Page 289 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 275

SGi
PDN-GW Internet

S5 Gn

S6a
MME

LTE core
HSS /
HLR UMTS / GSM core
Gn Gn
(SGSN–SGSN) (SGSN–GGSN)
Gr
SGSN

GSM / UMTS
radio network GSM BTS
UMTS NodeB

Figure 4.23 Interconnection of LTE to GSM and UMTS networks.

3GPP TS 23.401 [18] are not used over external network interfaces. Figure 4.23 shows
how this interconnection looks from a logical point of view.

4.9.1 Cell Reselection between LTE and GSM/UMTS
The simplest way from a network and signaling point of view to move from LTE to
another RAT is cell reselection in RRC idle state. For this purpose, the eNode‐Bs broad-
cast information on neighboring GSM, UMTS and CDMA cells in their System
Information messages as described above. When a network‐configured signal level
threshold is reached, the mobile device starts searching for non‐LTE cells and reselects
to them based on their reception level and usage priority.
With LTE, an optional usage priority scheme for different frequencies and RATs was
introduced. It is, for example, preferable in practice to remain with an LTE cell with a
weak signal for as long as possible rather than to reselect to a GSM cell with a strong
signal but with a very low bandwidth compared to an LTE cell. If priorities are broadcast
in System Information messages or sent in dedicated signaling messages to the mobile
device, the device would, furthermore, try to reselect to frequencies and RATs with a
higher priority when a configured threshold is reached, even if the current serving cell
has a stronger signal. Details of this mechanism are described in 3GPP TS 36.304 [21]. At
this point, it should be noted that before the introduction of LTE, a usage priority was
not necessary for GSM and UMTS, as UMTS cells were generally preferred to GSM cells.
With three radio technologies and the rising number of frequency bands in use, however,
LTE cells in one band might in some situations be preferred to LTE cells in another band,
which are preferred over UMTS cells, which in turn are preferred to GSM cells.
Once the mobile device decides to move from an LTE cell to a GSM or UMTS cell, it
performs a location area update with the circuit‐switched side of the core network if it
has circuit‐switched voice and SMS capabilities. For details, see Section 4.12.