Abstract: The degree of freedom (DoF) is derived from the concept of spatial multiplexing introduced by the using of multi-antenna system,reflecting the utilization efficiency of communication system to the resources of space.In cognitive radio networks,the coexistence of primary network and secondary network will interfere each other and reduce the DoF of system.This paper studied the DoF of two point to point(PTP) coexisting networks and the corresponding method for interference elimination.We obtained the DoF range of secondary network for message transmission when it is avai-lable to use the spare space resource of primary network in the same frequency.Meanwhile,the maximum DoF of se-condary network was given,when the space resource of primary network is fully exploited.In addition,a new cooperation approach was proposed to increase the degrees of freedom of the whole network by some ways to borrow the spare receive antennas from primary network.Analysis and comparison show that when M₁≤N₁ or N₁≥M₁₊M₂,cooperation at receivers can increase the degrees of freedom.

Key words: Degrees of freedom,Cognitive radio,Interference cancellation,Cooperation