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Active cooperation is used to improve the security of wiretap channel. Especially, the number of relay antennas has an important effect on the security of wiretap channel. However, in previous work it is seldom comprehensively considered how to make full use of the relay antennas in the transmission process. In this paper, we investigate the influence of the number of relay antennas on the secure degrees of freedom (s.d.o.f.). We consider the situation that there is no direct link between the source and the destination. Firstly, we study the condition that the relay between the source and the destination adopts an amplify-and-forward (AF) strategy. Under this situation, the theoretic estimation is given to verify that the number of antennas at the relay have an influence on the s.d.o.f. in the case of  N r   ≤ min{ N t  ,  N d  ,  N e  } and  N d   ≥  N e   ≥  N r   ≥  N t   . Otherwise, it almost has no influence on the s.d.o.f. Meanwhile, if the relay adopts a decode-and-forward (DF) strategy. We can prove that if  N e   ≤ ½( N t   +  N d  ), the s.d.o.f. is ½ min( N r  ,  N d  ) and if ½( N t   +  N d  ) &lt;  N e   ≤  N d   + ½ N t   , the s.d.o.f. is ½( N t   +  N d   − 2 N e   + min( N r  ,  N d  )). The simulation results are consistent with the theoretical results.

The Influence of the Number of Relay Antennas on the Secure DoF with Active Cooperation