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Prior Planning Can Ensure a Smooth Running Simulcasting Operation.

There are several techniques that are in use today to increase the effectiveness and efficiency of systems that use the scarce resource we all use: the spectrum. One of thse, usually misunderstood and misused, is simulcasting.

Simulcasting is a highly effective technique when used properly. Without it, antenna sites might as well be chosen with a dart board. So, let's review a little about simulcasting, what it is, why people use it, what advantages it as as well as its weaknesses, and how to overcome the weaknesses to apply this techniques properly.

Simulcasting is the process of transmitting the same signal, on the same frquency, in the same general area and from more than one location to achieve broader coverage areas and better penetration of a signal. This increase in penetration will yield improved coverage into difficult-to-service areas.

Typically, when people think of simulcasting, they associate it with paging. There are other applications for this technology, but for purposes of this article, we will concentrate on paging.

Here is an example of an application for simulcasting: In low-band paging systems, unless a transmitter is close to a hard-to-penetrate area, it is likely that the signal strength of a distant transmitter will be insufficient to service because of clusters of buildings, buildings of particular types of construction, and areas of high electrical interference.

The solution to these problems of insufficient service in many cases could be to locate another transmitter close to trouble area, or, in technical terms, the non-capture area. The simplest means of doing this is to transmit the signal from a "main" site and then, after the transmission is completed, to re-transmit that same signal to the trouble area through another, more closely located antenna after the signal has been transmitted from the first antenna. This technique is referred to as "sequencing."

The sequencing concept works fine, but in practice it's a "resource bog." Why? Because it doubles the required amount of airtime.

Several Transmitters Require More Air Time

An exaggerated example of the effect of this is more visible when there are several transmitters. This would require retransmitting the same information on each transmitter separately and would consequently use more airtime than is really necessary.

If sequencing is such a waste of the only resource we have in the radio business, airtime, what is the solution to these penetration problems without sequencing? The solution to these problems can be offered through the technology of simulcasting.

In simulcasting, the same information is transmitted at the same time regardless of the number of transmitters or sites that are in the system. This saves the resource, airtime, and allows more users into the same amount of spectrum.

If this is such a great idea, what's the problem? Why doesn't everybody simulcast all the time and really increase the availability of the airtime resource? The answer is simple: simulcasting causes interference. By its very definition, simulcasting is co-channel interference.

When two signals are transmitted in the same area, at the same time, on the same frequency, interference occurs. This interference can either distort or destroy the originally intended signal in certain areas that are encompassed by both transmitters' coverage contours.

The key to a quality simulcasting system is to isolate where the signal is degraded due to the co-channel interference. By locating, isolating moving or planning for where these areas of debilitating inerference will be located, a simulcast system can be installed with confidence.

Through the engineering techniques described here, the non-capture area could be moved to a plaace where there is little need for a pager to receive a signal--swamps or forest areas, the middle of a body of water (although this can cause other problems due to a gain in signal strength over water), or over the runway system of a local airport.

This would offer the benefits of simulcasting without suffering greatly (or perhaps at all) from the effects of interference. There are, of course, equipment manufacturers who can provide certain types of equipment that will facilitate simulcasting and control the areas of interference (the non-capture areas). This equipment usually contains high-stability oscillators, high-stability modulators, high-stability interconnect circuitry (not just any old line from the phone company wil do), or special timing equipment to make sure that the transmitters modulate at precisely (within very small parameters) The same moment.

You don't have to be an engineer, a market research or a simulcast system operator to figure out that all that high-stability stuff costs a lot of money. what then, can be done to get the benefits of a simulcast system without the exhorbitant cost? With the proper engineering tools, and using a few simple steps, it is possible to design, install a d successfully operate a simulcast system.

One of the problems in spending all of the money to purchase the beste simulcasting gear is that it doesn't consider some of the most important factors in engineering a system. Usually, sites are found, equipment is ordered, installation is performed and then everybody sits around trying to figure out why the system won't work the way that it's supposed to.

Consider Lay of the Land When Predicting Interference

The source of this problem is simple, when the system was engineered, standard 43 dBu contours were used, applying the Carey method. No one considered the "Lay of the land"--the terrain. Traditionally, when preparing the engineering, a point equidistant from both sites is taken up map measurement and is assumed to be the overlap area. It does not consider terrain, building or foliage loss. It hasn't taken the interaction between the sites surrounding the two overlapping sites into effect in predicting interference.

What happens is that the terrain, and o ther factors, impact the signal of each site so dramatically that you could spend all the money in the world on the fanciest simulcasting equipment and still have debilitating interference.

How are the modern tools of technology to be used? It requires first a computer-readable data base of elevation contours of USGS maps. To create this data base, we use a process called digitizing. It allows the computer to do a simulated sweep around a transmitter site and calculate the signal loss at thousands of points based on the terrain surrounding the site.

Then the software compares the modulation and timing of the various sites transmitting the signal and calculates, based upon known engineering parameters, the level of signal in the areas that are overlapped by more than one site. This allows us to determine where there will be sufficient signal strength from one of the two overlapping sites to "capture" the receiver that the signal is intended for. If there is less than a small (6 dB) difference in signal strength, most receivers, or pagers in this case, would not be able to discriminate between the two signals and would receive both at the same time, causing interference.

The steps to properly engineer a simulcast system include defining coverage area, combined with marketing data; locating hard to penetrate areas; determining ideal site locations, considering terrain as a signal attenuator; determining impact of terrain on overlapping signals; adjusting sites to maximize coverage and penetration; and defining (and moving if necessary) areas of noncapture.

Inferference Can Be Controlled

So what is so impressive about performing the engineering in this fashion? The figures at the end of a year will show little or no expenses under "high-stability equipment," "special conditioned telephone lines," "technicans overtime," an others. The point being that by considering terrain the interference area, or the area of non-capture, can be controlled. How else is this a benefit? Obviously, by saving money. But even more, through customer satisfaction.

By simply planning the simulcast system properly from the start, using the right engineering tools, it is possible to avoid complaints before they happen; bypass expensive simulcasting equipment; establish coverage in the critical areas; move interference to unimportant areas; and make the system easier to maintain, yielding lower costs for maintenance.

Simulating is a powerful tool when used properly. When used improperly, not only is it useless, it's harmful to the overall system. Simple steps, a minimal amount of prior planning and a small investment can ensure a smooth-running continuous and profitable simulcasting operation.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Goldman, S.
Publication:Communications News
Date:Aug 1, 1985
Previous Article:Utility Speeds Communications to and from Field with Mobile-Radio Controller.
Next Article:RF Field-Service Monitor Spans Gap between Analog and Digital.

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