# Interference-Robust Transmission in Wireless Sensor Networks
## 1 Introduction  
Recent advances in wireless communications and electronic device technology make it possible to realize low-power and low-cost wireless sensor networks (WSNs). The low cost of deployment of low-power WSNs can make various IoT services quite feasible [1]. However, low-power WSNs operating in unlicensed spectrum bands may seriously suffer from unavoidable co-channel interference generated by other radio systems (e.g., IEEE 802.11b/g/n wireless local area networks (WLANs)) [2; 3; 4; 5]. It is of great concern to deploy low-power WSNs that can provide stable transmission performance in the presence of co-channel interference. A number of techniques has been proposed for the coexistence of various radio systems operating in unlicensed spectrum bands. It is well recognized that WLAN is one of major interference sources that seriously hamper the operation of low-power WSNs [6; 7]. A simple technique is to allocate WSN and WLAN devices spectrum resource orthogonal to each other or to exploit the utilization of white space with channel switching [8; 9; 10; 11; 12]. Recently, channel hopping mechanisms have been applied to low-power WSNs to mitigate the interference. For example, IEEE 802.15.4e deterministic synchronous multi-channel extension (DSME) employs a channel hopping and channel adaptation scheme for channel diversity [13]. Bluetooth low energy (BLE) employs an adaptive frequency hopping (AFH) mechanism [14]. However, these approaches may not be effective as the available spectrum is getting sparse due to the increase of active WLAN devices [15; 16]. The performance of the AFH mechanism may considerably be degraded in the presence of multiple WLAN interference mainly due to its slow adaptation process [17]. Although WLAN devices employ a carrier sense multiple access with collision avoidance (CSMA/CA) scheme, they may not detect the presence of WSN signalswhose transmission power is much lower than theirs [18]. As a consequence, they may continue the signal transmission even in the presence of low-power WSN signals, significantly deteriorating the operation of low-power WSNs. Some works have considered placing a special device to help low-power WSNs share a channel with WLANs. A special device, referred to as a signaler, may help other ZigBee devices to access the channel by sending a busy tone, forcing WLAN devices to defer their transmissions [18]. However, the coordination between the signaler and other ZigBee devices may be too complex, making it impractical in WLAN operation environments. Another special device, referred to as the arbitrator, can schedule the activity of ZigBee and WLAN devices through communication among them [19]. However, the scheduling may not be easy in dynamic interference environments, since it may require the arbitrator to re-initiate a spectrum scanning and re-allocate the parameters. Some previous works have also considered performance improvement of low-power WSNs in the presence of co-channel interference. Two approaches, the collision-recovery and collision-avoidance approach, have mainly been considered. The collision-recovery approach mainly considers the use of forward error correction (FEC) techniques to mitigate the co-channel interference. For example, BuzzBuzz employs a Hamming(12,8) code-based FECscheme for ZigBee devices [20]. However, it may not be effective in dynamic interference environments. A real-time adaptive transmission (RAT) scheme makes WSN devices choose an appropriate FEC coding scheme to maximize the throughput [21]. However, these collision recovery schemes may not be effective unless the signal-to-interference-plus-noise ratio (SINR) is sufficiently high. Figure 1 illustrates the SINR of IEEE 802.15.4 in the presence of co-channel interference. Note that IEEE 802.15.4 requires a minimum SINR of 0.4 dB for the transmission of a 20-byte packet (refer to Appendix A). However, the SINR may not be high in practical operating environments, making the collision-recovery approach ineffective for reliable signal transmission.