Across the world, 5G networks have popped up to deliver faster speeds and lower latency to power new consumer and enterprise applications, with ubiquitous coverage already reached in a number of countries.

Over the past four years, some 260 operators deployed the technology around the world. The GSMA forecasts by 2025 5G networks will cover about one-third of the world’s population.

While the adoption of 5G has been much faster than LTE, with some 1.5 billion connections worldwide in just over four years, indoor coverage in subway stations and trains has proven to be more challenging.

In China, for example, where 5G outdoor coverage tops 95 per cent, the percentage in the country’s commuter lines is less than 55 per cent. The nation has 308 metro lines, spanning 10,287km, with nearly 90 per cent built five or more years ago, which is before 5G networks were first deployed.

Fan Yingying, ZTE RAN product planning manager, explained this gap is caused by a number of factors, such as limitations in the type of cable deployed in metros for cellular systems.

The country’s 5G networks make use of the 2.6GHz and 3.5GHz bands, but existing cables only can handle frequencies between 800MHz and 2.3GHz. New cables are required to support 5G frequencies, she noted.

Conventional leaky coaxial cable coverage faces significant obstacles, with network signals severely weakened inside the metro due to signal attenuation caused by the dense metal carriages and the Doppler effect resulting from the rapid movement of the train.

A second obstacle is space limitations inside subway systems. The installation of new cables requires a minimum of 1.2 metres from the tunnel to the train carriage, but the majority of existing metros only have about 1 metre. 

Insufficient power is a third factor. Each station equipment room has a power supply capacity of about 60kW. For cables supporting 5G service as well as the addition of new signal sources (5G RRUs), the power supply needs to be raised to at least 100kW. This necessitates remodelling equipment rooms to upgrade the power supply system.

Fan noted there is little time for building and remodelling because metro lines run an average of 18 hours a day. “Compared with outdoor coverage, these conditions considerably slow the process of deploying 5G coverage enhancements in metros, leading to a much lower coverage rate within metros.”

She explained underground cellular systems are impacted by two kinds of signal loss: cable penetration and carriage penetration. 

Boosting capacity
ZTE developed a custom system for metros with mmWave backhaul installed in tunnels, reducing the impact of cable loss. The backhaul can deliver up to 15Gb/s on the downlink and 2Gb/s on the uplink.

The advantage of using mmWave for backhaul is it has extremely large bandwidth and high transmission capacity. “This makes it suitable for applications that require ultra-fast data transfer, such as backhaul for multiple operators,” she stated.

To eliminate the frequency shift and Doppler effect caused by the high-speed movement of trains, the mmWave backhaul system compensates when the train access unit (TAU, which is a type of CPE) receives frequency signals.  All passenger and control information in the carriages is packaged and transmitted to the TAUs via routers with L2TP protocol.

For cable-based solutions, when the metro moves at high speed, signal Doppler shift will occur. But when Qcells, combining multi-band and multi-mode capabilities, are deployed inside the metro carriage, the signal shift is eliminated. Cell capacity and user experience are maintained using ZTE’s super-MIMO solution, ensuring coverage inside the carriage is the same as typical indoor coverage.

The 3GPP-based system makes data security better than on Wi-Fi and LTE-M platforms, Fan noted.

Since its Smart 5G-Powered Metro can be shared by all operators in a city, the number of signal source points and the multiple non-shareable frequency band RRUs can be decreased. 

Other benefits are low interference since mmWave has a shorter range compared with lower-frequency bands, reducing the likelihood of interference with neighbouring networks and allowing for more efficient use of the available spectrum and enhancing the overall network performance.  

Fan added mmWave device antenna arrays can accommodate more units in a smaller area because of the narrower wavelength, which improves beam-forming gain for both uplink and downlink. “Even with a very low output power, mmWave still offers a good signal quality and coverage range.”

China deployments
ZTE worked with China Telecom in April to deploy its Smart Giga 5G system for Shanghai Metro Line 4. The 34km line now has full 5G coverage. Energy consumption was lowered by 75.6 per cent, with the total investment cost reduced by 20 per cent, or about $2 million.

Based on ZTE’s new solution, the line maintains the 160 RRUs available in the conventional cable system, but only 96 AAUs are needed. The company said the system delivers peak download speeds of up to 1.7Gb/s in the metro and transportation capacity of up to 15Gb/s in tunnels.

In 2020, the company partnered with China Mobile to roll out the new system on Guangzhou’s metro, guarantying service performance at peak speeds of 160km/h. The average data rates on trains is 600Mb/s.

Using flexible frame structure adaptation, ZTE’s metro solution has the ability to quickly modify downlink and uplink frame structures in response to unforeseen circumstances arising from 5G industry applications. This allows mmWave to more effectively utilise its wide bandwidth and cover a wide range of scenarios and service changes.

The upgraded network is for use by both passengers and metro operations. A dedicated app as well as screens on station platforms display arrival timings for all lines, the real-time occupancy levels of carriages and carriage temperature. After disembarking, the sub-metre-accuracy positioning capability of Qcell directs passengers to the closest path to another line or the exit.

The installation of the mmWave backhaul system in tunnels takes about six months, while carriages can concurrently undergo modifications, taking a few weeks.

Fan noted its solution can be deployed in more than 90 per cent of China’s metro lines, and has been tested on some 190 metro models. 

“Within two years, China can achieve the same 5G outdoor coverage level in metros through the ongoing rollout of our solutions along with retrofitting of traditional cables,” she predicted.