Tracking a large number of tokens: a customised prioritised medium-access protocol for target responses

An organisation is interested in tracking a large number of potentially moving "targets", each fitted with a simple tracking-assisting device, and roaming over a known, limited geographical area. To assist in tracking, a well-designed net- work of fixed "anchors", with plentiful energy and computing power, and a reliable (say wired) communication channel for coordination is available. Anchors take turns (with spatial reuse as possible) sending a "ranging request" (RR) and receiving re- sponses from targets. A target response is "heard" by several anchors, which enables the system to estimate the target's current location. The key issue is to coordinate the targets responses. At a given moment some targets may be inactive, asleep, or simply static, while others may be moving at various speeds. Potentially, many active targets may congregate in certain subarea. A simple time-division solution is inappropriate because many time-slots would be needed to accommodate the maximum number of targets, and many would be wasted at a given time. A simple ALOHA-style solu- tion would be problematic when many active targets are near each other. Below, we provide a customised priority-based medium-access solution, which works for a large (or small) number of targets under the assumed scenario. The present report covers conceptual/qualitative aspects only.