Tuesday, September 1, 2009

LTE Handovers

Below are possible handover scenarios in LTE. Any more scenarios you can think of? Let me know.

X2 Based handover without SGW change


X2 Based Handover with SGW changed


S1 Based Handover


S1 Based Handover -II


The above are the scenarios I can think when it comes to handovers in LTE. However all the scenarios may not make any sense during initial LTE deployments. My guess is that we will mostly see the first scenario during the initial stages, followed by the fourth scenario. Considering the first scenario, it will be very important to see how many sessions can a MME and SGW/PGW can handle. What would be performance numbers of the devices? These performance numbers are what drives the number of devices. My guess would be 1 SGW should be able to take care of say a Million general users. If we consider corporate networks and etc it may go beyond 1. I have no clue this is just a guess. Can anybody throw some light in this direction?

Excuse me for just posting the figures, I am working on something and its taking longer than I expected. More to come soon.


Kostubh Sharma said...

I think the below scenario is also possible:

UE --- eNB_1 --- MME_1 --- S-GW_1 --- P-GW

moved to new eNB2 which is served by new MME_2 but old S-GW_1:

UE --- eNB_2 --- MME_2 --- S-GW_1 --- P-GW

Santosh said...

Yes! I will update the post with above scenario tonight. Thanks Kostubh!

Anonymous said...

could you please tell me what happen to RRC connection in the target enodeB in case where the MME is also changing. Does the UE make a RRC connection and default radio bearer as soon as it detects the second enodeB or it makes this connection after receiving Hadnover command. in this case, is there a possibility that two RRC connection exists in the UE simultaneously?


Santosh said...

Hi Kaushik

Considering an X2 based handover where both target and source eNB are communicating using X2AP, RRC Reconfig command will be sent by source eNB to UE only when Source eNB receives handover request ack from target eNB. Once UE receives the RRC reconfig command it will directly establish or modify the RRC connections to the target eNB. I believe same is the case when MME and SGW is changed. After UE established the RRC connection with target eNB it can start sending the data. Target eNB will buffer the data untill the EPS bearers are established/modified accordingly.

And I guess there will be only one RRC connection in UE. This will be modified or created again when UE enter new eNB. 3GPP TS 36.300. Refer to section 10.1.2.

Thanks, Santosh

Moh Yan said...

LTE intra-RAT handovers between BS are
qualified by the following properties :

1. BS have an X2 connection
2. The MME serving the BS
3. The SGW serving the BS

So, with #1 = Yes / No, #2/#3 =
Same/Different, you have 2 x 2 x 2 = 8
different scenarios :

T1. (Yes,Same,Same)
T2. (No,Same,Same)
T3. (Yes,Different,Same)
T4. (No,Different,Same)
T5. (Yes,Same,Different)
T6. (No,Same,Different)
T7. (Yes,Different,Different)
T8. (No,Different,Different)

Your diagrams appear to cover the
following : T1, T6, T8, T4