Iphone 3gs - 4.3.5 - Baseband 05.16.02 - Serial number **114 - Need help unlocking it

Hi everyone,I have been searching google and forums trying to unlock my iphone 3gs that I just bought on ebay. (clearly I don't have any files SHSH saved from previous versions). I wanted to ask if somebody can provide exact information how to I unlock this phone. I read that I have to update baseband to 06.15.00 (ipad baseband) and then do some other things, but I wasn't sure if that was applicable for 4.3.5 3gs. So, your help would be greatly appreciated.Thanks!