Page 396 - From GMS to LTE

382 From GSM to LTE-Advanced Pro and 5G

MCPTT
SGi (voice packets + Server
floor control) and
MME Database
S-GW
PDN-GW SGi (IMS signaling)
LTE
Gx ISC
UE with
MCPTT Client Cx
PCRF Rx P-CSCF Mw I/S-CSCF HSS
ISC

MMTEL
(VoLTE)

Figure 5.19 MCPTT application server in IMS.

3GPP to transmit a single IP multicast data stream from an LTE cell in which many
subscribers are located. This can significantly reduce the amount of bandwidth required
in a cell. As described in 3GPP TS 23.179 chapter 5.2.7, the locations in which eMBMS
is used for the downlink speech path of a group call do not have to be predetermined.
Based on the current location of group members, the MCPTT server can decide to use
eMBMS only in those cells in which many of the group subscribers are located while in
other cells in which only few group members are located, per‐subscriber unicast IP data
streams are used. Using eMBMS in a flexible way requires group members to inform the
MCPTT server of cell changes. This is part of the specification and the MCPTT server
can request group members to report the cells they are currently connected to based on
a number of trigger criteria such as, for example, cell changes.
For reasons that include the high bandwidth capacity of an LTE cell, the significant
complexity and cost of implementing eMBMS in the network and the fact that eMBMS
is a specialized feature not used on consumer devices, it remains to be seen if eMBMS
will be used by many public safety organizations for MCPTT group calls and under
which circumstances. The datarate of a typical voice data stream including IP, UDP and
RTP overhead is around 15 kbit/s if Robust Header Compression (RoHC) is used in the
radio network as described for VoLTE above. Even for 100 MCPTT users in a cell com-
municating in the same group, the overall datarate required in the downlink direction
would only amount to around 1.5 Mbit/s. Such an aggregated datarate can easily be
supported even by a bandwidth‐limited LTE radio cell and even if many group partici-
pants are experiencing bad radio conditions. In addition, scheduling such a number of
users nearly simultaneously should also be possible, as in the LTE radio network, sched-
uling assignments can be made for a high number of devices every millisecond.
When a participant leaves a cell in which the downlink voice packets are delivered via
eMBMS, it can inform the MCPTT server, which will then send the downlink voice