Treatise on radio paths or some marketing misconceptions about radio communications
Write this article prompted the presence in the network of a large number of marketing misconceptions, information such as "radio equipment operating at a distance of 100km", "radio equipment operates on completely closed routes," etc. In principle, marketing exists to embellish one or another product, but now I come across more and more often with the fact that many colleagues take these marketing statements at face value. In the article I will try to voice and substantiate some statements that concretize some "marketing information."
Before we begin, it makes sense to recall the generally accepted classification of radio wave bands in radio communication theory.
Actually, no one forbids anyone to come up with their own classification of radio wave bands, but all classifications in radio communication theory are usually based either on the frequency type (UHF - ultrahigh frequencies) or on the wavelength (VHF - ultrashort waves). Usually they use the combined classification (when they say “VHF”, they imply a certain frequency range). Some general classification of the radio wave bands is given in the table here - well, where would it be without Wikipedia.
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Addition to it is in the VHF range given in the upper table here .
According to these data, all the equipment common on the market, with which radio channels are built in the range of 30 MHz and above (it makes sense to talk about the upper limit to 90 GHz), refers to the UHF band. Those. All class and family protocols such as GSM, WiFi, WiMax, LTE, ShBD, RRL, etc., work in this category with all the features inherent in this range. Radio communication in the VHF range is organized by the surface wave, so the range within the Earth is not very large and is limited to the "curvature of the Earth." When they talk about the radio, often all the arguments are reduced to a connection between 2 points. Therefore, in order not to complicate our article with “point-to-multipoint”, “multi-access” schemes, etc. we confine ourselves to the “point-to-point” or “radio channel” scheme.
So let's get started.
The maximum practical range for which you can organize a radio channel in the VHF range is 50-60km. I will explain the term "practical". It means only one thing: it is possible to organize a radio route over long distances, but only if there is a suitable height of the equipment suspension.
The maximum range of the radio path in the VHF range is reduced to the calculation according to the following formula:
D = 3.57 * [√√ (H1) + ²√ (H2) + 10 * ³√ (λ)]
Where:
3.57 is the refraction coefficient taking into account the curvature of the surface wave at frequencies above 1 GHz. If the frequencies are less than 1 GHz, then the coefficient will be equal to 4.12 or 4.56 (at frequencies less than 500 MHz).
D is the radio path distance, in km.
1, 2 - suspension height of receiving and transmitting antennas, in m.
Λ is the length of the working radio wave in m. However, this parameter is often neglected in calculations of the VHF radio path because of its small size.
There is an example of a calculated table explaining this relationship. According to this table, in order to organize a radio path for 50 km, it will be necessary to hang the antennas on both sides higher than 49 meters. For comparison, 53m is the top of a modern 16-storey building. The table is correct if the surface of the Earth on the track islike a billiard ball ideally even. If there are different elevations of the relief, buildings or trees - it is necessary to raise the antenna above, at least the magnitude of these obstacles. Next, you still need to compensate for the value of the 1st Fresnel zone, but more on that below. In modern conditions on the radio route, building is usually unexpected, or some kind of forest, or it all comes down to "the reluctance of the owners of high-altitude points to let you into their patrimony."
In my practice, there have been cases of organizing VHF radio relay communication for a distance of approximately 400 km. The communication session was organized at the old analog radio relay stations working on antennas with a circular pattern, one of which was in a flying plane (altitude over 1000m).
Satellite communications operate in the same range, but there is always visibility when the communications satellite is above the horizon.
Note that the values of transmitter power and receiver sensitivity are not involved in the formula. The fact is that the effect of power on the radio communication range in the VHF range is not significant, because the range is high frequency. The capacities of the existing equipment on the market are measured in dozens, less often in hundreds of milliwatts (values of 20, 63, 150 mW were encountered), and the gains at the antennas also vary. It is clear that more powerful radio equipment with a greater antenna gain will give better channel power, but this mainly affects the channel speed and its stability in the presence of interference, obstacles, etc., but this is somewhat lower.
