Installation of the GSM repeater. Allowance.

Cellular operators use two frequency bands: 900 and 1800 MHz. In populated areas with high-rise buildings, cellular communication most often operates in the 1800 MHz band, and outside the city and in low-rise buildings in the 900 MHz band. This is due to the peculiarities of the propagation of radio waves and the number of free channels.

The lower frequency (900 MHz), the better and then apply radio waves in rough terrain. Due to this, the base station can be located far from each other. Another important property of the signal of low frequencies is that they penetrate buildings better than signals of higher frequencies.

High-frequency radio waves (1800 MHz) quickly fade out over long distances and penetrate buildings and rooms worse. However, these seemingly negative properties become an advantage when using higher frequencies in cities: at each point in the city, each subscriber unit, roughly speaking, “sees” only a small number of base stations. Thus, base stations almost do not “interfere” with each other.

In the GSM-1800 standard, the quantity of channels is significantly more than in the GSM-900 standard, which allows more subscribers to work simultaneously. This is another reason why cities use the GSM-1800 standard, and in sparsely populated areas - mostly GSM-900.

Inspection of the object.

First you need to find out from the customer exactly what problems of cellular communication he wants to solve. In this article, we will look only at enhancing voice communication and will not consider enhancing the mobile Internet (however, installing repeaters for the mobile Internet is different from installing repeaters for mobile communication only in small details).

The customer may want to amplify the signal of one operator as well as the signals of several or all operators at once.
The distribution of radio frequency bands ( channels) between operators varies between regions, this information can be seen in the following tables: for 900 MHz and 1800 MHz.

From these tables you can see at what frequencies the operators of interest can operate. Therefore, already at this stage of the work, one can estimate their complexity and cost.

The facility survey should be carried out using a spectrum analyzer. Netmonitor on the phone can only be used as an auxiliary tool, because Netmonitor is not a measurement tool and its readings can serve as a basis for making any decisions. However, Netmonitor can still be useful, as it, unlike a spectrum analyzer, shows the channel number and the operator name. Using Netmonitor and good control of the correctness of the measurements performed by the spectrum analyzer.

Before going to the site, the engineer should be familiar with the map of the location of the base stations. Basically, these cards are created by amateurs and are not an official source of information. But they can help at least roughly navigate the situation. These maps are easily available on the Internet and are constantly updated.

Upon arrival at the facility, the engineer must inspect it and determine the places where it will be possible to install external and internal antennas.

The determination of possible antenna installation locations should take into account several factors:

• directions to the required base stations for external antennas;
• required indoor coverage for indoor antennas;
• mutual arrangement of internal and external antennas (to prevent self-excitation);
• ease of installation;
• location of antennas, amplifiers, cables should suit the customer.

The definition of RF environments.

At the next stage, it is necessary to make measurements of the Downlink signal levels ( from base stations) at each point of possible installation of external antennas. Measurements should be carried out using a spectrum analyzer and an external measuring antenna. It is necessary to make a table of dependences of the signal level on the frequency of reception.
For example, the task is to strengthen the three operators (Beeline, Megafon and MTS) in the Voronezh region. The figure (this is a top view of the roof of the house) shows three points where there is access to the roof and where it is therefore possible to install external antennas (points A1, A2 and A3).

The procedure is as follows:

1. In the radio frequency distribution table, we find that in the Voronezh region operators operate at the following frequencies:

2. With the help of Netmonitor, each operator in each frequency range selects the channel on which the downlink channels are located. Channels should be chosen so that the frequencies from different operators are separated as much as possible from each other (at least by 5-10 MHz). It should be noted that the base station can operate on only a few channels, and these channels can sometimes change.

3. Using the following formulas calculate the frequency that corresponds to each selected channel (n - channel number).

4. Write the data in the table. For example, you can get the following values:

5. It turned out six frequencies at which we will conduct measurements. At each point of the possible installation of an external antenna, it is necessary to find, firstly, the directions from which the signal levels at each of the six frequencies have maximums, and, secondly, record the values of these maximums. All obtained data must be recorded in the table and put on the figure. In the following figure, the arrows indicate the directions in which the signals have maximums. For clarity, the length of the arrows is proportional to the levels of the received signals.

