Indoor optical wireless power transfer to small cells at nighttime

The application of wireless backhaul communication and power transfer to outdoor small cells (SCs) could significantly decrease their installation cost. In this paper, the concept of indoor optical wireless power transfer to SCs is investigated in the absence of ambient light, i.e., during darkness hours. An experimental study is conducted by the use of up to four red laser diodes (LDs), a crystalline silicon solar panel and cell placed at 5.2 m. A value of 69% is measured for the fill factor of the solar panel. Also, a total power efficiency of 3.2% is measured for an optical wireless (OW) link with an average efficiency of two LDs of 26.8%, a solar cell efficiency of 13.3% and only 10.6% of geometrical losses. A comparison of this link with a state-of-the-art inductive power transfer system shows an improvement of the total power efficiency by 2.7 times. Another OW link is implemented with a divergence of full width at 36.8% of the peak intensity of 3 and 5.75 mrad along the small and large axes of the beam, respectively. The experimental levels of harvested power are in the order of mW, whereas approximately 1 W is required for the operation of a SC. Therefore, a 42 laser-based transmitter is designed both analytically and by the use of the simulation tool Zemax. The respective results show the feasibility of delivering 7.2 W of optical power to a solar cell of up to 30 m distance with geometrical losses of only 2%, but a beam enclosure is also required due to eye safety restrictions.