Inter-Operator CS Only National Roaming

Dear Experts,

In Network Rollout scenario, if one operator has only 4G / 5G network and it want to get CS based voice services from another operator over National Roaming then what are the possibilities available for seamless CS voice service. i.e.

  1. 4G / 5G ( Own Network – Operator A )
  2. 2G / 3G ( National Roaming with another operator for CS Voice – Operator B )

The major challenge which I see in this case for Operator-A is terminals not supporting VoLTE or VoLTE not activated. In any case these terminals has to go throughput CSFB process to establish call. Under this situation, can CS Calls get established to 2G / 3G and also after call get terminated it can get return to 4G network without any break.

On RAN / Core level via RIM based CSFB and Fast Return we achieve this process to get optimized, can these features help in such case of inter-operator seamless CS call initiation and return.

VoLTE could be best tried within own network and Devices to be upgraded by subs.
National roaming would be administrative challenge and trust is the main concern many times.
Else 2/3G to be a neighbour relation and seamless overlay of 4G atleast or 4/5G to be planned to keep as much as possible its forward compatibility rather a backward integration towards 2/3G which may not commercially viable.
Technically Yes its possible.
RIM to be supported by both network and ensured.
This is similar to 2/3G and 4G as new nwtwork with VoLTE.
And making little spectrum for 2G VOICE and making VOLTE full city/country wide in given time.
India had 10% VoLTE in 2018 and today it’s 90%, leaving 10% for 2G in 3 years. Traffic in Erlang.
And 3G is shut now.

Technically need to understand how is it possible all definition mostly as external neighbours for IRAT are done intra-plmn.
How can we do inter-plmn definition.
Especially taking case of 2G / 3G back to 4G.

Has to be equal PLMN for those overall areas of coverage.

We had new operator in Egypt working with this scenario during its first launch.
It is applicable, yes.
4G in one PLMN and CS services fallback to other PLMN.

4G/5G (Own Network - Operator A) is FDD or TDD?

It is FDD.

Let’s assume Operator A Only have 4G and Operator B have 2G3G.
Most important thing, we have to take is to defined link Sgs interface between MSC to MME in case of CSFB to Operator B and Redirection to operator A.

Secondly, at Operator A eNodeB end: You have Add of Operator B PLMN (MCC,MNC) definition for Inter operator Handover and also Defined Inter-Rat neighbor for Operator B.
-TAC LAC mapping also performed.

How about Core ends PLMN definition?
First UE latch to Opt A by default but when UE moved to Operator B due to CSFB or IRAT handover due to coverage then they also have Authenticate UE, you also have to defined UE profile at HSS end as well.

Fast Return or Fast Reelection is cell level feature and can be based on IMSI. So that when IMSI is for another PLMN, the RRC Release message with redirection on Operator B will have the LTE frequency of Operator A, UE will then camp back on home PLMN. The drawback is the time taken to perform the Release if it’s blind or with measurements and if UE is in multi-RAB or single CS call.
Keep in mind that TAC-LAC Mapping will also be needed on both core sides of both operators.
Not sure how parameters are configured on core side.

Dear Moh,
Thanks, yes it is good reply & idea.

But as I understand there is no configuration parameters in SIB-19 that defined another operator PLMN. So by default Fast Return always work within same operator network. Now if we change frequency in SIB-19 definition for fast return and add operator-A bands then is it not going to drop the session because there is no IRAT defined between 3G of operator-B and 4G of operator-A. Also both are managed by different cores. Moreover in fast return - SIB-19 setting during gap period terminal will read LTE frequency, but in that case it apparently will read only operator-B frequencies.

Hi @Kaleem,
I didn’t configure Fast Return myself before, but I’m sure that, based on IMSI, frequency can be configured, will look again into documentation.
Even with Fast Return, depending on vendor, drop may occur. Keeping in mind that UE is not listening to SIB19 except in 2 cases, either connected and inactive or Idle. Depending on Vendor, it may force the UE to be inactive to allow it to listen to SIB19 without going idle. But in all means, UE is re-camping again on home PLMN from Idle mode, no handover is taking place, so no inter-core interface is used.