Every day, millions of passengers descend underground to quickly get to work or home, and almost everyone has a smartphone in their hands. The instinctive movement of a thumb to the settings icon or lock screen occurs automatically, even if we know there's no service in the deep tunnel. However, upon exiting the platform or while in the train, we often see the network indicators change, allowing us to scroll through social media feeds or watch videos.
Few people realize the colossal engineering effort behind this seemingly simple operation. The underground environment is an extreme environment for radio wave propagation, where concrete walls, metal reinforcement, and the curvature of the tunnels create powerful interference. Radio signal In such conditions it behaves unpredictably, reflecting and fading faster than on the surface.
To ensure a stable connection, engineers use complex signal distribution systems that are fundamentally different from a home router. In this article, we'll explore the physical principles of underground wireless communication, discover why your phone switches between towers in a split second, and understand why free Wi-Fi in the subway is often slower than mobile internet.
Physics of radio waves in a confined space
The main problem with organizing communication in the subway lies in the physics of electromagnetic wave propagation. Unlike open space, where the signal propagates spherically, in a subway tunnel it behaves like a giant waveguideThe tunnel walls, made of concrete with metal reinforcement, act as reflectors, creating a multitude of interference patterns.
The signal doesn't simply travel in a straight line; it bounces off walls, ceilings, and floors multiple times before reaching the receiver. This phenomenon is called multipath propagation. As a result, the signal may be amplified at one point in the tunnel, but completely disappear a few centimeters further away due to the out-of-phase addition of waves.
⚠️ Note: Multipath propagation is the main cause of speed drops. Even if your phone's screen shows full signal strength, the actual data transfer speed may be zero due to high interference.
Furthermore, metal train bodies create a Faraday cage effect, shielding the interior of the car from external radiation sources. This is why base station antennas are never simply placed on platforms—this is categorically insufficient to cover the entire train route.
DAS Systems: The Backbone of Underground Communications
To solve the problems of attenuation and shielding in modern metro systems, distributed antenna systems, known as DAS (Distributed Antenna System)This is a highly complex infrastructure, consisting of a network of hundreds of low-power antennas, evenly distributed along the entire train route. The key feature of DAS is that all antennas operate synchronously on the same frequency, creating a single information field.
Unlike surface cellular networks, where each tower has its own unique identifier and the phone constantly searches for the best one, the metro uses SFN (Single Frequency Network) technology. The signal from all antennas reaches the subscriber simultaneously, and the smartphone's receiver perceives it as a single, powerful signal with multiple reflections, significantly increasing connection reliability.
The DAS system consists of a headend that receives the signal from the telecom operator and remote modules scattered throughout the tunnels. Between them, kilometers of fiber-optic cables transmit the radio frequency signal in digital form without loss of quality over long distances.
Why can't you just install powerful amplifiers?
Using powerful amplifiers in the confined space of a tunnel would create extremely strong echo signals and interference, completely paralyzing communications. DAS antennas operate at low power, but there are many of them.
Data transmission technologies: from 2G to 5G
The evolution of metro communication standards proceeded at its own pace, often lagging behind terrestrial networks due to the complexity of infrastructure upgrades. For a long time, the primary standard remained GSM-900, which, due to its low frequency, penetrates obstacles better, but has low throughput.
With the advent of smartphones and video content, operators were forced to implement 3G and 4G (LTE) standards. This required the installation of significantly more antennas, as high-frequency signals are less able to bypass obstacles and fade more quickly. Fifth-generation networks are currently being actively tested and implemented. 5G, which require even denser placement of equipment.
- 📡 GSM-900/1800: Basic coverage for voice communication, works even in deep soil layers.
- 🚀 4G LTE: The main standard for mobile internet, requiring frequent cell changes while moving.
- ⚡ 5G NR: A future metro that delivers gigabit speeds but requires antennas every few tens of meters.
It's important to note that the transition between standards (for example, when 4G is lost and only 3G remains) occurs seamlessly for the user thanks to the operator's network resource redistribution algorithms. However, during sudden load changes, such as on a crowded platform during rush hour, the network may prioritize voice calls at the expense of internet access.
Secrets of free Wi-Fi in train cars
The free Wi-Fi system, often found in metro cars in major cities, deserves special attention. Unlike cellular service, this Wi-Fi isn't broadcast from the platforms. The signal is sourced from dedicated routers installed in each car or every few cars.
