I am not sure about simulating 802.11n/ac using ns-2, but certainly 802.11b/g can be simulated. Alternatively, OPNET and other simulation tools can be used to simulate 802.11n but not sure about 802.11ac.
NetSim from http://www.tetcos.com/ has an implementation of 802.11 n and 802.11 ac along with channel models covering pathloss, fading and shadowing. The code is also open for user customization.