  The backhaul of hundreds of light fidelity (LiFi) base stations (BSs)
constitutes a major challenge. Indoor wireless optical backhauling is a novel
approach whereby the interconnections between adjacent LiFi BSs are provided by
way of directed line-of-sight (LOS) wireless infrared (IR) links. Building on
the aforesaid approach, this paper presents the top-down design of a multi-hop
wireless backhaul configuration for multi-tier optical attocell networks by
proposing the novel idea of super cells. Such cells incorporate multiple
clusters of attocells that are connected to the core network via a single
gateway based on multi-hop decode-and-forward (DF) relaying. Consequently, new
challenges arise for managing the bandwidth and power resources of the
bottleneck backhaul. By putting forward user-based bandwidth scheduling (UBS)
and cell-based bandwidth scheduling (CBS) policies, the system-level modeling
and analysis of the end-to-end multi-user sum rate is elaborated. In addition,
optimal bandwidth scheduling under both UBS and CBS policies are formulated as
constrained convex optimization problems, which are solved by using the
projected subgradient method. Furthermore, the transmission power of the
backhaul system is opportunistically reduced by way of an innovative fixed
power control (FPC) strategy. The notion of backhaul bottleneck occurrence
(BBO) is introduced. An accurate approximate expression of the probability of
BBO is derived, and then verified using Monte Carlo simulations. Several
insights are provided into the offered gains of the proposed schemes through
extensive computer simulations, by studying different aspects of the
performance of super cells including the average sum rate, the BBO probability
and the backhaul power efficiency (PE).
