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4G/5G spectrum sharing for enhanced mobile broad-band and IoT services

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5G has been developed for supporting diverse services, such as enhanced mobile broadband (eMBB), massive machine type communication (mMTC) and ultra-reliable low latency communication (URLLC). The latter two constitute enablers of the Internet of Things (IoT). The new spectrum released for 5G deployments, primarily above 3 GHz, unfortunately has a relatively high path-loss, which limits the coverage, especially for the uplink (UL). The high propagation loss, the limited number of UL slots in a TDD frame and the limited user-power gravely limit the UL coverage, but this is where bandwidth is available. Moreover, the stringent requirements of eMBB and IoT applications lead to grave 5G challenges, such as site-planning, ensuring seamless coverage, adapting the TDD DL/UL slot ratio and the frame structure for maintaining a low bit error rate (BER) as well as low latency, etc. This paper addresses some of those challenges with the aid of a unified spectrum sharing mechanism, and by means of an UL/DL decoupling solution based on 4G/5G frequency sharing. The key concept is to accommodate the UL resources in an LTE FDD frequency band as a supplemental UL carrier in addition to the New Radio (NR) operation in the TDD band above 3 GHz. With the advent of this concept, the conflicting requirements of high transmission efficiency, large coverage area and low latency can be beneficially balanced. We demonstrate that the unified 5G spectrum exploitation mechanism is capable of seamlessly supporting compelling IoT and eMBB services.<br/
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Abstract—5G has been developed for supporting diverse
services, such as enhanced mobile broadband (eMBB), massive
machine type communication (mMTC) and ultra-reliable low
latency communication (URLLC). The latter two constitute
enablers of the Internet of Things (IoT). The new spectrum
released for 5G deployments, primarily above 3 GHz,
unfortunately has a relatively high path-loss, which limits the
coverage, especially for the uplink (UL). The high propagation
loss, the limited number of UL slots in a TDD frame and the
limited user-power gravely limit the UL coverage, but this is
where bandwidth is available. Moreover, the stringent
requirements of eMBB and IoT applications lead to grave 5G
challenges, such as site-planning, ensuring seamless coverage,
adapting the TDD DL/UL slot ratio and the frame structure for
maintaining a low bit error rate (BER) as well as low latency,
etc. This paper addresses some of those challenges with the aid
of a unified spectrum sharing mechanism, and by means of an
UL/DL decoupling solution based on 4G/5G frequency sharing.
The key concept is to accommodate the UL resources in an
LTE FDD frequency band as a supplemental UL carrier in
addition to the New Radio (NR) operation in the TDD band
above 3 GHz. With the advent of this concept, the conflicting
requirements of high transmission efficiency, large coverage
area and low latency can be beneficially balanced. We
demonstrate that the unified 5G spectrum exploitation
mechanism is capable of seamlessly supporting compelling IoT
and eMBB services.
Index Terms—5G-NR, coverage enhancement, NR/LTE
frequency sharing, DL/UL decoupling, supplemental uplink,
internet of things
Introduction
The fifth-generation (5G) concept has been developed by
the International Telecommunication Union (ITU) known as
“IMT-2020” since 2012. Diverse 5G use-cases have been
envisioned, spanning from enhanced mobile broadband (eMBB)
to massive machine type communication (mMTC) as well as
ultra-reliable and low latency communication (URLLC) [1-4].
The latter two use-cases constitute major components of the
Internet of Things (IoT). Accordingly, the 5G radio interface
has to have quite diverse capabilities, including 20 Gbps peak
data rate, 100 Mbps user-rate, up to 500 km/h velocity, less
than 4 ms latency, and 100-fold improved network energy
efficiency to enable the seamless delivery of large amounts of
data for eMBB. Additionally, it also has to be capable of
supporting 1,000,000/km2 connection density, low power
consumption for mMTC and at least 99.999% reliability within
1 ms latency for URLLC. Vehicular communications which are
referred to as Vehicle to Everything (V2X) also constitute a
compelling 5G application. V2X communications defined in
3GPP include V2N (Vehicle-to-Network), V2V (Vehicle-to-
Vehicle), V2I (Vehicle-to-Infrastructure), V2P (Vehicle-to-
Pedestrian), complemented by the integrated cellular interface
and the direct link interface [15].
In the 3rd Generation Partnership Project (3GPP), 5G New
Radio (5G-NR) has been developed relying on a common air-
interface aiming for addressing such diverse requirements. The
first version of NR specifications was frozen in December 2017.
