Research on Coverage Enhancement of Narrowband M2M
Communications Based on Unlicensed Spectrum
Shaoyi Xu
1
, Dong Wang
1
, Feng Gao
2
, Hantao Li
2
1. School of Electronics and Information Engineering, Beijing Jiaotong University, Beijing, China
2. Huawei Technologies Co., Ltd., Beijing, China
Email: {shyxu, 14120126}@bjtu.edu.cn; {stephen.gao, lihantao}@huawei.com
Abstract—It can be seen that Machine-to-Machine (M2M)
devices will be widely deployed in the near future. However,
because of the scarce spectrum, it is difficult to support such a
large number of devices only depending on the licensed spectrum.
On the contrary, unlicensed spectrums are a better choice for
many enterprises which prefer to build M2M networks
independently. In this paper, we present a narrowband M2M
communications system on the unlicensed spectrum (U-NB-M2M).
The key principles of the system design are reduced energy
consumption, low complexity, high feasibility and reliability. In
addition, this article provides several schemes on how to improve
the coverage of our proposed M2M system. An experimental
platform is built to evaluate the performance of the proposed
system by various metrics and the simulation results prove that
our proposed schemes achieve significant coverage enhancement.
Keywords—M2M; system design; narrowband M2M
communications; unlicensed spectrum; coverage enhancement;
I. I
NTRODUCTION
Machine-to-Machine (M2M) communications enables
machines, vehicles and devices to share and collect data with
little or no human intervention. As the key technology of
Internet of things (IoT), M2M communications can be applied
into many areas, such as in transportation, finance, smart home,
environmental monitoring and smart grids. A steady growth is
forecasted for M2M communications markets that there are
approximately embedded 20 billion M2M devices connections
to offer various M2M applications by the end of year 2020 [1].
Today, most M2M applications use Global System for
Mobile Communication/General Packet Radio Services
(GSM/GPRS) networks. GPRS is a low-cost and well-
established technology and designed for the bursty traffic [2].
Therefore, a GPRS network was suitable for M2M
communications. But the capacity of a GSM/GPRS network
limits the increase of M2M devices in the future. LTE which
offers higher capacity and more flexible radio resource
management (RRM) has been a candidate for M2M
communications. However, LTE has been designed for
broadband applications, while many M2M applications
transmit small amounts of data, may lead to severe network
congestion. Moreover, the characteristics of low power
consumption and diverse QoS requirements should be
considered for M2M communications. Therefore, efforts have
been exerted by 3
rd
Generation Partnership Project (3GPP) to
overcome the deficiencies and coexistence problem between
LTE and M2M communications.
3GPP in Release 13 [3] proposed a narrowband M2M (NB-
M2M) system which uses a minimum system bandwidth of 200
kHz on both uplink and downlink transmission. NB-M2M
achieves good energy efficiency and the 20 dB coverage
improvement compared with that by using GPRS. NB-M2M
can be deployed by reframing a duplex GSM, using a single
Resource Block (RB) in LTE systems, or using a part of the
guard band in LTE systems. However, because of the limitation
of capacity and high cost of licensed spectrums and the
motivation to build up a network independently, many
enterprises tend to build a M2M communications network
based on unlicensed spectrums. Consequently, applications on
the unlicensed spectrum open a huge market for wireless
communication and academic research.
In this paper, to enable enterprises to deploy M2M networks
with a lower cost, we propose a new NB-M2M communications
system on the unlicensed spectrum (U-NB-M2M). Different
from the traditional NB-M2M proposed by 3GPP which is a
Frequency Division Multiple Access (FDMA) based Frequency
Division Duplex (FDD) system, the proposed system is a
FDMA/Time Division Duplex (FDMA/TDD) system in the
unlicensed spectrum. Specifically, several schemes on
designing in transmitters and receivers to further improve the
performance of system coverage are proposed. The
experimental platform is built to evaluate the proposed U-NB-
M2M system. The measurement results prove the efficacy of U-
NB-M2M by comparing with the current NB-M2M and LTE
systems.
The rest of the paper is organized as follows. In Section II,
the physical layer design of the system is proposed. In Section
III, the coverage enhancement performance of the system is
discussed. Simulations setup and performance evaluation of the
proposed system are given in Section IV. Finally, Section V
concludes the paper.
II. S
YSTEM
D
ESIGN
A. Channelization Design
In the unlicensed spectrum, the transmit power is much lower
than that of licensed spectrums (for example, in China it is
below 17 dBm/200 kHz in most free spectrum). Therefore the
system DL transmission is more restrictive. Thus, as shown in
Fig. 2.1, in order to realize power boosting, multiple distributed
channels are proposed where subchannels are divided into
wider bandwidth and the spacing is widened.
978-1-5090-5935-5/17/$31.00 ©2017 IEEE