Page 293 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 279

4.10.1 Cell Reselection between LTE and CDMA2000 Networks
When the mobile device is in RRC idle state, it can decide on its own when to move
between the two radio access network technologies. On the LTE side, it is supported by
neighbor cell information in the SIB 8 message.
For the mobile to keep the IP address when changing between the two radio technolo-
gies and core networks, it is necessary that the PDN‐GW is selected as a mobile IP
home agent (HA) by the CDMA2000 network when the connection is first established.
For this purpose, the S2a interface is used as shown in Figure 4.24.
When reselecting from LTE to CDMA2000, the mobile device logically disconnects
from the LTE network and establishes a signaling connection with the legacy network.
The network then contacts the PDN‐GW via the S2a interface and moves the subscrib-
er’s context to the CDMA2000 side.

4.10.2 RRC Connection Release with Redirect between LTE and CDMA2000
While in LTE RRC connected state, the network decides as to when to move a connec-
tion to another LTE cell or another radio access system in case the user is at the edge of
the LTE coverage area. If supported by the mobile device, the network can be aided by
signal strength measurements. The simplest form of such a transfer is to issue an RRC
connection release with redirect information to a CDMA2000 cell. Two variants are
described in 3GPP TS 23.402 [23]:
Non‐optimized. The mobile device changes the RAT and performs the same signaling
●
message exchange as described above for cell reselection.
Optimized. If supported by the network, the mobile device performs a preregistra-
●
tion with the CDMA2000 access network at the time it initially registers with the LTE
network. This way, a session is already established in the CDMA2000 access network
and then put into dormant state until such time as the mobile device reselects into the
CDMA network. This minimizes the delay during the reselection process, as fewer
actions have to be taken at the time the mobile changes from one radio network tech-
nology to the other. As shown in Figure 4.24, the S101 interface has to be in place for
this purpose. This interface is used to transparently tunnel CDMA messages between
the mobile device in the LTE network over the S1 interface to the MME and from
there to the CDMA access network.
4.10.3 Handover between LTE and CDMA2000

To further reduce the interruption time when changing between the two RATs, it is also
possible to perform a handover of the ongoing data session from LTE to CDMA2000
and vice versa. As in the previous section, the network can instruct the mobile device to
perform signal strength measurements, for example, once the signal strength or quality
of the current cell reaches a certain threshold. Once a handover has to be made, the
eNode‐B sends an Evolved‐UMTS Terrestrial Radio Access (E‐UTRA) handover prepa-
ration request message to the UE. The UE then communicates with the CDMA radio
network over the S101 interface to prepare the other radio network for the incoming
connection. As part of this preparation phase, the MME prepares a transparent indirect
user data tunnel between the current eNode‐B, a Serving‐GW and the CDMA2000
radio network via the S103 interface to forward any incoming data from the Internet to