These devices are connected to the external network via a high-speed connection using Long Range Wi-Fi technology or dedicated LTE/5G lines laid along the tunnel. Antennas on the roof of the train continuously receive signals from access points installed on the tunnel ceiling at intervals of several hundred meters.
A local network is created inside the car, to which passengers connect. Since the communication channel with the outside world is limited by the capacity of the equipment on the roof, speeds often drop when the car is fully loaded. Furthermore, the Wi-Fi signal inside a metal car is less effective than a cellular signal due to the shielding properties of the structure.
⚠️ Warning: Connecting to open Wi-Fi networks on public transport puts your data at risk. Attackers can exploit protocol vulnerabilities to intercept traffic. Use a VPN when working with sensitive information.
The system works on the cellular principle, but on a miniature scale. As the train moves, its external antennas quickly switch between access points in the tunnel. This process, called handover, must occur within milliseconds to ensure the video stream is uninterrupted. If the train is traveling at high speed, the number of switching errors increases, leading to video buffering.
Network congestion issues during peak hours
Why is internet speed so great in the metro during the day, but images barely load at 8 a.m. and 6 p.m.? The answer lies in the physical limitations of radio channel bandwidth. Each base station or DAS sector has a fixed frequency resource, which is divided among all active users.
During rush hour, the subscriber density per square meter on a platform or in a train car reaches critical levels. Thousands of smartphones simultaneously attempt to establish a connection, send data, or update their feeds. Telecom operators use complex resource scheduling algorithms to distribute airtime fairly, but the physical bandwidth limit cannot be exceeded.
The situation is exacerbated by the fact that many background apps constantly try to synchronize data, creating a signaling storm. The network wastes enormous resources simply servicing connections, without transmitting useful traffic.
How to improve connection on the metro
Comparison of coating technologies
To understand the scale of engineering solutions, it's useful to compare various approaches to communications. The metro uses hybrid schemes that combine the advantages of different technologies. Below is a table illustrating the differences between the main coverage methods.
As the comparison shows, there is no perfect solution. Operators are forced to combine these technologies to balance implementation costs, coverage quality, and throughput. New metro lines are typically equipped with fiber-optic backbones for DAS, while older lines often use repeaters.
| Parameter | Cellular communication (DAS) | Wi-Fi in the carriage | Repeaters |
|---|---|---|---|
| Signal source | Telecom operator | Local router | Signal from the surface |
| Coating | Platform + Tunnel | Only inside the carriage | Depends on the location |
| Stability | High | Average (depending on external connection) | Low |
| Speed | Up to 100+ Mbps | Up to 20-50 Mbps | Unstable |
The Future of the Underground Internet
Technology is advancing, and metro systems around the world are preparing to implement new solutions. One promising area is the use of millimeter waves (mmWave), which can transmit enormous amounts of data but have a very short range. Their operation requires antennas installed every 10-20 meters.
Li-Fi technology, which transmits data via modulated LED light, is also being considered. In dark tunnels, this could be an effective complement to radio, although it requires a clear line of sight between the transmitter and receiver.
Another trend is the integration of metro networks with Smart City systems. Antennas will not only distribute internet but also monitor train occupancy, monitor safety, and control station climate control systems using a unified communications infrastructure.
⚠️ Please note: Network specifications and available services may change depending on operator equipment upgrades. Up-to-date coverage information can always be found in the operator's official apps.
Frequently Asked Questions (FAQ)
Why is there internet in one carriage, but not in the next one?
This is due to uneven distribution of Wi-Fi antennas within the train or the equipment in a particular car being faulty. The signal may also be blocked by the large number of passengers in adjacent cars.
Does train speed affect connection quality?
Yes, at high speeds (over 80 km/h), handovers between base stations become more frequent. If the equipment isn't configured perfectly, micro-connection interruptions may occur, which are noticeable during video calls.
Can the metro completely block cell phone service?
Theoretically, yes, for security or emergency purposes. However, in normal operation, operators strive to maximize coverage, as it is the main source of traffic during peak hours.
Why does Wi-Fi in the metro require SMS authorization?
This is a legal requirement for identifying users of public Wi-Fi networks. The operator is obligated to know who is using their channel, even if the service is provided free of charge.