Achievable Rate Region of Non-Orthogonal Multiple Access Systems With Wireless Powered Decoder

Non-orthogonal multiple access (NOMA) is a candidate multiple access scheme in 5G systems to simultaneously accommodate tremendous number of wireless nodes. On the other hand, RF-enabled wireless energy harvesting is a promising technology for self-sustained wireless devices. In this paper, we study a NOMA system where the near user harvests energy from the strong radio signal to power the information decoder. Both constant and dynamic decoding power consumption models are considered. For the constant decoding power model, the achievable rate regions for time switching and power splitting are characterized in closed-form. A generalized scheme is proposed by combining the conventional time switching and power splitting schemes, and its achievable rate region can be found by solving two convex optimization subproblems. For the dynamic decoding power model where the decoding power consumption is proportional to data rate, the achievable rate region can be found by a low-complexity search algorithm. Numerical results show that the achievable rate region of the generalized scheme is larger than those of the time switching scheme and power splitting scheme, and rate-dependent decoder design helps to enlarge the achievable rate region.

I. Introduction

With the tremendous increase of mobile users for high-speed video services and machine-type communication nodes for Internet-of-Things (IoT) applications in the coming years, the next generation mobile communication systems (5G) will encounter severe scarcity of radio resources. Non-orthogonal multiple access (NOMA), in which the users access the network in the same frequency band and are distinguished in the power domain, is expected as one of the candidate solutions to greatly enhance the spectral efficiency [1]. The basic idea of NOMA (as proposed in [2]) is that two users, a far user and a near user, access in the same frequency band, and are allocated with higher and lower transmit power, respectively. The far user directly decodes its desired information, while the near user adopts successive interference cancellation (SIC) to firstly decode and subtract the signal of the far user, and then decode the desired information for itself. Inspired by the fact that the near user receives strong radio signal (including desired signal and interference), it is promising to apply RF-enabled wireless energy harvesting technology [3], [4] for this near user, which facilitates the application of self-sustained devices in the IoT networks [5], such as sensor nodes and RFIDs.