Due to the broadcast nature of radio propagation, wireless transmission can be readily overheard by unauthorized users for interception purposes and is thus highly vulnerable to eavesdropping attacks. To this end, physical-layer security is emerging as a promising paradigm to protect the wireless communications against eavesdropping attacks by exploiting the physical characteristics of wireless channels. This article is focused on the investigation of diversity techniques to improve physical-layer security differently from the conventional artificial noise generation and beamforming techniques, which typically consume additional power for generating artificial noise and exhibit high implementation complexity for beamformer design. We present several diversity approaches to improve wireless physical-layer security, including multiple-input multiple-output (MIMO), multiuser diversity, and cooperative diversity. To illustrate the security improvement through diversity, we propose a case study of exploiting cooperative relays to assist the signal transmission from source to destination while defending against eavesdropping attacks.
We evaluate the security performance of cooperative relay transmission in Rayleigh fading environments in terms of secrecy capacity and intercept probability. It is shown that as the number of relays increases, both the secrecy capacity and intercept probability of cooperative relay transmission improve significantly, implying there is an advantage in exploiting cooperative diversity to improve physical-layer security against eavesdropping attacks.
In wireless networks, transmission between legitimate users can easily be overheard by an eavesdropper for interception due to the broadcast nature of the wireless medium, making wireless transmission highly vulnerable to eavesdropping attacks. In order to achieve confidential transmission, existing communications systems typically adopt the cryptographic techniques to prevent an eavesdropper from tapping data transmission between legitimate users [1, 2]. By considering symmetric key encryption as an example, the original data (called plaintext) is first encrypted at the source node by using an encryption algorithm along with a secret key that is shared only with the destination node. Then the encrypted plaintext (also known as ciphertext) is transmitted to the destination, which will decrypt its received ciphertext with the preshared secret key. In this way, even if an eavesdropper overhears the ciphertext transmission, it is still difficult for the eavesdropper to interpret the plaintext from its intercepted ciphertext without the secret key. It is pointed out that ciphertext transmission is not perfectly secure, since the ciphertext can still be decrypted by an eavesdropper through an exhaustive key search, which is also known as a brute-force attack. To this end, physical-layer security is emerging as an alternative paradigm to protect wireless communications against eavesdropping attacks, including brute-force attacks.
Physical-layer security work was pioneered by Wyner in , where a discrete memoryless wiretap channel was examined for secure communications in the presence of an eavesdropper. It was proved in  that perfectly secure data transmission can be achieved if the channel capacity of the main link (from source to destination) is higher than that of the wiretap link (from source to eavesdropper). Later on, in , Wyner’s results were extended from the discrete memoryless wiretap channel to the Gaussian wiretap channel, where a so-called secrecy capacity was developed, and shown as the difference between the channel capacity of the main link and that of the wiretap link. If the secrecy capacity falls below zero, the transmission from source to destination becomes insecure, and the eavesdropper can succeed in intercepting the source transmission (i.e., an intercept event occurs). In order to improve transmission security against eavesdropping attacks, it is of importance to reduce the probability of occurrence of an intercept event (called intercept probability) through enlarging secrecy capacity. However, in wireless communications, secrecy capacity is severely degraded due to the fading effect.
As a consequence, there are extensive works aimed at increasing the secrecy capacity of wireless communications by exploiting multiple antennas  and cooperative relays .
Specifically, the multiple-input multiple-output (MIMO) wiretap channel was studied in  to enhance the wireless secrecy capacity in fading environments. In , cooperative relays were examined for improving the physical-layer security in terms of the secrecy rate performance. A hybrid cooperative beamforming and jamming approach was investigated in  to enhance the wireless secrecy capacity, where partial relay nodes are allowed to assist the source transmission to the legitimate destination with the aid of distributed beamforming, while the remaining relay nodes are used to transmit artificial noise to confuse the eavesdropper. More recently, a joint physical-application layer security framework was proposed in  for improving the security of wireless multimedia delivery by simultaneously exploiting physical-layer signal processing techniques as well as upper-layer authentication and watermarking methods. In , error control coding for secrecy was discussed for achieving the physical-layer security.
Additionally, in [12, 13], physical-layer security was further investigated in emerging cognitive radio networks. At the time of writing, most research efforts are devoted to examining the artificial noise and beamforming techniques to combat eavesdropping attacks, but they consume additional power resources to generating artificial noise and increase the computational complexity in performing beamformer design.
Therefore, this article is motivated to enhance the physicallayer security through diversity techniques without additional power costs, including MIMO, multiuser diversity, and cooperative diversity, aimed at increasing the capacity of the main channel while degrading the wiretap channel. For illustration purposes, we present a case study of exploiting cooperative relays to improve the physical-layer security against eavesdropping attacks, where the best relay is selected and used to participate in forwarding the signal transmission from source to destination. We evaluate the secrecy capacity and intercept probability of the proposed cooperative relay transmission in Rayleigh fading environments. It is shown that with an increasing number of relays, the security performance of cooperative relay transmission significantly improves in terms of secrecy capacity and intercept probability. This confirms the advantage of using cooperative relays to protect wireless communications against eavesdropping attacks.
The remainder of this article is organized as follows. The next section presents the system model of physical-layer security in wireless communications. After that, we focus on the physical-layer security enhancement through diversity techniques, including MIMO, multiuser diversity, and cooperative diversity. For the purpose of illustrating the security improvement through diversity, we present a case study of exploiting cooperative relays to assist signal transmission from source to destination against eavesdropping attacks. Finally, we provide some concluding remarks.