10 mW per hundred kilometers.
Historically, Russia is the most active player in the global market of radio channel security and fire systems.
Unlike Europe, which is entangled in wires, reliable wired communications are quite rare in Russia, so in most cases it is advisable to use a radio channel for communication.
This is justified both technically and economically, since many real estate objects do not have reliable wired communications, and organizing them is a troublesome and expensive matter.
This is why the technology of transmitting an alarm signal via a radio channel is increasingly used to protect facilities.
We will not dwell on the radio channel used to connect detectors with a control panel or other receivers inside the premises.
There are quite a few articles on this subject, and almost any manufacturer of fire alarm systems offers such solutions to the market.
Progress does not stand still, and transmitting information over a hundred or two meters is now not particularly difficult and does not require large development costs, and the components of such transceivers are more than affordable. Let's pay attention to the radio channel, which allows transmitting information over kilometers, or even tens of kilometers.
Currently, in Russia, without special permission, you can use only four officially permitted radio wave ranges — 27/433/868/2400 MHz with low transmission power.
This is regulated by the RF Government Resolution of October 12, 2004, No. 539 «On the procedure for registering radio electronic equipment and high-frequency devices», on the basis of which frequencies and powers are limited.
Due to the technical features of radio wave propagation, the most widely used ranges for fire alarm systems are 433 MHz and 868 MHz, therefore, the equipment of most manufacturers operates at these frequencies.
The specified ranges are also used for other additional devices operating on radio channel technology: amateur radio stations, car alarm key fobs, baby monitors, etc.
As a result of active use, the specified frequency ranges are heavily overloaded. Therefore, the question of how to ensure the range and noise immunity of radio channel systems designed to ensure security with the least cost is still relevant.
One of the options for solving the problem is to increase the transmitter power. But even here a number of problems arise.
Firstly, there is a need to obtain permission for the frequency, secondly, the question of paying for the frequency registration arises, and thirdly, there are no guarantees of the extension of the permission for the next year.
Cellular communications is a direction that is developing by leaps and bounds.
Some decades ago, a cell phone was a luxury unaffordable for most people.
Now a first-grader pokes at the buttons, telling his mother that he got an A in math. Of course, it is quite convenient and functional, not to mention the huge coverage area.
But between you and the object there will always be a third party — the cellular operator — which does not guarantee the transmission of messages.
As a result of a failure or repair of the base station, the transmission channel may be disconnected.
The «New Year» effect, when all lines are overloaded, has not been cancelled either, and you will have to pay. Of course, less than for a dedicated frequency, but you will still have to.
But this is your choice, we are just considering solutions.
So, let's leave aside phones, smartphones, GSM modems and other delights of cellular communication for the next article and consider «authorized» systems.
And here we can bow our heads to the developers of such systems, who find a way out of a seemingly hopeless situation. Let's make a comparison.
Dedicated frequencies, i.e. requiring registration, have virtually no interference. This can be visualized as a personal free high-speed highway, where our cars (packets with information) do not encounter difficulties.
Then free frequencies that do not require registration are nothing more than busy streets of a big city during rush hour.
Here are auto pagers, amateur radio stations, alarm key fobs, and children's toys — and all this in one range. And the notification transmission system, like a good driver, must overcome these obstacles and deliver information.
What to do, how to proceed?
And the developers solve this problem in a set of measures.
First, it is necessary to increase noise immunity. We are in a free range, and it is necessary to reach the receiver.
The possibilities of adjusting the sensitivity of equipment elements are not unlimited, and another solution must be found.
In order to obtain stable communication over long distances, it is necessary to narrow the information transmission channel several times (Fig. 1).
Figure 1
The essence of the effect can be illustrated using the LMCH method (modeling by little people) from the TRIZ system (theory of inventive problem solving), developed in the USSR by the Soviet inventor G. S. Altshuller.
This method allows for a better understanding of physical processes and phenomena occurring at the micro level. The LMCH method consists of processes being depicted as little people acting according to the laws of the simulated environment. In our situation, the little people will model the operation of a receiver and transmitter in a radio channel.
Let's imagine that there is a flat field, at one end of which you are (conditionally the receiver), and at the other end there are several little people who will send you signals with red flags.
Moreover, only one little person will send the necessary signals, and the others will simply wave the flags, fuss, and thus imitate the behavior of radio interference.
For simplicity of description, let us agree that the person we need is dressed in a blue sweater, and the interference people are dressed in green (Fig. 2).
Figure 2
First, let us imagine the picture that the receiver “sees” without dividing the radio range into communication channels. This picture is illustrated by Figure 3.
Figure 3
From a distance, it is difficult to understand what signal the blue man is sending, because the further you are from the picture, the more the flags merge.
In order to understand the signal, you need to zoom in on the picture. A similar situation occurs on the radio — the further the receiver is from the transmitter, the greater the probability of error.
To get a confident reception, you need to get closer.
Figure 4
In Figure 4, the columns and chandeliers illustrate the division of the range into narrow communication channels, and attention is focused on one of the channels by narrowing it several times.
Please note: despite the fact that all the little people remained in the same position, it became easier to determine the position of the necessary flags in the selected channel No. 2, and this can be done from a greater distance.
The first interference man is clearly obscured by our transmitter man and does not interfere, and the second interference man got into channel #3, and we simply do not pay attention to him.
The same thing happens in the radio channel — narrowing the width of the communication channel reduces the power of the interference acting on the useful signal.
Figure 5
This technology is called Hopping (the dictionary translates this word as «jump»).
In Figure 5, our transmitter has “jumped” to free channel #1 (nobody interferes with it there at all), and we can determine the position of the flags with even greater confidence.
This figure demonstrates that by dividing the allocated range into narrower channels and switching them according to a certain algorithm, it is possible to transmit information on the air on free frequencies, minimizing the impact of interference affecting another part of the range.
Of course, the MMC method greatly simplifies real processes, but it is enough for a general understanding of the technical features of the system's radio channel.
What else does Hopping technology provide?
Let's give an example from the life of a Russian summer resident who uses an ordinary rubber hose to water his garden.
Obviously, if the end of the hose is not clamped, the water will flow out abundantly, but the irrigation radius will be quite small, and we have no way to increase the water pressure in the water supply system.
All we can do is reduce the outlet.
After all, if you squeeze the end of the hose with your fingers, the water will spray out strongly in different directions, which will allow you to water even remote areas of the garden.
The jets, of course, will be quite thin, but the task of increasing the watering range will be achieved.
Let's imagine that with the same amount of water flowing out of the hose (constant transmitter power), we narrow the outlet by 512 times.
The jet will not only reach the borders of our earth worker's garden, but will also fly far beyond it.
But the amount of water supplied has not changed!
Figure 6 graphically shows this principle in relation to the radio channel.
Figure 6
So, a standard transmitter operating at the permitted power over a long distance is unlikely to be heard by the receiver.
It is impossible to increase the power, say, hundreds of times without registration.
Retransmitting a signal is often a very expensive operation, since expensive components are required to simultaneously collect information from several hundred channels.
When using Hopping technology alone, the transmission range of information, even at 10 mW of power, is limited only by direct visibility.
Of course, the volume of transmitted information is reduced.
In such a narrow channel, video and audio data cannot be transmitted, but it is easy to transmit a dozen or two bytes with information about events at the protected facility.
With a correctly placed receiver antenna, a range of 100 km is more than possible, and eight years of successful operation of these systems is the best confirmation of this.