You can use the Omnet++ simulator with the MiXiM framework. Since the version 4 the omnet++ simulator is easy to use and it's easy to get statistics from data.
Thank you Damian Rusinek and Angelo Trotta, i try now with NS2 its very good and with last version we can simulate wireless networks. see this article http://www.ijarcce.com/upload/june/3-Network%20Simulation%20Tools%20Survey.pdf
In my thinking, the NS2 is the best simulator to simulate the Wireless Sensor Network. As you mentioned you use the MATLAB and it will be more easy to simulate Wireless Sensor Nodes with NS2 as you're good in Coding.
OMNet++ and NS2 are the best choices for event-driven WSNs' applications. But, based on my experience, OMNet++ has a better GUI. In the case of data-centric applications, you are free to use an arbitrary programming environment.
I actually just got 6 or more sensor nodes in a wireless network using RF band switching frequency and the proper working together I'm sure the network.
It would be better if the software GUI programming because it takes time to learn a language that I do not. At least it's free, and you could have a lot of training database