LTE Aggregation and Unlicensed Spectrum

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LTE Aggregation & Unlicensed Spectrum
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TABLE OF CONTENTS
Preface ..........................................................................................................................................................................1
1 Introduction ..............................................................................................................................................................1
2 Deployment Scenarios ............................................................................................................................................2
2.1
LTE-WLAN Radio Level Aggregation (LWA) ................................................................................................2
2.2
LTE in Unlicensed Spectrum ........................................................................................................................6
2.2.1
Indoor Deployments .................................................................................................................................8
2.2.2
Outdoor Deployments ..............................................................................................................................9
3 Data Offloading with LTE-WLAN Aggregation..................................................................................................... 10
3.1
Architectural Aspects .................................................................................................................................. 10
3.2
User and Control Plane .............................................................................................................................. 12
3.3
Performance Evaluation ............................................................................................................................. 14
3.3.1
Simulation Methodology and Assumptions ............................................................................................. 14
3.3.2
Deployment Scenarios ........................................................................................................................... 14
3.3.3
WLAN Offloading and LWA Solutions Compared................................................................................... 14
3.3.4
Metrics for Optimization.......................................................................................................................... 15
3.3.5
System Performance Results ................................................................................................................. 15
3.3.6
TCP Performance Results ...................................................................................................................... 18
3.3.7
Discussion of Results ............................................................................................................................. 18
3.4
Interworking with Previous Offloading Solutions ......................................................................................... 19
4 LTE in Unlicensed Spectrum................................................................................................................................. 19
4.1
4.1.1
Data Offloading for Operators ..................................................................................................................... 19
Overview ................................................................................................................................................ 19
4.2
Coexistence with WLAN in Adjacent Channels .......................................................................................... 20
4.3
Performance Evaluation for Co-channel LAA and WiFi .............................................................................. 21
5 Conclusion.............................................................................................................................................................. 21
Appendix: LWA Performance Evaluation - Simulation Details .............................................................................. 22
Acronym List .............................................................................................................................................................. 24
Acknowledgements ................................................................................................................................................... 25
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November 2015
PREFACE
A key industry discussion concerns the efficient use of spectrum to serve societies’ connectivity
requirements. A recent solution to the rising wireless data demands, and the topic of this paper, is data
offloading from licensed LTE networks to unlicensed spectrum. Recent work by the 3rd Generation
Partnership Project (3GPP) on offloading to the Wireless Local Area Network (WLAN) via LTE/WiFi
Aggregation and using LTE directly in unlicensed spectrum are specifically discussed.
WiFi offloading is driven by the rapid adoption of smartphones which are Wi-Fi enabled and the increase
in data demand from consumers. This has led to mobile operators quickly moving to offload data traffic
from capacity constrained licensed networks onto available Wi-Fi networks. 3GPP has defined several
WLAN offloading mechanisms which rely on the connection between the LTE core network and WLAN.
The recent work on data aggregation at the LTE base station, which is discussed in this paper, allows for
better control of offloading with improved system and user performance while leveraging the existing LTE
features of Carrier Aggregation (CA) and dual connectivity.
The option of using LTE in unlicensed spectrum was motivated by the possibility of a more seamless and
spectrally efficient method of offloading while using the same core radio technology across both licensed
and unlicensed spectrum. The coexistence of deploying both LTE and Wi-Fi in the same way was
studied extensively by 3GPP which concluded that the two technologies can coexist, thus allowing mobile
operators the option to use either or both LTE and Wi-Fi to offload data to unlicensed spectrum.
1 INTRODUCTION
In recent years, mobile data usage has been increasing at an exponential rate by almost doubling every
year and it is expected to continue. Even though advances in cellular technology have increased the
performance and capacity of mobile networks, this alone will not be sufficient to meet the mobile data
demand. The usage of unlicensed spectrum provides an attractive opportunity for operators to help
support their subscribers by increasing network data capacity.
The conventional method for data offloading to unlicensed spectrum by operators has been using 802.11
based WLAN networks. Such networks have been deployed by either operators themselves or their
partners. The residential and campus WLAN networks have also been used by end users, especially for
connecting to the Internet. In order to provide an architectural framework and standardization for WLAN
offloading, 3GPP has developed several solutions such as ANDSF, IFOM, and SaMOG which enable
interworking with WLAN and provide data offloading through switching of data bearers to WLAN.
There have been recent methods developed within standardization bodies and industry for data offloading
to unlicensed spectrum:
LWA: A new alternative for LTE and WLAN interworking is data aggregation at the radio access network,
where an Evolved NodeB (eNB) schedules packets to be served on LTE and Wi-Fi radio links. This is
similar to the carrier aggregation and dual connectivity features defined in Release 10 and Release 12.
The advantage of this solution is that it can provide better control and utilization of resources on both
