Page 291 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 277

of cells, the types of radio networks and possibly also to the current throughput required
by the user. The more frequent and longer the gaps, the faster can the neighboring cells
be found and reported to the network.
Once the signal level reaches a point where a data transfer cannot be sustained any
longer, the easiest method for sending the mobile device to a UMTS or GSM cell is to
release the RRC connection with a redirection order. If inter‐RAT measurements have
taken place the network can select the cell with the best radio conditions. If no meas-
urements have taken place, the network simply informs the mobile device of the UMTS
channel number on which to search for suitable UMTS cells. In practice, this takes
somewhat longer compared to a release for which measurements have taken place
before. However, it can be observed that the reselection is still performed quickly.
Once the redirection order has been received, the mobile device terminates the com-
munication, changes to the new frequency and RAT and tries to find and synchronize
to the new cell. If the new cell is not found, the mobile device performs a general cell
search and selects a suitable cell on the basis of signal strength and the priority given for
certain bands and radio technologies in the system information of the last cell.
Once the mobile device has synchronized to the new cell, it establishes a signaling
connection, and a location area update and routing area update procedure will be per-
formed as was previously described for idle state inter‐RAT cell reselections. If the
intended cell is found immediately, the redirection procedure will typically take less
than 4 seconds under ideal conditions.
A similar procedure is used to redirect a mobile device currently served by a UMTS
Node‐B while it is in Cell‐DCH state. To find neighboring LTE and GSM cells, the RNC
instructs the mobile device regarding what to search for and where and defines a measure-
ment gap pattern. In GSM, the procedure is a bit different. Here, the mobile device searches
autonomously for the inter‐RAT cells it knows about from SIB 2‐quarter messages even
while in GPRS ready state and reselects to LTE or UMTS without the help of the network.
In all cases, the procedure takes several seconds in which the data transfer is interrupted.
In addition, data that was buffered in the network during the interruption is discarded
and the TCP/IP layer or the applications have to ensure that the packets are repeated.

4.9.3 Handover from LTE to UMTS

As the outage time during an RRC Release With Redirect procedure can take several sec-
onds, 3GPP has also specified a much smoother handover procedure between LTE and
UMTS in TS 23.401 [18]. From a network point of view, handovers are much more com-
plicated than a release with redirect and hence some but not all networks use it in prac-
tice today.
In general, an inter‐RAT handover procedure from LTE is performed in a way similar
to a handover from one LTE cell to another as described in the section ‘S1 Handover’
and Figure 4.21. In addition to the steps discussed for an intrafrequency LTE handover,
the following actions are required when handing over an ongoing connection to UMTS:
For measurements on other frequencies, the eNode‐B needs to reconfigure the radio
●
connection so that reception and transmission gaps for measurements on other channels
can be inserted.
Instead of an RRC Reconfiguration message that contains the parameters of the
●
target LTE cell, a Mobility From E‐UTRAN Command message is sent by the