The organization of cellular communications in the metro - the experience of St. Petersburg
Typical placement of antennas on the tunnel arch
In the transition
For us, signalmen, one of the complex features of the St. Petersburg metro is the presence of closed stations , the so-called "horizontal elevators." At such stations, the train drives up to the platform, “joins” the doors of the train to the platform doors, then the pairs of doors open simultaneously.
Such metal doors protect not only from monsters from hauls and suicides, but also perfectly shield the cellular signal. Therefore, when the base station of the platform is opened on the uplink, several hundred subscribers simultaneously connect and a kind of DDoS attack is obtained.
The second problem is long escalators that often do not go into the lobby, but through a small hall into the “dull” underground passage.
Cellular communication in the subway is needed, and is needed quite strongly. The subways of various cities are, as a rule, closed strategic objects, therefore they have rather strict restrictions on the possibilities of organizing communications, installation, and so on. In our case, in the subway there is one provider that provides the transport channel, and one certified contractor involved in installation works and maintenance. This is a fairly favorable situation, because these two companies have all the necessary approvals and can carry out the same emergency work very quickly. In other cities, problems may arise with the fact that for any movement you need to get approval in 5-6 days. Our routine maintenance is usually coordinated much faster, it is possible to quickly carry out repairs.
Communication in the subway is not implemented as in the city. The city uses the so-called macrocells - powerful devices designed for many subscribers over a large area. In the metro at the stations there are the following features of radio planning:
- The coverage areas, as a rule, are quite small - this is the platform, transitions and escalators.
- These areas are often elongated (for example, a high escalator), and the possibility of installation can only be on one side of the object.
- The number of subscribers at rush hour per cell can be as many as in the “large” cell on the surface.
- There are almost no difficulties with interference and intersections with neighboring cells - there are no corny neighbors except for the junction with the surface. The exception is open hauls, but we have very few of them.
- In many places, people are close enough to radiating equipment, so special attention is paid to its safety.
- In the last six months, the need for metro subscribers in Internet traffic has sharply increased.
- Our metro is quite deep: in contrast to the “shallow” European metro, there are almost no situations where city cells can be used to cover platforms or when their signal “breaks through” into a stage or transition.
Antennas at the escalator
Due to the specifics of our metro, an antenna cable is laid in the tunnels (in fact, a long piece of iron with antenna holes) To establish communication in the tunnels, you need access to this cable, which we do not have yet, but may appear in the future.
The first cycle of work was carried out in 2004-2005: then 2G stations and repeaters were installed at the metro station to cover the crossings. The control modules of the base stations themselves (everything that is not antennas) were installed in special technical rooms (server) at the stations - you probably saw these always forbidden doors at the edge of the platform or in the transitions. One server is used by all four city operators, and the subway contractor is responsible for its operation. If you need to touch something with your hands, our specialist is assigned a special escort who opens all the doors and monitors the work.
At the beginning of this year, we began a large-scale equipment replacement in St. Petersburg. One of the main points is the transition from one vendor to another. The first new 3G stations were tested in the test zone, then we put one on the stations and conducted a beta test, and then we upgraded all the base stations of the subway. In just three months, the new equipment was aired at 64 stations.
As a result, the following work was done:
- Repeaters were removed and replaced with regular sectors.
- 2G stations have been dismantled and replaced with 2G-3G stations.
- The transport channel was expanded to 30 Mbps per station. We have the opportunity to expand it further as necessary, but in practice this is enough for voice and surfing. If you are alone at the station and standing near the antenna - you can get 42 Mbit / s according to the standard, but in fact it will be about 30 due to transport restrictions. If there are a lot of people, respectively, the channel is divided between them. At the same time, we carried out a set of works on channel reservation for our part - we are talking about all points of data forwarding to our core network, independent of the subway provider. In particular, for example, we take separately the data of 2G and 3G stations, which, in fact, gives some protection against failure of the feeder or unit on the BS. The remaining failures are associated with transport, which does not depend on us, and applies to all operators at once.
- At the same time, we reconfigured the BS, a little lower in more detail.
- We pledged a reserve in case of connecting to a radiating cable in stages - when it can be used, we can increase the service in a matter of days without changing the BS hardware.
Installation of a 19 ”rack and combiner unit.
