MCPTT: Is eMBMS Really Needed For Public Safety Communication?

3GPP Release 13 finally brings all bits and pieces together for Mission Critical Push To Talk (MCPTT) services, i.e. the cornerstone for Public Safety Communication. I’m going through 3GPP TS 23.179 which contains Stage 2 of the functional architecture description of the service. MCPTT services can be implemented over traditional IP unicast, i.e. a copy of the speech data is sent to each subscriber of a push to talk group call or via eMBMS, i.e. over a single IP multicast transmission to all group participants in a cell. But is eMBMS really needed?

The reason I’m asking is because voice data streams are transmitted pretty efficiently. AMR-WB requires around 12 kbit/s without IP, UDP and RTP overhead. RoHC decreases the overhead significantly but let’s assume for a moment that the overall bandwidth requirement per participant is 20 kbit/s for the times there is actually data flowing in the downlink direction, i.e. somebody is talking. Even if 500 users listening to the same group call are in the same cell, the aggregated data rate is still ‘only’ 10 Mbit/s. That’s significantly less than the overall capacity of an LTE cell, even if “only” a 10 MHz carrier is used. 500 simultaneous users per cell might perhaps be a bit challenging, put probably not according to these thoughts.

The second reason I’m asking is because without MBMS, MCPTT services can be implemented on mobile devices much more easily, much cheaper and much more independent from the device manufacturer as no multicast specific extension needs to be put in place between the baseband and the part of the device that runs the user operating system (e.g. Android). The only thing that is required is that the MCPTT application running on a mobile device has access to the IMS stack that is also used for VoLTE.

So I wonder if eMBMS will be used for MCPTT. What do you think?