  Visible light communication (VLC) is envisioned as a core component of future
wireless communication networks due to, among others, the huge unlicensed
bandwidth it offers and the fact that it does not cause any interference to
existing radio frequency (RF) communication systems. Most research on RF and
VLC coexistence has focused on hybrid designs where data transmission to any
user could originate from either an RF or a VLC access point (AP). However,
hybrid RF/VLC systems fail to exploit the distinct transmission characteristics
of RF and VLC systems to fully reap the benefits they can offer. Aggregated
RF/VLC systems, in which any user can be served simultaneously by both RF and
VLC APs, have recently emerged as a more promising and robust design for the
coexistence of RF and VLC systems. To this end, this paper, for the first time,
investigates AP assignment, subchannel allocation (SA), and transmit power
allocation (PA) to optimize the energy efficiency (EE) of aggregated RF/VLC
systems while considering the effects of interference and VLC line-of-sight
link blockages. A novel and challenging EE optimization problem is formulated
for which an efficient joint solution based on alternating optimization is
developed. More particularly, an energy-efficient AP assignment algorithm based
on matching theory is proposed. Then, a low-complexity SA scheme that allocates
subchannels to users based on their channel conditions is developed. Finally,
an effective PA algorithm is presented by utilizing the quadratic transform
approach and a multi-objective optimization framework. Extensive simulation
results reveal that: 1) the proposed joint AP assignment, SA, and PA solution
obtains significant EE, sum-rate, and outage performance gains with low
complexity, and 2) the aggregated RF/VLC system provides considerable
performance improvement compared to hybrid RF/VLC systems.
