27 November 2013

Line of sight propagation – bane or boon?

By Dr Manas Kumar Haldar

Imagine looking at an object. The straight line joining your eye to the object is called the line of sight. Light travels from the transmitter (object) to the receiver (your eye) along this straight line. This type of travel is called line of sight propagation. Radio waves with frequencies greater than about 2MHz have line of sight propagation characteristic. If the earth were flat, everything would be fine with this type of propagation. But the earth is round. So the radio transmission at high frequency can reach a maximum distance at which a straight line from the transmitter just touches (i.e. becomes tangential to) the round surface of the earth. Beyond this distance, the wave shoots out into space and is no longer available to a receiver on earth. So, only a circular region centred at the transmitter with a radius equal to this distance can receive the transmission. The distance increases with the height of the transmitter above ground. This is why TV transmitting antennas are put so high up on top of a TV tower. The tower is often put on a hill if there is one nearby, as in Singapore. But the problem remains – line of sight propagation is a bane to large area coverage.

One solution before the advent of satellites was to construct repeaters receiving and then re-transmitting radio signal. This may be uneconomic inside a country and impossible to do when the broadcast is intended for other countries. To solve this problem of limited range, short wave radio was developed. These waves have frequencies in the range 3 to 30MHz. The name “short wave” arises from their wavelengths smaller than waves of lower frequencies. These waves travelling towards the sky are reflected back to earth by the ionosphere (the ionosphere is part of the upper atmosphere consisting of ions of atmospheric gases such as oxygen produced by sun rays expelling an electron from an atom). As a result, a much greater area can receive transmission. The ionosphere starts from about 75km above the ground during the day and is much higher up at night. This means short wave radio can cover much larger area during the night than during the day. If you are a short wave radio enthusiast, you must have found many more radio stations coming alive at night.

Line of sight propagation can be a boon. It is used by radars to find the distance of objects. A radio pulse is sent towards the object and the time taken for the echo to return is measured. Then the distance of the object is given by half the product of this time and the velocity of the radio wave.

The biggest boon is of course to use the limited area coverage as an advantage – such is human ingenuity. Communication signals – speech, video and data (e.g., text) – are carried by radio waves from point to point without using wires. For more people to talk, more radio frequencies are needed to avoid interference. The range of radio frequencies is rather limited but money could be made by selling them. This is where the problem of limited coverage is turned into an advantage. So long as two transmitters are well separated they can use the same radio frequency for communication. This is called frequency reuse and is one of the main principles behind mobile phone service. Areas are divided into cells with each cell having a transmitter and receiver. Adjacent cells do not use the same frequency, but cells further away can use the same radio frequency. In this way, a very large number of users – the whole of human kind – can use mobile phones.

I would have stopped here but for an attentive reader of the distance of coverage. He or she could wonder if the cell radius has to be the same as the maximum distance of coverage considered earlier. The answer is that it can be smaller. While increasing the transmitter power does not in any way increase the maximum distance as the signal travels skywards after this distance, the distance of coverage can be reduced by lower transmitter power as the received power decreases with distance from the transmitter. The power received in a cell from a distant cell using the same frequency will be too low to cause appreciable interference. Thus there is no limit to reduction of cell radius by reducing transmitter power.

The explanation of mobile communication given in terms of line of sight propagation is rather rudimentary. Although the waves do travel in straight lines, they get reflected from buildings. There can be many reflections. The direct and reflected waves can be subtractive. When this occurs at some location, the power received by the mobile is reduced. So, propagation models more complex than the line of sight propagation are considered in mobile communication system designs.

Dr Manas Kumar Haldar is Associate Professor with the Faculty of Engineering, Computing and Science at Swinburne University of Technology Sarawak Campus. He is contactable at mhaldar@swinburne.edu.my