links. This can increase the aggregate throughput for all users and improve the total system capacity by
better managing the radio resources among users. In contrast to the previously developed offloading
solutions which rely on policies and triggers, scheduling decisions for each link can be made at a packet
level based on real-time channel conditions and system utilization. Furthermore, data aggregation at the
Radio Access Network (RAN) can be implemented without any changes to the core network since the
WLAN radio link effectively becomes part of the Enhanced Universal Terrestrial Radio Access Network
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(E-UTRAN). 3GPP has approved a Release 13 (Rel-13) Work Item on LTE/WLAN Aggregation which is
1
to be completed according to the Rel-13 schedule. The term Radio Aggregation will be used to be
synonymously with LTE/WLAN Aggregation at the RAN Level and also as the abbreviation for LWA (LTE
WLAN Aggregation).
LAA: Another alternative for using unlicensed spectrum to complement mobile networks is the
deployment of LTE itself in unlicensed spectrum. LTE has been successfully deployed in providing
wireless broadband data and its usage in both licensed and unlicensed spectrum can enable operators
and vendors to leverage the existing or planned investments in LTE/Evolved Packet Core (EPC)
hardware at both the radio and core network and simplify User Equipment (UE) architecture.
3GPP has completed a Rel-13 Study Item on License Assisted Access LTE (LAA-LTE) where the
unlicensed carrier can be used as a Secondary Component Carrier in the LTE Carrier Aggregation
2
framework. The study took into account different regulatory requirements for the usage of unlicensed
spectrum around the world, and in particular, the Listen-Before-Talk (LBT) schemes. Furthermore, fair
coexistence with other technologies which use unlicensed spectrum, particularly WiFi, was an important
part of the study. A Work Item is now ongoing to include LAA-LTE in the Rel-13 specifications.
LTE-U: Another effort in expanding the usage of LTE to unlicensed spectrum has been occurring outside
3
3GPP in the LTE-U Forum which has published specifications and studies for minimum base station and
UE requirements and co-existence with Wi-Fi. The focus of the LTE-U Forum based solution is for
deployment options and regions where LBT is not required. For the purpose of this document, LTE-U will
refer to the LTE-U Forum defined solution.
Others: In the industry, other options for multi-connectivity/aggregation occurring at the network level
such as Multipath Transmission Control Protocol (MP-TCP) and Quick User Datagram Protocol Internet
Connections (QUIC) are also emerging. These multi-connectivity approaches can work with any
combination of licensed and unlicensed band technologies including future ones (e.g., 5G), but these are
not within the scope of this paper.
2 DEPLOYMENT SCENARIOS
2.1
LTE-WLAN RADIO LEVEL AGGREGATION (LWA)
The deployment scenarios for LTE-WLAN radio level aggregation can be grouped into two categories as
shown in Figure 2.1; collocated scenarios and non-collocated scenarios. The collocated scenario would
be most common for an integrated LTE small cell plus one or multiple WLAN Access Points (AP(s)). The
non-collocated case corresponds to when the eNB connects to WLAN via a standardized interface. The
end-point of this interface on the WLAN side is a logical entity which can reside at the AP, Access
Controller (AC) (as shown in Figure 2.2) or at a new WLAN entity. The non-collocated scenarios would
represent situations where there are WLAN APs within a macro or small cell coverage area, but not
collocated with the macro or small cell.
1
“LTE-WLAN Radio Level Integration and Interworking Enhancement”, RP-151022.
“Licensed-Assisted Access using LTE”, RP-151045.
3
http://www.lteuforum.org/index.html.
2
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Figure 2.1. Deployment scenarios for LWA-WLAN radio aggregation.
The main deployment scenario for LTE-WLAN Radio Level Aggregation is WLAN networks deployed and
controlled by an operator and its partners. The 3GPP Work Item does not consider interworking with other
types of WLAN networks.
From a coverage point of view, two basic deployment scenarios are shown in Figure 2.2. The collocated
scenario considers the integrated eNB and WLAN AP, such as small cells or when eNB has an ideal
backhaul, to a WLAN logical entity which is connected to the APs. The non-collocated scenario is when
such backhaul is not ideal.
Figure 2.2. Basic Deployment options for LWA.
In LWA, eNB is the anchor node for both data and control planes and connects to the Core Network (CN)
via regular S1 interfaces (S1-C and S1-U). Since data packets have to traverse the eNB before being
transmitted over WLAN, an interface between eNB and WLAN is needed. This interface is being
standardized by 3GPP in Rel-13 and will be used for both control signaling to enable aggregation
operation and feedback from WLAN on network, channel and user conditions to help with LWA
scheduling operation.
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The following use cases are covered by LWA and shown in Figure 2.3:
A. UE is within E-UTRAN coverage, using 3GPP and goes into WLAN AP coverage (scenario 3).
B. UE is within E-UTRAN and WLAN coverage, using WLAN in aggregation mode and goes out of
WLAN AP coverage (scenario 3).
C. UE is within E-UTRAN and WLAN coverage, using WLAN in aggregation mode and changes
association to another WLAN AP (scenario 2).
D. UE is within E-UTRAN coverage of a first eNB and using 3GPP, then leaves coverage for the first
eNB and goes into E-UTRAN coverage from a second eNB and WLAN AP coverage (scenario 1).
E. UE is within E-UTRAN and WLAN coverage and in aggregation mode and goes out of E-UTRAN
coverage and changes association to another WLAN AP.
F. UE is within E-UTRAN and WLAN coverage, UE using LTE and WLAN in aggregation mode, all,
none, or a subset of the traffic of a data bearer for the UE should be routed via E-UTRAN where
subset could be empty.
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Figure 2.3. Mobility scenarios for LWA.
It should also be noted that alternative architectures to LWA are still being discussed in 3GPP. For
instance, a recent proposal has been made using Internet Protocol (IP) tunneling above the Packet Data
Convergence Protocol (PDCP) layer packets between the eNB and UE over WLAN as shown in Figure
2.4. The advantage of this approach is that it can be supported by legacy WiFi nodes (no upgrades
needed). Some of the challenges of this approach are that the IP tunnel must be re-established for every
WiFi AP change and the IP address of the eNB is exposed to the UE.
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Figure 2.4. IP Tunneling above PDCP layer.
2.2
LTE IN UNLICENSED SPECTRUM
Both LAA and LTE-U use carrier aggregation between a primary LTE component carrier in a licensed
band and one or more low power secondary component carriers deployed in the unlicensed spectrum.
The deployment scenarios span with and without macro coverage, outdoor and indoor small cell
deployments, co-location and non-co-location (with ideal backhaul) between the licensed and unlicensed
carriers. Figure 2.5 shows four possible LAA deployment scenarios. Though the backhaul between small
cells can be ideal or non-ideal, the unlicensed small cell only operates in the context of the carrier
aggregation through ideal backhaul with a licensed cell. In scenarios where carrier aggregation is
operated within the small cell with carriers in both the licensed and unlicensed bands, the backhaul
between macro cell and small cell can be ideal or non-ideal.
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Figure 2.5. LAA deployment scenarios.
Scenario 1