Installation of multiband operators of telecom operators.
Installation of blocks RRU 900 and 2100 ranges
We practically did not change the positions, therefore the principles of installation remained the same as in 2004-2005:
- The platforms are covered by antennas located on the tunnel arch. By the way, nothing can be installed in the tunnel itself: there is a special train that measures the dimensions - it will knock down everything that sticks out.
- Escalators are covered with antennas on the arch below, pointing up. The exception is particularly long escalators like a position on the Admiralteyskaya (the deepest station of the St. Petersburg metro to date) - there is coverage on both sides.
- Transitions - from one of the points, most often - the one closest to the server end.
- BS equipment is located at stations in technical rooms in conventional 19-inch racks, most often - two units.
- Transport - 30 Mbps Ethernet.
19 ”rack with installed base station blocks, power supply and transport router.
At the same time, in the same way as with the usual procedure for launching a BS on the surface, when the radiation level is measured several times for different organizations, the metro has a special procedure. It is checked not just the radiation of the sector, but the total electromagnetic flux from all emitters (and there may be 4-8 in one place, along a pair of antennas per operator).
In fact, this means another thing that is difficult for us - 3G stations give an output power of about 2 watts, which means a small signal propagation area. Here we are faced with the problem of horizontal elevators. Fortunately, the 2G signal has better penetrating power and is able to pass into the tunnel for a certain distance (pay attention to the slots under the doors on the platform - just 2G “shines” in them).
When the train approaches the station, at first some of the subscribers receive 2G, then the rest “break up” to the 3G station, interrupting the uplink, then due to interference the part falls off or leaves for 2G, and only after 15-20 seconds (or up to 30 seconds in rush hour) all connected. This is precisely the problem of simultaneous requests, not BS resources - there are plenty of them there, given the possible transition to LTE in the future.
Change in traffic by day for 2 months of summer in MB blue by day, black - average.
When replacing, the BS manufacturer's engineers phoned the entire feeder path, measuring the SWR and, if necessary, the contractor replaced the connecting cables. They scored the “blank” of settings for the metro, which we then reconfigured for each specific station. For the most part, the work is automated, but at first I had to dig deeper. The fact is that in alpha and beta tests we did not reveal one specific problem that occurs only during rush hour loads. I had to tinker a bit with the transport equipment to solve this controller problem - but, in general, after two months of operation, everything went smoothly. We were waiting for big surprises from the new hardware.
Seamless handover on urban macro cell
City honeycombs “blow out” most often into the lobby as in a normal room, and the signal can be normally caught right up to the start of the escalator. If the signal power from the bottom of the mini-cell and the signal from the macro-cell is enough, they quite normally transfer the subscriber to each other using a standard handover. By the way, we have cells from different manufacturers, but, surprisingly, there were no problems with the configuration - there is a standard protocol, and there were no surprises.
There are situations when the signal power for a handover is not enough. These are stations where access is made to the underground passage: the transition architecture itself is directly created to shield the signal at cellular frequencies. In this case, of course, there will be a network loss for a few seconds while the subscribers leave. Colleagues in other cities sometimes solve the problem by installing mini-hundreds or additional sectors in the lobby or walkways, but in St. Petersburg this is not done, at least for now.
So, when we updated the BS, we did the standard indoor tests. Based on the tests, neighbors were removed or added by hands, which allowed achieving a very high-quality seamless transition.
At rush hour, the number of subscribers usually doubles from the normal cell load, and at some points, for example, exits from the metro, the density of subscribers is very high. Colleagues in Moscow at some positions have solutions that turn the antennas of the macro cell to exit the metro at 9-10 in the morning and 18-19 in the afternoon, and in the afternoon they raise the antennas a little higher, in traffic jams. We do not use such motors. Management of electric tilt angles is possible (there are motors), but programmatically changing angles for the CNN metro are not performed.
Drive tests are held regularly, as our network is changing “from above”. The results often come optimized parameters. The visiting team writes us a service note about what is worth changing, our scripts analyze the note and collect the configs, plus immediately wrap them in batch files. It looks like this: during the day the engineers go for tests, in the evening they give out a memo, in the morning we arrive at the office and see the finished packages on the deploy. We check the parameters and just roll onto the BS.