Project 25 Technology Information Group

Tuesday, January 22, 2019

Project 25 Technology Information Group

Project 25 Technology Interest Group Releases New Whitepaper:

Project 25 Trunking Control Channels

The paper was authored by:

Dominick Arcuri: DVA Consulting

Andy Davis: Chairman TIA TR-8, Senior Resource Manager-Motorola Solutions

The New Project 25 Technology Interest Group Whitepaper defines the use and benefits of the current P25 Frequency Division Multiple Access (FDMA) Trunking Control Channel used in most P25 Trunking Systems today as well as application and benefits of the new, recently released, P25 Time Division Multiple Access (TDMA) Trunking Control Channel.  Planning and User Interoperability considerations required for transition from FDMA to TDMA P25 Trunking Control Channels are also covered.  These include a thorough inventory of the subscriber base and roaming requirements for all agencies supported by the system, including visiting radios from interoperability and roaming partners. 

The P25 suite of standards is evolving and continuing to grow as technology and user requirements evolve.  The standardization of the TDMA control channel will enable multiple manufacturers to support this capability with interoperable equipment.  Additionally, the TDMA control channel will provide another option for system managers and user agencies to effectively support their users and to improve the spectrum efficiency of their network both low density and high density situations. 

A copy of the Full whitepaper can be found on the Project 25 Technology Interest Group Website using the following link:


Whitepaper: P25 Trunking Control Channels



A P25 trunked system can be deployed with either FDMA or TDMA traffic channels or a mixture of FDMA and TDMA traffic channels.  The FDMA Common Air Interface (CAI) is designed to provide 12.5 kHz spectral efficiency and meet FCC (Federal Communications Commission) narrow-banding requirements, while the TDMA CAI is designed to provide 6.25 kHz “equivalent” spectral efficiency by providing two virtual channels within a 12.5 kHz channel.  For TDMA, the two virtual channels are commonly referred to as “TDMA slots”.  When a 12.5 kHz channel is operating in TDMA mode, the infrastructure is using both slots for outbound signaling.  Depending on how the TDMA channel is being used, radios are instructed to use one or the other slot for inbound signaling (radio to infrastructure) and radios will listen to one or both slots for outbound signaling (infrastructure to radio).

Prior to the development of the TDMA control channel, both FDMA and TDMA trunking traffic channels utilized an FDMA 12.5 kHz control channel.  A new addition to the P25 suite of standards defines a TDMA control channel for P25 trunking operation.  Similar to the FDMA trunking control channel, the TDMA control channel includes an inbound (radio to infrastructure) channel which is used for individual or group service requests for voice, data or supplementary service.  It also includes an outbound (infrastructure to radio) channel which broadcasts system information, control signaling, and provides call assignments.





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