6. From the table and figure, it became clear that external antennas for amplifying Beeline and MTS signals can be placed at point A1 or A2, and at the point of amplification of Megafon only at point A3.

In the example we have considered, all base stations operate both in the 900 MHz band and in the 1800 MHz band. In practice, this is likely to never happen, since each BS operates in any one range

The choice of location of internal antennas

We solved the first task - we determined the direction to the necessary base stations for external antennas. The next task is to determine the necessary indoor coverage areas for internal antennas:

• the location of the internal antennas depends on the configuration of the interior and on which premises the customer wants to amplify the signal;
• it is necessary to take into account of what materials the external walls of the building, floors and internal partitions are made. Solid metal roof attenuates the signal by 40-60 dB; thick brick, reinforced concrete walls and ceilings attenuate the signal by 20-40 dB; plasterboard, wooden partitions weaken the signal by 10-20 dB. It should be borne in mind that higher frequencies in the 1800 MHz band are attenuated more strongly than frequencies in the 900 MHz band
• the mutual arrangement of the internal and external antennas must be such that the amplifier is not self-excited, that is such a situation in which a signal from an internal antenna is picked up by an external antenna is amplified, fed to an internal antenna, and so on.

If competently and reliably amplify the Downlink signal (i.e., from the BS to the phone), the problems with the Uplink signal (i.e., the BS) should not be. When you use a quality repeater such a signal is also amplified. Typically, the Uplink channel gain of the repeater is doing with the gain on 5...10 dB less than the gain of the Downlink channel. To phone securely receiving the signal Downlink, it needs to be at least -80 dBm (at the point in space where the phone). To the base station is reliably receiving the signal Uplink , it needs to be at least -95 dBm (at the point in space where the reception antenna of the base station). This is because the base station uses a low-noise channel filters and amplifiers, which can amplify very weak signals from phones. This amplifier narrows the band of the reception signal up to 100 times, reducing to 10...20 dB self noise of the receiver of the base station.

However, the phone can only take on strong signals, as in telephones, there are a number of technical limitations to the possibility of receiving the signal with greater sensitivity. This is due to the technical complexity of the implementation in the dimensions of the phone receiving antenna with high gain amplifier and the input of the filter - preselector of the receiver.

Modes of phone operation when receiving Downlink signals of different power:

The calculation of the Downlink signal

We will calculate the Downlink signal for each point of possible installation of external and internal antennas. To do this, add:

• signal level at the installation point of the external antenna;
• external antenna gain;
• losses in connectors and cable;
• repeater gain;
• loss in connectors and cable;
• losses in the signal splitters (if any);
• internal antenna gain;
• attenuation of the indoor signal.

It is necessary to make sure that in the room where we want to enhance cellular communication, the Downlink signal level is not below -80 ... -75 dBm.

The table below shows the attenuation of the signal in free space, depending on the distance to the antenna. Using this table, it is necessary to make an amendment to the attenuation inside the room (the maximum distance from the internal antenna to the far corner of the room is taken).

Example of calculation of the Downlink signal from BS Beeline in the range of 1800 MHz with the antenna installation at the point A2:

-81 dBm (BS signal Downlink from Beeline at point A2 in the range of 1800 MHz)
+14 dB (external antenna gain)
-4 dB (the external antenna at point A2 may be sent between Beeline and MTS base stations, so the antenna gain will be slightly less than the nominal)
-7 dB (approximate loss in connectors and cable 10 m in length between an external antenna and a repeater)
+Rg (repeater gain, we just have to find it)
-7 dB (approximate losses in connectors and cable 10 m in length between the repeater and the internal antenna)
+8 dB (internal antenna gain)
-58 dB (approximate attenuation of a signal in a room at a distance of 10 meters from the antenna to the far corner of the room)

Do: -81+14-4-7+Rg-7+8-58

It turned out (Rg-135) dB.

