GSM Network Basics

An Introduction to GSM Networks





Main Components of a Microwave link

Components of a Microwave link

 The radio equipment configuration of a microwave link consists of,

  • Indoor Unit (IDU)

  • Outdoor Unit (ODU)

  • Antenna Unit

  • ODU to IDU cabling (IF cable)

  • Digital Distribution Frame (DDF)

Indoor Unit (IDU)

The IDU contains much of the intelligence of the system. Main functions of the IDU include Providing the Data interface, Error correction, Modulation and Demodulation, Alarm status monitoring and Site-to-site communications. An IDU located in an equipment shelter to interface with the operator interface and is connected to a close coupled ODU Antenna assembly on the tower by a single coaxial cable. The IDU is independent of any frequency band and will operate over all frequency ODUs. But it is capacity dependant and there are separate modules capable of handling different capacity inputs.  Configuring and monitoring of link performance can be done through the IDU front panel. Display of Local & Remote radios and Alarm Relay Status LEDs are there in the IDU front panel for fault identification. Memory backup inside the IDU ensures that it maintains its configuration in the event of a DC power loss.



Outdoor Unit (ODU)

The ODU converts data from the IDU into an RF signal for transmission. It also converts the RF signal from the far end to suitable data to transmit to the IDU. ODUs are weatherproofed units that are mounted on top of a tower either directly connected to a microwave antenna or connected to it through a wave guide. Here ODUs are full duplex configured. The ODU receives its power from the IDU through a coaxial cable. ODU parameters are configured and monitored through the IDU. Each ODU is designed to operate over a predefined frequency band. For example 21.2 - 23.6GHz for a 23GHz system, 17.7 - 19.7GHz for a 18GHz system and 24.5 - 26.5GHz for a 26GHz system as for DMC XP4 ODUs. Suitable ground wire should be connected to the ODU ground lug to an appropriate ground point on the antenna mounting or tower for lightning protection. It should be noted that this unit is electronically controlled. Transmitted power is controlled by adjusting a value on the IDU which instructs the ODU to adjust the drive voltage on its Tx PIN diode attenuator. The configuration of a DMC XP4 ODU is shown below.


The polarization of the transmitted signal is determined by the position of the polarization rotator fitted within the ODU mounting collar. The rotator is an integral part of the antenna-mounting bracket. The default setting is for vertical polarization. The ODU must be mounted on the collar to match the chosen polarization. 

Adjustments for polarization

Antenna Unit

Antenna unit maintains the connection to the other end via a microwave signal path, provided that the line of sight condition is satisfied. In order to have an uninterrupted communication, the two antennas should be placed face to face, without any obstruction between.




Chathuranga Perera,

Electrical Engineer

Ceylon Electricity Board