QAM modulation in Uplink and Downlink Carrier Waveform Generation

When performing a simulation in Uplink and Downlink Carrier Waveform Generation When analyzing the representation of subcarriers/symbols, with QAM modulations, What is the cause of these results (in QPSK the blocks are uniformly coloured)?

Answer 1

[Sriram Tadavarty]

Hi Javier,

Based on the plots, i see it is placed for an uplink waveform generation example.

The uniform color for QPSK with green color, represents the DM-RS symbols locations. Since in the DM-RS OFDM symbol locations, the modulated symbols are QPSK, it has uniform color in that symbol (same amplitude). In another OFDM symbols, the modulated symbols are not having the same amplitude.

From the image of BWP 1, it can be inferred that there are 4 DM-RS locations for your shared channel setup.

Hope this helps.

Regards,

Sriram

[Javier Sola morras]

In testing, I have seen that this is also the case with QPSK modulation (Uplink Carrier Waveform Generation example) .

There are also empty subcarriers

[Sriram Tadavarty]

Ya. For QPSK modulation, magnitude of all the modulated symbols is the same. So, you will observe the uniform color.

The reason for empty subcarriers in those OFDM symbols carrying DM-RS, come from the fact of number of CDM groups that are unavailable for data transmission.

For example, if the DMRSPortSet is set to 0 (i.e. CDM group number 0), and all the resource elements that are unused for DM-RS are not available for data transmission, then you will witness empty subcarriers.

To control the number of CDM groups that are unavailable for data transmission, use NumCDMGroupsWithoutData field. To observe no empty subcarriers and to be filled with data, set NumCDMGroupsWithoutData value to 2 (for DM-RS configuration type 1). If you want to observe empty subcarriers, set NumCDMGroupsWithoutData value to 1 (for DM-RS configuration type 1). It is basically this parameter that controls the empty subcarries in the DM-RS locations.

Hope this helps.

Regards,

Sriram

[Javier Sola morras]

Thank you, I understand about the empty subcarriers.

Sorry, but I don't understand why in some QPSK modulations the green color is not uniform, it means that the symbols are not having the same amplitude?

[Sriram Tadavarty]

HI Javier,

I am not sure when you say you didn't have same amplitude for QPSK symbols in DM-RS symbol locations. Can you post an image showing this and if possible attaching the PUSCH configuration of it?

It provides me some insights of what the configuration might be.

Thanking you.

Regards,

Sriram

[Javier Sola morras]

Using the example of the Uplink Carrier Waveform generator (pucch(1).Modulation = 'QPSK' and pusch(1).Modulation = 'QPSK'; ) the PUCCH in subcarriers/symbols graph

[Sriram Tadavarty]

Hi Javier,

For PUCCH format 3, after symbol modulation 'QPSK', there is transform precoding stage. So, though the modulated symbols are uniform, after transform precoding stage, the magnitude of symbols wont be uniform.

Hope this helps.

Regards,

Sriram

[Javier Sola morras]

Thank you for the information. It was very helpful.