Carrier aggregation between licensed macro cell (F1) and unlicensed small cell (F3)
Scenario 2

Carrier aggregation between licensed small cell (F2) and unlicensed small cell (F3) without macro
cell coverage
Scenario 3

Licensed macro cell and small cell (F1), with carrier aggregation between licensed small cell (F1)
and unlicensed small cell (F3)
Scenario 4

Licensed macro cell (F1), licensed small cell (F2) and unlicensed small cell (F3)
o
Carrier aggregation between licensed small cell (F2) and unlicensed small cell (F3)
o
If there is ideal backhaul between macro cell and small cell, there can be carrier
aggregation between macro cell (F1), licensed small cell (F2) and unlicensed small cell
(F3)
o
If dual connectivity is enabled, there can be dual connectivity between macro cell and
small cell.
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In the study to support LAA and deployments in unlicensed spectrum for the above scenarios, CA
functionalities are used as a baseline to aggregate PCell/PSCell on a licensed carrier and SCell on an
unlicensed carrier. When non-ideal backhaul is applied between a macro cell and a small cell cluster in
Scenarios 3 and 4, a small cell on an unlicensed carrier has to be aggregated with a small cell on a
licensed carrier in the small cell cluster through ideal backhaul. One of the goals of this report is to identify
the need of and, if necessary, evaluate needed enhancements to the LTE RAN protocols applicable to
carrier aggregation in all of the above scenarios.
Operators can potentially utilize the entire available unlicensed spectrum in addition to licensed spectrum
while using the same:




Core network
Unified authentication mechanisms
Unified operations and management systems
Unified acquisition, access, registration, paging and mobility procedures
At the Physical Layer/Media Access Control PHY/MAC layer, several key functionalities have been
identified for an LAA system to operate in unlicensed spectrum. The Listen-Before-Talk (LBT) procedure
using energy detection is the primary mechanism by which equipment assesses the availability of the
unlicensed channel before transmission. Apart from regulatory requirements, carrier sensing via LBT is
one way for fair sharing of the unlicensed spectrum and hence it is considered to be a vital feature for fair
and friendly operation in the unlicensed spectrum in a single global solution framework. Discontinuous
transmission by limiting the transmission during each channel access opportunity to a few milliseconds
(ms) is another vital feature to ensure that all nodes obtain channel both fairly and frequently. A third vital
feature for unlicensed operation is carrier selection which enables LAA eNBs to select channels with low
interference levels and achieve effective coexistence with other unlicensed networks. Several design
enhancements in Discovery Reference Signals (DRS) design,
cell selection, Radio Resource
Management (RRM) measurements, Channel State Information (CSI) feedback, Hybrid Automatic
Retransmission Request (HARQ) enhancements, newer waveforms and flexible multi-carrier transmission
have been proposed to be further improve operation in unlicensed spectrum and studied as part of the
3GPP Study Item (SI).
The SI concluded that for Downlink (DL) transmission, a Cat 4 LBT scheme with random backoff and
variable contention window sizes would enable fair channel access and good coexistence with WiFi and
other LAA networks. The key parameters of such a scheme would be discussed in detail as part of the
ongoing Work Item (WI). For UL, it was recommended to support an Uplink (UL) LBT scheme which is
different from the DL LBT as the UL access is scheduled and controlled by the eNB.
The two main deployment options that were being considered under the carrier aggregation framework
are:
1) Indoor Deployments
2) Outdoor Deployment
2.2.1 INDOOR DEPLOYMENTS
The indoor deployment is based on a primary component carrier in the licensed band, with LTE in the
unlicensed band as the secondary component carrier. While the primary component carrier remains the
operator licensed channel, LTE could operate in UNII-1, UNII-2A, UNII-2C or UNII-3 set of channels 17
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decibel-milliwatts (dBm) subject to mandatory Dynamic Frequency Selection (DFS) to detect incumbent
radar operation and Transmit Power Control (TPC) to protect the Earth Exploration-Satellite services.
Since LTE carrier aggregation protocols are used for the small cell operation, seamless interworking and
mobility is guaranteed, even if the licensed and unlicensed cell footprints are not similar. The initial attach,
authentication and security are performed on the primary carrier, with LTE in the unlicensed spectrum
used only for data offloading. All other time/jitter sensitive applications such as Voice over LTE (VoLTE)
can be supported on the licensed primary channel that has a more predictable availability and Quality of
Service (QoS). The indoor small cell can operate in a dedicated licensed channel (Scenario 4) or in a
Heterogeneous Network (HetNet) mode with a common licensed channel (Scenario 3) shared with the
macro cell, while the LTE in unlicensed component is used only for offloading data traffic to LTE users
nearby or in indoor hotspots. Indoor small cells can be connected directly or most likely through a
gateway to the core network.
2.2.2 OUTDOOR DEPLOYMENTS
An LTE Evolved NodeB (eNB or eNodeB) can be augmented to support operating LTE in unlicensed
spectrum for outdoor small cell deployments. While the primary component carrier remains the operator
licensed channel, LTE in unlicensed spectrum could operate in Unlicensed National Information
Infrastructure (UNII)-1 or UNII-3 set of channels that allows a maximum Transmit (Tx) power of 30 dBm,
with an additional 6 decibels-isotropic (dBi) antenna gain. Since LTE carrier aggregation protocols are
used for the small cell operation, seamless interworking and mobility is guaranteed, even if the licensed
and unlicensed cell footprints are not similar. The initial discovery, authentication and security are
performed on the primary carrier, with LTE in the unlicensed spectrum used only for data offloading. All
other time/jitter sensitive applications such as VoLTE can still be supported on the licensed channel which
