I. Introduction
WIRELESS communication is inherently insecure owing to the broadcast nature of radio propagation. In particular, any receiver within the cover range can unobstructively listen and analyze the transmission, which raises the security concerns. Due to the unreliability of the wireless links, physical layer (PHY) security is attracting considerable attentions recently. Traditionally, spread spectrum techniques and information theoretic security scheme are two of the most well-developed techniques for PHY security. The information theoretic security scheme, an innovative and provably secure approach, is first proposed in [1] and has been extended to many common scenarios, such as Gaussian wiretap channels [2], fading channels [3], and MIMO scenarios [4]– [7]. However, these existing approaches have to face following challenges: a) with the development of detecting methods, spread spectrum techniques alone can not satisfy the secrecy request; b) some assumptions, such as the channel state information should be well known by the transmitter or the legitimate receiver occupies a better channel than eavesdroppers, are adopted in the information-theoretic secrecy schemes but impractical to be implemented.