  Physical layer security has been recently recognized as a promising new
design paradigm to provide security in wireless networks. In addition to the
existing conventional cryptographic methods, physical layer security exploits
the dynamics of fading channels to enhance secured wireless links. In this
approach, jamming plays a key role by generating noise signals to confuse the
potential eavesdroppers, and significantly improves quality and reliability of
secure communications between legitimate terminals. This article presents
theoretical limits and practical designs of jamming approaches for physical
layer security. In particular, the theoretical limits explore the achievable
secrecy rates of user cooperation based jamming whilst the centralized, and
game theoretic based precoding techniques are reviewed for practical
implementations. In addition, the emerging wireless energy harvesting
techniques are exploited to harvest the required energy to transmit jamming
signals. Future directions of these approaches, and the associated research
challenges are also briefly outlined.
