Degrees of Freedom of Wireless Networks - What a Difference Delay Makes

We explore the impact of propagation delay on the degrees of freedom of wireless interference networks. For K > 2 user interference channel we show through an example that propagation delays can increase the degrees of freedom by up to a factor of K/2. We provide an example of node placement for a 4 user interference network such that the propagation delays for line-of-sight communication allow perfect interference alignment. We show that even if nodes are randomly placed, one can almost surely achieve sufficient interference alignment to approach the upperbound on the degrees of freedom by choosing the basic symbol duration small enough. An analogy with deterministic channel models is pointed out as an interesting mechanism to translate propagation delay based interference alignment schemes to delay-free Gaussian channel models.