has a predictable availability and QoS. The Macro and Outdoor Pico cells can operate in separate
dedicated licensed channels (Scenario 4) or in a HetNet mode (Scenario 3) with a common licensed
channel, while LTE in unlicensed is only used by Pico cells for offloading nearby users or users in a
hotspot. LTE in unlicensed enabled Pico cells is connected to the core network directly without the need
for any additional network elements.
A key design goal, as defined in the 3GPP Study Item for LAA, was that considering different fair sharing
metrics, LAA should not impact Wi-Fi services (data, video and voice services) more than an additional
WiFi network on the same carrier; these metrics could include throughput, latency, jitter, etc. 3GPP has
identified that in order to comply with region specific regulations mandating LBT in unlicensed frequency
bands and ensure fair coexistence of LAA with incumbent co-channel WiFi deployments, an LAA eNB
should perform LBT prior to DL/UL transmission over an unlicensed SCell.
To enable a single global solution deployable in all regions and effective coexistence with WiFi and LAA
deployments, the SI recommended that LAA will use a variable backoff-based contention window
adaptation scheme to access the channel. In the WI phase, several schemes which vary the size the
contention window dynamically based on either: (1) Acknowledgement/Negative Acknowledgement
(ACK/NACK) reported by the UEs or (2) eNB sensing outcomes have been considered. The exact
mechanism to trigger the contention window update is still under discussion and is expected to be
finalized soon.
In addition, adapting the Energy Detection Threshold (EDT) for sensing whether the medium is idle or
busy has been considered a key variable to ensure effective coexistence with WiFi and performance of
LAA networks. 3GPP RAN1 has identified several criteria for adapting the EDT and some detailed rules
for triggering such an adaptation are expected to be discussed soon. Finally, there is an ongoing
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discussion in 3GPP on how to test whether eNBs are following the expected channel access mechanism
and correctly varying the window size and EDT.
Considering the likelihood of discontinuous transmission due to LBT, there has been intent to enable as
flexibly as possible resource partitioning between UL and DL traffic. Due to the LBT requirement for both
UL and DL LAA operation, there is strong consensus in 3GPP that the existing LTE Time Division Duplex
(TDD) UL/DL configurations cannot be utilized in LTE Unlicensed operation as such. Therefore, some
new definitions or at least understanding will be needed on the interpretation of UL and DL sub-frames;
however, the related discussions in 3GPP are still ongoing.
In the Rel-13 time-frame, only downlink LAA operation will be specified, while studies on LAA uplink in
3GPP would be restricted only to identify (not specify) the principles of channel access for forward
compatibility during a later release. Due to discontinuous transmission from LBT in both DL and UL
directions, synchronous UL HARQ process retransmission (as in licensed carrier LTE operation) may not
be guaranteed. Therefore, 3GPP has proposed to base the UL operation of LTE LAA on asynchronous
HARQ (similar as current LTE DL HARQ operation) which may require additional information on HARQ
information to be transmitted as part of the UL scheduling grant. All the LAA Physical Uplink Shared
Channel (PUSCH) retransmissions will likely be adaptive (i.e., scheduled by separate grants). This also
potentially renders usage of the Physical HARQ Indicator Channel (PHICH) to be unnecessary.
The European Telecommunications Standards Institute (ETSI) requires a transmission of a node to
occupy at least 80 percent of the nominal bandwidth and that requirement also applies to LTE LAA UL
operation. As a single global solution for LAA is envisioned, there is a need to guarantee that the PUSCH
waveform should be in compliance with the ETSI channel occupancy requirement. The 3GPP LAA Study
Item has identified that for the PUSCH, extending the current single and dual cluster allocation to allow
multi-cluster (>2) allocation has been identified as a candidate waveform that satisfies ETSI regulatory
requirements and maximizes uplink coverage.
As the amount of unlicensed spectrum aggregated with licensed spectrum increases, there is an
increasing need to offload UL control channels to the unlicensed spectrum as well. For example,
supporting Physical Uplink Control Channel (PUCCH) operation in the UL might enable significantly more
spectrum to be aggregated without increasing the burden on the licensed carriers. In addition, supporting
control channel signaling on the unlicensed spectrum provides an opportunity to use the dual-connectivity
framework in the future instead of the carrier aggregation framework to ensure that unlicensed spectrum
can be integrated into the core network using more flexible deployment models which address several
diverse business cases.
3 DATA OFFLOADING WITH LTE-WLAN AGGREGATION
The main use case is WLAN deployments either by a mobile network operator and/or its partners. It is
possible to deploy LTE-WLAN Aggregation for enterprise and residential networks; however, 3GPP will
not define specific solutions for these use cases.
3.1
ARCHITECTURAL ASPECTS
The user plane architecture aggregation builds upon the 3GPP Rel-12 Dual Connectivity (DC) split-bearer
which is illustrated in Figure 3.1 below (taken from 3GPP Technical Report (TR) 36.842):
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S1
Xn
PDCP
PDCP
RLC
RLC
RLC
MAC
MAC
MAC
MeNB
SeNB
Figure 3.1. Dual connectivity split-bearer architecture.
In the aggregation architecture, WLAN AP will be similar to the Secondary eNB (SeNB) in the user plane.
A new interface between eNB and WLAN, called Xw, is defined whose functionality is similar to Xn in the
figure for DC but also includes some functionality tailored for the Rel-13 WLAN interworking. (Xn has
been standardized as part of X2). The termination point of Xw at WLAN is a logical node, called WT
(WLAN Termination), and it can be implemented at an Access Point, Access Controller, or another
physical entity.
There are several differences of aggregation from the DC split-bearer architecture:

