  In this paper, we propose a new paradigm in designing and realizing energy
efficient wireless indoor access networks, namely, a hybrid system enabled by
traditional RF access, such as WiFi, as well as the emerging visible light
communication (VLC). VLC facilitates the great advantage of being able to
jointly perform illumination and communications, and little extra power beyond
illumination is required to empower communications, thus rendering wireless
access with almost zero power consumption. On the other hand, when illumination
is not required from the light source, the energy consumed by VLC could be more
than that consumed by the RF. By capitalizing on the above properties, the
proposed hybrid RF-VLC system is more energy efficient and more adaptive to the
illumination conditions than the individual VLC or RF systems. To demonstrate
the viability of the proposed system, we first formulate the problem of
minimizing the power consumption of the hybrid RF-VLC system while satisfying
the users requests and maintaining acceptable level of illumination, which is
NP-complete. Therefore, we divide the problem into two subproblems. In the
first subproblems, we determine the set of VLC access points (AP) that needs to
be turned on to satisfy the illumination requirements. Given this set, we turn
our attention to satisfying the users' requests for real-time communications,
and we propose a randomized online algorithm that, against an oblivious
adversary, achieves a competitive ratio of $\log(N)\log(M)$ with probability of
success $1 - \frac{1}{N}$, where $N$ is the number of users and $M$ is the
number of VLC and RF APs. We also show that the best online algorithm to solve
this problem can achieve a competitive ratio of $\log(M)$. Simulation results
further demonstrate the advantages of the hybrid system.
