On-Time Communications Over Fading Channels

We consider the on-time transmissions of a sequence of packets over a fading channel. Different from traditional in-time communications, we investigate how many packets can be received delta -on-time, meaning that the packet is received around an expected slot with a deviation no larger than delta slots. In this framework, we first derive the on-time reception rate of the random transmission scheme when no packet-controlling is used. Our analytical and simulation results show that the on-time reception rate of random transmissions decreases (to zero) with the sequence length. To improve the on-time reception rate, we further propose to schedule the packets by delaying, dropping, or repeating the transmissions. Specifically, we model the packet scheduling problem as a Markov decision process (MDP) and then obtain the optimal scheduling policy using an efficient iterative algorithm. By using the optimal packet scheduling, the on-time reception rate converges to a much larger constant, thus ensuring better on-timeliness. Moreover, we show that the on-time reception rate increases if the target reception interval and/or the deviation tolerance delta is increased, or the randomness of the fading channel is reduced. By optimally scheduling the packets and choosing these parameters, therefore, wireless channels will be more suitable for the latency-sensitive applications.