# High-Throughput Cross-Technology Communication via Chip-Level Side Channel
## 1 Introduction  
In the recent decade, the dramatic proliferation of wireless devices has offered us convenience in most aspects of our daily life. These wireless devices (e.g., smart phones and ZigBee sensors) are deployed densely in typical scenarios such as offices, hospitals, etc. However, coexistence of heterogeneous devices equipped with fundamentally different physical-layer protocols would cause severe cross-technology interference (CTI) [1, 2] since these devices work on the shared ISM band. Cross-technology interference is a double-bladed sword. On one hand, it declines the performance of wireless communication or even interrupts ongoing transmission. An extensive number of researches have focused on coordination of heterogeneous devices to avoid CTI [3, 4] and mitigate the effects of CTI on data transmission [5, 6]. On the other hand, it creates opportunities for cross-technology communication (CTC) via sensing the energy of received signal. Esense [1], HoWiES [7], FreeBee [8], and LongBee [9] were proposed a CTC frameworks to enable wireless devices with different physical layers to communicate through modulating CTC bits over the length of packets, number of consecutive packets, and packet interval. To significantly improve the throughput of CTC, WEBee [10], SymBee [11], X-MIMO [12], and SDR-lite [13] were proposed to enable high-throughput CTC between WiFi and ZigBee via exploring the specific physical-layer information of WiFi and ZigBee. Although SymBee [11] has already enabled 31 Kbpshigh-throughput cross-technology communication from ZigBee to WiFi, it sacrifices the existing ZigBee traffic, resulting in the interruption of ZigBee communication. Hence, to reduce the large overhead caused by CTC, a high-throughput ZigBee-to-WiFi CTC, which is transparent to ongoing ZigBee communication, is needed. In order to achieve high-throughput ZigBee to WiFi CTC, we still face many critical challenges as follows. First, ZigBee and WiFi have totally different packet formats. A ZigBee preamble without modification cannot pass the packet detection module of WiFi. Second, the bandwidth of WiFi is much wider than that of ZigBee. It means that there will be information loss when ZigBee node send the packet to WiFi. Third, ZigBee and WiFi adopt different modulation schemes. It is not possinle to directly decode a ZigBee signal on WiFi. In over to address the above challenges, we propose a cross-technology communication framework to improve the throughput of ZigBee-to-WiFi communication without introducing any additional burden to the existing ZigBee network. In this paper, we modify some of ZigBee chips to carry CTC messages and, theoretically, this modification would be tolerated by ZigBee communication because of high redundancy in the ZigBee protocol. At the WiFi receiver, the Schmidl-Cox algorithm is applied to detect whether the received signal is WiFi or not in preamble detection [14]. Even ZigBee signal could not pass WiFi preamble detection, we can obtain some useful information--for example, phase offset--to demodulate CTC messages in the WiFi physical layer. In detail, ZigBee signal would generate a strong pattern at the phase offset value calculated in WiFi preamble detection. In our design, we can generate different patterns (extending the duration of stable phase offset values) in WiFi preamble detection through modifying ZigBee chips. We implement our design on USRP/GNURadio platform to explore the feasibility of our scheme. The experiments show that the throughput of our CTC design is 31.25 Kbps, while the ZigBee packet error rate is \(\leq\)5%, which means that our design is transparent to ZigBee communication. The main contributions of this paper are listed as follows:  
* We investigate the impact of ZigBee signal on WiFi preamble detection. The preliminary experiment shows that ZigBee signal generates the stable periods in preamble detection, which has a strong pattern and is different with noise. * We propose our chip-level side channel design to embed the CTC bits.