On the other hand, regional regulators invested a lot of efforts
in 5G spectrum planning for the first wave of 5G-NR
deployments, including the C-band (3 GHz-5 GHz) and
millimeter wave (mmWave) bands around 26 GHz and 39 GHz.
The mmWave bands have very large available bandwidth and
usually adopt time division duplexing (TDD) for exploiting the
channel-reciprocity to support both multiple-input multiple-
output (MIMO) techniques and asymmetric downlink
(DL)/uplink (UL) resource-allocation. However, those high
frequency bands also experience high propagation loss and are
typically configured to have a small number of UL
transmission slots in a 10 ms time-frame due to the heavy DL
traffic load, which will result in a limited UL coverage. Hence
a high infrastructure cost is imposed by the dense base station
(BS) deployment required for continuous coverage.
4G/5G Spectrum Sharing for Enhanced
Mobile Broad-Band and IoT Services
Lei Wan, Huawei Technologies Co., Ltd.
Zhiheng Guo, Huawei Technologies Co., Ltd.
Yong Wu, Huawei Technologies Co., Ltd.
Wenping Bi, Huawei Technologies Co., Ltd.
Jinhong Yuan, University of New South Wales.
Maged Elkashlan, Queen Mary University of London.
Lajos Hanzo, University of Southampton.
Additionally, the limited UL coverage also hampers both the
low latency of URLLC and the massive connection
requirements of mMTC, especially in the light of cost-
efficiency. Several challenging issues such as large coverage
and low latency, have to be tackled in order to support robust
vehicular communications, especially for autonomous driving
applications. As it will be analysed later, critical challenges are
experienced by the TDD wideband operation above 3 GHz in
efficiently delivering 5G services in wide-area coverage.
Hence innovative air interface design is expected for the
efficient support of 5G-NR eMBB and IoT services. Given that
the majority of operators are expected to deploy 5G as an over-
sailing layer on top of their existing long term evaluation (LTE)
network using frequency division duplexing (FDD) below 3
GHz, there is an opportunity to share the low frequency band
with part of the 5G-NR users or devices, as a complementary
band to the TDD band above 3 GHz. LTE/NR frequency
sharing, also known as “DL/UL decoupling”, was consequently
proposed during the standardization of 3GPP and was accepted
in Release 15. The concept of LTE/NR frequency sharing
detailed in Section III is to employ a part of the existing LTE
frequency band (most of them are below 2 GHz and are
allocated as paired spectrum) into NR operation in addition to
the new un-paired NR bands above 3 GHz. Since the lower-
frequency bands experience a lower propagation loss, by
exploiting this concept, the coverage can be substantially
extended and the challenges involved in 5G deployments can
also be conveniently circumvented. The frequency sharing
mechanisms can also be used jointly with previous studies [5-7]
for further enhancing the coverage for frequency bands above 3
GHz. This paper focuses on the standardization progress of the
first version of NR, thus it does not include the mMTC part.
However, it is clear that most IoT applications (low power
wide area, mMTC and even URLLC) need a large continuous
UL coverage. In this sense the LTE/NR UL sharing will indeed
benefit diverse IoT applications.
In this paper, the potential benefits of 4G/5G spectrum
sharing are investigated. Firstly the challenges ahead for 5G
deployments are analysed in Section II, and LTE/NR frequency
sharing is proposed for addressing these challenges. Their key
concepts and benefits are briefly introduced in Section III,
while the technical enablers of LTE/NR frequency sharing are
discussed in Section IV. The standardization process as well as
the regulatory framework of LTE/NR frequency sharing are
summarized in Section V. Finally, Section VI concludes the
paper.
5G Spectrum and Challenges
5G candidate spectrum
The IMT spectrum identified in the ITU’s World
Radiocommunication Conferences (WRC) 2015 and 2019
(which are below 6 GHz and above 24 GHz, respectively) are
applicable for 5G deployments. 3GPP defines frequency bands
for the 5G-NR interface according to the guidance both from
ITU and from the regional regulators, with prioritization
according to the operators’ commercial 5G plan. According to
[8], three frequency ranges are identified for 5G deployments
for both eMBB and IoT applications, including the new
frequency ranges of 3-5 GHz and 24-40 GHz, as well as the
existing LTE bands below 3 GHz.
As illustrated in Figure 1, generally, a triple-layer concept
can be applied to the spectral resources based on different
service requirements. An “over-sailing layer” below 2 GHz is
expected to remain the essential layer for extending the 5G
mobile broadband coverage both to wide areas and to deep
indoor environments. This is especially important for mMTC
and URLLC applications. On the other hand, the “coverage and
capacity layer” spanning from 2 to 6 GHz can be used for
striking a compromise between capacity and coverage.
