## 1 Introduction  
Ultra-high energy efficiency (UHEE) is a new paradigm for massively populated and densely composed communication networks at the heart of the massive growth of Internet-of-thing (IoT) applications, and constitutes a complex challenge for beyond 5G (B5G) wireless systems. B5G refers to the network specifications and requirements represented by the era beyond the launch of 5G and before the anticipated launch of 6G [1]. In this context, the need of protocols that require minimum overhead and simplified access mechanism has pushed forward the design of uncoordinated random access solutions [2] based on slotted-ALOHA strategies in combination with the Non-orthogonal Multiple Access (NOMA) paradigm [3; 4; 5]. Moreover, ultra-high energy efficiency for B5G systems is achieved by considering the design of extremely simple and passive device architectures, and backscattering communication has emerged as one of the most promising technologies [6].  
The backscattering communication principle has found wide diffusion in the context of Radio Frequency Identification (RFID) applications, which were originally supported by a mono-static architecture consisting of an active device (i.e., carrier emitter or reader), with physically co-located transmitting and receiving antenna, and passive transponders (i.e., passive devices or tags) that have data to be collected by the reader. The passive nature of the tags (i.e., the absence of a dedicated and durable power source) imposes a very simple design and almost-zero computational capabilities. The reader transmits a continuous wave signal to the tags that in turn scavenge a portion of the incident power to operate an antenna's load impedance switching modulator [7]. Nevertheless, mono-static systems suffer from very limited coverage due to the inherent inefficiencies of the scavenging circuitry at the device side [8]. To overcome coverage limitations, multi-static architectures have been proposed [9], where a set of networked carrier emitters/readers support communication with backscattering devices either cooperatively or concurrently. Albeit the increased system complexity, multi-static solutions guarantee robustness in terms of both energy provisioning and communication reliability.  
Furthermore, the inherent simplicity of passive devices' architecture poses substantial limitations to the design of the system protocol stack. For instance, the devices' inability to perform channel sensing restricts the applicability of complex collision resolution mechanisms. At the same time, the inability to handle complex synchronization mechanisms for the devices make orthogonal multiple access protocols, such as Orthogonal Frequency Division Multiple Access (OFDMA), not attractive. In general, promising channel access for passive devices is based on simple random access protocols, such as ALOHA, characterized by very low overhead and high energy efficiency.  
With the objective of extending the backscattering communication paradigm to the context of ultra-high energy efficient communication, system design is naturally oriented at multi-static solutions due to the offered extended coverage and flexibility. In addition, the energy required to power up the passive nodes can be harvested from non-dedicated ambient sources [10], thus avoiding the deployment of dedicated power beacons. More importantly, the presence of multiple receivers inherently gives rise to macroscopic diversity that can be effectively exploited to enhance system performance and take advantage of the propagation features that characterize the communication environment. Specifically, the difference in the relative distance among passive devices and backscattering receivers, and the different fading realizations observed at different detection points, can concur at reducing the detrimental effect of interference and allow for simultaneous detection of multiple packets [11; 12].