Cellular Test Base Station: What's Inside

You know, after switching to fiber, we began to take out less hunting bullets from the cables.

About this, as well as about what else has changed after the transition to new technologies, I will describe below. Now we will go to the base station in our office. It is not quite ordinary: on the one hand, it is a functioning base station, on the other hand, it is also used to test and test new equipment.


This is how BS antennas look, the position of which can be controlled remotely.
Antenna motor control controller (upper rack unit)
Caution , there are a lot of photo traffic below.

Brief educational program


Many people think that roof mounted antennas are the base station. This is not entirely accurate - they represent one of its elements, specifically, devices for receiving and transmitting a cellular signal from subscriber to subscriber, then to the base station itself, to the controller and other control devices.

Here is the signal path (very simplified):
1) Your signal is received by antennas to one of the sectors of the base station;
2) Passes through the amplifier, where the level of the received and transmitted signal is aligned (from a weak transmitter of the phone);
3) Amplified by remote controlled radio module;
4) Through the base station it goes further to the controller, where processing and switching take place;
5) Data is processed and transmitted to voice and packet traffic switches, connecting subscribers between themselves and the Internet.

Photos


Below is a photo of the room with base station equipment. Typically, these rooms are much smaller than ours, and in containers for installation in an open area, and even less space. In addition to the base station itself, there are many auxiliary equipment in the room, such as a cooling system, an alarm, a fire extinguishing system and uninterruptible power supplies, which can ensure the base station is operational for a while in the absence of an external power supply.

The “dinosaur” of the cellular network is looking at us to the right - a cabinet with a 2G station. In the center - more modern equipment that supports 3G standard.



Here are the empty controller cells:



Another “exhibit” is fixed on the wall - one of the first base stations designed for organizing indoor communications, the so-called microcells.





Another old device is a packet switching unit, DDF for two-megabit streams.



This is where he got the “elephants” connecting traffic flows. The device really is a bit like an elephant.



But let's look at the working equipment. This is how old base stations look from the inside:
On this equipment, amplifiers are located in the base station itself, signal transmission to the antennas via thick radio frequency cables is used. More modern solutions use fiber and blocks at the antennas themselves.

Looking at this figure, you can understand what the difference is between the modern base station device (right) and what it was.



In the second case, the radio modules are installed next to the antennas:
This is exactly the same radio module in the room. Here it is for the convenience of testing:
Previously, the feeder path rose to the roof. He is about two fingers thick and practically does not bend his hands. The connectors are large, often oxidized, get wet. Here it is:
But this is a new connection, fiber optic in an armored cable, it is very flexible and easy to install and maintain:



With the introduction of precisely fiber, several important benefits appeared: energy consumption decreased by several percent throughout the network (because the unit began to cool naturally on the street), copper consumption for cables decreased, plus we began to drive less often to take bullets out of the cable. Yes, it sounds ridiculous, but as soon as the hunting season begins, we begin to regularly fail because of the "shot through the feeder path." Getting into the fiber is much more difficult, because snipers have not yet grown, apparently.

And here is the conclusion again without an armored braid, look how thin:



It’s not always possible to use remote controlled modules (RRU) near the antennas themselves - in the southern regions free cooling may be lacking (but this is quite rare). In the north, winter helps a lot. Here, for example, a photo of my colleague from Arkhangelsk, this is a block with an open lid.



Now let's go back to Moscow, again we will go into our room and look at the next closet. These are power supplies.



Below are uninterruptible power supply (UPS) batteries, on which the base station can operate for several hours if there are problems with electricity. If this problem occurs, we use the energy of a diesel generator.



Also, the base station can be powered by other sources, such as windmills or solar panels. During the restoration of communications after the flood in Krymsk, generators helped a lot: the emergency team connected them to the station for several hours. During this time, the UPS batteries were charged to a sufficient level to allow the generator to “rest”. The cycle was repeated until the restoration of the city grid.

Base stations use 48 V.
Opposite these cabinets is a standard 19-inch rack in which several units are installed. So now a modern base station looks like, in this case 3G and LTE. In addition, the antenna motor control unit we are familiar with can be used to remotely change the electric angle of inclination and transmit information to the control server.



The motor control device has been used for several years and has established itself as a reliable device; it does not require much attention and time for maintenance.



You can connect to each motor at any base station remotely via the web interface or console and receive data on its status or issue control commands. Previously, the angle of the antennas could only be changed manually using the device shown in this photo.



This is the LTE base station - its control part (the second unit is on top):
And this is the LNA control unit - a low-noise amplifier. The phone has a weak transmitter, so the signal coming from it must be amplified, if possible, without distortion. This unit is just designed for this.



This is one of the devices that is used to test equipment - the coordinated load (a test device that allows you to check some parameters of the base station without putting it on the air):
Now back to the top! This is how the base station antennas usually look at the city.





In addition to antennas for transmitting a cellular signal, there is an antenna for transmitting information flows between base stations. This is a radio relay antenna. It is used where there is no possibility of transmitting information between base stations using optical communications. There are several ways to transmit information channels. It can be transmitted in any way - starting from an optical cable and ending with a two-way satellite antenna with a satellite modem (as we have in the port of Dixon). Here is this radio relay antenna: The
optical fiber comes from the BS from below and looks like this:



Further, the optical fiber goes into the remote control units (transceivers) already familiar to you. This is, for example, a tri-band antenna. Three antennas of the ranges 900, 1800 and 2100 are installed in its case at once. It differs from other antennas in its large size and weight (which makes its maintenance a little difficult). This antenna has a triple control motor.



Above the antennas, you can see the pins. These are lightning rods:



Typically, base station repairs and scheduled maintenance are done by contractors. But our department constantly takes part in the repair and maintenance of base stations and antennas, so we also have our own equipment for measuring. In the photo - equipment. Thick black cables lie below it - these are feeder jumpers. The feeder is heavy, bends badly, it is difficult to connect it. Therefore, such an elastic short adapter is used here.



Sometimes it happens that on one cable you need to transmit signals to the antenna in several ranges, for example, in 900 and 2170. Where a second cable cannot be laid for various reasons, a combiner is used. The principle of operation is a multiplexer and a demultiplexer. The quality of the line suffers somewhat, but you have to sacrifice something.



This is what the connectors on the feeders look like.



A very important thing for installation: a large suitcase with all kinds of fasteners and fittings for various purposes.



And this is a very useful device for measuring the standing wave coefficient in antenna-feeder systems. With it, we check the performance of antennas and feeder paths. He helps us with troubleshooting and repairs.



References