As already mentioned, the level of the Downlink signal must be below -80 ... -75 dBm. Therefore, Rg=135-75=60 dB. Thus, the minimum gain of the repeater must be at least 60 dB. Since each repeater has an error in specifying the gain (approximately 3 dB), uneven frequency response (approximately 3 dB) and noise figure (approximately 6 dB), you should choose a repeater with a condition of approximately 70 dB. The specific characteristics of each repeater must be viewed in its passport, but do not forget that repeaters from unfamiliar or unreliable manufacturers may have a very high gain on the passport, uneven frequency response and noise figure.

Thus, when installing a repeater with a gain of 70 dB indoors in its far corner, the Downlink signal from Beeline will be -65 dBm.

Similarly, you must calculate the level of the Downlink signal from BS MTS and Megafon for the same system configuration: the same external antenna at point A2, the same amplifier, the same internal antenna:

• the level of signal from MTS BS = -55 dBm
• the level of signal BS from a Megafon = -87 dBm (we subtracted another 10 dB, because the external antenna with a gain of 14 dB at point A2 will receive a signal from the BS Megaphone with a gain of about 0 dB, since it will be directed away from the direction of the maximum).

Calculation of the isolation of the external antenna relative to the internal

Then, it is necessary to estimate approximately what parasitic power will go from the internal antenna to the external one (the reverse parasitic signal from the external antenna to the internal one can be ignored, since, as mentioned earlier, the Uplink gain channel has a lower gain). Optimally, when you have a scalar network analyzer available that allows you to measure the signal flow from an external antenna to an internal one. The maximum possible power from the BS MTS will be radiated on the internal antenna: -71 + 14-4-7 + 65-7 + 8 = 3 dBm. The internal antenna is 20 meters away from the external one (this is -64 dB attenuation, from the table) and, moreover, there is a reinforced concrete wall between them (this is about -40 dB attenuation). In total, this will give up to -101 dBm.
Thus, the difference between the spurious signal (from the internal to the external antenna, -101 dBm) and the weakest downlink signal (from the Beeline BS, -81 dBm) is -20 dBm. This difference should be at least -15 ...- 20 dBm. If this difference is less than -15 ... -20 dBm, then the following remedies can be applied:
- spread the internal and external antennas to an additional distance;
-change the polarization plane of the internal antenna from vertical to horizontal (this will give a gain in the junction of 15 ... 30 dB);
- use a repeater with less gain;
- start the system at the facility in test mode, since some parameters were taken only approximately during the calculation (for example, the value of the attenuation value of the signal as it passes through the reinforced concrete wall). In practice, things may be a little different.

Calculations for decoupling the internal and external antennas should be carried out for all signals of the same range. If one operator works in one range, and the second in another, then their mutual influence in this case will be minimal.

Choosing a repeater based on radio frequency conditions

Thus, after studying the radiofrequency situation at the facility, we received information about the Downlink signals from various operators, namely the channel levels of each operator. Depending on which operator, on which channels and with which levels it transmits signals, it is necessary to choose one or another gain scheme.

Briefly about the gain of the repeater

Consider a repeater with a 50 dB gain. Everything is simple: by applying a signal with a level of -83 dBm to its input, we will get -83 + 50 = -33 dBm at the output. However, for each repeater, in addition to the gain, the maximum output power is indicated (this value is also called the “compression point”). This is the maximum possible output signal of the amplifier, above which distortions begin to grow and the overall gain of the repeater falls. Up to this value, the gain can be considered linear.

But the fact is that all the manufacturers of repeaters, without exception, are cunning, indicating the amplification and amplitude-frequency response for one channel. And we need to strengthen the many channels.

When several channels are amplified, the gain of the repeater decreases by one channel. But more on that later.
The repeater should not operate in the mode close to the compression point (for the considered repeater it is 20 dBm - this value is indicated in the passport of the repeater and is called the maximum output power), since intermodulation distortion will increase. Therefore, the repeater should operate in a mode in which the output power is less than the compression point by 10-15 dB.
That is, in the repeater in question, we can apply a maximum of -40 dBm to the input. Then the output of the repeater will be 10 dBm, which is just 10 dB less than the compression point for this repeater.

