# Performance Analysis of Ambient Backscatter NOMA Systems
## 1 Introduction  
Given the explosion of connected wireless devices and data traffic, human society has evolved into the age of the internet of things (IoT), the ubiquitous and intelligent connectivities between people, machines, and things [1, 2]. According to predictions, the number of mobile terminals accessing networks worldwide will exceed 125 billion by 2030 [3]. Such a large number of connected devices poses two challenges for future mobile communications. Firstly, the limited availability of wireless frequency resources is an important bottleneck, which prevents wireless communications' further development [4]. Non-orthogonal multiple access (NOMA), with its high spectrum utilization, is the solution to enabling large-scale transmission networks for the IoT [5]. Secondly, energy efficiency (EE) is a key factor in the sustainability of mobile transmission networks. Backscatter technology enables low-power communication, by absorbing the energy of surrounding radio frequency (RF) signals [6]. NOMA enables multi-user connectivity through a non-orthogonal resource allocation method. Compared to orthogonal multiple access (OMA), NOMA is characterized by superior spectral efficiency, link density, and user fairness [7, 8, 9]. The fundamental tenet of NOMA lies in transmitting the superposed signals to numerous users within the same time/frequency resource blocks, so that the receivers can then distinguish various signals by utilizing successive interference cancellation (SIC) technology [10, 11, 12]. The authors in [13] optimized the performance of the distributed uplink NOMA, by finding the optimal decoding order. In [14], the authors surveyed the achievable rate region, by analyzing theoutage probability (OP) of NOMA users, and proved the optimal SIC order. Considering the vulnerability of signals in wireless channels to eavesdropping, the authors of [15] examined cooperative NOMA networks' effectiveness, in terms of secrecy, at providing valuable security properties. On a parallel avenue, backscatter communication is capable of absorbing energy from nearby RF signals. It has evolved into a potential remedy for the problem of energy limitation of wireless communication networks [16]. Traditional backscatter communication networks consist of one reader and a backscatter device (BD) [17]. It is worth pointing out that a BD has no extra active components (i.e., oscillators and amplifiers). The typical commercial application of the BD is radio frequency identification [18]. The BD allows wireless devices to transmit data passively, and modulates the RF signal through mismatched antenna impedance [19]. The authors of [20] investigated a relay collaboration scheme that maximized system throughput by optimizing time allocation. However, traditional backscatter communications may not be suitable for IoT devices, as these devices consume energy in active transmission mode [21]. To resolve this issue, in [22], the authors evaluated a new transmission scheme: the ambient backscattering (AmBC) system. The RF signal of AmBC can make use of the signal from the surrounding environment (e.g., bluetooth signal or cellular signals) as the source of the RF, instead of the unmodulated RF signals [23; 24]. In [23], the authors surveyed a system model of two devices utilizing ambient RF as the sole power source for communication and a prototype of an ambient BD. The authors in [24] studied the adoption of AmBC for RF power, where a secondary receiver receives data from a secondary transmitter via an AmBC signal. To further reduce power consumption, the authors of [25] considered the combination of both backscatter communication and wireless communication technologies. In [26], the authors proposed a collaborative AmBC system based on spread spectrum and spectrum sharing, which allowed the reader to decode information from the BD. To increase the system's actual performance gain, the authors of [27] optimized the BD's reflection coefficient, to improve the system throughput.