Energy-efficient retransmission strategies under variable TDMA scheduling in body area networks

We consider a 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. Our previous work has established the benefits of variable TDMA scheduling in this setting, where the order of communication with nodes in each TDMA round is determined on the fly by the hub, based on the actual outcomes (success or failure) of their recent transmissions. This technique takes into account the very slow fading nature of wireless channels in BANs, and can minimize the expected rate of transmission losses merely by ordering nodes according to their most recent channel state information, without any additional energy overhead. In this paper, we focus on an extended TDMA setting where the length of each TDMA round is greater than the number of nodes, i.e. a number of ´spare´ slots are reserved for retransmissions of lost packets. We consider delay-constrained data transmissions based on a Markov model of the wireless channels. We evaluate several variable scheduling strategies in terms of the trade-off between the energy consumption due to the additional retransmissions and the reduction of the loss rate. We show how the best strategy depends on the energy budget of the sensor nodes, and demonstrate the considerable reduction in the loss rate (up to 18%) that can be achieved over a naive approach where static TDMA scheduling is combined with retransmission slots at the end of the round.