Underground Transit RF Design & Optimization Considerations

Introduction: The smartphone revolution spanning in first two decades of 21st century has changed the way subscribers consume data on mobile devices. The very definition of mobile device has expanded to include tablets, wearables and other connected accessories including machine to machine. With Internet of Things (IoT) being major growth area for data demand, the coverage and capacity requirements of wireless networks will grow for foreseeable future.

One of the mass gathering places that presents peculiar challenges for coverage with exceptional demands of capacity is underground transit systems. It is the most common mode of transportation in many metropolitan centers across the world. The infographic below depicts annual ridership of major transit systems across the world. We will take a look at high-level guidelines for designing and optimizing a wireless network for underground transit system in order to meet the demands imposed.

Figure 1: Underground Transit System Riders

Background:Wireless connectivity is increasingly becoming one of the key requirements for passengers while using mass transit. For most consumers their choice of the wireless service provider depends on their ability to get seamless coverage including underground transit stations and rail systems. The subway transit system while being vastly different from one another catering to the needs of the cities they serve, have some key commonalities. These transit systems almost always serve core urban areas and heavily commercial clutter. The busy hours are usually morning hours and the evening hours where people travel for business commute. The macro coverage is usually limited due to majority of the system being underground.

These challenges are not limited to single wireless service provider. Some transit systems are targeting to generate additional revenue monetizing access to such service providers while enticing end users with better connectivity. This makes it difficult for individual operators to establish contracts to lay equipment. Most of these transit systems have restrictions on putting equipment due to contractual, structural, environmental and safety reasons. This makes it more challenging from design point of view. Based on these and other prevailing issues an effort has been made to compile best practices to follow while undertaking such projects.

Design Study:The design of such complex system would require establishing certain assumptions and ground rules. The system will be designed as neutral host allowing equal power to different operators in the bands they operate. It is upto individual operator to determine if they want to further divide available power between different technologies. This will also mean the desing has to include wireless technologies (e.g. GSM, CDMA, UMTS, LTE, Public Safety and Wi-Fi) and bands (700 MHz, 800 MHz, 850 MHz, 1900 MHz, 2100 MHz and 2400 MHz). The design criteria would be to provide coverage across all bands and technologies, most importantly the public safety network inside the trains and tunnels. The usual target RF signal strength values are -85 dBm for GSM BCCH and UMTS CPICH and -95 dBm for LTE RSRP.

Figure 2: Target Coverage Areas for Underground Design

Design Considerations:

While the DAS deployed in the subway transit system is often a neutral host system the Emergency services and the Wireless Service Providers (WSPs) may be converged or separate. The decision on the choice of the DAS architecture (discrete or converged) would be based on the bands and technologies deployed.

The type of antenna to be used will be based on the location. Directional antennas can be used to provide coverage for platforms and high ceilings. In entry halls and low ceiling walkways Omni-directional antennas are can be used. If tunnels are smaller, directional antennas can be used, for longer tunnels radiating cables are the best. Even with radiating cables there is distance limitation. If the tunnels are longer than a mile we will require cascading RU’s to extend coverage. Below is a calculation showing the distance limitation of the radiating cable.

Table 1: Radiating Cable Limitations

So at 6.4 dB loss per 100 meter the max loss of 51 dB would be reached at 800m. Two cable runs from each end may provide coverage for a 1.5km tunnel. Any tunnel longer than 1 mile will need cascading RU’s. The limitation however in using cascading RU is the increase in uplink noise and must be taken into consideration during link budget calculations.

Figure 3: Coverage Challenges Inside Tunnels

Capacity Consideration: Capacity dimensioning determines the number of sectors required to support peak traffic at the venue. The number of passengers during rush hour needs to be calculated based upon the number of trains arriving at the station and adding passengers who may be in waiting lounges or commercial establishments inside the station. Once the total number is obtained WSP level classification can be done using the market share rate. This number will provide individual WSPs enough information to determine the number of sectors required. Additionally, if a DAS is only covering platforms and not the tunnels additional capacity considerations need to be made. Assume every device coming in on the next train will pop into coverage, register, start downloading background data, and then drop as it leaves the platform. This excessive instant loading needs to be accounted for beyond just passengers waiting on the platform.

