1. Introduction
With the rapid development of smart phones and mobile
internet applications, mobile users require ever increasing
wireless transmission data rates. According to the CISCO
Visual Network Index (VNI) report, the global mobile data
traffic will increase 13-fold between 2012 and 2017, growing
at a compound annual growth rate of 66%, and reaching 11.2
exabytes per month by 2017 [1]. To meet the explosive
mobile data traffic demand, various measures have been
taken to enhance the system capacity, such as more
spectrum, smaller cells, employing advanced wireless technologies
such as orthogonal frequency division multiplexing
(OFDM) [2] and multiple-input multiple-output (MIMO).
Nevertheless, the peak data rate from 2G to 4G system
has only increased at a compound annual growth rate of 55%
[1]. There is a big gap between the traffic demand and
network capacity and the gap is further growing.
In future 5G mobile communication systems, a number of
promising techniques have been proposed to support a three
orders of magnitude higher network load compared to what
operators are experiencing today. One possible approach is
to increase the density of wireless access points [3].
However, if the conventional mobile network architecture
based on distributed base stations (BSs) is employed, more
and more BSs will be needed, which brings mobile operators
big challenges such as