Thursday 3 March 2011

LTE to 3G Handover Procedure and Signalling

It may be worthwhile brushing up the LTE/SAE Interfaces and Architecture before proceeding.

1) Overview of Handover Operation

With EPC, continuous communication is possible, even while the terminal switches from one type of radio access system to another.

Specifically, in order to achieve the internal network path switching required to change radio access systems, the S-GW provides a mobility management anchor function for handover between 3GPP radio access systems, and the P-GW provides the function for handover between 3GPP and non-3GPP radio access systems. In this way, the IP address does not change when the terminal switches radio access systems, and communications can continue after handover.

In handover between the 3GPP radio access systems, LTE and 3G, handover preparation is done before changing systems, including tasks such as securing resources on the target radio access system, through cooperation between the radio access systems (Figure 3 (a)(A)). Then, when the actual switch occurs, only the network path needs to be switched, reducing handover processing time (Fig.3 (a)(B)). Also, loss of data packets that arrive at the pre-switch access point during handover can be avoided using a data forwarding function (Fig.3 (b)).

In this way, through interaction between radio access systems, fast handover without packet loss is possible, even between radio access systems such as LTE and 3G which cannot be used simultaneously.

2) Handover Preparation Procedure (Fig.3 (a)(A))

The handover preparation procedure for switching radio access from LTE to 3G is shown in Figure 4.

Step (1):The terminal sends a radio quality report containing the handover candidate base-stations and other information to the eNodeB. The eNodeB decides whether handover shall be performed based on the information in the report, identifies the base station and RNC to switch to, and begins handover preparation.

Steps (2) to (3): The eNodeB sends a handover required to the MME, sending the RNC identifier and transmission control information for the target radio access system. The MME identifies the SGSN connected to the target RNC based on the received RNC identifier and sends the communication control and other information it received from the eNodeB to the SGSN in a forward relocation request signal. The information required to configure the communications path between the S-GW and SGSN, which is used for data transmission after the MME has completed the handover, is sent at the same time.

Steps (4) to (5): The SGSN forwards the relocation request to the RNC, notifying it of the communications control information transmitted from the eNodeB. The RNC performs the required radio configuration processing based on the received information and sends a relocation response to the SGSN. Note that through this process, a 3G radio access bearer is prepared between the SGSN and RNC.

Step (6): The SGSN sends a forward relocation response to the MME in order to notify it that relocation procedure has completed. This signal also includes data issued by the SSGN and required to configure a communications path from the S-GW to the SGSN, to be used for data forwarding.

Steps (7) to (8): The MME sends a create indirect data forwarding tunnel request to the S-GW, informing it of the information issued by the SSGN that it just received. From the information that the S-GW receives, it establishes a communications path from the S-GW to the SGSN for data forwarding and sends a create indirect data forwarding tunnel response to the MME.

Through this handover preparation, target 3G radio-access resources are readied, the radio access bearer between the SGSN and RNC is configured, and the data forwarding path from the
S-GW to the SGSN configuration is completed.

3) Handover Procedure for Radio Access System Switching (Fig. 3(a)(B)):

The handover process after switching radio access system is shown in Figure 5.

Steps (1) to (2): When the handover preparation described in Fig.4 is completed, the MME sends a handover command to the eNodeB. When it receives this signal, the eNodeB sends a handover from LTE command for the terminal to switch radio systems. Note that when the eNodeB receives the handover command from the MME, it begins forwarding data packets received from the S-GW. Thereafter, packets for the terminal that arrive at the S-GW are forwarded to the terminal by the path: S-GW, eNodeB, S-GW, SGSN, RNC.

Steps (3) to (6): The terminal switches to 3G and when the radio link configuration is completed, notification that it has connected to the 3G radio access system is sent over each of the links through to the MME: from terminal to RNC, from RNC to SGSN, and from SGSN to MME. This way, the MME can perform Step (10) described below to release the eNodeB resources after a set period of time has elapsed.

Step (7): The MME sends a forward relocation complete acknowledgement to the SGSN. A set period of time after receiving this signal, the SGSN releases the resources related to data forwarding.

Step (8): The SGSN sends a modify bearer request to the S-GW to change from the communications path before the handover, between the S-GW and eNodeB, to one between the S-GW and SGSN. This signal contains information elements required to configure the path from S-GW to SGSN, including those issued by the SGSN. When the S-GW receives this signal, it configures a communications path from the S-GW to the SGSN. In this way, the communications path becomes: S-GW, SGSN, RNC, terminal; and data transmission to the target 3G radio access system begins.

Note that after this point, data forwarding is no longer needed, so the S-GW sends a packet to the eNodeB with an “End Marker” attached, and when the eNodeB receives this packet, it releases its resources related to data forwarding.

Steps (9) to (10): The S-GW sends a modify bearer response to the SGSN, indicating that handover procedure has completed. The MME also releases eNodeB resources that are no longer needed.

Through this handover procedure, data is forwarded during the handover, the switch of radio access bearer is completed, and the communications path from the P-GW to the terminal is updated.

In the examples above, we described the handover procedure between 3GPP radio access systems in which the S-GW did not change, but handovers with S-GW relocation are also possible. In these cases, the P-GW provides the anchor function for path switching, as with switches to non-3GPP access systems.


Anchor function: A function which switches the communications path according to the area where the terminal is located, and forwards packets for the terminal to that area.

Relocation: Switching communications equipment such as area switches during communication.


Anonymous said...

Very nice.........

Asad kM said...

Hi Zahid,

I am new to learning LTE and alot of things have become clear after reading your blog. I have one question regarding this handover, is the handover for a IP based call or for a normal mobile call? Since LTE has to use other methods (e.g. VoLGA or CSFB) to communicate with 3G if its not IP based. Could you clear this part? Thanks.


