Page 402 - From GMS to LTE

388 From GSM to LTE-Advanced Pro and 5G

SG-imb
M1 MBMS SG-mb BM-SC MB2
eNodeB GW MCPTT
Server
Sm and
MCE M2 MME Database
M3

eNodeB
UE with SGi (unicast voice packets +
MCPTT Client S-GW floor control)
Unicast streams PDN-GW
to eNodeBs
Figure 5.21 MCPTT and eMBMS network nodes.

Multicast Traffic Channel. If the next subframe contains IP multicast data for the TMGI
it subscribed to, it decodes the respective data blocks and forwards the IP multicast
packets to the IP stack and from there to the application that has requested to join a
multicast group, i.e. the MCPTT client application. This means that the radio imple-
mentation and the new radio channels are completely transparent to the application
using multicast as it connects to the same IP stack as unicast applications.
Another multicast challenge not present in fixed‐line networks is the user’s mobility.
If the user moves outside an eMBMS service area that can be comprised of one or more
cells, they no longer receive the multicast stream. For MCPTT this means that the
device has to send a request to the MCPTT server to forward a unicast downlink audio
stream again. As described above the MCPTT server can configure mobile devices to
send location information, for example, after a cell change, so it can decide to include or
remove cells from the eMBMS service area for a group call as required.
A major advantage of distributing the downlink audio stream of a group call in several
cells via eMBMS multicasting is that the cells can transmit the multicast stream in the
same timeslots and in the same Physical Resource Block assignments. This way mobile
devices at the cell edge can receive the downlink audio stream from several cells simul-
taneously, which significantly improves cell‐edge performance. Not only is the signal
from neighboring cells not seen as noise but it is actually seen as useful signal data. To
achieve this neighboring cells need to closely time‐synchronize themselves.
Figure 5.21 shows which network components are required for eMBMS in practice. In
the core network the BM‐SC (Broadcast and Multicast Service Center) is the ingress
node for the multicast data and is connected to the Push To Talk Server (MCPTT
Server). The data stream and control information (e.g. which cells should distribute
which multicast streams) is then forwarded to the MBMS‐Gateway node, which then
distributes the multicast data to the radio base stations that have requested a particular
stream. Note that the multicast data streams are not flowing via the SGi interface, the
P‐GW and the S‐GW to the eNodeBs, but directly to the eNodeBs outside the LTE
unicast infrastructure. In addition, data sent via the Sm interface to the MME (Mobility
Management Entity) and from there via the M2 interface to the Multicell Coordination