Reducing transmission losses in body area networks using variable TDMA scheduling

We consider a typical body area network (BAN) setting in which sensor nodes send data to a common hub regularly on a TDMA basis, as defined by the emerging IEEE 802.15.6 BAN standard. We explore variable TDMA scheduling techniques that allow the order of transmissions within each TDMA round to be decided on the fly, rather than be fixed in advance. Our approach is related to opportunistic scheduling used in other multiuser wireless systems, which aims to maximize the system throughput by allocating transmission opportunities to users with the `best´ instantaneous channel. However, the energy overheads of opportunistic scheduling, which requires the continuous reporting of channel states to the scheduler, make it incompatible with body area networks where nodes turn their radio off (`sleep´) until it is their turn to transmit, as an energy-saving measure. Accordingly, we consider variable TDMA scheduling strategies that decide on the transmission order at the start of each TDMA round based on the last known channel state of each sensor. We formulate the optimization problem of maximizing the expected throughput (or, equivalently, minimizing the expected loss rate) based on a Markov model of the wireless channel, and present some simple approximate heuristics that can be computed quickly yet perform well in practice. We evaluate the performance of these strategies and heuristics numerically in a wide range of scenarios, and demonstrate the marked improvement they achieve over a fixed TDMA allocation.