The control plane is only at the eNB and not shared with WLAN; therefore, the functionality
supported for SeNB RRC is not needed

Radio Link Control (RLC) is not supported at WLAN and thus PDCP Packet Data Units (PDUs)
are delivered by WLAN MAC

An adaptation layer to deliver PDCP PDUs over WLAN is needed (as discussed in Section 3.2)
Since the eNB is the anchor point for both the user and control plane, aggregation is only feasible when
the UE is in LTE coverage and a connected state with LTE eNB. The UE can also use other WLAN
offloading schemes defined prior to Rel-13 which do not require LTE connection; however, it is preferable
to enable aggregation when available since it provides better performance than other schemes as shown
in Section 3.3.
3GPP WLAN offloading solutions always allow the user preferences to have higher priority over
standardized mechanisms offered by the network. The same principle continues also on aggregation.
When a user, for example, wants to connect to a home WLAN network, aggregation data flow can stop
and has to be communicated by the UE to the network.
Figure 3.2 shows the network architecture for aggregation. An important element of aggregation is that it
does not require any new CN nodes, interfaces and signaling. In Figure 3.2, the connection of WLAN to
the CN is shown to illustrate the fact that the same WLAN network can be used to provide offloading
services using such connections (which were standardized by 3GPP before Rel-13).
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Core Network
S1-MME
eNB
S10
S1-U
MME
E-UTRAUu
HSS
S5
PGW
S11
Xw
SGW
E-UTRA
E-UTRA UE
UE
S6a
SGi
Internet
802.11
WLAN
ePDG/
TWAG
Figure 3.2. Network architecture for LTE/WLAN aggregation.
3.2
USER AND CONTROL PLANE
As described in Section 3.1, the data plane aggregation for LTE and WLAN happens at the PDCP layer
located at the eNB and UE. On the downlink, the eNB scheduler decides to send a PDCP PDU on either
LTE or WiFi. On the uplink, eNB schedules data transmissions for LTE link per current LTE MAC
specifications while the transmissions on WLAN are initiated by the WiFi Station (STA) part of the UE.
This can still allow the flexibility for eNB to have control on the total uplink transmissions.
A PDCP PDU transmitted on LTE for either downlink or uplink goes through the same LTE procedures
without any modifications to the LTE PHY/MAC/RLC layers. On the WLAN side, the PDCP PDU is
encapsulated in a WiFi MAC packet by the AP or STA to be transmitted over WiFi. For downlink, STA will
forward the received WiFi packet payload to the UE PDCP layer. For the STA to easily determine that the
WiFi packet contains a PDCP PDU, a new identifier will be specified by 3GPP.
LWA will support having multiple bearers per UE to be served over WLAN. For the receiver PDCP layer to
determine the correct logical channel a PDCP PDU belongs to, a header containing the logical channel
(bearer) information should be attached to the PDCP PDU before transmitting over WiFi. This header will
be inserted by the eNB on downlink and UE on uplink and transparent to the WLAN.
Since PDCP PDUs can be transmitted on both LTE and WiFi, they may arrive at different times at the
receiver. Therefore, a reordering mechanism is needed in order to deliver the PDCP SDUs to the upper
layers. In Rel-12 Dual Connectivity, such mechanism was adopted for the split-bearer option. The same
mechanism will also be used for LWA.
When eNB and WLAN are not collocated, the PDCP PDU has to be forwarded between the eNB and WT.
A standardized data interface similar to Dual Connectivity MeNB and SeNB interface for data forwarding
is also defined between eNB and WLAN Termination (WT). The transport protocol for this interface is
General Packet Radio Service Tunneling Protocol (GTP) tunneling over UDP/IP where the data for each
bearer is exchanged in a different GTP tunnel. Flow control mechanisms from eNB to WT are also
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implemented to minimize buffer underflow and overflow at WLAN side to improve the performance. In
addition, the eNB can use UE feedback in controlling the amount of traffic forwarded to WLAN. If
LTE/WLAN aggregation is enabled on the uplink, a similar flow control from WT to eNB side may also be
useful to prevent buffer overflow at the eNB.
In LTE, the eNB uses UE feedback such as Channel Quality Indication (CQI) and other RRM functionality
in deciding which cells to use and how to schedule data packets. A similar functionality is also needed for
LWA for the eNB to decide the amount of traffic to forward to WLAN. To this end, feedback for WLAN
channel conditions (e.g., signal strength, loading) can be used. The eNB can configure the UE to report
such statistics but may can also receive them from WT via the Xw interface. Other WLAN statistics such
as backhaul conditions and hardware load can also be similarly received at least via the Xw.
The main functionality of the control plane for LWA is the selection of WLAN to configure or de-configure
aggregation for data bearers. The signaling between eNB and UE for this is done by Radio Resource
Control (RRC) procedures carried on the Signaling Radio Bearers (SRBs). The eNB also communicates
with WT for the addition, removal and change of the WT.
The main steps for initial configuration of LWA are: 1) determining UE and network capability for LWA, 2)
selection of WLAN AP for aggregation and 3) configuration of aggregation for specific data bearers.
These steps can be repeated as needed due to UE mobility and/or changing radio and load conditions.
Since the network should be aware of the UE capability in supporting LWA before initiating any of the
procedures, the first step of the eNB-UE signaling for LWA will be the UE reporting such capability (e.g.,
via conventional RRC messages such as UECapabilityEnquiry and UECapabillityInformation exchanged
during normal UE capability reporting procedure (e.g., during initial attach). The UE can also include
WLAN related information such as its MAC address and supported WLAN bands and channels; the MAC
address can be used on the eNB-WLAN interface for UE identification in enabling data transfer and
exchanging control information.
The second step is WLAN selection to determine a suitable AP which supports aggregation and the radio
and channel conditions. In current WLAN implementations, finding WLAN networks (scanning and
association) are done by the STA. For LWA, eNB will also be part of this decision by indicating which APs
should be targets for scanning so that UE doesn’t need to report APs that are not capable for LWA. In
many WLAN deployments, using 3GPP-like mobility based on UE measurement and eNB triggers is not
optimal for all WLAN mobility. This is especially true when the UE moves between APs controlled by the
same AC where current WLAN deployments include many mobility optimizations. In such scenarios,
controlling every AP change of the UE by eNB will require signaling exchange between eNB and UE for
measurement reporting and eNB actions. This will cause significant overhead and delay and can result in
early or late handovers between APs considering the fact that coverage area of single AP is relatively
small. For these reasons, 3GPP adopted a UE-based mobility scheme wherein a “WLAN mobility set” is a
group of APs (identified via WLAN identifiers) determined by the eNB. The eNB signals to the UE such
mobility set where UE is allowed to switch APs without informing the eNB. It is also assumed that all the
APs within the mobility set are controlled by the same WT. For APs outside the mobility set or for APs
within different WTs, the decision to change AP is made by the eNB based on e.g., UE measurements.
The information of UE moving to another AP can be provided to eNB by either UE or WT. Such
information may not always be necessary, e.g., for mobility within the mobility set or when the WT does
not change, but in other cases the information is needed for the eNB to direct the backhaul (Xw) data and
control plane towards the correct WT. For example, mobility between two WTs requires signaling
exchange between eNB and the source and target WT similar to dual connectivity SeNB change.
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3.3
PERFORMANCE EVALUATION
LWA can help improve user quality of service and overall system capacity, through efficient management
of radio resources across both links. A simulation analysis based on 3GPP HetNet methodology and
modeling of WLAN contention based access is used to illustrate these benefits. Accordingly, we evaluate
system performance gains in terms of layer 2 throughput enhancements. Additionally, TCP layer
performance for a “representative user” is also evaluated to benchmark the impact of reordering delays
and protocol overhead on the overall gain from aggregation.
3.3.1 SIMULATION METHODOLOGY AND ASSUMPTIONS
System level simulations to characterize layer 2 throughput enhancements are based on 3GPP
methodology as captured in 3GPP TR 36.814, 36.819 and 36.842. The methodology is extended to
include the 802.11n interface and the contention based MAC protocol. Application layer performance is
modeled assuming the best effort File Transfer Protocol (FTP) traffic model. We focus on the downlink
performance to be consistent with Rel-13 LWA priorities.
TCP performance characterization is based on the modeling of full LTE and WLAN protocol stacks. The
simulations track the performance of a representative user, whose link throughput is obtained from the
system simulation analysis.
Detailed simulation assumptions are described in the Appendix.
3.3.2 DEPLOYMENT SCENARIOS
We focus on outdoor, multi-tier heterogeneous deployments, wherein a 3-sectored LTE macro cell tier is
overlaid with a tier of small cells according to the following configurations:
1. Collocated Deployments are based on deployments of integrated WLAN-LTE small cells,
supporting collocated eNB and WLAN AP as described in Figure 2.2. Small cell LTE operates on
the same frequency as the macro-cell.
2. Non Collocated Deployments comprise a tier of WLAN only small cells connected to the eNB over
a non-ideal backhaul.
3.3.3 WLAN OFFLOADING AND LWA SOLUTIONS COMPARED
We compare the following solutions in our evaluation:

WLAN Preferred: Conventional “WLAN preferred if in coverage” scheme, implemented by most
current devices. Here, a device always connects to a WLAN AP if a minimum UE-specific signal
quality threshold is satisfied.

