312 Radio Engineering for Wireless Communication and Sensor Applications

Figure 12.2 Typical footprint of a GEO TV satellite beam directed at Central Europe.

Solution

The beam area is approximately VA = (p/4)(3p /180)2 = 2.15 × 10−3 steradians (see Section 9.1). The directivity is D = 4p /VA = 5,840. The power density at a distance of r is S = (DPt )/(4pr 2 ). Thus, S = (5,840 × 100)/[4p × (4 × 107 )2 ]W/m2 = 2.90 × 10−11W/m2, that is, −105.4 dBW/ m2. The atmosphere is assumed to be lossless. The effective area of a 0.5-m antenna having an aperture efficiency of 0.6 is A ef = 0.6p × 0.252 m2 = 0.118 m2. The power received is Pr = A ef S = 3.42 × 10−12 W, that is, −114.7 dBW or −84.7 dBm.

12.2 Radio Link Systems

Radio link systems convey telephone and data traffic, TV and radio signals to broadcasting stations, and so on. Stations may be terrestrial or space borne. In this section, only fixed links are considered. Short communication links allowing user mobility are the topic of Section 12.3.

12.2.1 Terrestrial Radio Links

A radio link system typically consists of terminal stations and repeater stations, as shown in Figure 12.3. There are several frequency bands reserved for

Figure 12.3 A point-to-point radio link system (horizontal distances not in scale).

fixed terrestrial point-to-point radio links, including 7/8, 13, 15, 18, 23, 38, and 58 GHz. Bands below about 2 GHz are nowadays reserved for other applications, such as mobile communication. Below 10 GHz the curved surface of the Earth or the height of masts (60m to 80m) limits the length of a hop to about 50 km. At frequencies higher than 10 GHz, the atmospheric attenuation and especially the attenuation due to rain limits the hop length. For example, at 23 GHz the maximum hop length is about 10 km and at 58 GHz (near the oxygen resonance) about 1 km.

The reliability of a radio link system is important. Between two stations, the first Fresnel ellipsoid should be free from obstacles to avoid excessive propagation loss. The curved surface of the Earth, the ground profile between the stations, and the bending of rays have to be taken into account when planning the heights of antennas. The statistical nature of the bending of rays and rain attenuation must be considered in the link budget. Diversity techniques can reduce the adverse effects of multipath propagation; space diversity in which two antennas are at different heights is a common technique.

Radio link antennas are usually parabolic reflectors. At millimeter-wave range, low-profile array antennas are also used [1]. The half-power beamwidth is typically 1° to 3°. Often antennas are protected against weather with a radome. Repeater stations have transponders, which change the frequency so that successive hops do not interfere with each other.

Radio links are either analog or digital. Analog links use FDM; digital links use TDM. Standard capacities of digital links are 2, 8, 34, 140, and 155 Mbit/s, or multiples of these bit rates. A 2-Mbit/s signal is composed of the content of 32 channels with 64 kbit/s. Lowand medium-capacity (34 Mbit/s or less) long-haul links below 10 GHz often use the QPSK modulation. In high-capacity links more complicated modulation methods such as 16QAM and 64QAM are needed. The modulation method of shorthaul links above 10 GHz is often 4FSK.