A distributed scheduling algorithm for heterogeneous cache-enabled small cell
networks using ADMM
Zhilong Zhang, Danpu Liu
Beijing Key Laboratory of Network System Architecture and Convergence
Beijing University of Posts and Telecommunications, Beijing, P. R. China 100876
jerome.zl.zhang@gmail.com, dpliu@bupt.edu.cn
Abstract—We consider designing a distributed scheduling
algorithm for multi-homing users in heterogeneous cache-
enabled small cell networks. The aim is to minimize the total
operation cost on condition that all users’ QoS requirements
are satisfied. The proposed algorithm is based on alternating
direction method of multipliers (ADMM) and can be carried
out in each small cell base station (SCBS) independently.
Simulation results show that the operation cost converges to the
global optimum after a few iterations and the proposed algo-
rithm outperforms a conventional distributed method largely.
Keywords-Distributed scheduling, cache-enabled small cell,
multi-homing, ADMM
I. INTRODUCTION
Recently, mobile traffic grows dramatically in wireless
networks and is predicted to increase 13-fold from 2012
until 2017 [1]. It has been observed that a great amount of
the transmitted data is a small portion of popular videos.
To relieve the burden of networks, operators deploy cache-
enabled small cell base stations (SCBSs). With the help
of cache-enabled SCBSs, popular files can be downloaded
directly from the caches with a certain probability, rather
than downloaded from the costly backhaul links. Recent
years have also witnessed an increasing growth of user
equipments (UEs) which are equipped with multiple wireless
interfaces. They are usually called multi-homing UEs [2–
4]. When moving into an area covered by heterogeneous
small cell networks, they can be possibly associated to and
download from multiple SCBSs simultaneously.
Many previous academic literature focuses on the caching
strategies and aim at maximizing some utilities of networks
or users, such as maximizing the cache hit rate [5] and
enhancing the users QoS by offloading traffics to local
device-to-device links [6]. Power saving is also considered
in cache-enabled networks [7]. In most of the works, it
is usually assumed that a UE’s average downloading rate
is pre-known and fixed during the file downloading period
[4, 8]. However, in fact, the rate can be variable and depends
on the scheduling strategy adopted by the SCBSs. For
example, if a requested file is cached by a SCBS but the
cost to transmit the file through wireless channels is too
large, then it is better to offload the traffic to a proper SCBS,
even though the requested files have to be downloaded from
backhaul links. Therefore, scheduling strategies should be
considered in addition to the caching methods.
Moreover, when the cache-enabled small cell networks
meet multi-homing UEs, distributed scheduling methods
considering both the cache-enabled and multi-homing char-
acteristics of network nodes are needed. Although several
researches have studied the resource allocation algorithm for
multi-homing service in heterogeneous networks [2–4], none
of them aim to minimize the network operation cost in the
presence of caches at SCBSs.
In this paper, our problem arises as how to allocate
wireless resources to multi-homing UEs in heterogeneous
cache-enabled small cell networks. We consider a scenario
that a group of UEs are served by a set of cache-enabled
SCBSs from different operators. An optimization problem is
formulated in terms of minimizing the total operation cost.
A distributed optimal scheduling strategy for the SCBSs is
proposed based on alternating direction method of multipli-
ers (ADMM) [9] which is a distributed algorithm widely
discussed recently.
The rest of the paper is organized as follows. Section
II describes the system model and elaborates the problem
setting. In Section III, a distributed algorithm based on
ADMM is proposed by solving the optimization problem.
Simulation results verify our proposal in Section IV and a
summary concludes the paper in Section V.
II. SYSTEM MODEL AND PROBLEM FORMULATION
A. Network model
We consider a region covered by a group of cache-enabled
SCBSs denoted by M = {1, 2, ..., M}. They are connected
to the core network via limited backhaul links. In this region,
there exists a set of multi-homing UEs N = {1, 2, ..., N}
which are able to access multiple SCBSs simultaneously as
shown in Fig. 1. Assume that the nth UE has been associated
with a set of SCBSs denoted by M
n
, where M
n
⊆ M, and
the mth SCBS serves a set of UEs denoted by N
m
, where
N
m
⊆ N . Obviously, if n ∈ N
m
, then m ∈ M
n
, and vice
versa.
If all the resources of SCBS m are allocated to UE n, the
downloading data rate of UE n is calculated by
r
m,n
= W
m
log
2
(1 + SIN R
m,n
),
(1)