# 10.3 Communication-Link Power Budget

The uplink and downlink segments of a satellite communication link (Fig. 10-7) are each governed by the Friis transmission formula (Section 9-6), which states that the power P r received by an antenna with gain G r due to the transmission of power P t by an antenna with gain G t at a range R is given by

P r=P t G t G r λ 4 π R 2. (10.7)

This expression applies to a lossless medium, such as free space. To account for attenuation by clouds and rain in Earth’s atmosphere (when present along the propagation path), as well as absorption by certain atmospheric gases (primarily oxygen and water vapor), we rewrite Eq. (10.7) as

P r i=Υ(θ)P r=Υ(θ)P t G t G r λ 4 π R 2. (10.8)

# Figure 10-7 Satellite transponder.

Figure 10 7 Full Alternative Text

Now, P r i represents the input power at the receiver with atmospheric losses taken into account, and Υ(θ) is the one-way transmissivity of the atmosphere at zenith angle θ. In addition to its dependence on θ, Υ(θ) is a function of the frequency of the communication link and the rain-rate conditions along the propagation path. At frequencies below 10 GHz, which include the 4/6 GHz band allocated for satellite communication, absorption by atmospheric gases is very small, as is attenuation due to clouds and rain. Consequently, the magnitude of Υ(θ) is typically on the order of 0.5 to 1 for most conditions. A transmissivity of 0.5 means that twice as much power needs to be transmitted (compared to the free-space case) in order to receive a specified power level. Among the various sources of atmospheric attenuation, the most serious is rainfall, and its attenuation coefficient increases rapidly with increasing frequency. Consequently, atmospheric attenuation assumes greater importance with regard to transmitter power requirements as the communication-system frequency is increased toward higher bands in the microwave region.

The noise appearing at the receiver output, P n o, consists of three contributions: (1) noise internally generated by the receiver electronics, (2) noise picked up by the antenna due to external sources, including emission by the atmosphere, and (3) noise due to thermal emission by the antenna material. The combination of all noise sources can be represented by an equivalent system noise temperature, T sys, defined such that

P n o=G rec K T sys B, (10.9)

where K is Boltzmann’s constant, and G rec and B are the receiver power gain and bandwidth, respectively. This output noise level is the same as would appear at the output of a noise-free receiver with input noise level:

P n i=P n o G rec=K T sys B. (10.10)

The signal-to-noise ratio is defined as the ratio of the signal power to the noise power at the input of an equivalent noise-free receiver. Hence,

The performance of a communication system is governed by two sets of issues. The first encompasses the signal-processing techniques used to encode, modulate, combine, and transmit the signal at the transmitter end and to receive, separate, demodulate, and decode the signal at the receiver end. The second set encompasses the gains and losses in the communication link, and they are represented by the signal-to-noise ratio S n. For a given set of signal-processing techniques, S n determines the quality of the received signal, such as the bit error rate in digital data transmission and sound and picture quality in audio and video transmissions. Very high-quality signal transmission requires very high values of S n; in broadcast-quality television by satellite, some systems are designed to provide values of S n exceeding 50 dB (or a factor of 10 5).

The performance of a satellite link depends on the composite performance of the uplink and downlink segments. If either segment performs poorly, the composite performance will be poor, regardless of how good the performance of the other segment is.