When organizing radio communications on modern digital equipment on long radio paths (more than 30 km), they often encounter restrictions that are imposed by high speeds in the channel. When organizing high-speed radio paths, radio equipment is used, which uses quite complex methods of signal manipulation / modulation, several signal polarizations simultaneously, and works with an increased channel bandwidth. If the length of the radio path is large and the transmitter power and receiver sensitivity are insufficient, then the natural delay of the signal in the channel, the reflected signals begin to influence strongly, this leads to losses and repetitions, adaptive reduction in speed and loss of communication, i.e. radio channel corny often "falls."
Once colleagues from one cellular company told how they organized a radio relay route for 50 km. According to the behavior of the energy of this route, from their words, "it was possible to predict the weather."
On the "fully closed routes" to organize VHF radio is almost impossible. Traditionally, I will explain the term "practically." This can be done, but rather problematic - we need repeaters (reflectors), some other ways of avoiding obstacles or increasing the energy in the channel. Usually, when this is approved by any vendor, then it is either a “half-closed highway” or the connection is organized on a “reflected signal”.
The theory says that the model of the radio path is described not by a beam, but by figures that resemble acucumber and a glass of an elongated ellipsoid of rotation. These are the so-called Fresnel zones, the 1st is critical for us, and we'll talk about it. When calculating, the radio paths look at whether there are obstacles on it, and how they close the Fresnel zone of this route. Practice shows that visibility between two points, between which you need to organize a radio path, may be, but with partial blocking of the Fresnel zone by obstacles, the radio path power will be weak, and organizing a radio channel with the necessary speeds will be problematic. The route with a 20% closure of the Fresnel zone will already be "half-closed" and will lose energy. Equipment that can actually work on semi-closed routes (30-40% of the overlap of the Fresnel zone) is quite expensive on the market, and is represented by a very small number of vendors (I will not mention, for advertising). When the Fresnel zone is closed by 50% or more, the normal operation of the radio path without losses is practically unrealistic.
The radius of this Fresnel zone in a particular part of the radio path or the minimum clearance to an obstacle is calculated by the formula:
h = ² √ [1/3 * D * λ * X / D * (1 - X / D)], where
h - the value of the "lumen",
D is the length of the radio path in m.,
Λ is the length of the working radio wave in m,
X - the distance to the point of obstacles, in m.
As a result, if we substitute the data for the 10km highway in the 2.4 GHz range into the formula, we will get the maximum radius of the Fresnel zone in the middle of the highway more than 10 meters At 50km this value will be more than 23m.
You can compensate for the effects of obstacles on the Fresnel zone either by raising the antenna's suspension point by the required amount of lumen - see the reasoning on Statement 1, or by increasing the energy in the channel — new, usually extra-budgetary, power costs, antenna gains, proprietary protocols of individual vendors.
The speed in the radio channel, declared by the seller of the equipment, the real speed in the radio channel fullduplex, packet performance - completely different characteristics, with each other often do not coincide.
Usually, when the speed of the radio channel being organized is mentioned in the characteristics of the radio equipment, it turns out thatnot all yogurts are equally useful as the total theoretical speed in the radio channel. Those. This is the total speed in the radio channel "there" and "back." Let me explain by example. Somehow we were faced with radio equipment, about which it was claimed that a radio channel of 350 Mbit / s could be organized on it. A detailed study showed that we have, at best, only theoretical 175Mbit / s FullDuplex. Actually, at best, it was about 140 Mbit / s, and then it turned out that the radio equipment had a 10 / 100BaseTx interface, i.e., the manufacturer limited the actual speed to 70-80 Mbit / s for some good reasons.
Current hardware implementations of virtually all radio equipment are in general a network IP router (of varying degrees of functionality) with a radio receiving-transmitting module / modules. All this is placed in one box. It works like this - the network protocol is processed with iron for the specifications of the radio protocol (usually 802.11) and is broadcast. Naturally, this requires some power hardware stuffing. Hence the limitations that are characterized by batch performance.
Packet performance is a characteristic of radio equipment, which shows what the speed in the channel will be if “heavy” traffic goes. “Heavy traffic” usually refers to traffic that is generated by applications using small packets (VoIP, TV, etc.). There is an old article in which a detailed analysis of the packet performance of WiFi equipment used in BWA networks is carried out. The article, of course, is aimed at advertising a specific type of equipment and a specific company, but the analysis used by the author is correct in my opinion. The result of the article, if you omit all advertising, is as follows. If you do not know the packet performance of a specific radio equipment, then you can get an unpleasant surprise in the form of the maximum speed on the radio channel in 16% of the declared. I know only one vendor, in which the batch performance of the equipment really coincides with the declared speed on the radio channel. This vendor has achieved using only proprietary protocols in the operation of its equipment. Much more often it happens that packet performance is not a documented characteristic.
