Possible bug in DCF functions?

[Alvin SW]

Hi folks,

I tried to create a simple wireless ad hoc simulation using 2 node,
where the distance between those two nodes is 10 meters. I used 11a
standard with Non-QOS MAC (the default ad hoc mac).

n1 --------------- n2
10m

And then I scheduled both n1 and n2 to broadcast a packet at the same
time. The result is a packet collision occurs, so neither n1 or n2
receive any packet.
If I am not mistaken, according to the DCF, there should be a random
backoff period before when a packet is sent. So for the above
scenario, if n1 get smalller backoff time, then it will transmit the
packet, n2 then can receive the packet. After the channel is idle, n2
will take its turn to transmit, which can be received by n1. In short,
there should not be any packet collision for such simulation setup.
Is this a bug or is it just me that have incorrect understanding?

Cheers,

Alvin