Radio Interworking: Rel-12 RAN-assisted WLAN interworking with optimum thresholds or Rel13 radio interworking enhancements with measurement reporting. This scheme may be
considered reflective of radio interworking schemes that do not employ aggregation with bearer
split.

Rel-13 LWA (with bearer split): Two variations are considered. The first is suitable for
collocated WLAN and LTE small cells.
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o
Joint Queue/Scheduling is based on packet level scheduling across LTE and WLAN. The
solution assumes a shared transmission queue across independent but cooperative
WLAN and LTE schedulers, which are capable of exchanging per bearer throughput
history periodically.
o
Multi-user Bearer Splitting (MUS) is designed to also work with non-collocated
deployments with non-ideal backhaul delays. The eNB employs a splitting algorithm
based on minimizing the logarithm of sum throughput across all users configured for
aggregation. Here, the WLAN/LTE schedulers use independent transmission queues but
still cooperate to exchange per bearer throughput history information.
3.3.4 METRICS FOR OPTIMIZATION
User perceived throughput enhancements are used to characterize layer 2 system performance gains.
TCP throughput gains are also considered to characterize the application layer performance for a
representative user.
3.3.5 SYSTEM PERFORMANCE RESULTS
Figures 3.3 are illustrative of LWA gains for collocated small cell deployments. The LWA bearer split
algorithm is based on the joint queue/scheduling algorithm. Results are reported for all users as well as
the users that are associated with the small cell. As the macro cell users do not perform aggregation, the
performance across users associated with the small cell is of interest. It can be seen that LWA improves
the average as well as the cell edge user perceived throughput across all small cell users in the system
when compared to the Rel-12/Rel-13 radio interworking scheme. When considering medium system
load, LWA gains in average user throughput of up to 70 percent are observed. The cell edge gains for
small cell users, which exploits aggregation, also increases substantially.
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Figures 3.3. LWA performance gains for collocated Het-Net deployments with same licensed carrier being used across
macro and small cell tiers. No interference coordination is assumed. 1 AP/9 UEs per macro cell sector are considered.
System utilization of Low, Med and High correspond to 20-25%, 40-50% and 60-70% utilization levels, respectively.
Figures 3.4 illustrate the LWA performance gains for non-collocated deployments (macro cell and WiFi
only small cell) with ideal and 20 millisecond backhaul delay. The LWA bearer split scheme is based on
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the MUS algorithm. Considering the scenario with ideal backhaul, it can be seen that LWA based on
MUS algorithm outperforms radio interworking solution, with average and cell edge gains across all users
of 30 percent and 85 percent, respectively, at medium utilization level. LWA MUS gains are still available
(especially for cell-edge users) even when considering non-ideal backhaul with a 20 millisecond delay,
with gains of 24 percent and 45 percent in average user throughput across all users in the system.
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Figures 3.4. LWA performance gains for non-collocated Het-Net deployments, comparing average and cell-edge
performance results across all users with ideal backhaul delay (0ms) with a non ideal delay scenario of 20 millisecond.
Five WLAN APs per macro cell sector are considered. System utilization of Low, Med and High correspond to 20-25%, 4050% and 60-70% utilization levels, respectively.
3.3.6 TCP PERFORMANCE RESULTS
Table 1 shows LWA performance gains considering TCP throughput. TCP performance is simulated
using WLAN and LTE link throughput experienced by representative median and cell edge users to
illustrate the characteristic of LWA and radio interworking performance. The scenario shown, corresponds
to a non-collocated case with ideal backhaul at low system utilization levels. A simpler algorithm based on
per user buffer equalization is used for LWA. TCP performance characterization focuses on whether layer
2 performance gains translate to corresponding gains in TCP throughput, as protocol overhead as well as
the impact of reordering delays must be considered for overall application layer TCP performance.
Results show that while TCP throughput is reduced to some extent due to the overhead considered, LWA
layer 2 gains still translate to gains in overall TCP throughput.
Table 1. LWA TCP performance gains compared to layer 2 throughput gains over radio
interworking solution for representative users.
3.3.7 DISCUSSION OF RESULTS
The results shown in this section are illustrative of substantial performance benefits for Rel-13 LWA
solutions with bearer split. The results show up to 70 percent system gains in average user throughput for
users associated with a collocated WiFi/LTE cell at medium load. The cell edge user experience
substantially improved throughput (about 2x gains at medium load levels).
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LWA with bearer split also performs well for non-collocated deployments with non-ideal backhaul delays.
Our results show an average/cell-edge gains of more than 30 percent and 80 percent, respectively, with
LWA multi-user splitting algorithm at medium load. Results also show that LWA gains are preserved for
reasonable backhaul delays.
Although not covered here, it can also be shown that gains in user throughput also results in system
capacity improvements, in that LWA can support higher number of users for the same target user quality
of service when compared to the WLAN/3GPP interworking solution.
We also investigated LWA TCP performance for a representative user accounting for TCP overhead and
reordering delays which showed that layer 2 gains translates to TCP layer gains.
3.4
INTERWORKING WITH PREVIOUS OFFLOADING SOLUTIONS
LWA solutions can be deployed by an operator along with its already existing WLAN networks that use a
different offloading scheme (e.g., Evolved Packet Data Gateway (ePDG) based). In this case, it is
necessary to specify how a UE operates when it has access to both options. Such scenarios can also
happen while the user is roaming, for example, in an LWA network while it was configured by home
operator Access Network Discovery and Selection Function (ANDSF) policies. 3GPP has not specified
solutions to address these coexistence problems. It is envisioned that LWA should have higher priority by
default since the UE has to follow LWA traffic steering commands just like any LTE configuration by the
network. However, it is possible that the prioritization can be left to the operator choice which can provide
the priority list to the user; in this case for example, the network can assign higher priority to a non-LWA
WLAN network for certain users depending on their subscription levels.
4 LTE IN UNLICENSED SPECTRUM
4.1
DATA OFFLOADING FOR OPERATORS
4.1.1 OVERVIEW
An alternative technique to WLAN offloading was recently introduced to use LTE-Advanced (LTE-A) in
the same unlicensed bands that WiFi traditionally occupied, sharing the spectrum alongside WiFi.
Deploying LTE-A in unlicensed bands instead of utilizing WiFi offloading offered a more seamless and
spectrally efficient method of offloading data. Deploying both LTE-A and Wi-Fi in the same band presents
coexistence issues that are currently under study in 3GPP RAN. Results to date indicate that the two
technologies can coexist, thus allowing mobile operators the option to use both LTE-A and WiFi to offload
data.
LTE-A can operate in the unlicensed bands in the following configurations:
1. Supplemental Downlink (SDL)
2. Carrier Aggregation (CA)
3. Dual Connectivity (DC)
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4.1.1.1
SUPPLEMENTAL DOWNLINK (SDL)
Most of the data demand increase has been in the DL, driving operators to utilize additional download
capacity that includes use of unlicensed bands, including both LTE-A (LAA, LTE-U) and WiFi, to offload
data traffic in order to supplement DL data capacity. SDL offers the DL capacity for utilization of
download-intensive services such as video/music streaming and web browsing. SDL is typically used to
refer to an unpaired spectrum band used to supplement download traffic.
4.1.1.2
CARRIER AGGREGATION (CA)
Due in part to the opportunistic nature of unlicensed spectrum, it is not as suitable for control channels, so
the initial use of LTE in unlicensed spectrum is as a secondary channel in a carrier aggregation scheme.
This allows better management of data traffic between the licensed LTE carrier and an LAA carrier.
3GPP is currently studying coexistence and specification issues for use of unlicensed carriers in LTE CA
combinations. CA using unlicensed carriers will allow the offload to be seamless and also accommodate
load-balancing between licensed carriers and unlicensed carriers. The current LAA work in 3GPP allows
unlicensed carriers to be limited to SCell, or secondary carriers. In addition to LAA, a newly-approved
3GPP work item for CA between LTE and Wi-Fi is also in progress. This will allow tighter integration
between LTE and WiFi and give LTE carriers the option to do CA with both LAA and WiFi. The study item
for LAA coexistence was completed in June 2015. A work item to incorporate the SI findings into the
3GPP specs was approved in June 2015, with work scheduled for completion as part of Rel-13.
4.1.1.3
DUAL CONNECTIVITY (DC)
Dual Connectivity is a relatively new LTE architecture developed by 3GPP RAN to allow a UE to connect
to two distinct eNBs simultaneously, allowing a UE in a HetNet scenario to be simultaneously connected
to both the macro eNB and small cell eNB. This architecture allows aggregation of user-plane radio
resources to improve UE throughput. In a DC scheme, one eNB, a macro-cell, is defined as the Master
eNB, the other, a small cell, is defined as Secondary eNB. The two eNBs are connected via non-ideal
backhaul.
4.2
COEXISTENCE WITH WLAN IN ADJACENT CHANNELS
3GPP RAN4 considered adjacent channel coexistence studies between LAA and WLAN. Companies
provided results from adjacent channel coexistence studies using scenarios and methodology for
evaluating adjacent channel coexistence between different networks in the unlicensed band.
The following scenarios for adjacent channel coexistence were taken into account:


