SiP, If it is a fully RF SiP and uses same process technology we can use ADS I guess. Usually System in package contains digital portions also which are fabricated using differnt process technology (may be 65 nm CMOS) different from RF. I have read some where the processing speed required by each is different and it is difficult to simulate the system as a whole. We can do part by part simulation inside the SiP when compared to full simulation of SOC (system on chip which uses single wafer and Technology).
Hi Khouly
I am attaching one IEEE paper which will give you good idea about SIP & its applications.
Also See the toshiba's pdf it is also has good explantions
Regards,
Abhishek