How do intermodulation distortions appear.

If it happens that two signals with approximately the same power and frequencies that differ by a small amount arrive at the repeater input, then the output of the repeater will be not only these two signals, but also signals with frequencies that differ from the frequencies of the input signals by their difference.
For example, signals with frequencies f1 = 952 MHz and f2 = 953 MHz are input. Then, when the repeater operates in the mode above the compression point, the output of the repeater will be not only amplified signals f1 = 952 MHz and f2 = 953 MHz, but also signals that differ by their difference ∆f = f2-f1 = 1 MHz, namely f3 = f1 -∆f = 951 MHz and f4 = f2 + ∆f = 954 MHz. These parasitic signals will be greater, the closer to the maximum output power is the working point of the repeater.

So, we repeat once again: the repeater should not work in the mode close to the maximum output power.

What does “repeater gain” mean?

Consider a repeater with a gain of 50 dB and a maximum output level of 20 dBm. This means that the repeater will amplify ONE incoming channel by a factor of 50 dB, provided that there are no other incoming signals in the repeater gain range anymore. This is an ideal case and this is almost never the case. There are always several channels at the entrance and some noise and interference. Each additional channel reduces the gain by an amount of 10Lg (N / 2). This formula is valid when the signal levels are about the same and differ by no more than 1 dB. For example, for 10 channels at the input, the repeater gain will drop by about 7 db. And accordingly, the maximum output level will be 20-7 = 13 dBm. If at the repeater input there are not only useful signals, but also interference, they will also reduce the gain in terms of useful channels.

Thus, the gain coefficient of the repeater depends on the number of channels to be amplified. It is important to know that at night the base station can work only on one or two channels, since the number of calls is small. Accordingly, the gain of the repeater will be large. In the afternoon, the number of active subscribers increases, base stations automatically increase the number of channels, and the gain of the repeater naturally decreases.

AFC repeaters

AFC - amplitude-frequency characteristic - the dependence of the gain of the repeater on the frequency. The frequency response of the repeaters is always non-linear, has a blockage at the boundaries of the ranges of 3-8 dB. In addition, the frequency response of each specific repeater differs from the frequency response of another repeater
The frequency response of the ideal band repeater GSM-900 with a gain of 50 dB should look like this:

But the world of electronics is not ideal, and the real frequency response of the repeater looks like this (the figure shows the frequency response of repeater VECTOR R-610):

As you can see, the repeater will amplify some channels better, and some worse. Especially need to be careful when increasing the channels located on the borders of the ranges.
In view of the specific circuitry of cheap repeaters, the higher the gain of the repeater, the more uneven the frequency response it has. The authors of this article came across the Chinese and Russian copies of the repeaters having a flatness in the range up to 18 dB. It is also necessary to take into account when installing.

You need to pay attention

• If the frequency difference between the incoming channels is 5-10 MHz with the difference in their levels of 10 dBm, all these channels will amplify without distortion.
• If the frequency difference between the incoming channels is of the order of 1 MHz when the difference in levels is 10 dB, the weak signal can be suppressed stronger. In this case, you must either increase the channels separately, through different repeaters, or try to find other channels harder spaced from each other.

Ways to improve the quality of gain

• You can use channel repeaters. They amplify only a narrow band with the desired channels. Such repeaters are somewhat more expensive than conventional ones, but sometimes only their use can enhance the signal in a complex radio frequency environment.
• You need to buy equipment (repeaters, GSM antenna, cables, connectors, filters, etc.) only from reliable suppliers, preferably those who are responsible for installing repeaters, because only then the risk of running into the left equipment with unknown characteristics will be minimal.
• You can use Uplink and Downlink signal filters for the frequencies that we want to boost. This is a way that only very experienced installers are able to directly interact with the manufacturers of repeaters.