Castalia is very good platform. It is based on OMNeT++. Its free and has good support group. You can find it here https://castalia.forge.nicta.com.au/index.php/en/
It also has the IEEE802.15.6 BAN implementation, which could help you a lot.
If you want to do network level simulation then OPNET, OMNET etc. can solve your purpose but if you want to work on few aspects of it like developing algorithms etc. MATLAB or C++ can solve the purpose. For electromagnetic field simulations COMSOL may be used.