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Top1. Introduction
Wireless sensor networks are used today for a wide range of applications, from animal tracking (Juang et al., 2002) to military threat detection (Simon et al., 2004). Network requirements and characteristics, such as topology, power consumption, throughput, security, and protocols employed, are just as diverse (Yick, Mukherjee, & Ghosal, 2008). A new class of wireless sensor networks is emerging that places network security, scalability, and reliability at the forefront of design considerations. Applications such as the industrial Internet of Things (IoT) (D. Chen, Nixon, & Mok, 2010; Rondon, Gidlund, & Landernas, 2017), vehicular networks (Rahman, 2014), military (Kott, Swami, & West, 2016), and Wireless Avionics Intra-Communications (Technical characteristics and spectrum requirements of Wireless Avionics Intra-Communications systems to support their safe operation, Dec. 2013) will monitor or control highly critical systems and therefore require resilient physical layer communication.
An emerging technology that can be applied to these highly critical wireless sensor networks is the use of receiver-assigned code division multiple access (RA-CDMA) as a multiple access technique. RA-CDMA networks use the target destination’s, i.e. the intended receiver’s, assigned spreading code to modulate messages for transmission. CDMA offers many key benefits such as built-in interference mitigation, resilience against multipath interference, and the ability to receive multiple messages simultaneously. These benefits are of particular interest in secure IoT networks.
Most current IoT networks employ other multiple access schemes such as time division multiple access (TDMA), carrier sense multiple access (CSMA), carrier sense multiple access with collision avoidance (CSMA-CA), or some hybrid combination of the three (Kredo II & Mohapatra, 2007), all of which have the potential drawbacks of synchronization requirements or undesirable behavior in the presence of collisions. For example, networks using CSMA-CA protocols like IEEE 802.15.4 have the potential for cascading backoffs that contribute significantly to poor latency performance during high network activity (Misic, Shafi, & Misic, 2006; Petrosky, Michaels, & Ridge, 2018). RA-CDMA, enabled by developing technology (Michaels, 2018), could offer a promising alternative with benefits over these traditional multiple access techniques, yet few published techniques exist for Medium Access Control (MAC) layer protocols optimized for RA-CDMA.
This paper focuses on a new MAC protocol that supports networks using RA-CDMA as a multiple access technique. A MAC layer is one of two sublayers that form the data link layer, which falls directly above the physical layer in the OSI telecommunications model. The MAC layer’s responsibilities include packet framing for data, successful and correct data transmission, and prevention of collisions in time, space, or frequency (Kurose & Ross, 2013).
The proposed MAC design is applicable to wireless sensor networks that meet the following criteria:
- 1.
The network employs a receiver-assigned CDMA scheme for multiple access.
- 2.
The network’s topology is a hierarchical star-of-stars with a relatively large number of wireless sensor edge nodes, a relatively small number of wired access points, and a central processing hub. Data relay nodes are supported, but not optimized.
- 3.
Nodes associated with the network are IoT-caliber devices with low power requirements.
- 4.
Data rates are bounded to ≤100 kbps per node in most cases.
An overview of existing MAC layer protocols and multiple access techniques that are commonly employed in wireless sensor networks is presented, along with an explanation of RA-CDMA, including key motivating factors for its use in IoT wireless sensor networks. Key design features and a design comparison to existing MAC options are also presented. Conclusions and future work are then provided.