LTE-A Pro for Public Safety Services – Part 2 – Advantages over PMR in 2G

LTE for Public Safety Services, also referred to as Private Mobile Radio (PMR) is making progress in the standards and in the first part of this series I’ve taken a first general look. Since then I thought a bit about which advantages a PMR implementation might offer over current 2G Tetra and GSM PMR implementations and came up with the following list:

Voice and Data On The Same Network: A major feature 2G PMR networks are missing today is broadband data transfer capabilities. LTE can fix this issue easily as even bandwidth intensive applications safety organizations have today can be served. Video backhauling is perhaps the most demanding broadband feature but there are countless other applications for PMR users that will benefit from having an IP based data channel such as for example number plate checking and identity validation of persons, access to police databases, maps, confidential building layouts, etc. etc.

Clear Split into Network and Services: To a certain extent, PMR functionality is independent of the underlying infrastructure. E.g. the group call and push to talk (PTT) functionality is handled by the IP Multimedia Subsystem (IMS) that is mostly independent from the radio and core transport network.

Separation of Services for Commercial Customers and PMR Users: One option to deploy a public safety network is to share resources with an already existing commercial LTE network and upgrade the software in the access and core network for public safety use. More about those upgrades in a future post. The specific point I want to make here is that the IP Multimedia Subsystem (IMS) infrastructure for commercial customers and their VoLTE voice service can be completely independent from the IMS infrastructure used for the Public Safety Services. This way, the two parts can evolve independently from each other which is important as Public Safety networks typically evolve much slower or and in fewer steps compared to commercial services as there is no competitive pressure to evolve things quickly.

Apps vs. Deep Integration on Mobile Devices: On mobile devices, PMR functionality could be delivered as apps rather than built into the operating system of the devices. This allows to update the operating system and apps independently and even to use the PMR apps on new devices.

Separation of Mobile Hardware and Software Manufacturers: By having over-the-top PMR apps it’s possible to separate the hardware manufacturer from the provider of the PMR functionality except for a few interfaces which are required such as setting up QoS for a bearer (already used for VoLTE today, so that’s already taken care of) or the use of eMBMS for a group call multicast downlink data flow. In contrast, current 2G group call implementations for GSM-R require deep integration into the radio chipset as pressing the talk button required DTAP messages to be exchanged between the mobile device and the Mobile Switching Center (MSC) which are sent in a control channel for which certain timeslots in the up- and downlink of a speech channel were reserved. Requesting the uplink in LTE PMR requires interaction with the PMR Application Server but this would be over an IP channel which is completely independent from the radio stack, it’s just a message contained in an IP packet.

Device to Device Communication Standardized: The LTE-A Pro specification contains mechanisms to extend the network beyond the existing infrastructure for direct D2D communication, even in groups. This was lacking in the 2G GSM-R PMR specification. There were attempts by at least one company to add such a “direct” mode to the GSM-R specifications at the time but there were too many hurdles to overcome at time time, including questions around which spectrum to use for such a direct mode. As a consequence these attempts were not leading to commercial products in the end.

PMR not left behind in 5G: LTE as we know it today is not likely to be replaced anytime soon by a new technology. This is a big difference to PMR in 2G (GSM-R) which was built on a technology that was already set to be superseded by UMTS. Due to the long timeframes involved, nobody seriously considered upgrading UMTS with the functionalities required for PMR as by the time UMTS was up and running, GSM-R was still struggling to be accepted by its users. Even though 5G is discussed today, it seems clear that LTE will remain a cornerstone for 5G as well in a cellular context.

PMR On The IP Layer and Not Part of The Radio Stack (for the most part): PMR services are based on the IP protocol with a few interfaces to the network for multicast and quality of services. While LTE might gradually be exchanged for something faster or new radio transmission technologies might be put alongside it in 5G that are also interesting for PMR, the PMR application layer can remain the same. This is again unlike in 2G (GSM-R) where the network and the applications such as group calls were a monolithic block and thus no evolution was possible as the air interface and even the core network did not evolve but were replaced by something entirely new.

Only Limited Radio Knowledge Required By Software Developers: No deep and specific radio layer knowledge is required anymore to implement PMR services such as group calling and push to talk on mobile devices. This allows software development to be done outside the realm of classic device manufacturer companies and the select few software developers that know how things work in the radio protocol stack.

Upgradeable Devices In The Field: Software upgrades of devices has become a lot easier. 2G GSM-R devices and perhaps also Tetra devices can’t be upgraded over the air which makes it very difficult to add new functionality or to fix security issues in these devices. Current devices which would be the basis for LTE-A Pro PMR devices can be easily upgraded over the air as they are much more powerful and because there is a broadband network that can be used for pushing the software updates.

Distribution of Encryption Keys for Group Calls: This could be done over an encrypted channel to the Group Call Server. I haven’t dug into the specification details yet to find out if or how this is done but it is certainly possible without too much additional work. That was not possible in GSM-R, group calls were (and still are) unencrypted. Sure, keys could be distributed over GPRS to individual participants but the service for such a distribution was never specified.

Network Coverage In Remote Places: PMR users might want to have LTE in places that are not normally covered by network operators because it is not economical. If they pay for the extra coverage and in case the network is shared this could have a positive effect when sharing a network for both consumer and PMR services. However, there are quite a number of problems with network sharing one has to be careful when proposing this. Another option, which has also been specified, is to extend network coverage by using relays, e.g. installed in cars.

I was quite amazed how long this list of pros has become. Unfortunately my list of issues existing in 2G PMR implementations today that a 4G PMR system still won’t be able to fix is equally long. More about this in part 3 of this series.

LTE-A Pro for Public Safety Services – Part 1

In October 2015, 3GPP has decided to refer to LTE Release 13 and beyond as LTE-Advanced Pro to point out that LTE specifications have been enhanced to address new markets with special requirements such as Public Safety Services. This has been quite long in the making because a number of functionalities were required that go beyond just delivery of IP packets from point A to point B. A Nokia paper published at the end of 2014 gives a good introduction to the features required by Public Safety Services such as the police, fire departments and medical emergency services:

  • Group Communication and Push To Talk features (referred to as "Mission Critical Push To Talk" (MCPPT) in the specs, perhaps for the dramatic effect or to perhaps to distinguish them from previous specifications on the topic).
  • Priority and Quality of Service.
  • Device to Device communication and relaying of communication when the network is not available.
  • Local communication when the backhaul link of an LTE base station is not working but the base station itself is still operational.

Group Communication and Mission Critical Push to Talk have been specified as IP Multimedia Subsystem (IMS) services just like Voice over LTE (VoLTE) that is being introduced in commercial LTE networks these days and can use the eMBMS (evolved Mobile Broadcast Multicast Service) extension in case many group participants are present in the same cell to only send a voice stream in the downlink once instead of separately to each individual device.

In a previous job I've worked on the GSM group call and push to talk service and other safety related features for railways for a number of years so all of this sounds very familiar. In fact I haven't come across a single topic that wasn't already discussed at that time for GSM and most of them were implemented and are being used by railway companies across Europe and Asia today. While the services are pretty similar, the GSM implementation is, as you can probably imagine, quite different from what has now been specified for LTE.

There is lots to discover in the LTE-A Pro specifications on these topics and I will go into more details both from a theoretical and practical point of view in a couple of follow up posts.