Hi Mohit,
One quick thing to check is the number of floquet modes you are using. The cell looks to be large and for that we would need more mode. Use Mode calculator in Floquet Port setup to check how many modes are needed. You can find more information about it in HFSS online help.
You may instead use a PML boundary and use incident plane wave with the proper angles and proper scan angle for coupled boundary.
Also, note that in comparison with a paper, it is important to do the exact simulation as they did. Usually not all the information is available in the paper. Check every step of what you did which was not exactly given in the paper. This may show where the differences are coming from.
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