Why SPS is only using for VoLTE?

Hi experts,
I want to clarify my doubts.
Why SPS is only using for VoLTE?
Why Non Persistent Scheduling is not used?

There is non persistent - it’s dynamic scheduling.
SPS is rarely used by operator.
Dynamic is preferred as it gives quality but in congestion SPS is preferred.
Depends on strategy and VoLTE traffic.
If we are using SPS we should expect that quality will be comprised and there are features to enable and check mos of VoLTE.
If it is less than 3.5 then quality of VoLTE is not meeting KPI.

MOS value is better above 3.5 to 5.
For reaching better Quality.

So Dynamic Scheduling also using for VoLTE?
If there is no any congestion?

Yes, it depends on Operaror preference.
But the criteria is if VoLTE users are 5 to 10 percent of non GBR then SPS is used as it depends on VoLTE Erlang and Traffic.
It’s all about operator strategy.

I think its GBR.

VoLTE is GBR only.

What is Dynamic Scheduling?
Is it some Huawei feature for VoLTE?

It is nothing but eNodeB allocate the UE to be scheduled the data.
Based on the data it can schedule the resources for the UE through the PDCCH.

1 ms scheduling, same as non GBR.

What eNodeB allocate you mean?
UL grants?

SPS is 20 ms and dynamic is 1 ms, so VoLTE PDCCH will be scheduled fast, which helps quality but ultimately it can lead to other issues.

But SPS is still not preferred.
In Huawei, most operators and I see good MOS in Dynamic.
Ultimately it’s Quality which customer demand.
But Packet Loss, Mute can be high, that’s drawback and in Congestion scenario it’s risky.

SPS is a feature that significantly reduces control channel overhead for applications that require persistent radio resource allocations such as VoIP.

In LTE, both the DL and UL are fully scheduled since the DL and UL traffic channels are dynamically shared channels. This means that the physical DL control channel (PDCCH) must provide access grant information to indicate which users should decode the physical DL shared channel (PDSCH) in each subframe and which users are allowed to transmit on the physical UL shared channel (PUSCH) in each subframe.

Without SPS, every DL or UL physical resource block (PRB) allocation must be granted via an access grant message on the PDCCH. This is sufficient for most bursty best effort types of applications which generally have large packet sizes and thus typically only a few users must be scheduled each subframe. However, for applications that require persistent allocations of small packets (i.e. VoIP), the access grant control channel overhead can be greatly reduced with SPS.

Great, SPS for VoLTE understood .
Now, what is this Dynamic Scheduling?

As said before: Dynamic Scheduling is 1 ms scheduling for non GBR services which is defaulty used for PDSCH to go thu DCI.
Non GBR wants to enjoy PDSCH so we have to release PDCCH at fast rate.

Dynamic Scheduling is 1 msec normal scheduling.

Got it. It’s no special feature. I thought it’s some feature for Huawei. :grin:

No, they have VoLTE rate control which is one of best feature for VoLTE in huawei and compensation scheduling apart from RIHC and TTI.
Compensation helps to substain calls.
In dynamic scenario, somewhat.

Dynamic means:

In LTE that PRB allocated depend on radio conditions of users, also we have flexibility to use different channel bandwidth. Scheduling happens in 1ms TTI.

In 5G it’s like scheduling can be done per slot basis.

I see most of VoLTE networks do not use SPS. That may be because:

  • In SPS: unlike normal scheduling, UE must read DCI every slot, SPS scheduling information is fixed every 20ms. But SPS scheduling, MCS is always fixed at about 15, fix no of RB, so it’s not good for MOS.
  • transmission link between SGW - enb always has jitter, can not be assured that packet from SGW to/from enb always 20ms.