How to adapt OFDM HDL Coder example from 72 to 36 active subcarriers (ASc)?

Hello,

I am working with the OFDM HDL Coder MATLAB/Simulink example.

In MATLAB script-based simulation, I successfully changed the number of active subcarriers from 72 → 36, and both transmitter and receiver chains work correctly (headers are encoded/decoded properly, variables are validated, and frames are synchronized).

However, when I move to Simulink implementation, the situation is different:

• Frames are synchronized,
• But header is never detected (no CRC pass/fail, nothing happens after sync).

I already adapted the following in the MATLAB side:

• txParam.ASc = 36;
• txParam.NHeader = ceil(72/txParam.ASc); % = 2 when ASc=36
• Verified headerIntrlvMaxBlkSize = ASc * NHeader = 72 and headerIntrlvNColumns = headerIntrlvMaxBlkSize/4 = 18, so block length consistency is preserved.
• All OFDM-Tx/Rx functions (OFDMTx, OFDMRx, tx rx parameters etc.) run fine in script mode.

In Simulink, I suspect the issue is in one of the HDL-optimized blocks (Header Formatter / Header Interleaver / Header Parsing). Some blocks still assume a single header symbol (NHeader=1), or are hard-coded to ASc=72. For example:

• Generate Output Control Signals inside Header Interleaver Read Logic subtracts fixed numbers (8, 11) from block length.
• IndexGen functions (Reference Signal Parsing, Header & Data Parsing) may still be tied to fixed symbol counts.
• Thresholds in Frame Sync may need retuning for reduced subcarriers.

[Andreas Goser]

Hello Vaibhav, this is quite a specialized question and I suppose the community may not help out here. I also am under the impression that might be an important and new workflow for you. You will be contacted by a MathWorks support engineer directly.

[Vaibhav BHATNAGAR]

Hello Andreas,

Many thanks for your comment.

I was able to get it working by using two header symbols instead of one, while keeping the block size and other parameters unchanged.

The remaining issue is that when I try to reduce the CP size from 32 to 16, the header at the receiver does not decode properly. I will wait for further information from the MathWorks support team on this.

Best regards,

Vaibhav

[Vaibhav BHATNAGAR]

I found the solution of my problem the problem is due to : Params.OFDMDemodLatency for shorter CP it requires a adjustment. The same latency wont work.