Optimization Study: Optimization of neutral host underground transit system has to be taken up by dual approach. One portion is overall DAS performance as a whole and second portion is how system is performing for each wireless service provider. This is a key demarcation point for all parties involved. There has to be an agreement that allows for remedy of common issues as well as operator specific issues. Minimizing the downtime to allow such remedies is key limiting factor for all operators involved.

A good system is capable of providing access to be able to identify problem areas quickly and accurately. It is prudent that a joint operations center be established to handle issues affecting performance of DAS system.

Parameter Settings: Some parameter tweaks are required when using radiating cable. For UMTS the search window parameter needs to the set at 256 chips for the sector that connects to a radiating cable longer than 1500 m. The reason being at 256 chips the max resolvable path difference is 10km while at 40 chips it is 1,560m. So any multi-path higher than the max resolvable path difference would be treated as interference. Hence to combat the interference caused by excess signal delay the search window needs to be adjusted accordingly. Similarly for LTE the cycle prefix value should be set to extend CP for any radiating cable longer than 1500 m. Also cascaded radiating cables longer than 5,000m should not be used for LTE as that will be the max allowable CP setting.

Handover Considerations: Handover management is a critical part of the design consideration. Once the traffic evaluation is completed the WSP comes up with the number of sectors required to serve its customers. It is a best practice to try and confine the handover areas to walkways and corridors to make sure there is not a lot of soft handoff overhead. Handover boundaries in tunnels should have sufficient overlap for the handover to be successful. This requires the radiating cables from both the sectors to overlap to provide the UE with enough time to successfully handover.

It is important to manage the handover between macro networks and DAS cells. This will be highly dependent on different operators’ macro network strength at the handover boundaries of the system. Because DAS changes will impact all operators equally it is best practice to optimize these handover boundaries with soft parameters.

External Factors Study: The physical and environmental considerations play a vital role in the subway transit design. Different regions present different factors that drive key decisions during design and optimization process.

• Equipment location choice should take ease of access into account. For example RUs residing inside the tunnel are harder to get to and should be avoided as much as possible even if it means installing two RUs are tunnel ends. This will save operating cost over the longer run.
• Ensure RU locations are safe from vandalism.
• Battery backup is a must for each RU location as power outage can be common occurrences in the tunnels.
• Radiating cable installation should withstand vibrations generated due to train traffic.
• Water leaks are common inside tunnels and hence passive components with high ingress protection is preferred.
• Antennas should be placed away from metallic objects and away from train tracks to avoid PIM.
• During natural disasters or any other calamity, first responders need to be able to establish communication lines inside these systems to be able to aid in relief work.

Figure 4: Physical and Environmental Design Limitations

Conclusion:In conclusion the subway transit design provides with a lot of unique challenges and physical limitations to consider. A precise 3D modeling of the venue is necessary to make accurate predictions and in turn an appropriate design. Environmental factors play and major role in the design often limiting the equipment location areas. Antenna locations need to be carefully planned to avoid PIM causing metallic objects and the RUs need to be in accessible locations thereby saving huge operational costs in the future. The design needs to be futuristic making sure all the technologies and incorporated with a scope for easy expansion and upgrade.

Optimization needs to be addressed at two separate levels. One is for combined DAS system and other is service provider specific. Handover between different sectors of the system and between macro network and DAS sectors has to be optimized with utmost care to provide seamless experience to end customer.

Telecom Technology Services, Inc has provided extensive design and optimization work on systems across major US cities including the NYC Subway system for major operators. Throughout these experiences they've have grown in experience and understanding to ensure a quality final product or determine the capital expansion requirements to get there. Each transit system has unique challenges that must be understood and overcome. It is with this understanding that we have shared our experiences in these systems.

References:

• iBwave Webinar : “Wireless Network Design in Subways”, June 2015
• Eupen Technical Data sheet, Radiating Cable Specification Sheet.
• iBwave : “Subway Design Case Study”, June 2015
• Time Out New York: “The NYC Subway Versus Subways Around the World”