In our country, the radio frequency spectrum is owned by the state. For supervision of its correct use, supervisory and regulatory bodies have been created. Civilians and organizations can use a specific radio frequency for the operation of their radio equipment, but, first, on a secondary basis, i.e., if they do not interfere with the work of radio equipment of state organizations (MO, FSB, MIA, etc.) secondly, if they do not interfere with their radio-electronic means to the existing radio routes of the same civil organizations, thirdly, only in the range permitted for this. For this purpose, a procedure has been introduced for obtaining and registering a radio frequency resource. Radio frequency organs permitted range for use, divided into radio channels. For some reason, which is not known to me, apparently, historically, the width of these radio channels, and in fact the bandwidth of the allocated radio channel, is 20 MHz. Those. if we get to use the frequency / frequency, then beyond + 10MHz and -10MHz from the carrier, we cannot step. If we need a band of 40 MHz or 80 MHz, then, respectively, we need to obtain a resolution of 2 or 4 adjacent carrier frequencies. In practice, this is not realistic, because it turns out that something is already occupied by someone, because the allowed range is not empty. It would seem, well, okay, why do we need duplex spacing at 40 or 80 MHz? The answer is simple - for the organization of radio channels with a speed of more than 50 Mbit / s fullduplex bandwidth is required at 40 MHz, and with an increase in appetite over 150 Mbit / s - 80 MHz.
On the topic, about who needs to get permission to work in the permitted range, and who does not, I’ll stop very briefly, because this is a topic for another conversation. The procedure for obtaining a permit may be permissive or informative. It is rather complicated, it all depends on whether we are a legal or an individual, we organize a radio channel for commercial purposes or for our own use, what power we use equipment, we use indoors or outside it, to what height we hang, in what range we work.
In principle, one can be guided by the fact that if we use indoors with or in the street in the 2.4-2.4835 GHz band, equipment with a capacity of up to 100 MW, we do not provide commercial services through it, then we may not receive permits. The same is true for the range of 5.15-5.25 GHz for equipment with a capacity of up to 200mW, but only indoors. But again, you need to understand that in all such cases, as always, there are nuances, for example, the power of 100mW is the power at the output of the antenna, i.e. it is obtained from the power of the transmitter itself, taking into account the antenna gain.
For the sake of fairness, it is necessary to add that it is not necessary to get permissions to any radio channels of the range 76-86 GHz.
The bottom line is theobvious banality of the following result. From the point of view of price-quality ratio, taking into account the voiced statements:
1. It makes sense to consider the organization of the radio channel in the conditions of urban development, if the required distance does not exceed 10km. Usually effectively used at a distance of 3-5km.
2. It makes sense to consider the organization of a radio channel in an open area if the required distance does not exceed 20km. It is usually effective to use at a distance of 12-15km.
Before we begin, it makes sense to recall the generally accepted classification of radio wave bands in radio communication theory.
Actually, no one forbids anyone to come up with their own classification of radio wave bands, but all classifications in radio communication theory are usually based either on the frequency type (UHF - ultrahigh frequencies) or on the wavelength (VHF - ultrashort waves). Usually they use the combined classification (when they say “VHF”, they imply a certain frequency range). Some general classification of the radio wave bands is given in the table here - well, where would it be without Wikipedia.
')
Addition to it is in the VHF range given in the upper table here .
According to these data, all the equipment common on the market, with which radio channels are built in the range of 30 MHz and above (it makes sense to talk about the upper limit to 90 GHz), refers to the UHF band. Those. All class and family protocols such as GSM, WiFi, WiMax, LTE, ShBD, RRL, etc., work in this category with all the features inherent in this range. Radio communication in the VHF range is organized by the surface wave, so the range within the Earth is not very large and is limited to the "curvature of the Earth." When they talk about the radio, often all the arguments are reduced to a connection between 2 points. Therefore, in order not to complicate our article with “point-to-multipoint”, “multi-access” schemes, etc. we confine ourselves to the “point-to-point” or “radio channel” scheme.
