Sunday 19 July 2015

Small Cells on the Train - A 2 hop solution

An Ericsson blog post some time back talked about the 2 hop solution for trains. Thinking about it, I quite like the idea. The post talks about 3 main challenges on high speed trains:

There are mainly three reasons communication services on high-speed trains is challenging:

  • First, large penetration loss via the shield of the train. This penetration loss is expected to be 20 to 30 dB.
  • Second, large numbers of handovers in very short time. This is due to hundreds or thousands of users needing handover from one site to another concurrently/sequentially. This phenomenon affects system stability and eats up capacity.
  • Third, high power consumption of user equipment (UE). This is because UE-s on the train need higher power to overcome the large penetration loss in uplink as well.

  • A common currently adopted solution for high speed trains is to densify the network along the railway to combat the large penetration loss. However, this will make the second issue more severe, as handover frequency is increased due to smaller site- to-site distance. Another way is to increase the transmission power of the base stations, which helps to solve the large penetration loss as well. However this cannot solve the third issue. And neither of these solutions are cost-effective.

    Another solution I have discussed before is the Mobile Relay Node which was designed with avoiding multiple handovers when the vehicle moves between different macro cells. Not sure about its status in the standardisation process right now.

    Anyway, coming back to the Ericsson post on Small cells on the train, while the Macro cells provide the TD-LTE backhaul outside, Radio Over Fiber (ROF) is used inside the tunnels to provide the same coverage.

    Within the train Small cells (I guess multiple small cells will be needed in practical deployments, one for each carriage) can provide good coverage to the users and avoid the need for handovers.

    Embedded is the video from Ericsson Taiwan that provides more details about this trial


    1. Mobile Relay Node in standards is stalled - Technical Report complete, but not enough support to move forward. You may speculate on the companies expressing concern about aspects of moving forward

    2. Thanks for the information Zahid.
      It would be interesting to know more about the way they optimize the handovers betweent the pico cell and the macro cell.

      As much as this solution improves power consumption, it also create a single point of failure.
      I.e. if the roof antenna fails to handover, then all the users attached to the pico cell will drop.
      So handover optimization between the roof antenna and the macro network is key to finalize the solution.

    3. Nico, good question. I dont know the details. I assume these problems can be ironed out during test phase. I am hoping on hear Ericsson speak about this next month, hopefully they will provide more details.

    4. Cheers Zahid.
      Looking forward to hearing more about this.
      Thank you for your blog as well, it's a very nice resource.

    5. Any relay (or repeater) node adds a single point of failure unless you carry out multiple routes from the intermediate node - e.g using different mobile providers. There are some on-train solutions for shuttles that use this technique