passing a string type parameter to veriloga through schematic based design

hello experts, 

please share some clue:

I would like to pass a file name to my veriloga code so it can dump to different log files as needed. but somehow the cdf parameter taking the intended variable itself as the string name instead parsing it as an variable. here's how I did:

1) declare string parameter in veriloga model and use it : 

    parameter string CALDAC1_LOG="CALDAC1.dat";

    ....

    fd1 = $fopen(CALDAC1_LOG  , "a");

2) instantiate that veriloga block and put variable name there. 

3) from ocean, define this variable by desVar:

       desVar(  "CALDAC1_LOG"  "XXX.dat") 

BUT, instead an "XXX.dat" file being written, it gets a file called "CALDAC1_LOG". 

how can I make it right?

P.S. I have no problem passing integar or real numbers through by this way. Also I've tried this CALDAC1_LOG parameter " parse As CEL" true or false in CDF editor too.  

thanks a lot,

David