Problem with the OFDM CARRIER ALLOCATOR

Dear GNU Radio community,

I am working with the OFDM Carrier Allocator block and facing an issue when setting up a system based on LTE subcarrier allocation. Specifically, I want to allocate 1024 subcarriers, with only 600 being occupied, while avoiding pilot symbols and pilot carriers to simplify the setup.

The problem arises when I visualize the spectrum:

• When setting the FFT length to 256, the spectrum appears correctly.

  
  

• However, when increasing the FFT length to 1024, the spectrum becomes shifted, and I am unsure why this is happening. 

  
  

Has anyone encountered a similar issue? Could this be related to how the carriers are mapped internally within the OFDM Carrier Allocator block? Any insights or suggestions would be greatly appreciated.

Here is my flowgraph: