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A Survey and Comparison Of Device-To-Device
Architecture Using LTE unlicensed Band
Bushra Ismaiel
∗
, Mehran Abolhasan
†
, David Smith
‡
, Wei Ni
§
, Daniel Franklin
‡
∗
School of Computing and Communications Faculty of Engineering and Information Technology (FEIT),
University of Technology, Sydney, Australia
bushra.ismaiel@student.uts.edu.au,
†
mehran.abolhasan@uts.edu.au
†
david.smith@data61.csiro.au,
§
Wei.Ni@csiro.au
†
daniel.franklin@uts.edu.au
Abstract—Due to the rapid increase in data traffic, one of the
solutions provided by mobile operators is to operate Long Term
Evolution (LTE) in the unlicensed 5GHz band, as the licensed
spectrum is becoming scarce. Mobile operators can expand their
network capacity by operating LTE in the unlicensed band at
lower cost when compared with using other licensed bands.
Device to Device (D2D) communication, proven to be another
effective way to enhance the capacity of a network, enables direct
data exchange of localized traffic of users in proximity. Applying
D2D communication to LTE unlicensed 5GHz band will further
improve the network performance and user experience. In
this article, we will discuss the new type of solutions that
have been proposed for LTE operating in an unlicensed 5GHz
band that includes; LTE-Unlicensed (LTE-U), LTE-License
Assisted Access (LTE-LAA), LTE WiFi Link Aggregation
(LWA), and MuLTEfire. We will discuss the important features
along with their advantages and disadvantages and compare
these technologies as well. We simulate LTE-LAA, LWA and
MuLTEfire technologies in the presence of Wi-Fi hotspot
and compare their results. Furthermore, we apply D2D
communication to these technologies and from the results
we conclude that MuLTEfire can increase the throughput
drastically but network saturates quickly. Whereas, applying
D2D communication with LWA is beneficial for a scalable
network as it will not only increase the network throughput but
will increase the network capacity as well.
Index Terms—Long Term Evolution (LTE), LTE Unlicensed
(LTE-U), LTE-License Assisted Access (LTE-LAA), LTE WiFi
Link Aggregation (LWA), MuLTEfire, Wi-Fi, Device to Device
Communication (D2D).
I. INTRODUCTION
The usage of unlicensed spectrum has provided an excel-
lent opportunity for the mobile operators to meet the data
requirement of mobile users. Mobile operators can expand the
network capacity and provide a better quality of service (QoS)
by using LTE in unlicensed 5GHz band at a lower cost [1].
The concept of LTE in the unlicensed spectrum was intro-
duced first time in the TV white space [2]. In this scheme
frequency hopping and time hopping was used by LTE small
cells in TV white space band to reduce interference between
other devices in the band. Currently, the crucial issue for
LTE networks to exploit 5GHz unlicensed spectrum is the
coexistence problem with Wi-Fi technology [3]. LTE adopts
scheduling based access technology in licensed band, whereas
Wi-Fi works on contention-based access mechanism and car-
rier sensing technology. The LTE unlicensed proposal has been
a big concern for the Wi-Fi vendors and service providers,
which have used 5GHz band for a long time and want to
continue to do so without being unfairly penalized by the
introduction of LTE [1].
Recently new types of LTE-Wi-Fi aggregation solutions
have been proposed at the radio link, as well at the TCP
link [4]. One is carrier Wi-Fi, in which network operators
can deploy their own Wi-Fi to offload traffic and reduce
congestion [5]. LTE unlicensed (LTE-U), is the first version
of LTE unlicensed which was initially proposed by Ericsson
and Qualcomm in 2013, based on 3GPP release 10-12 [1].
LTE-U works on the mechanism of carrier sensing adaptive
transmission (CSAT) and can only be used in China, USA,
South Korea and India [6]. For worldwide deployment,
LTE unlicensed needs to deploy Listen-Before-Talk (LBT)
mechanism. LTE-Licensed Assisted Access (LTE-LAA)
has been standardised by 3GPP in release 13 for downlink
operation, which uses the mechanism of LBT and is the
modified version of LTE-U [7]. It is believed that if LTE-LAA
and Wi-Fi use the same spectrum, LTE-LAA will have more
chance to dominate the spectrum and Wi-Fi devices will keep
deferring to LTE-LAA transmissions [4]. Wi-Fi users will
easily be interfered by LTE-LAA users as soon as LTE-LAA
signal becomes stronger, hence degrading the performance
of both networks. An alternative solution was proposed to
LTE-U/LTE-LAA, called as LTE Wi-Fi link aggregation
(LWA). A part of LWA is released by 3GPP in release 13
[8]. LWA increases the capacity by offloading some of the
LTE traffic through Wi-Fi using CSMA protocol. Some
researchers believe that LWA will perform better in terms of
co-existence than LTE-U/LTE-LAA in the presence of Wi-Fi
as it offloads the traffic using the same protocol as Wi-Fi,
hence sharing the spectrum fairly [4]. Also Wi-Fi companies
are much in favour of LWA as it boosts the LTE user data
without affecting pre-existing Wi-Fi users. Alcatel-Lucent is
collaborating with Qualcomm to demonstrate LWA at Mobile
World Congress [9]. Currently, Qualcomm proposed another
LTE based technology known as Multifire that solely operates
in an unlicensed band and does not require an anchor in the
licensed spectrum. [10].
LTE Anchor
Primary Cell
WiFi AP
Secondary Cell
Macro Cell
Licensed Band
Unlicensed
Band @5Ghz
LTE and WiFi
Modem
WiFi
LTE
LTE Anchor
Primary Cell
Macro Cell
Licensed Band
Unlicensed
Band
@5Ghz
LTE Modem
Small Cell
Link
Aggregates
Carrier
Aggregates
LTE
LTE
(Uses LBT to
access channel)
LTE WiFi Aggregation
(LWA)
LTE Licensed Assisted Access
Aggregation (LAA)
Unlicensed
Band @5Ghz
Uses LBT to
access channel
No Anchor Node in licensed
band require.
Co-exist with WiFi and LAA
MuLTEfire
(a)
(b)
(c)
Fig. 1: LTE in unlicensed bands: LWA, LAA, MuLTEfire
Device to Device (D2D) communication an integral part of
the future 5G standard, aims to improve the capacity, spectral
efficiency, and coverage by enabling direct communication
between the devices with minimal involvement from the base
station (BS) in infrastructure assisted cellular system. D2D
communication has been a central topic of research for quite
some time and it has been shown that D2D communication
increases the network throughput while minimizing the
interference and power usage [11]. D2D communication
can be very beneficial for the mobile operators when they
combine D2D with LTE technologies operating in unlicensed
5GHz band.
Our main contribution in this paper is that we give a detail
overview of LTE technologies operating in an unlicensed
5GHz band that includes; carrier Wi-Fi, LTE-U, LTE-LAA,
LWA, and MuLTEfire. To the best of our knowledge, no
paper has been published yet that discussed a detail overview
of these technologies that includes; architecture, working
mechanism, co-existence with Wi-Fi and overall cost. We also
compare these technologies and conclude their advantages
and disadvantages. Furthermore, we also present a detail
simulation for LTE-LAA, LWA and MuLTEfire technologies
in the presence of Wi-Fi hotspot and compare their results.
We then investigate the performance of D2D communication
over these strategies. It is concluded from the results that
MuLTEfire can increase the throughput of users drastically
but network saturates quickly. For a scalable network it will
best to use D2D with LWA as it increases the throughput and
can increase the capacity of the network as well.
II. STATE OF ART: LTE NETWORKS OPERATING IN
UNLICENSED BANDS
In this section, we will investigate the different solutions
provided for LTE operating in an unlicensed 5GHz band.
We will discuss their architecture, working mechanism, cost
and co-existence problem with Wi-Fi. Furthermore, we will
compare these technologies with their advantages and disad-
vantages.
A. Carrier Wi-Fi
Carrier Wi-Fi is considered to be the first step for mobile
operators to utilize the unlicensed bands. Network operators
can deploy their own Wi-Fi access points (AP) to further
offload traffic, increase coverage and reduce congestion.
Carrier Wi-Fi can lead to ineffectiveness of network man-
agement and low spectrum efficiency as it adopts different
access and management mechanisms from LTE networks [5].
B. LTE-Unlicensed (LTE-U)
LTE-U uses the same LTE channels as LTE CA but unlike
LTE CA LTE-U operates in unlicensed band at 5GHz [4]. In
LTE-U, only LTE channel can work as the primary channel
whereas the unlicensed channel can work as the secondary
channel. LTE-U works on the mechanism of carrier sensing
adaptive transmission (CSAT), where the channel is sensed by
the LTE devices for a longer period of time (up to 200ms) and
based on the activities define an ON and OFF duration for a
duty cycle [5]. LTE devices can transmit when the duty cycle
is in ON mode and remains silent when the duty cycle is in
OFF mode. In the OFF mode, LTE-U allows the WiFi devices
to access the system.
LTE-U requires a new installation of 5 GHz LTE-enabled
hardware on the device and small cells. LTE-U will only be
implemented in the regions where regulation does not require
LBT, such as China, Korea, India, and USA [6]. The biggest
issue of LTE-U is its intrusion to Wi-Fi, as it can block the
access to Wi-Fi users by giving priority to LTE-U users.
Carrier Wi-Fi LTE-U LTE-LAA LWA MuLTEfire
Features
Deploy operator
own Wi-Fi
LTE operates in
unlicensed 5GHz
band using CSAT
LTE operates in
unlicensed 5GHz
band using LBT
LTE operates in
unlicensed 5GHz
band aggregated
with carrier Wi-Fi
LTE only
operates
in unlicensed
5GHz band
using LBT
Advantage Easy to deploy
Unified
management
mechansim
Unified management
mechanism.
Global standard.
Easy to implement.
Quick for
commercialization
Unified
management
mechansim in
unlicensed band
Disadvantage
Lack the
performance
benefits from
LTE in
unlicensed band
Contention
problem with
WiFi.
Applicable
where CSAT
mechanism is
applied. No global
standardisation
Contention problem
with Wi-Fi.
Commercialization
will take longer time
For better QoS
latency
of the link between
the LTE eNB
and Wi-Fi AP
should be kept
low.
Contention
Problem
with Wi-Fi.
Need to replace
or upgrade
old Wi-Fi AP
to get LTE
performance.
Access
Network
Cost
Medium
(New Wi-Fi
AP)
High (New LTE
enabled
cell with LTE-U)
High (New LTE
enabled cell with
LTE-LAA)
Medium (New
small Cell LWA
aware Wi-Fi AP)
High (New
LTE enabled
Cell)
New Hardware
Support
No
UE and eNB
(5GHz)
UE and eNB
(5GHz)
No
UE and eNB
(5GHz)
LTE-Wi-Fi
Coexistence
Problem
No
Contention
Problem
Contention
Problem
No
Contention
Problem
Global Problem No
Global
Harmonization
Required No
Global
Harmonization
Required
Standardisation 3GPP
LTE-U
forum/3GPP
3GPP 3GPP 3GPP
TABLE I: Comparison between Carrier Wi-Fi, LTE-U, LTE-LAA, LWA and MuLTEfire
C. LTE-Licensed Assisted Access (LTE-LAA)
LTE-LAA is the modified version of LTE-U and adopts LBT
mechanism for the coexistence of LTE-LAA and Wi-Fi, as
shown in Figure 1(a). In LBT, LTE device first listens to check
for ongoing transmission. If the channel is clear it will transmit
for a period of time and then back-off to re-check the channel
availability, but if the channel is busy it will not transmit and
keep on listening unless the channel is available [5]. The LTE
device examine another channel if the current channel is busy
after several attempts [5].
Two options for LBT schemes have been suggested by
the European Union; Frame-Based Equipment (FBE) and
Load-Based Equipment (LBE) [4]. In FBE,the transmit/receive
structure is not directly demand-driven but has fixed timing.
After every frame, clear channel assessment (CCA) is checked
if the channel is free the data is transmitted otherwise it has to
wait for another frame period. In LBE LBT, CCA is performed
whenever there is data to transmit. If the channel is available
data is transmitted but if the channel is busy it will retransmit
the data after the back-off time during extended CCA (eCCA)
[4].
LTE-LAA is developed with the single global solution
framework and is replacing the current terminology LTE-U
[12]. LTE-LAA is the version of LTE in an unlicensed band
that 3GPP standardizes in Release 13 and is set to become a
global standard as it strives to meet regulatory requirements
worldwide [6]. As LTE-LAA is the modified version of LTE-
U, it also requires an additional installation of 5GHz LTE-
enabled hardware.
The biggest concern for LTE-LAA is the co-existence with
Wi-Fi technology. Qualcomm and Ericsson tested and claimed
that LTE-LAA operates with little interference with Wi-Fi and
can increase the system capacity with faster mobile broadband
speed [13]. Whereas, some of the companies like Google are
not in favour of LAA as it can block the access to Wi-Fi users
and will degrade the overall network throughput of Wi-Fi by
giving priority to LTE-LAA users [14].
D. LTE WiFi Link Aggregation (LWA)
LWA emerged as an alternative technology to LTE-U and
LTE-LAA and is capable of leveraging existing Wi-Fi access
points (AP) to improve the network performance [5]. LWA
architecture is shown in Figure 1(b).
For the transmission of LTE traffic, LWA uses unlicensed
band similar to LTE-U and LTE-LAA but the transmission
is done through Wi-Fi. Unlike LTE-U/LAA, LWA does not
require a new LBT based protocol, but LWA uses the current
Wi-Fi protocol to transmit the LTE traffic. LWA base station
performs scheduling of packets at PDCP layer and transmits
some over LTE and other on Wi-Fi after encapsulating in
Wi-Fi frames. All the packets received from LTE and Wi-
Fi are then aggregated at PDCP layer of LWA UE. LWA base
stations can improve the LTE performance by managing the
radio resources according to the load and RF conditions [4].
During this transmission, Wi-Fi APs can use LTE core network
for authentication, security, billing, etc, without a dedicated
Gateway (GW) and without disturbing the native Wi-Fi APs.
LWA uses LTE on LTE band and Wi-Fi on Wi-Fi band
which is not the case in LTE-U/LTE-LAA. LWA requires the
deployment of small cells and other WiFi APs near it can get
a software upgrade to support LWA. The Wi-Fi APs can also
support non-LWA traffic by using separate service set identifier
(SSID) [5]. According to some researchers, LWA is a solution
that leverage’s the existing Wi-Fi APs and does not impact on
an unlicensed band, hence improving the network performance
[4].
E. MuLTEfire
MuLTEfire is based on 3GPP LTE-LAA and LTE-eLAA
(LTE-enhanced licensed Assisted Access). Figure 1(c) shows
the architecture for MuLTEfire. MuLTEfire performs like LTE-
LAA and LTE-eLAA only the difference is that it operates in
an unlicensed band without a licensed anchor node.
According to Qualcomm, MuLTEFire will benefit mobile
operators with new deployment opportunities for offloading
their mobile network traffic [10]. The primary goal for MuL-
TEfire is to provide a seamless user experience in a hyper-
dense network by combining the benefits of LTE technology
with the simplicity of WiFi like deployments. It is a solution
that may be attractive to network operators and cable operators
that lack licensed spectrum, although this mode has not been
discussed in 3GPP [6].
A comparison among the different LTE technologies op-
erating in an unlicensed band; Carrier Wi-Fi, LTE-U, LTE-
LAA, LWA, and MuLTEfire is shown in Table 1. It is easier to
deploy carrier Wi-Fi as the network operator can easily deploy
their own Wi-Fi to offload traffic but the spectrum efficiency
cannot be same as of LTE as Wi-Fi and LTE works on different
mechanism. LTE-U/LTE-LAA carrier aggregates the licensed
and unlicensed band to further improve the performance,
whereas LWA aggregates the link of licensed and unlicensed
band and MuLTEfire solely operates in unlicensed band. The
major disadvantage of LTE-U, LTE-LAA and MuLTEfire is
the contention problem with Wi-Fi as compared to LWA.
LTE-LAA and MuLTEfire are still in the process of getting
standardised.
III. SIMULATION
In our simulation, we consider a single LTE macro cell
with small cell deployed in it. This small cell is LTE-
LAA/LWA/MuLTEfire, taking one at a time for simulation. We
have also considered a single Wi-Fi AP deployed in the same
LTE macro cell whose coverage area overlaps with the same
small cell; LTE-LAA/LWA/MuLTEfire. The small cell (LTE-
LAA/LWA/MuLTEfire) and Wi-Fi share the same 20 MHz
unlicensed sub-band within the 5 GHz band. The number of
Wi-Fi users are fixed and are associated with Wi-Fi AP. It is
assumed that the number of LTE-LAA/LWA/MuLTEfire users
are randomly distributed around the small cell. No handover
are considered in this paper.
Offered Load (Mbps)
5 10 15 20 25 30 35
Throughput (Mbps)
20
40
60
80
100
120
140
160
LTE
LWA
LAA
MULTE
Fig. 2: Total Network Throughput vs Load
Throughput graph versus load is shown in Figure 2. MuL-
TEfire has higher throughput as it operates entirely in an
unlicensed band using LBT and has more chance to capture the
channel and transmit data. MuLTEfire saturates quickly as can
be seen from the graph and might not be suitable for a scalable
network as it can not accommodate more users. Whereas it
can be observed from the graph that LWA can provide higher
throughput when the load increases as compared to LTE-LAA.
LTE-LAA and MuLTEfire can be a better solution for small
loads as the throughput is higher whereas for higher load LWA
can perform better.
No. of workstations
20 30 40 50 60 70 80 90
Wi-Fi Throughput (Mbps)
5
10
15
20
25
30
35
40
45
50
55
LWA
LAA
MULTE
Fig. 3: Effect of Wi-Fi vs No of users
Figure 3 shows the effect of Wi-Fi users in the presence of a
small cell; LTE-LAA, LWA, and MuLTEfire. The throughput
of Wi-Fi users degrades in the presence of LTE-LAA small
cell, as preference is given to LTE-LAA users over Wi-Fi
users. This is because LTE has a continuously as well as
a periodically transmitting protocol to transfer a variety of
control and reference signals whereas Wi-Fi is designed to
coexist with other technologies through random backoff and
channel sensing. Due to this Wi-Fi users will have little chance
to sense a clear channel and transmit. Similarly, in case of
MuLTEfire, the Wi-Fi throughput degrades further as it is LTE
that is operating solely in an unlicensed band. Whereas, in
