Routing Protocol Design in Tag-to-Tag Networks with Capability-enhanced Passive Tags Chang Liu ∗ and Zygmunt J. Haas ∗† ∗ Erik Jonsson School of Engineering and Computer Science, The University of Texas at Dallas, Richardson, TX 75080 † School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853 {chang.liu@enmu.edu, zhaas@cornell.edu} Abstract—Radio frequency identification (RFID) is a technology that incorporates the use of electromagnetic fields to uniquely identify objects. Among different types of RFID tags, passive tags have some salient features such as light weight, low cost, small size, etc. However, the downside of passive RFID systems is very limited reading range due to lacking their own energy sources (passive RFID tags communicate solely by backscattering the reader’s power). A novel concept of passive RFID tag-to-tag (T2T) communication has been recently proposed, via which passive tags in proximity (at centimeter level) can directly communicate with each other with the existence of an external energy source. Utilizing this concept, we proposed a Network of Tags (NeTa) that passive tags can connect with each other through multiple hops, using a the novel concept of turbo backscattering operation. This significantly enhances the scalability of such a T2T network. However, to implement the proposed NeTa architecture, one of the main issues is the inter-tag interference, which brings challenge to the routing protocol design. In our previous work [1], we introduced protocol design for both tag-to-reader routing and tag-to-tag routing, considering basic hardware capability of tags, i.e., tags cannot measure the strength of received signals. In this paper, we extend upon the results in [1] and focus on tag-to-tag routing for two distinct types of tags with different hardware capabilities – tags can measure and attenuate the received signal before backscattering. These functions can greatly reduce the inter-tag interference and therefore enhance the network throughput. The protocol design is based on solutions of two mixed integer non-linear programming (MINLP) problems, respectively. The performances of the proposed protocols are analyzed and the impacts of several network factors (e.g., tag density, the transmit power of the reader, etc.) are investigated. Index Terms—RFID; Tag-to-Tag Communication; Backscattering; Routing Protocols; Internet of Things; I. INTRODUCTION RFID tags, are small-size and low-cost wireless devices that help identify objects and people ([2]). Each RFID tag has a unique identification code, which differentiate the tags as part of an RFID reader interrogation operation. RFID has applications in various sectors, such in retail, manufacturing, healthcare, agriculture, etc. We can generally divide RFID tags into three classes: active, passive, and semi-passive tags. Active tags are powered by batteries, while passive tags don’t require on-board energy sources. Instead passive tags use the radiation of a reader as an energy source to power the electronics and for communication through backscattering. The operation principle of a typical passive RFID link between a tag and the reader is as follows: the reader sends an activation signal to a passive RFID tag in its coverage area, which energize the chip circuit of the RFID tag. The tag can then respond by backscattering the received waveform signal. Due to the salient features, such as low cost, small size, physical flexibility, theoretically infinite lifetime, and environmental safety, passive RFID tags are extremely attractive as an enabling technology of many novel applications. Tag-to-tag (T2T) communication ([2], [3]) has been recently proposed, via which passive tags can communicate with each other within close distance (at the centimeter level). With the introduction of a Network of Tags (NeTa) architecture proposed in our previous work [1], a tag which is too distant from another tag will attempt to reach the destination via relaying by a sequence of other tags. This is mainly achieved by the introduced novel mode of operation, which we refer to as turbo backscattering, and which relies on refreshing the backscattering energy at each hop in a sequence of tags that relay the information. The basic principle of the turbo backscattering is as follows: each tag in the sequence first receives and decodes the transmission, modulates the received signal with its own information, and then backscatters the “fresh” power waveform from the reader. To implement such a NeTa architecture, one of the main technical challenges is the routing protocol design, due to inter-tag interference and possibly complicated topology of the tags. As a prerequisite for routing, the discovery and identification of tags in the coverage of a reader for a traditional RFID network has been widely investigated (e.g., [4] – [7]). However, there is inadequate existing literature on routing protocols specifically related to passive T2T networks. The design of routing protocol for T2T networks has unique challenges because of the significantly different connectivity/coverage requirements, which is due to the nature of backscattering communication. To the best of our knowledge, the only prior efforts on routing protocol design for T2T communication are [8] and [9]. In Ref. [8], an algorithm is designed to identify and define uplink paths in networked active tags. However, the proposed algorithm cannot be applied to NeTa, mainly due to the asymmetry between downlink (i.e., reader to tag) and uplink (i.e., tag to reader) communication. Ref. [9] develops a fully distributed optimal link cost multipath routing protocol in the network 978-1-5386-3531-5/17/$31.00 ©2017 IEEE C. Liu and Z.J. Haas, “Routing Protocol Design in Tag-to-Tag Networks with Capability-enhanced Passive Tags,” IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Montreal, QC, Canada, October 8-13, 2017