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AHybridApproach for Fair LTE-U and Wi-Fi Coexistence:
ALiterature Review
Arin Thaokloy
athaoklo@masonlive.gmu.edu
Telecommunications, Volgenau School of Engineering
George Mason University
Abstract
LTE-U is an upcoming technology to increase the
mobile network capacity. The main issue of this
technology is how LTE-U traffic coexists
harmoniously with other Radio Access Technologies
(RAT), particularly, how to maintain the Wi-Fi QoS
while LTE-U is coexisting in the same shared
spectrum. This paper reviews the existing proposed
solutions for dealing with the projected impact of
LTE-U on Wi-Fi performance, analyzes each
solution and points out some drawbacks of each
solution. This paper also offers a hybrid fair
approach which could achieve each solution’s
weakness
I. Introduction
Mobile broadband operators across the world are
facing the challenge of supporting the exponentially
increasing demand for mobile data communications as
well as Internet of Things (IoT), thus trying to occupy
spectrum as much as possible. However, licensed
spectrum for mobile communication carriers becomes
more competitive and expensive. As a result, there have
been attempts to utilize unlicensed spectrum, such as Wi-
Fi bands, for the mobile broadband network. For now,
LTE-U coexistence is considered as a key technology that
could enhance the mobile broadband network, gaining
capacity, roaming between outdoor cellular networks and
indoor networks seamlessly, thus improving the
satisfaction of user experience [1]–[3]. According to [1],
[4]–[6] for LTE-U co-existence, LTE traffic can operate
not only its licensed bands but also other unlicensed
bands, especially 5 GHz being utilized for Wi-Fi
networks.
However, the main issue of LTE-U coexistence is
how LTE operates on the same shared bands with the
Wi-Fi fairly without the Wi-Fi degradation [1], [2], [6].
To avoid the impact on Wi-Fi systems while LTE traffic
co-exists in unlicensed spectrum, according to [1], [5],
LTE-U systems need to overlay either the Listen-Before-
Talk (LBT) technique or the Carrier Sensing Adaptive
transmission (CSAT) mechanism on Dynamic Channel
Selection (DCS). [2] proposed another similar mechanism
called LTE muting which limits LTE transmission on
shared unlicensed bands only in a fraction of time.
Interestingly, [7] proposed the adaptive user and
bandwidth allocation (AUBA) framework for LTE
mobile data offloading in unlicensed spectrum. In this
framework, Wi-Fi users could be transferred into LTE-U
systems based on selective criteria while the Wi-Fi
system needs to relinquish reasonably some unlicensed
time slots to serve the transferred Wi-Fi users [7]. Despite
the fact that many research studies [1]–[3], [5], [7], [8]
proposed multiple approaches to protect the quality of
service in the Wi-Fi network, there are rarely any studies
that consider these approaches by basing on practical
implementations and realistic situations
The section II explains how LTE-U can affect the
Wi-Fi performance. The section III reviews different
existing proposed solutions for dealing with the projected
impact of LTE-U coexistence on Wi-Fi performance and
investigates the limitations of each solution. Also, this
article considers the feasibility of a hybrid approach, as
discussed in the section IV, which could be for most
promising with significantly increasing LTE-U
throughput yet the most minimal negative impact on Wi-
Fi performance in realistic scenarios.
II. The Impact of LTE-U on Wi-Fi Performance
Operating LTE in an unlicensed band (5GHz)
becomes challenging among the Wi-Fi network since
LTE and Wi-Fi use different Media Access Control
(MAC) protocols [2], [3]. LTE can access channels in
accordance with a non-contention MAC protocol which
allows all LTE traffic to continuously occupy entire band

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with multiple orthogonal subcarriers simultaneously. That
means there is rarely any idle period in LTE transmission
channels. However, Wi-Fi depends on a contention-based
MAC protocol which allows Wi-Fi traffic from only one
user to occupy a channel with a fraction of time while
other user traffic needs to wait until the channel becomes
idle. This idle period opens opportunity up for all user
traffic to compete for channel transmission in the next
fraction of time. Consequently, when LTE coexists with
Wi-Fi in an unlicensed band, Wi-Fi can hardly discover
an idle state, thus leading to less Wi-Fi transmission
probability [2], [3], [7], [9].
According to the simulation result in [2] as
shown in figure 1, When we put both systems on the
same shared frequency band, we observe that when the
load is increased, LTE performance suffers only a minor
served load degradation, while WLAN performance drops
significantly. This illustrates clearly that bringing both
systems to the same shared frequency band without
handling the co-existence has a huge negative impact on
the WLAN system performance.
III. Existing Proposed Solutions and Limitations
Even though 3GPP release12, 13, and 14 have
specified protocols for LTE-U, there have been still
discussing to enhance the LTE-U system. Most research
studies tend to focus on how to mitigate the impact of
LTE-U on Wi-Fi performance. This section reviews four
solutions discussed mostly in many relevant research
articles in [1]–[5], [7]–[9] along with investigating the
limitations of each solution.
A. Dynamic Channel Selection (DCS)
In 12/23/2016 3:10:00 AM, DCS mechanism is
prioritized as the first-step method to avoid interference
and sharing a channel with other users. Due to very wide
bandwidth with a large number of channels in the
unlicensed 5 GHz band, there are probably vacant
channels in which no user is occupying [1]. To find a
vacant channel for data transmission, thus, DCS method
helps LTE-U devices to scan multiple channels in the
unlicensed shared band , even periodically detect and
dynamically switch to a new channel with the least
interference [1], [2], [5]. However, [5] points out that in
highly dense access networks with a large number of
users and all-time demand of data traffic, there could be
no vacant channel, so other methods are also needed to
allow LTE-U to fairly share channels with Wi-Fi.
B. Listen-Before-Talk (LBT)
As the limitation of the previous solution, DCS,
in the 5 GHz band highly-dense deployment, [2], [4], [5]
proposes LBT as an additional method that is utilized
after DCS to avoid collision between LTE-U traffic and
Wi-Fi traffic. Under LBT, both LTE-U and Wi-Fi devices
need to sense a channel whether there is another
transmission in the channel or not and begin data
transmission in a selected channel only when the channel
is idle—no other activities in that channel [2], [4], [5].
However, [10] views some concerns in this
Figure 1: compare LTE and WLAN performance in standalone and shared band
cases.
Figure 2: illustrate DCS and LBT mechanism

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solution. Namely, when a lot of LTE-U traffic contends
all-time against Wi-Fi to occupy a channel, it tends to
oppress Wi-Fi such that Wi-Fi has little chance to access
the channel since LTE is more efficient than Wi-Fi to
utilize the unlicensed spectrum [10].
C. Carrier Sensing Adaptive Transmission (CSAT)
Instead of the LBT mechanism, [2], [5] propose
the CSAT mechanism to reduce having an advantage of
LTE-U over Wi-Fi. In other words, CSAT decreases the
opportunities of LTE-U to use shared channels. CSAT is
based on the Time Division (TD) Resource Management
for which LTE-U is switched into ON and OFF state in
each cycle. At the ON state, the LTE-U has an
opportunity to utilize the shared unlicensed band while at
the OFF state, the LTE-U cannot transmit data over the
unlicensed band yet can still offload data on licensed
bands, allowing only Wi-Fi traffic over the unlicensed
band. The period of ON and OFF state are adaptively
adjusted based on the sensed channel activities of Wi-Fi
during OFF period [2], [5].
However, by considering CSAT mechanism in
the realistic small cell in which there are several mobile
operators providing LTE-U service in the same small cell,
this research study observes that CSAT might not
preserve enough space for Wi-Fi transmissions because
different LTE-U operators are asynchronous in CSAT.
Assume that two mobile providers (A and B) operate
LTE-U in the same building, the LTE-U ON period of
operator A might overlaps with the LTE-U OFF period of
operators B, thus blocking Wi-Fi transmissions during the
LTE-U OFF period of operator B
D. Adaptive Transferred User and Resource Allocation
(AURA) framework
Basically, this framework proposed by [7] can be
considered as a user distribution management. It also
needs some above mechanisms that are DCS and either
LBT or CSAT to avoid collision and interference. In this
framework, when hybrid users are transferred from a Wi-
Fi connection into an LTE-U connection, some
unlicensed resources (time slots) are relinquished to the
LTE-U network simultaneously in order to support
transferred users [7]. This method can bring mutual
benefits for both LTE and Wi-Fi users since LTE is more
efficient than Wi-Fi to utilize the unlicensed spectrum,
thus boosting overall throughput and reducing Wi-Fi
congestion. According to the simulation by [7], this
solution shows very impressive results. It improves both
individual and overall throughput, and the Wi-Fi
performance is absolutely protected.
By examining the AURA framework when
implemented in a complex small cell, however, this
solution is quite not suitable for realistic implementation.
To achieve this AURA framework, all Wi-Fi networks in
a small cell need to belong to any mobile network
provider operating LTE-U network in the small cell [4],
[8]. In a complex small cell, such as shopping centers,
there are many individual Wi-Fi networks in the small
cell. It is impossible that mobile operators will transfer
un-subscribers using any individual Wi-Fi network into
their LTE-U network. Another real manner is that most
users don’t want to connect to an LTE network if any Wi-
Fi network is available because of the limitation of
maximum data subscription; most users subscribe a
mobile network with an affordable payment that limits
their data usage. Thus, a few hybrid users would be
transferred from a Wi-Fi network into an LTE-U network.
Figure 3: illustrate CSAT mechanism
Figure 4: illustrate AURA framework

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Therefore, this framework is not suitable for practical
implementation.
IV. Discussion for A Hybrid Approach and Suggestion
for Future Works.
As pointed out in the previous section, LBT,
CSAT, and AURA have some gaps that could drop the
Wi-Fi performance. LBT might not absolutely preserve
Wi-Fi performance when a lot of LTE-U and Wi-Fi
traffic competes each other to use any channel. CSAT
mechanism is not suitable for when there are more than
one LTE-U networks coexisting with Wi-Fi networks in
the same small cell. AURA framework is inappropriate
for realistic situations when there are many individual
Wi-Fi networks in the same small cell.
To fill the gaps of each solution, some solutions
should rely on each other—interworking together. LBT
and CSAT mechanism should not be operated solely.
Therefore, resolving the drawbacks of the existing
proposed solution, this article offers a hybrid approach in
which LBT is integrated into CSAT for better
maintaining the QoS of Wi-Fi. LBT should be embedded
in CSAT in order to preserve the opportunities of Wi-Fi
to contend to occupy any channel while overlapping each
other of CSAT from different LTE-U operators.
Figure 5: illustrate the hybrid approach
The methods in this hybrid approach are visually
explained in figure 6. LTE-U devices employ DCS first to
find an unused channel. If there is no unused channel and
LTE-U traffic need to share a channel with Wi-Fi traffic,
the shared channel’s bandwidth need to be distributed for
both LTE-U traffic and Wi-Fi traffic by using CSAT
mechanism allocating a LTE-U OFF period for only Wi-
Fi traffic and a LTE-U ON period in which both LTE-U
and Wi-Fi traffic can contend to use the channel for data
transmission.
However, this offered hybrid approach has not
proved yet by any numerical analysis and modeling
simulation. To ensure this approach, future research
studies should simulate this approach with
systematic models for possible realistic situations.
Moreover, telecom authorities need to standardize
this technology for globally practical
implementation.
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