Zahid Ghadialy said...

Hi Asad,

The handover is for a normal IP Data call.

When voice comes into picture, things get more complicated and probably not exactly covered by this example signalling.


hiphophoe said...

I am a OEM for Sprint devices and wanted to know what is the key performance indicators we should see when switching from 3G to 4G and vice versa? Is there certain RSSI levels that determine to switch for handing up and handing down?

Asad kM said...

Hi Zahid,

Do you know if there are any theoratical values for the handover (in terms of delay) between 3G to LTE (or LTE to 3G)? I know verizon said they had 2 min delay, but i am talking in terms the expected delay based on the theory for the handover?

Asad kM

rsid said...

Hi, I am new tho this LTE, handover is explained nicely. I have one doubt, when UE is connected to PDN through E-UTRAN, the bearer is called EPS-Bearer, but after handover, UE uses UTRAN for PDN connection, so can we still call the bearer as EPS-Bearer or by different name.

Unknown said...

hello, everybady, i am a new member,I am very happy to make friends with everybady,
i come from Viet Nam, a developing country!
i am having a small problem is :
I'm participating in a project it built an eNodeB, my part is to understand and design the block communication between 4G and 3G systems in the eNodeB that, but now I do not know where to start, for all people comment to me, so thanks for listening!

Anonymous said...

Hi Zahid,
Can we have a flow diagram from 3G to LTE handover. Everywhere I can see only Lte to 3G handover discussion.

Anonymous said...

I am optimizing the LTE IRAT at the moment, equipment was NSN.
my parameters are the following with not so desirable improvement.

Serving RSRP should be below -107 dBm

Target EcN0 should be greater than -12dB (25 (internal value)-49)/2

LTE start measure threshold2 = -106dBm while stops 2a = -101dBm
and the triggerring was changed from RSCP to EcN0 triggered.

BTW, the results was an increase in Inter_RAT_HO_Preparations
from ~1000 ATT to ~28000 while succ rates was degraded.

any comments or help are welcome


Vinutha said...

In 4G to 3G handover, will the 4G PS call be translated to HSPA call in 3G or its not mandatory that it has to be a HSPA call.Can it be translated to a R99 call?

Unknown said...

i Zahid,
Can you help with this situation? I am testing two phones on how they make the data handover from 4G to 3G. Test is done using a Skype video call. One of the phones stays on 4G despite 4G low signal and quality of the video call. The other one makes the handover to 3g without dropping the video call. Why the first phone doesn't try to jump to 3G?
If we make the test using video streaming, the phone makes the HO. In IDLE, he jumps too. But why it remains on 4G with a video call test?

Let me know your comments.


Zahid Ghadialy said...

Victor, its difficult to comment without looking at logs. The decision to handover is taken by the network while in the connected mode so why is the network not initiating this handover? Is the UE reporting the right measurements?

Yeni84 said...

Hello. Which already released commercially UEs at this date support LTE to UTRAN handover (not redirection)? Does Galaxy S5, iPhone5 support? Is a category of UEs to support LTE to UTRAN handover (not redirection)? Because one UE brand and model for one country can have LTE to UTRAN handover working but for other countries not. What is this UE category which support LTE to UTRAN handover? Thank you. Yener

Zahid Ghadialy said...

The Qualcomm chipset was supporting Inter-RAT handover from LTE to 3G a few years back so my guess would be that most modern devices should be supporting this feature. I cant be certain for sure though.

imane said...

Hi Zahid ,
i was wondering , if that procedure of handover between LTE to 3G will be the same , if we have two system or two operator ,the first who has enodeB and the second nodeB(don't have yet LTE) !!

Unknown said...

Hi Zahid
Can you help to explain on what condition UE will start doing measurement on 3G technology and which events will happen during the IRAT handover procedure (4G to 3G) ?

lalit sharma

Anonymous said...

hi ,

how to use 3G APN when Gn interface is there if subscriber is in 3G coverage only he will not perform combined Lup and only 3G lup will be performed and as we are using 3G APN then DNS will select GSGSN instead of PGW ?


Ashutosh Kaushik said...

@ Lalit ,

UE will start doing measurement on 3G when measured RSRP (i.e. actually RSRP+Hyst) of current 4G cell goes below B2threshold1RSRP. When 3G's neighbor cell's measured RSCP (actually RSCP-Hyst) becomes better than B2threshold2RscpUTRA , UE enters B2 event and after some time (trigger timer setting) UE starts sending measurement reports to eNB.

Event B1 is also for IRAT but i.e. for IRAT Offloading basically dealing with load criteria.

Unknown said...

there is a relocation failure between sgsn to rnc...what could be the possible reasons for it?

Unknown said...

Hi there is reallocation failure between sgsn and rnc on the attempt 4g to 3g handover.what are the possible reasons for the issue?

Unknown said...

Dear expert !
I have case handover 4G to 3G successful,after handover finish speed download 3G very slow (handover buy Load balancing)
- Test only 4G: Speed normal
- Test only 3G: Speed normal
- Handover PS 4G to 3G, speed on 3G very slow for Iphone, Hawei, Nokia, this Samsung test speed normal.

Help support

Thank you very much.

San D said...

Hello Zahid,

Does UE IP change when HO is completed from 4G to 3G?


Zahid Ghadialy said...

Hi Sandeep, it's been a while since I looked at this. Not sure but the IP address should not change otherwise any secure connections, VPNs, etc. might get affected. I guess it also depends on what is the operator policy regarding IP address allocation, etc.