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LTE Signalling Messages

With Long Term Evolution (LTE) comes a new and exciting attributes. One of these is the LTE call flow itself. In fact, call flow and signaling is unique for LTE, and is driven by 3GPP standards. Call flow is how signaling and sessions are established across an LTE network. We will have a look at the call flow diagrams about different procedures in LTE which includes basic registration, Carrier Aggregation (band addition and removal), 4x4 MIMO activation, SMS etc.

LTE Registration

lte registration



Tracking Area Update

Tracking Area Update

CS Fallback to 3G (when UE is in idle mode on LTE)

LTE is a Packet only technology which is designed for handling data traffic. There are two options of handling voice(CS) call in LTE.

  • CS Fallback - call redirected to legacy 2G/3G

  • VoLTE - Packet based voice call

CS Fallback is normally done via the existing technology such as WCDMA and GSM where the calls are redirected to legacy networks. For example, if UE wants to make a voice call, the network redirect the call to one of the existing technology like WCDMA or GSM. This technology that enables to redirect connection to other technology is called 'CS Fallback'. The basic principle for the CS Fallback(CSFB) solution is that the UE is registered to CS domain while it is in LTE(E-UTRAN). UE's capable of CS fallback should perform a combined attach, i.e. attach to both PS and CS services despite being on a PS-capable only network.


The UE in RRC_IDLE mode is prepared to receive a paging message with a CN domain indicator pointing to CS. The UE in RRC_CONNECTED mode it is prepared to receive a CS SERVICE NOTIFICATION sent as a NAS message. The UE sends back an EXTENDED SERVICE REQUEST (in both cases). After receiving a CSFB request, the UE transitions to a CS-capable RAT. The UE should return back to E-UTRAN after the CS call using normal idle or active mode procedures.


  • A SERVICE REQUEST is used to request PS services.

  • An EXTENDED SERVICE REQUEST is used to initiate a CS fallback call or respond to a mobile terminated CS fallback request from the network.

Difference between handover and redirection:

CSfallback from LTE to 2G/3G can either be via Handover or Redirection.

In case there if there is an interface between LTE eNB and UMTS NodeB (or) GSM BTS then handover is done. If there is no interface then redirection takes place. In case of redirection the RRC connection on LTE network is released by the UE and establishes a new connection with UMTS or GSM network.

CSfallback to 3G




lte MO SMS

LTE to LTE Handover

Handover in LTE is complicated as it has to handle Intra-LTE handovers as well as handover between LTE-3G, LTE-2G and other mobile networks.

The basic objective of handover procedures are:

  • UE should able to continue it’s services before and after handover

  • QoS should be maintained at all times

  • Seamless handover between 2G/3G/LTE/CDMA technologies

Normally there are two ways in which handover could be triggered in mobile networks.

  • Blind - The network will make handover decisions and will direct the UE accordingly.

  • Measurement based - The network will configure measurements for the UE which has to be performed on the target cell. When the measurement criteria is satisfied the UE sends measurement report to the network. The network will then make a decision based up on the radio resource available in target cell.(but in LTE measurement based handover is used).

There are 3 types of Handover in LTE network,

  • Intra-LTE: The source and the target cell belong to the same LTE network.

  • Inter-LTE: The source and the target cell belong to a different LTE network. The LTE nodes MME and SGW are different for the source and target cell.

  • Inter-RAT: The source cell belongs to LTE and the target cell belongs to 2G/3G/CDMA technology.

For handover between 2 LTE cells, measurement values for event A2/A3 is configured by the network in "RRC Connection Reconfiguration" which is then sent to the UE. The UE performs measurement and when the criteria for event A2/A3 is reached, it sends "Measurement Report" to the network. The network will then send the mobility control information which shall contain target cell information such as Physical cell ID, UL/DL bandwidth, PRACH parameters etc.


lte handover
lte handover

LTE SCell addition and removal for Carrier Aggregation

Carrier Aggregation is an important topic in LTE where the carriers belonging to different/same frequencies are combined to increase the downlink/uplink throughput for the user. The SCell addition is done using the IE "sCellToAddModList" and removed using the IE "sCellToReleaseList" which is always sent in "RRC Connection Reconfiguration". The SCells are added either blindly or based on measurement event A4 whereas SCells are removed blindly when the UE moves to idle or the data transfer is completed. The SCells are added from SCell index 1, 2, 3,...., n and removed using the same SCell index. Multiple SCells could be added and removed using a single reconfiguration message.

SCell Addition.png
SCell Removal.png
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