3 research outputs found
Outage Performance in Secure Cooperative NOMA
Enabling cooperation in a NOMA system is a promising approach to improve its
performance. In this paper, we study the cooperation in a secure NOMA system,
where the legitimate users are distributed uniformly in the network and the
eavesdroppers are distributed according to a homogeneous Poisson point process.
We consider a cooperative NOMA scheme (two users are paired as strong and weak
users) in two phases: 1) Direct transmission phase, in which the base station
broadcasts a superposition of the messages, 2) Cooperation phase, in which the
strong user acts as a relay to help in forwarding the messages of the weak
user. We study the secrecy outage performance in two cases: (i) security of the
strong user, (ii) security of both users, are guaranteed. In the first case, we
derive the exact secrecy outage probability of the system for some regions of
power allocation coefficients and a lower bound on the secrecy outage
probability is derived for the other regions. In the second case, the strong
user is a relay or a friendly jammer (as well as a relay), where an upper bound
on the secrecy outage probability is derived at high signal-to-noise-ratio
regimes. For both cases, the cooperation in a two-user paired NOMA system
necessitate to utilize the joint distribution of the distance between two
random users. Numerical results shows the superiority of the secure cooperative
NOMA for a range of the cooperation power compared to secure non-cooperative
NOMA systems
Performance of Cooperative NOMA Systems under Passive Eavesdropping
A key feature of the non-orthogonal multiple access (NOMA) technique is that users with better channel conditions have prior information about the messages of other users. The technique to exploit the prior knowledge of strong users in order to improve the performance of weak users is known as cooperative NOMA. In this paper, we study the physical layer security in such a cooperative NOMA system. In order to reduce the complexity, the considered system in this paper has two users. Through the cooperative NOMA concept, the performance of the weak user is enhanced by the strong user. Given that there is an eavesdropper in the system that can hear all transmissions, we study the secrecy rate of the strong and the weak users. More specifically, we make an attempt to derive the secrecy outage probability (SOP) of both the users. Due to the intractable nature of the exact analysis for the weak user, we provide the closed form expression for the SOP of this user in high SNR regime while keeping the exactness for the strong user. Through numerical simulations, we verify the correctness of our analytical derivations under different scenarios. Besides, we provide the insights of achieving optimal secrecy performance in such a system.acceptedVersionPeer reviewe