Microwave Communication – An Introduction

The objective of microwave communication systems is to transmit information from one place to another without interruption, and clear reproduction at the receiver. Fig. indicates how this is achieved in its simplest form.

Above 100 MHz the waves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using a parabolic antenna (like the satellite TV dish) gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. Before the advent of fiber optics, these microwaves formed the heart of the long distance telephone transmission system.

In its simplest form the microwave link can be one hop, consisting of one pair of antennas spaced as little as one or two kilometers apart, or can be a backbone, including multiple hops, spanning several thousand kilometers. A single hop is typically 30 to 60 km in relatively flat regions for frequencies in the 2 to 8 GHz bands. When antennas are placed between mountain peaks, a very long hop length can be achieved. Hop distances in excess of 200 km are in existence.

The "line-of-sight" nature of microwaves has some very attractive advantages over cable systems. Line of sight is a term which is only partially correct when describing microwave paths. Atmospheric conditions and certain effects modify the propagation of microwaves so that even if the designer can see from point A to point B (true line of sight), it may not be possible to place antennas at those two points and achieve a satisfactory communication performance.

In order to overcome the problems of line-of-sight and power amplification of weak signals, microwave systems use repeaters at intervals of about 25 to 30 km in between the transmitting receiving stations. The first repeater is placed in line-of-sight of the transmitting station and the last repeater is placed in line-of-sight of the receiving station. Two consecutive repeaters are also placed in line-of-sight of each other. The data signals are received, amplified, and re-transmitted by each of these stations.

Microwave Transmitter and Receiver

Fig. below shows block diagram of microwave link transmitter and receiver section

The voice, video, or data channels are combined by a technique known as multiplexing to produce a BB signal. This signal is frequency modulated to an IF and then up converted (heterodyned) to the RF for transmission through the atmosphere. The reverse process occurs at the receiver. The microwave transmission frequencies are within the approximate range 2 to 24 GHz.

The frequency bands used for digital microwave radio are recommended by the CCIR. Each recommendation clearly defines the frequency range, the number of channels that can be used within that range, the channel spacing the bit rate and the polarization possibilities.

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