineering, 2nd ed., New York: IEEE Press, 2001.

[2]Gardner, F. M., Phaselock Techniques, 2nd ed., New York: John Wiley & Sons, 1979.

[3]Manassewitsch, V., Frequency Synthesizers: Theory and Design, 3rd ed., New York: John Wiley & Sons, 1987.

[4]Mumford, W. W., and E. H. Scheibe, Noise Performance Factors in Communication Systems, Dedham, MA: Horizon House—Microwave, 1968.

[5]‘‘IRE Standards on Methods of Measuring Noise in Linear Twoports, 1959,’’ IRE Proc., Vol. 48, No. 1, 1960, pp. 61–68.

[6]Ra¨isa¨nen, A. V., ‘‘Experimental Studies on Cooled Millimeter Wave Mixers,’’ Acta Polytechnica Scandinavica, Electrical Engineering Series, No. 46, Helsinki, 1980.

[7]Kraus, J. D., Radio Astronomy, 2nd ed., Powell, OH: Cygnus-Quasar Books, 1986.

[8]Bhargava, V. K., et al., Digital Communications by Satellite, New York: John Wiley & Sons, 1981.

[9]Carlson, A. B., Communication Systems, 3rd ed., New York: McGraw-Hill, 1986.

[10]Haykin, S., Communication Systems, 4th ed., New York: John Wiley & Sons, 2001.