On optimum time division multiple access for energy harvesting channels

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In this paper, we consider a multiple access channel, where multiple users equipped with energy harvesting batteries communicate to an access point. The users are supposed to share the channel via Time Division Multiple Access (TDMA). In many existing works, it is commonly assumed that the users' energy harvesting processes and storage status are known to all the users before transmissions. In practice, such knowledge may not be readily available. To avoid excessive overhead for realtime information exchange, we consider the scenario where the users schedule their individual transmissions according to the users' statistical energy harvesting profiles. We first show that in the case when each node has an infinite-capacity battery, equal-power TDMA is optimal for throughput maximization. Using Markov chain modeling, we then study the system performance for the finite-capacity battery case under the equal-power TDMA framework. We also consider an equal-time TDMA scheme, which assigns equal-length subslots to each user. It is found that equal-power TDMA always outperforms equal-time TDMA in the infinite-capacity battery case, while equal-time TDMA exhibits compatible or even slightly better performance in some scenarios when the batteries have finite capacities.