How Many Small Cells Can Be Turned off via Vertical Offloading under a Separation Architecture?

IEEE Trans. Wireless Commun.
Published Date:2015-6
To further improve the energy efficiency of
heterogeneous networks, a separation architecture called
hyper-cellular network (HCN) has been proposed, which
decouples the control signaling and data transmission functions.
Specifically, the control coverage is guaranteed by macro base
stations (MBSs), whereas small cells (SCs) are only utilized for
data transmission. Under HCN, SCs can be dynamically turned
off when traffic load decreases for energy saving. A
fundamental problem then arises: how many SCs can be
turned off as traffic varies? In this paper, we address this
problem in a theoretical way, where two sleeping schemes (i.e.,
random and repulsive schemes) with vertical inter-layer
offloading are considered. Analytical results indicate the
following facts: (1) Under the random scheme where SCs are
turned off with certain probability, the expected ratio of
sleeping SCs is inversely proportional to the traffic load of
SC-layer and decreases linearly with the traffic load of
MBS-layer; (2) The repulsive scheme, which only turns off the
SCs close to MBSs, is less sensitive to the traffic variations; (3)
deploying denser MBSs enables turning off more SCs, which
may help to improve network energy-efficiency. Numerical
results show that about 50% SCs can be turned off on average
under the predefined daily traffic profiles, and 10% more SCs
can be further turned off with inter-layer channel borrowing.

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