https://www.researchgate.net/post/Which_one_is_better_for_simulating_wireless_sensor_network_omnet_or_Ns-2
omnet++ and Ns-2. are good candidates for Wireless Sensor Networks (WSN)?. Both have different features and characteristics. omnet++ is better choice on physical layer. If you want to implement something on DDL, Network Layer and Transport, then go with NS2.
If you need to simulate a full protocol stack the NS2 or NS3 are usually a better choice. If you want a simple or more abstract model, with your protocol the OMNeT++ is usually better because its easier to add a new protocol here.
The OMNeT ++ MiXiM is a good framework providing tools for simulating radio signal propagation. If you need to implement your own protocol I think this is good choice.
If you want to only modify existing protocol, like e.g. RPL, I would start with NS3. Modification of the protocol is possible and there is a good tutorial how to do this in NS3. But in my opinion writing a new protocol from scratch is more complicated in NS3
than in OMNeT++. If you need to take existing one and just modify it slightly NS3 is a good choice.
It depends on your project requirements. For example, if you need to simulate mobility in WSNs, I suggest NS-2. Compared with Omnet++, NS-2 provides more realistic mobility models.