Indoor scenario
Outdoor scenario
The coexistence cases for adjacent channel evaluations that have been studied are as follows:



WLAN to WLAN, which could be the baseline to evaluate case of LAA to WLAN
WLAN to LAA
LAA to WLAN
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Based on the simulation results provided for 3GPP RAN4, it was concluded that LAA and WLAN can
coexist in adjacent channels. According to simulation results, LAA causes less adjacent channel
interference to a WLAN system compared to another WLAN system. In other words, LAA is a better
neighbor than another WLAN system in terms of adjacent channel coexistence with WLAN system.
4.3
PERFORMANCE EVALUATION FOR CO-CHANNEL LAA AND WIFI
The key focus of the LAA SI in 3GPP was to determine if an LAA network could coexist with a WiFi
network and other LAA networks. Extensive evaluations on several DL and UL schemes using various
4
LBT categories were carried out as part of the study. The following observations have been made in the
LAA SI technical report. For a LAA network carrying only DL traffic, it was noted that “a majority of
sources that evaluated an LAA network operating a category 4 DL LBT scheme based on ETSI Option B
with modifications including at least defer periods and variable (exponential) contention windows showed
that it can operate without impacting WiFi more than an equivalent WiFi network”. For networks carrying
both DL and UL traffic, it was similarly noted that “a majority of sources showed combinations of LAA DL
and UL LBT schemes that do not impact WiFi more than another WiFi network (offering the same traffic to
the same users) in any of the measured performance metrics. Category 3 and 4 were tested for the DL
and Categories 1 through 4 were tested for the UL. Within each LBT category, the LBT schemes and/or
parameters shown by different sources to not impact WiFi more than another WiFi network may be
different.” The conclusions in the Technical Report (TR) clearly demonstrate that LAA network supporting
both DL and UL traffic can coexist well with a WiFi network.
5 CONCLUSION
Both LTE/WiFi aggregation and LTE in unlicensed spectrum provides distinct advantages for data
offloading to alleviate the data capacity constraints in licensed LTE deployments. LWA can be leveraged
using the existing WiFi networks while providing better performance and control compared to other WLAN
offloading mechanisms. LTE in unlicensed spectrum can provide seamless and efficient offloading by
using the same core radio technology across both licensed and unlicensed spectrum. Both LWA and LAA
allow mobile operators effective mechanisms in using unlicensed spectrum to satisfy the increasing data
demands. The choice among LWA and LAA will depend on many factors, including an operator’s existing
infrastructure, technology road map and capital expenditure plans. However, it is expected that it will be
an option for consideration by operators worldwide.
4
“Feasibility Study on Licensed-Assisted Access to Unlicensed Spectrum”, 36.889.
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APPENDIX: LWA PERFORMANCE EVALUATION - SIMULATION DETAILS
Table A1. LTE simulation assumptions for system level results.
LTE
Topology
7 cell wrap-around (Het-Net deployment w/ collocated WiFi-LTE small
cells and WiFi-only small cells. Small cell LTE interface uses same
carrier as macro-cell. No ICIC is assumed.
Cell Association
Network controlled cell-association based on optimizing WLAN QoS and
RSRQ Thresholds for each deployment
UE dropping
Clustered
LTE Carrier Frequency
2 GHz
Channel/UE speed
[IMT] UMa Macro, UMi Pico, UE speed= 3 km/hr
LTE mode
Downlink FDD; 20 MHz for DL
No. antennas (macro, pico, UE)
(2, 2, 2)
Antenna configuration
macro, small cell: co-polarized, UE: co-polarized (||-->||)
Max rank per UE
2 (SU-MIMO)
UE channel estimation
Ideal
Feedback/control channel errors
No Error
PHY Abstraction
Mutual Information
Scheduler
Proportional-Fair Scheduler
Scheduling granularity
5 PRBs
Traffic load
Non full buffer with 3GPP FTP traffic model 3. Arrival rate, file sizes and number
of users are varied to generate Low = 20-25%, Med= 35-50% and High= 60-70%
load levels.
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LTE
Receiver type
Interference unaware MMSE
Feedback periodicity
10ms
CQI & PMI feedback granularity in
frequency
5 PRBs
PMI feedback
3GPP Rel.-10 LTE codebook (per sub-band)
Outer loop for target FER control
10% PER for 1 transmission
Link adaptation
MCSs based on LTE transport Format
HARQ scheme
CC (Chase Combining)
st
Table A2. WiFi simulation assumptions for system level results.
WiFi
WiFi Parameters
802.11n
WiFi Frequency, Channelization
2.4 GHz band, 3 frequency bands, 20 MHz channels; least power based
channel selection
AP Transmit power
20dBm outdoor, 18 dBm indoor
WiFi mode
Downlink only.
Scheduler
Proportional Fair and Round Robin
TX-OP
1ms
PHY Abstraction
RBIR
MPDU Size
1500 Bytes
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Assumptions for TCP Results

