comm.ViterbiDecoder with Trellis for HDL code generation

Hello, I want to use the comm.ViterbiDecoder in my design and want to generate HDL code out of my design. Unfortunatelly I get always error massages during the fixed point Types validation. It seems that poly2trellis is not supported for fixed point conversion. I tried to define the Trellis by my own, but 'struct' is also not supported for fixed point conversion. This is my code:

persistent hVitDec;
if isempty (hVitDec)
Trellis = poly2trellis(7,[133 171 145 133]);
hVitDec = comm.ViterbiDecoder('TrellisStructure',Trellis,...)
end
out_1 = step(hVitDec, c_in1);

Does someone know a workaround, so I can use the ViterbiDecoder with Trellis for HDL code generation? I'm using Matlab 2013a on Win7.

Thanks a lot, Stefan