# IRS-Enabled Ultra-Low-Power Wireless Sensor Networks: Scheduling and Transmission Schemes
###### Abstract  
Passive technologies, including intelligent reflecting surfaces (IRS), are gaining traction thanks to their ability to enhance communication systems while maintaining minimal cost and low complexity. They can assist a wireless sensor network (WSN) by achieving low power requirements for sensors and aid communication needs in many applications, for instance, environmental monitoring. In this paper, we propose an IRS-equipped WSN which describes sensors equipped with IRSs instead of active radio frequency (RF) electronics. The IRS sensor node (ISN) intercepts a dedicated signal from a power source such as a base station (BS) and modulates the transmission of that signal to an intended recipient. In order to enable multiple sensors to transmit to the receiver, we study opportunistic scheduling (OS) utilizing multi-sensor diversity while considering blind IRS operation, and compare it with round-robin (RR), proportional fairness (PF), and a theoretical upper bound. We study the effect of the choice of the number of IRS elements \(N\) and number of ISNs \(L\) on the average throughput of the system under OS. Finally, we provide pertinent comparisons for the different scheduling schemes and IRS configurations under relevant system performance metrics, highlighting different scenarios in which each scheme performs better.  
wireless sensor networks; intelligent reflecting surfaces; opportunistic beamforming; system outage probability; multi-user diversity; round robin; proportional fairness +
Footnote †: journal: 27  
and wireless energy transfer [5; 6]. IRSs often compete with existing helper technologies such as relays; for instance, in [7] the authors used relays in a way similar to the power-saving approach described in this paper in order to prolong the lifetime of WSN via simultaneous wireless information and power transfer (SWIPT).  
In certain WSN scenarios, sensors can be placed in remote, hazardous, or inaccessible terrain. In such cases, untangling the information source and the power source into two separate entities is desired to ensure that the sensors can monitor and survey the surroundings, thereby providing relevant real-time readings for a desired purpose while not equipped with a power supply dedicated to sending the readings. In this way, the realization of ISNs can reduce the need for battery replacement, minimize charging cost, and remove limitations due to inaccessible terrain. The ISN relinquishes the radio unit found in a transmitter-equipped sensor node, instead recycling and modulating intercepted waves from an outside dedicated power source. Figure 0(a),b provides an illustrated comparison between an an abstract model of a traditional transmitter-equipped sensor node and an ISN. In Figure 0(b), the IRS attached to the sensor can aid in transmitting information passively through backscattering. Backscattering has long existed and is ubiquitous in everyday life, as in the case of RFIDs [8].  
Similar in operation to [5], the power resource is exported, in a sense, to a nearby base-station. However, unlike [5], in which the IRS was used as a helper to improve the energy harvesting and information transfer performance of an RF-equipped sensor node, the role of the IRS in this paper is to recycle the reference signal received from a base-station (BS) and modulate it in order to transfer the information from the sensor to which it is attached, i.e., perform backscattering communication. For clarity, an ISN includes a sensor node equipped with an IRS, although we often use the terms IRSs and  
Figure 1: Two sensor node architectures. Common units are a sensing module, processor, and energy source. A transmitter unit is present in the transmitter-equipped sensor. Instead of a transmitter unit, we use an IRS controller and IRS in the ISN.