Article Preview
TopIntroduction
The most important and difficult consideration is reliability and security in the wireless networks. Due to the properties of the wireless medium, secrecy can be very difficult to obtain. The traditional cryptographic approach exits some problems: 1) The key agreement step is arguably the most challenging part and this step becomes even more daunting as network size grow (Koyluoglu, Koksal, & Gamal, 2012). 2) The adversary with infinite computing capacity can break the cryptographic scheme, which become worse with the development of high performance computing (e.g. quantum computing, cloud computing). 3) The more intelligent attackers can impersonate the legitimate nodes to eavesdrop the transmitted messages, which cannot be solved by the cryptographic approach (Tamilselvan & Sankaranarayanan, 2007).
The notion of information-theoretic secrecy was introduced by Shannon to study secure communication (Shannon, 1949). This work was later extended by Wyner (Wyner, 1975).Recently, a lot of works about secrecy capacity in wireless networks are presented (Csiszár & Korner, 1978; Goel, Aggarwal, & Yener, 2010; Ji, Song, Hu, & Chen, 2014; Koxias et al., 2015; Koyluoglu, El Gamal, Lai, & Poor, 2011; Laneman, Tse, & Wornell, 2004; Liang, Poor, & Ying, 2009; Pinto, Barros, & Win, 2009; Vasudevan, Goeckel, & Towsley, 2010a). The information theoretical analysis of secrecy capacity scaling in large wireless networks has been study which almost all of the nodes achieve a perfectly secure rate of 
(Koyluoglu, Koksal, & Gamal, 2012).
Recently, cooperative jamming has emerged as a promising technique to enhance wireless physical layer secrecy (Liu & Trappe, 2010). The basic idea is to send proper jamming signals in order to create interference at an eavesdropper. It is well understood that the use of relaying techniques can introduce significant benefits for wireless networks (Laneman, Tse, & Wornell, 2004). The relay transmits an artificial noise independent of the source signals in order to confuse the eavesdropper (Lai & Gamal, 2008). The secrecy rate of orthogonal relay eavesdropper channels is studied (Aggarwal, Sankar, Calderbank, & Poor, 2009). A relay is used for helping to jam eavesdroppers (Yuksel & Erkip, 2007). An extra jammer is introduced to enhance the secrecy performance (He & Yener, 2008). Artificial jamming noise is added to achieve secrecy (Negi & Goel, 2005). The work of (Dong, Han, Petropulu, & Poor, 2010) considers the case of a multi antenna relay in which the relay sends jamming signals based on a beam-forming strategy.
Especially, for two-top relay wireless networks, a protocol for secret communication between a source and destination using a messaging relay and artificial noise transmitted from a set of intervening relay is presented (Goeckel et al., 2011). This protocol does not require knowledge of eavesdropper channels and location. A. Sheikholeslami et al. proposed a protocol which multi source-destination (S-D) pairs transmit messages simultaneously, for two-hop wireless networks (Sheikholeslami, Goeckel, Pishro-Nik, & Towsley, 2012). The signal of a given transmitter is protected by the aggregate interference produced by the other transmitters.