Full LTE and WLAN protocol stack emulation via OPNET

20MHz WiFi 802.11n

Considers performance of a “representative user” from system level results (link throughput from
SLS)

Below PDCP layer bearer split modeled, with the following parameter settings:
o
o
o
o
o
Buffer equalization algorithm
PDCP flow control modeled
PDCP RX reordering time: 500ms
PDCP Discard Timer: 1s
Results computed across multiple FTP sessions of 50MB DL
ACRONYM LIST
3GPP
AC
ACK/NACK
ANDSF
3rd Generation Partnership
Project
HetNet
Heterogeneous Network
Access Controller
Acknowledgement/Negative
Acknowledgement
Access Network Discovery and
Selection Function
IP
Internet Protocol
LAA
License Assisted Access
LBT
Listen-Before-Talk
LTE-A
LTE-Advanced
LTE-U
LTE Unlicensed
LWA
LTE-WLAN Aggregation
MAC
Media Access Control
MP-TCP
Multipath Transmission Control
Protocol
AP
Access Point
CA
Carrier Aggregation
CN
Core Network
CQI
Channel Quality Indications
CSI
Channel State Information
dBi
decibels-isotropic
dBm
decibel-milliwatt
DC
Dual Connectivity
PDU
Packet Data Unit
DFS
Dynamic Frequency Selection
PHICH
Physical HARQ Indicator
Channel
DL
Downlink
PHY
Physical Layer
DRS
Discovery Reference Signals
PUCCH
Physical Uplink Control Channel
EDT
Energy Detection Threshold
PUSCH
Physical Uplink Shared Channel
eNB/eNodeB
Evolved NodeB
QoS
Quality of Service
EPC
Evolved Packet Core
QUIC
ePDG
Evolved Packet Data Gateway
RAN
Quick User Datagram Protocol
Internet Connections
Radio Access Network
Rel
Release
RLC
Radio Link Control
File Transfer Protocol
RRC
Radio Resource Control
General Packet Radio Service
Tunneling Protocol
Hybrid Automatic
Retransmission Request
RRM
Radio Resource Management
SDL
Supplemental Downlink
SeNB
Secondary eNB
European Telecommunications
Standards Institute
UMTS Terrestrial Radio Access
Network
ETSI
E-UTRAN
FTP
GTP
HARQ
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SI
Study Item
UL
SRB
Signaling Radio Bearers
UMTS
STA
WiFi Station
TDD
Time Division Duplex
TPC
Transmit Power Control
TR
Technical Report
Tx
Transmit
UE
User Equipment
UNII
Uplink
Universal Mobile
Telecommunications System
Unlicensed National Information
Infrastructure
VoLTE
Voice over LTE
WI
Work Item
WLAN
Wireless Local Area Network
WT
WLAN Termination
ACKNOWLEDGEMENTS
The mission of 4G Americas is to advocate for and foster the advancement and full capabilities of the
3GPP family of mobile broadband technologies, including LTE-Advanced and beyond to 5G, throughout
the ecosystem's networks, services, applications and wirelessly connected devices in the Americas. 4G
Americas' Board of Governors members include Alcatel-Lucent, América Móvil, AT&T, Cable & Wireless,
Cisco, CommScope, Entel, Ericsson, HP, Intel, Mitel, Nokia, Qualcomm, Sprint, T-Mobile US, Inc. and
Telefónica.
4G Americas would like to recognize the significant project leadership and important contributions of
Etienne Chapponniere and Ozcan Ozturk of Qualcomm and Gunjan Nimbavikar of T-Mobile, as well as
representatives from member companies on 4G Americas’ Board of Governors who participated in the
development of this white paper.
The contents of this document reflect the research, analysis, and conclusions of 4G Americas and may not necessarily represent
the comprehensive opinions and individual viewpoints of each particular 4G Americas member company.
4G Americas provides this document and the information contained herein to you for informational purposes only, for use at your
sole risk. 4G Americas assumes no responsibility for errors or omissions in this document. This document is subject to revision or
removal at any time without notice.
No representations or warranties (whether expressed or implied) are made by 4G Americas and 4G Americas is not liable for and
hereby disclaims any direct, indirect, punitive, special, incidental, consequential, or exemplary damages arising out of or in
connection with the use of this document and any information contained in this document.
© Copyright 2015 4G Americas
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