Spectral Efficiency Enhanced Cooperative Device-to-Device Systems With NOMA

This paper considers a cooperative device-to-device (D2D) system with non-orthogonal multiple access (NOMA). We assume that the base station (BS) can simultaneously communicate with all users to satisfy the full information transmission requirement. In order to characterize the impact of the weak channel and different decoding schemes, two novel decoding strategies are introduced: single signal decoding scheme and maximum ratio combining (MRC) decoding scheme, respectively. With the single signal decoding scheme, the users decode the received signals immediately after the receptions from the BS. On the other hand, the MRC decoding scheme jointly decodes the received signals via MRC until the corresponding phase comes and the users jointly decode the received signals by employing MRC. Considering Rayleigh fading channels, the ergodic sum-rate (SR), outage probability and outage capacity of the proposed D2D-NOMA system are analyzed. Moreover, approximate expressions for the ergodic SR are also provided with a negligible performance loss. Numerical results demonstrate that the ergodic SR and outage probability of the proposed D2D-NOMA scheme overwhelm that of the conventional NOMA schemes. Furthermore, it is also revealed that the system performance including the ergodic SR and outage probability are limited by the weak channel for both the single signal decoding scheme and conventional NOMA schemes, but not for the MRC decoding scheme.

I. Introduction

Non-orthogonal multiple access (NOMA) has been envisioned as a promising technique to realize the desired spectral-efficient massive access [1]–[3]. Unlike conventional orthogonal multiple access (OMA) [4] schemes such as frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA), etc., NOMA allows multiple users to simultaneously transmit signals in a superposition manner by using common orthogonal resources (i.e., time/frequencey/code) of respective power levels [5]. NOMA realizes multiple access by manipulating the power level of the superimposed user signals at the sender and successive interference cancellation (SIC) at the receivers for signal reception, where, in general, the user rates with better channels are determined according to the assigned bandwidth, while the user rates of worse channels are limited by the noise.