Multiuser Gain in Energy Harvesting Wireless Communications

In traditional wireless communications, multiuser diversity gain comes from the diversified channel power gains across different users. In energy harvesting wireless systems, the various energy levels at different transmitters may lead to another type of diversity gain. In this paper, <italic>multiuser gain</italic> with the emphasis on <italic>energy diversity</italic> is studied for multiuser energy harvesting communications, where the scaling law of the expected throughput over the number of users is investigated. Three access schemes are considered: two centralized schemes (fixed TDMA, where users transmit in a fixed order; and energy-greedy, where the user with the highest energy level is picked for transmission) and a contention-based distributed scheme (where each user contends for the channel with a certain probability). Under both centralized schemes, it is shown that the expected throughput scales on the order of <inline-formula> <tex-math notation="LaTeX">$\log (N)$ </tex-math></inline-formula>, where <inline-formula> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> is the number of users. For the distributed scheme, the scaling of throughput depends on the contention probability. Particularly, when each user contends the transmission with probability <inline-formula> <tex-math notation="LaTeX">$1/N$ </tex-math></inline-formula>, the throughput also scales on the order of <inline-formula> <tex-math notation="LaTeX">$\log (N)$ </tex-math></inline-formula> but with a discount factor <inline-formula> <tex-math notation="LaTeX">$1/e$ </tex-math></inline-formula>. Our analytical and numerical results reveal that compared with the point-to-point energy harvesting communication system, the multiuser throughput gain comes from two aspects: the power gain due to the increase of total energy arrivals; and the diversity gain due to the increase of energy arrival dynamics.