So let's get started.
1. First statement
The maximum practical range for which you can organize a radio channel in the VHF range is 50-60km. I will explain the term "practical". It means only one thing: it is possible to organize a radio route over long distances, but only if there is a suitable height of the equipment suspension.
The maximum range of the radio path in the VHF range is reduced to the calculation according to the following formula:
D = 3.57 * [√√ (H1) + ²√ (H2) + 10 * ³√ (λ)]
Where:
3.57 is the refraction coefficient taking into account the curvature of the surface wave at frequencies above 1 GHz. If the frequencies are less than 1 GHz, then the coefficient will be equal to 4.12 or 4.56 (at frequencies less than 500 MHz).
D is the radio path distance, in km.
1, 2 - suspension height of receiving and transmitting antennas, in m.
Λ is the length of the working radio wave in m. However, this parameter is often neglected in calculations of the VHF radio path because of its small size.
There is an example of a calculated table explaining this relationship. According to this table, in order to organize a radio path for 50 km, it will be necessary to hang the antennas on both sides higher than 49 meters. For comparison, 53m is the top of a modern 16-storey building. The table is correct if the surface of the Earth on the track is
In my practice, there have been cases of organizing VHF radio relay communication for a distance of approximately 400 km. The communication session was organized at the old analog radio relay stations working on antennas with a circular pattern, one of which was in a flying plane (altitude over 1000m).
Satellite communications operate in the same range, but there is always visibility when the communications satellite is above the horizon.
Note that the values of transmitter power and receiver sensitivity are not involved in the formula. The fact is that the effect of power on the radio communication range in the VHF range is not significant, because the range is high frequency. The capacities of the existing equipment on the market are measured in dozens, less often in hundreds of milliwatts (values of 20, 63, 150 mW were encountered), and the gains at the antennas also vary. It is clear that more powerful radio equipment with a greater antenna gain will give better channel power, but this mainly affects the channel speed and its stability in the presence of interference, obstacles, etc., but this is somewhat lower.
When organizing radio communications on modern digital equipment on long radio paths (more than 30 km), they often encounter restrictions that are imposed by high speeds in the channel. When organizing high-speed radio paths, radio equipment is used, which uses quite complex methods of signal manipulation / modulation, several signal polarizations simultaneously, and works with an increased channel bandwidth. If the length of the radio path is large and the transmitter power and receiver sensitivity are insufficient, then the natural delay of the signal in the channel, the reflected signals begin to influence strongly, this leads to losses and repetitions, adaptive reduction in speed and loss of communication, i.e. radio channel corny often "falls."
Once colleagues from one cellular company told how they organized a radio relay route for 50 km. According to the behavior of the energy of this route, from their words, "it was possible to predict the weather."
2. Second statement
On the "fully closed routes" to organize VHF radio is almost impossible. Traditionally, I will explain the term "practically." This can be done, but rather problematic - we need repeaters (reflectors), some other ways of avoiding obstacles or increasing the energy in the channel. Usually, when this is approved by any vendor, then it is either a “half-closed highway” or the connection is organized on a “reflected signal”.
The theory says that the model of the radio path is described not by a beam, but by figures that resemble a
The radius of this Fresnel zone in a particular part of the radio path or the minimum clearance to an obstacle is calculated by the formula:
h = ² √ [1/3 * D * λ * X / D * (1 - X / D)], where
h - the value of the "lumen",
D is the length of the radio path in m.,
Λ is the length of the working radio wave in m,
X - the distance to the point of obstacles, in m.
As a result, if we substitute the data for the 10km highway in the 2.4 GHz range into the formula, we will get the maximum radius of the Fresnel zone in the middle of the highway more than 10 meters At 50km this value will be more than 23m.
You can compensate for the effects of obstacles on the Fresnel zone either by raising the antenna's suspension point by the required amount of lumen - see the reasoning on Statement 1, or by increasing the energy in the channel — new, usually extra-budgetary, power costs, antenna gains, proprietary protocols of individual vendors.
