# Efficient Resource Scheduling for Multipath Retransmission over Industrial WSAN Systems
## 1 Introduction  
The process automation industry continuously works towards minimizing the cost of production and maintenance while improving the quality of their products. A step towards that goal is to utilize wireless technologies, which offer advantages such as less maintenance, more flexibility, and easy deployment in harsh industrial environments [1; 2]. As one of the key technologies, industrial Wireless Sensor-Actuator Networks (WSANs) will play a vital role in the Industry 4.0 framework and Cyber Physical System (CPS) and will also be important for future smart factories and intelligent manufacturing systems [3]. However, harsh conditions, such as complicated environments and metal interference, result in more packet loss in WSAN transmissions compared with traditional wireless sensor networks [4]. Additionally, application-driven industrial communication increases the requirements of stringent reliability as well as time efficiency [5]. Within process automation, standards such as WirelessHART, ISA100.11a, and IEEE (Institute of Electrical and Electronics Engineers) 802.15.4e have been operational for nearly a decade, and there exists several researches that address support for emerging applications, such as data aggregation [6], control, and safety [7; 8]. All of the above standards employ the Times-Slotted Channel Hopping (TSCH) Medium Access Control (MAC) protocol instead of pure contention-based MAC protocol Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA). The TSCH provides the capability of allocating a specific amount of bandwidth per node in a preknown pattern [9; 10]. Moreover, channel hopping addresses unreliability caused bymultipath fading and narrow-band external interference. However, these issued standards do not give specific link scheduling for efficient data transmission. An obstacle faced in guaranteeing deterministic communication in industrial environments, however, is that the sensor node hardware is usually equipped with an IEEE 802.15.4 compliant half-duplex single radio transceiver [11]. The resources such as slot and channel are restricted, since the transceiver supports only up to 16 different channels [12]. Communication ranges of most sensor devices are limited to short distances due to low power of transmission. Furthermore, obstacles in the wireless link or local interferences make single-hop topology WSANs difficult to deploy. Therefore, data packets originating from a source device are relayed to a destination device on end-to-end basis [13]. Thus, relaying packets in a multi-hop wireless network should be performed by reserving timeslots to provide on-time packet delivery. The unpredictable packet loss makes the retransmission scheme inevitably considered to achieve the desired levels of reliability. Multipath routing is one of the essential retransmission schemes to improve the robustness of end-to-end transmission in error-prone wireless communication environments [14]. However, multipath retransmission needs to be pre-allocated for more communication resources. Thus, the efficiency of resource scheduling is supposed to be considered. Moreover, the industrial WSAN is a technological paradigm that enables advanced and complex services by interconnecting a possibly large number of low-complexity resource-constrained embedded devices equipped with diverse sensors and actuators [15]. The incorporation that WSANs support the coexistence of different sampling rates is imperative for a better industrial solution. At such instances, resource scheduling needs to be considered for schedulability, since more collisions may exist in the process of resource allocation. Addressing the above problems, this paper proposes an efficient resource scheduling of CCA (clear channel assessment)-Embedded slot and Multiple sinks with Rate Monotonic scheme (CEM-RM) for multipath retransmission. The proposed CEM-RM scheme aims at periodic sensors, and we assume that multiple sampling periods exist on scheduled sensor nodes.