Modulation MCS used in LTE signaling: is it fixed to QPSK?

4G LTE
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1

Hello Huawei Experts.

Can you confirm if the modulation MCS used in LTE signaling is fixed to QPSK?

2

Do you mean during the attach stage or later?

For initial messaging, it’s fixed.

After that, I believe it uses the same MCS as PDSCH-A.

3

In NR, PDCCH uses QPSK only.

4

Thanks! Yes, I mean for the initial stage.

Is it configurable?

Which parameter controls this, and in which message can we find it?

5

Check with the vendor.

From what I understand, it’s hard-coded and not configurable.

Are you looking to set it to a higher MCS or lower?

6

I would like to check because the MCS is 100% QPSK.

I want to verify if this is due to initial access.

7

Ah, okay. If the entire traffic in the cell is 100% QPSK, it could be a licensing issue. Most phones now support higher MCS, so it’s likely that the license is limiting the higher data rates.

As soon as the phone switches to closed-loop power control, the MCS adjusts dynamically.

8

But is it true that the MCS is fixed at QPSK for initial access?

9

I believe so, yes.

10

Do you know which parameter controls this?

11

It’s hard-coded and not changeable. This is based on my own experience when I looked into this behavior. I even asked Huawei about it at the time.

1 Like
12
  1. Check TB size, small data → no need for higher modulation order.
  2. UE RF condition can be bad → need to maximize successful transmission rate