3. Statement Three
The speed in the radio channel, declared by the seller of the equipment, the real speed in the radio channel fullduplex, packet performance - completely different characteristics, with each other often do not coincide.
Usually, when the speed of the radio channel being organized is mentioned in the characteristics of the radio equipment, it turns out that
Current hardware implementations of virtually all radio equipment are in general a network IP router (of varying degrees of functionality) with a radio receiving-transmitting module / modules. All this is placed in one box. It works like this - the network protocol is processed with iron for the specifications of the radio protocol (usually 802.11) and is broadcast. Naturally, this requires some power hardware stuffing. Hence the limitations that are characterized by batch performance.
Packet performance is a characteristic of radio equipment, which shows what the speed in the channel will be if “heavy” traffic goes. “Heavy traffic” usually refers to traffic that is generated by applications using small packets (VoIP, TV, etc.). There is an old article in which a detailed analysis of the packet performance of WiFi equipment used in BWA networks is carried out. The article, of course, is aimed at advertising a specific type of equipment and a specific company, but the analysis used by the author is correct in my opinion. The result of the article, if you omit all advertising, is as follows. If you do not know the packet performance of a specific radio equipment, then you can get an unpleasant surprise in the form of the maximum speed on the radio channel in 16% of the declared. I know only one vendor, in which the batch performance of the equipment really coincides with the declared speed on the radio channel. This vendor has achieved using only proprietary protocols in the operation of its equipment. Much more often it happens that packet performance is not a documented characteristic.
4. Statement Four
In our country, the radio frequency spectrum is owned by the state. For supervision of its correct use, supervisory and regulatory bodies have been created. Civilians and organizations can use a specific radio frequency for the operation of their radio equipment, but, first, on a secondary basis, i.e., if they do not interfere with the work of radio equipment of state organizations (MO, FSB, MIA, etc.) secondly, if they do not interfere with their radio-electronic means to the existing radio routes of the same civil organizations, thirdly, only in the range permitted for this. For this purpose, a procedure has been introduced for obtaining and registering a radio frequency resource. Radio frequency organs permitted range for use, divided into radio channels. For some reason, which is not known to me, apparently, historically, the width of these radio channels, and in fact the bandwidth of the allocated radio channel, is 20 MHz. Those. if we get to use the frequency / frequency, then beyond + 10MHz and -10MHz from the carrier, we cannot step. If we need a band of 40 MHz or 80 MHz, then, respectively, we need to obtain a resolution of 2 or 4 adjacent carrier frequencies. In practice, this is not realistic, because it turns out that something is already occupied by someone, because the allowed range is not empty. It would seem, well, okay, why do we need duplex spacing at 40 or 80 MHz? The answer is simple - for the organization of radio channels with a speed of more than 50 Mbit / s fullduplex bandwidth is required at 40 MHz, and with an increase in appetite over 150 Mbit / s - 80 MHz.
On the topic, about who needs to get permission to work in the permitted range, and who does not, I’ll stop very briefly, because this is a topic for another conversation. The procedure for obtaining a permit may be permissive or informative. It is rather complicated, it all depends on whether we are a legal or an individual, we organize a radio channel for commercial purposes or for our own use, what power we use equipment, we use indoors or outside it, to what height we hang, in what range we work.
In principle, one can be guided by the fact that if we use indoors with or in the street in the 2.4-2.4835 GHz band, equipment with a capacity of up to 100 MW, we do not provide commercial services through it, then we may not receive permits. The same is true for the range of 5.15-5.25 GHz for equipment with a capacity of up to 200mW, but only indoors. But again, you need to understand that in all such cases, as always, there are nuances, for example, the power of 100mW is the power at the output of the antenna, i.e. it is obtained from the power of the transmitter itself, taking into account the antenna gain.
For the sake of fairness, it is necessary to add that it is not necessary to get permissions to any radio channels of the range 76-86 GHz.
The bottom line is the
1. It makes sense to consider the organization of the radio channel in the conditions of urban development, if the required distance does not exceed 10km. Usually effectively used at a distance of 3-5km.
2. It makes sense to consider the organization of a radio channel in an open area if the required distance does not exceed 20km. It is usually effective to use at a distance of 12-15km.
Source: https://habr.com/ru/post/256299/
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