## 1 Introduction  
The past decade has witnessed rapid innovations in low-power backscatter communication [1, 2, 3, 4]. These low-power designs effectively address the energy challenges posed by the large-scale deployment of devices, making it feasible for various large-scale applications to transition from the envisioned to practical deployments. Specifically, the widespread deployment of Distributed Photovoltaic Systems (DPVS) is currently limited by a shortage of data monitoring devices, which results in the inadequate coverage of measurement tools [5]. The mass deployment of low-power backscatter tags within DPVS can mitigate the issue of device scarcity while significantly reducing costs and providing substantial benefits. However, existing backscatter systems are hindered from large-scale deployment due to harmonic interference issues, as illustrated in Figure 1. This harmonic interference becomes increasingly severe with the expansion of the system, thereby obstructing the deployment of large-scale backscatter systems. Diving into this issue, it becomes clear that the process of using square waves for frequency shifting to facilitate the parallel transmission of tags generates numerous unwanted harmonic components. These harmonic components interfere with communications among neighboring tags, thus hindering the system's potential for large-scale deployment. Unfortunately, existing frequency-shifting backscatter systems [6; 7; 8; 9] do not take harmonic interference suppression strategies into account, making them unsuitable for large-scale deployment. To overcome this issue, we discover that the harmonic components in the spectrum of the square wave are related to its duty cycle. By utilizing duty cycle-adjustable square waves for the frequency-shifting design, it is possible to suppress specific harmonic components, thereby effectively addressing the issue of harmonic interference in backscatter systems. Then, we propose a harmonic interference resilient backscatter system that facilitates parallel communication between tags through frequency shifting while simultaneously enabling flexible control over the harmonic interference generated by this shifting. To achieve this system design, it is necessary to address several key challenges, as follows:  
* **Mitigating harmonic interference during frequency shifting. ** During the dense deployment of frequency-shifting tags, the various harmonic components generated by frequency shifting can interfere with the normal communication processes between readers and tags operating in different frequency bands in other areas, as illustrated in Figure 1. Therefore, our backscatter system should be capable of reducing harmonic interference during frequency shifting. * **Enhancing the system's compatibility with a common communication protocol. ** The communication processes of the numerous devices in a backscatter network require stringent adherence to standard protocol to ensure the quality of communication. Consequently, this requires our backscatter communication system to be compatible with a common protocol while maintaining a lightweight design, thereby making it more suitable for large-scale deployment. To address the first challenge, we propose the employment of an adaptive pulse-width frequency-shifting technique in the tag design. Unlike the traditional frequency-shifting backscatter systems [6; 9] that directly utilize standard square waves for frequency modulation, our innovative approach dynamically adjusts the pulse width of the square wave to compress various harmonic components, thereby minimizing harmonic interference during frequency shifting. Moreover, unlike existing schemes [10; 11] that rely on the IQ backscatter modulator or other specific hardwares for harmonic suppression, our approach performs harmonic suppression at the software level, without the need for custom hardware, thus better supporting the large-scale deployment of backscatter systems.