2 dB/km Attenuation due to rain about 5 dB/km at a rain rate of 20 mm/h Frequency band 55 GHz Maximum path length only few kilometres Rain fading Antenna diameter 15 cm Only vertical polarisation used Atmospheric attenuation 5 dB/km Attenuation due to rain about 7 dB/km at a rain rate of 20 mm/h Frequency band 58 GHz Maximum path length only one or two kilometres Rain fading Antenna diameter 15 cm Only vertical polarisation used Atmospheric attenuation 12 dB/km Attenuation due to rain about 7 dB/km at a rain rate of 20 mm/h Before an into explanation is given of various factors that affect the performance of the microwave links, an attempt will be made to explain the fundamentals of functioning of microwave links. 3.4.1 Microwave Link Figure 3.36 shows a simple microwave link. Major components that constitute a microwave link are: (a) Indoor unit (b) Outdoor unit (c) Antenna (d) Waveguide (e) Microwave tower Indoor Unit (IU) Also called IDU, it usually contains the radio modem; i.e. it acts as a termination point for the digital signals from the end user equipments and subsequently converts them into a signal that forms the basis of a radio signal sent across the microwave link, using modulation schemes. Also, it demodulates the carrier to the digital signal in the receive direction. Apart from the modulation/demodulation of the signal, other IU functions also include: forward error correction (FEC), multiplexing of user data, control unit (monitoring and controlling the radio unit through the NMS) and acting as a communication channel Microwave Link Planning nx2 Mbit/s, nx2 Mbit/s, Digital Signals Digital Signals Figure 3.36 Microwave link between the NMS and OU. Power to the microwave radio is fed through the IU. Indoor units are usually located in a protected environment, i.e. within cabinets inside a building or similar structure. They are not exposed to the environmental conditions like the outdoor units. Outdoor Unit (OU) Also called ODU, it converts the modulated low frequency digital signal into a high fr