Abstract
The two-way relaying (TWR) technique has been confirmed to achieve higher spectral efficiency and average sum rate compared with the one-way relaying (OWR) technique in ultra-dense next generation networks with limited spectal resources. In this paper, an enhanced secrecy cooperative TWR scheme for cooperative non-orthogonal multiple access (NOMA) networks against untrusted relaying is highlighted. Specifically, with the application of NOMA principle, a base station (Bs) communicates with two trusted users, i.e., namely a near and a far users, where the communication with the far user takes place only via an untrusted relay (UR) employing both TWR and analog network coding (ANC) mechanisms. To minimize information leakage at the UR, the far user transmits its uplink signal simultaneously with Bs's downlink signal to confuse the eavesdropping capability of the UR by increasing inter-user interference (IUI). To investigate the benefits of the proposed scheme, asymptotic lower bound expressions of the ergodic secrecy sum rate (ESSR) of uplink/downlink rates and their scaling law are derived to characterize the secrecy performance. The system parameters have been carefully studied to maitain the desired ESSR performance where we obtain an optimal value for the uplink power sharing coefficient with an arbitrary known value of the downlink one. Analytical and simulation results show that the proposed scheme can achieve scaling gain of 1/8 ln ρ of positive ESSR over the untrusted OWR scheme with adaptive uplink/downlink jamming and a significant gain over the other conventional NOMA uplink/downlink schemes in two time slots of communication.
Highlights
T HE ever-extending massive connectivity, low latency, high spectral efficiency demands makes non-orthogonal multiple access (NOMA) techniques having the urgent necessity to be performed to break the orthogonality requirement in time/frequency/code resources by partially overlapping wireless signals with a tolerable level of interference [1]
We have considered in the first phase the overestimated capability of eavesdropping at the untrusted relay (UR) which provides a performance lower bound for the proposed system [8, 48]
To save resources and prevent eavesdropping, two-way relaying (TWR) and inter-user interference (IUI) techniques were exploited at the UR to intensify the exchange of information signals and confuse the eavesdropping capability at the UR, respectively
Summary
T HE ever-extending massive connectivity, low latency, high spectral efficiency demands makes non-orthogonal multiple access (NOMA) techniques having the urgent necessity to be performed to break the orthogonality requirement in time/frequency/code resources by partially overlapping wireless signals with a tolerable level of interference [1]. In [23] a joint antenna and relay selection scheme was proposed to enhance the transmission reliability for multiple access and time division broadcast relaying strategies in a TWR NOMA based networks where the optimal transmit power allocation was obtained. The authors of [44] considered the case of multiple relay selection schemes to enhance the secrecy performance of a cooperative NOMA network of untrusted far user. As the emitting of AN-jamming signal may degrade the eavesdropping capability of the UR, it may cause a simultaneous degradation in the reception quality of the other elements of the network To overcome this issue, a novel secrecy-enhancing design for cooperative uplink/downlink NOMA with an UR was proposed in [48] where the near and far users send an adaptive jamming signal of a similar symbol structure. Sources rather than combat eavesdropping which leads to inefficient information decoding at receivers, and this motivates our work as follows
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