Extending your P25 System with LTE Interworking


January 16, 2024 by Dominick Arcuri, Subject Matter Expert

Televate recently contributed to a white paper published by the P25 Technology Interest Group (PTIG) which explores the methods and considerations for incorporating LTE Interworking into a P25 system. A link to that paper is provided below.

It is widely recognized that traditional Land Mobile Radio (LMR) systems and wireless broadband 4th generation (4G) and emerging 5G systems, based on Long Term Evolution (LTE) technologies, will likely coexist as key communications tools for public safety agencies for many years to come.  LMR systems have traditionally provided secure, quick-access, one-to-many communications that are reliable and designed to mission-critical grade (sometimes referred to as public safety grade) requirements.  On the other hand, broadband LTE systems provide high-speed wireless access to a wide variety of data driven applications, including Mission Critical Push-To-Talk (MCPTT) and Push-To-Talk over cellular (PTToC) applications, with commercial networks that can facilitate LMR over cellular coverage across the country.  Public safety communications professionals are investigating ways to maximize the benefits of each of these technologies to extend home LMR communications nationwide. 

One method of capitalizing on the complimenting advantages of these two technologies is Interworking. Interworking is commonly described as connecting systems that use different Radio Frequency (RF), or over-the-air technologies (i.e., broadband LTE and LMR).  This approach allows a single device (either an LMR radio or an LTE smartphone) to communicate through their respective systems by interconnecting those systems. The Interworking approach will permit users of the LMR network to communicate directly with members of their agency or other agencies—even if those agency members don’t have an LMR radio—by using a properly configured LTE device.

The white paper describes the activities within national and international standards organizations that are currently developing an Interworking study to describe how Interworking may be used to enable interoperability of voice services that are common between LMR and broadband LTE.  The paper also describes the specific groups involved, the goals of the group, the interoperability services being addressed, and the documents currently being developed.

What may be most interesting to public safety communications professionals are the various methods that can be used to realize the benefits of Interworking.  These include:

  • A basic means of interconnecting an LMR network with a cellular LTE network can be done by connecting a radio to an IP interface of the LTE network. The most common form of this interconnection utilizes a “donor” radio to provide the LMR audio via the P25 Common Air Interface (CAI).
  • The P25 Digital Fixed Station Interface (DFSI) can also be used as the P25 LMR network connection to support LTE Interworking.  This method eliminates the need for donor radios.
  • The P25 Inter-RF SubSystem Interface/Console SubSystem Interface (ISSI/CSSI), while developed to support interconnection of consoles and separate P25 LMR networks, can also be used to enable connectivity between a P25 LMR network and a broadband LTE network. This method provides a more advanced form of connection and supports additional features, such as:
    • User ID Information
    • Group Calls
    • Private Calls
    • Secure “Encrypted” Calls
    • Emergency Calls

Finally, the white paper provides a specific discussion of testing and performance characteristics in the context of LMR-LTE interconnected networks using the four different interface types described above: P25 Common Air Interface (CAI), P25 Digital Fixed Station Interface (DFSI), and P25 Inter-RF SubSystem Interface/Console SubSystem Interface (ISSI/CSSI).

The complete white paper can be found at: PTIG Publishes New Whitepaper: How to Extend your P25 System for LTE Interworking (project25.org).

Televate has assisted a number of our customers in defining Internetworking LMR and LTE communications requirements and with the implementation of these applications into the public safety operational toolkit. Contact us at info@televate.com to find out how we can help your agency.