Out of terrestrial network coverage

Updated 9.4.2016

Public safety terrestrial network coverage may not be always available for mission critical communication because of many reasons. Coverage may be missing by design in uninhabited areas, but temporary communication solution might be needed for example due to a forest fire. Some indoor or underground locations such as tunnels, basements and caves may not have coverage available. There could be also network outage due to natural disaster or broken transmission link. Still public safety users need communication solutions in case of an emergency.

Temporary coverage can be provided with deployable systems. Such a system can be fully standalone network without any connection to external networks or there can be a suitable transport connection for example over satellite to connect deployable eNB to a centralized core network. Deployable systems can also vary in coverage & capacity from large macro eNB with multiple sectors to a small micro or pico cell.

The figure below illustrates few options. The first on the left is a high capacity macro eNB including complete core network (at least MME, SGW, PGW and HSS). It can be a trailer or a truck installation with power supply and for example pneumatic mast antenna. In the middle there is outdoor micro eNB with single cell and satellite connectivity installed in a van. On the right there is a pico cell with embedded EPC and battery to be carried by one man easily to any location.

Deployable systems can be implemented today based on existing products, and the deployable systems can be used with all standard LTE devices. Though there is also 3GPP solution that can be used. 3GPP release 13 specifies isolated E-UTRAN operation for public safety (23.401 [Annex K] IOPS). IOPS can be the standalone solution for public safety users. Based on IOPS specification the main core functions are included in local EPC co-sited with eNB. In case of IOPS compliant solution, a dedicated PLMN identity is used and only UEs with access classes 11 - 15 can access the system.

Deployable base stations are quite common solutions for mobile operator, who can use them typically for temporary additional capacity. Below is a figure from millennial anniversary of Hanoi, where operators had to offer more capacity for the celebrating people.

Different companies have already deployable solutions targeted especially for public safety use cases. Few examples:

  • Nokia & Harris have launched deployable LTE in 2014 
  • Deployable cell on wheels by General Dynamics
  • Google's  project Loon is not specifically for public safety, but it is interesting concept with LTE basestation carried by a balloon 
  • Parallel Wireless offers 'instant LTE network for public safety' 
  • Star Solutions has LTE system in backpack 
  • Air-Lynx has compact deployable LTE network supporting different LTE bands
  • Oceus Networks offers compact rapidly deployable LTE system 

In the future public safety users will have also other option to communicate in areas without existing macro network coverage. The solution will be based on 3GPP Release 12 Proximity Services (ProSe) and especially on LTE direct communication, which is part of 3GPP proximity services.

LTE direct communication is based on direct LTE transmission between public safety UEs. This direct LTE connection is also called 'sidelink' in 3GPP radio specifications (see Rel-12 or Rel-13 36.331 RRC). When direct communication capable PS devices are out of coverage, they can use 'sidelink' for group communication. 3GPP Rel-13 mission critical push-to-talk (23.179 MCPTT) includes support for off-network communication based on ProSe direct communication.

Direct communication is specified in release 12 & 13 supporting selected LTE frequency bands and bandwidths (See 36.101 UE radio Tx and Rx):

  • Band 3 (1800 MHz), 10 MHz 
  • Band 7 (2600 MHz), 10 MHz
  • Band 14 (700 MHz), 10 MHz
  • Band 20 (800 MHz), 10 MHz
  • Band 26 (850 MHz), 10 MHz
  • Band 28 (700 MHz), 10 MHz
  • Band 31 (450 MHz), 5 MHz
  • new Rel-13 Band 68 (700 MHz), x MHz (info missing from version 13.3.0 specification)   

Direct communication UE uses the uplink frequencies of the FDD band for transmission and reception.


  1. Hi Mika,

    Excellent blog, thanks very much for sharing your insights into the subject. One questions around using Satellite Backhaul for MCPTT services, how do you view the latency requirements being met, as mouth to ear delay for Satellite BH can be around 500 to 600 msec. Would it be possible or simply won't work? Thanks


  2. Latency requirements are defined so that extreme conditions are excluded. Even extreme network load is accetable reason
    for longer mouth to ear latency. Satellite link is similar case.

  3. Hi Mika,
    I've been going around in circles searching for the media type for off-network group call but the only thing i could reference to is RTP (which is obvious) what i haven't found is if the stream is unicast or multicast. The other question is how the system provides a sense of user experience to the group call.. the floor arbitration is a new concept to me and i don't quite understand how one user is able to ear all the others without the streams being mixed !!!

    1. Hi Pedro,
      Push-to-talk is half duplex voice i.e. only one person can talk and this is managed with floor control procedures. Therefore no need to mix multiple voice stream like in conference calls. Off-network communication is supposed to use ProSe direct communication ("sidelink"), which is a broadcast channel to all who can hear. Then there are group specific encryption keys, so that only authorized group members can decrypt the messages. Off-network calls and floor control are defined in 3GPP 23.179. You can find more info about sidelink from 36.331.


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