Close-Coupled Chips Can Coordinate to Contain Collisions

Reducing collisions caused by the concurrent transmissions of RFID chips (tags) is a crucial research topic in designing efficient air-interface protocols in passive-tag RFID systems. Different anti-collision protocols are proposed to improve the tag reading efficiency, by ordering the tags´ participation over a period of time, by stochastic or deterministic manner. All existing anti-collision protocols follow a `single line of thought´; wherein the onus for mitigating collisions is always on the reader, with no active role being played by the tags. We propose a fundamentally different strategy of enabling communications among the tags, especially to assist the reader in further resolving collisions. Overthrowing the conventional wisdom of limited RFID communication allowed only between the reader and tags, recent experimental studies have shown the possibility of Tag-to-Tag (T2T) communication, in industry popular UHF RFID systems. Two tags within the Near-Field (NF) of each other, can electromagnetically couple with and therefore communicate, as long as they are receiving energy from the reader. A critical property of NF communication systems is their small reading range, about 30mm for a 900MHZ UHF RFID system. This provides a two spheres of communication, and enables concurrent T2T communications to happen within the long-range (around 6m) of a UHF RFID system. In this NF UHF passive tag RFID system, a tag can promiscuously listen to its NF neighboring tags´ collided-transmissions, and give them a second chance for identification. In this paper, we will augment two different variants of frame-slotted ALOHA protocols to support promiscuous learning of NF tags, and study the change in reading efficiency in terms of delay and energy costs. Our detailed performance evaluation study will demonstrate the efficacy of our proposed augmented protocols, both in terms of delay and energy costs.