 They convert digital data into radio waves and vice versa. They also perform supervisory functions for remote performance and fault monitoring from the network management center. Figure 4.36 illustrates the structure of a point-to-point radio relay system used in the telecommunications network. Microwave radio relay systems usually operate at radio frequencies in the range from 1 to 40 GHz. These frequencies are focused with parabolic dish antennas and applicable communication distances range from a few kilometers up to approximately 50 km depending on the frequency in use and the characteristics of the system. The radio waves at these frequencies travel along a straight line and therefore this kind of radio transmission is called line-of-sight transmission. The higher the frequency, the higher the propagation loss, as we saw in Section 4.2, and the shorter the communica- tion distance. At very high frequencies, weather conditions influence attenuation and transmission quality, which restricts the available frequency Section length 50 km Highly Radio frequency band directional 1 38 GHz parabolic antenna Radio transmitter/ receiver 1.5...155 Radio transmitter/ receiver 1.5...155 Frequency band: Typical distance: 7...8 GHz over 30 km 18 GHz 10-40 km 23 GHz 5-15 km 38 GHz less than 10 km Figure 4.36 Microwave radio transmission. Transmission band suitable for radio transmission and maximum transmission distance. Figure 4.36 shows examples of how communication distance depends on the radio frequency in use. 4.9 Problems and Review Questions Problem 4.1 How wide a bandwidth does a pulse with duration of (a) 1 ms and (b) 1 us require if only the strongest part of the spectrum needs to be transmitted? What is the bandwidth of a carrier wave with a duration of (a) 1 ms and (b) 1 us? Problem 4.2 What is continuous wave modulation and why is it often used in transmis- sion systems? Problem 4.3 (a) Draw the spectrum of a cosine wave at a frequency of 1 kHz. (b) Draw the spectrum of an AM signal when the carrier frequency i