However, compared to the range below 2 GHz, these bands
suffer from a higher penetration loss and propagation
attenuation. The “super data layer” above 6 GHz can be
invoked for use-cases requiring extremely high data rates but
relaxed coverage. Given this triple-layer concept, the eMBB,
mMTC and URLLC services that require different coverage
and rate-capability can be accommodated in the appropriate
layer. However, a service-based single-layer operation would
complicate the 5G deployments and it is inefficient in
delivering services that simultaneously require both good
coverage and high data rate as well as low latency, etc. To
accommodate these diverse services, the employment of joint
multiple spectral layers becomes a “must” for a meritorious 5G
network.
Figure 1 Multi-layer approach for 5G scenarios.
Coverage analyses for 5G spectrum
Let us define the coverage of a communication link as the
maximum tolerable power attenuation (in dB) of an
electromagnetic wave, as it propagates from the transmitter to
the receiver, whilst still guaranteeing the transmission rate
target, which is given by
= + + - - - - - - - - -
TX RX TX RX
coverage RE An t A nt RE m F CL C L pe SF f
CPGGNINγ
L
LLLL
(1)
where
R
E
P is the transmission power per subcarrier, γ denotes
the receiver sensitivity, TX
Ant
G and
R
X
Ant
G are the transmitter and
receiver antenna gains, respectively,
R
E
N and
F
N denote the
thermal noise and the noise figure of each subcarrier,
respectively. Furthermore, TX
CL
L
and
X
CL
L
are the cable-loss at
the transmitter and receiver side, while pe
L
, SF
L
, m
I
and
f
L
represent the penetration loss, shadowing loss, interference
margin and propagation loss difference due to the sub-carrier
frequency offset with respect to the reference frequency,
respectively.
According to Eq. (1), the coverage is affected by
numerous factors, including the transmission power,
propagation loss and receiver sensitivity. Since the propagation
loss varies with the frequency, the coverage substantially
differs in different frequency bands. Therefore, the provision of
a good performance in all frequency bands remains a key
challenge for 5G deployments. Furthermore, due to the limited
UL transmission power and higher path-loss in NR than in LTE,
the UL coverage is usually the bottleneck in 5G deployments.
In Figure 2, we portray the coverage performance of the
3.5 GHz TDD band and compare it to that of the 1.8 GHz FDD
band. Part of the parameters assumed for this comparison are
shown in Figure 2, while the rest are given in Table 1. In the
link budget, the UL coverage is calculated when the UL data
Figure 2 Link budgets for different frequency bands, where 1Mbps throughput is assumed
Parameters 1.8 GHz with 4T4R 3.5 GHz with 4T4R 3.5 GHz with 64T64R
PDCCH PUSCH PDCCH PUSCH PDCCH PUSCH
Tx antenna gain TX
Ant
G (dBi) 17 0 17 0 8.7 0
Tx Cable Loss TX
CL
L
(dB) 2 0 0 0 2 0
Rx antenna gain
X
Ant
G (dBi) 0 18 0 18 0 8.7
Rx Cable Loss
X
CL
L
(dB) 0 2 0 0 0 2
Penetration loss pe
L
(dB) 21 21 26 26 26 26
Receiver sensitivity γ (dBm) -129.44 -134.3 -129.44 -134.3 -141.02 -141.23
Shadowing loss SF
L
(dB) 9 9 9 9 9 9
Propagation loss due to frequency
f
L
(dB) 0 0 5.78 5.78 5.78 5.78
Interference margin m
I
(dB) 14 3 14 3 7 2
Thermal noise per subcarrier
R
E
N (dBm) -132.24 -132.24 -129.23 -129.23 -129.23 -129.23
Noise figure
F
N (dB) 7 2.3 7 3.5 7 3.5
Table 1 Parameters assumed in the link budgets.
rate is set to 1 Mbps for supporting typical uplink video traffic.
By contrast, the DL coverage is usually limited by the physical
downlink control channel (PDCCH) quantified in terms of the
block error rate of the primary PDCCH. It can be observed that
the UL coverage and DL coverage are balanced over the 1.8
GHz FDD band with the aid of 4 transmit and 4 receive
antennas. For the 3.5 GHz TDD band using the same transmit
and receive antennas as that of 1.8 GHz scenario, in excess of
10 dB coverage gap is observed. This is mainly due to the large
propagation loss, the penetration loss and the limited number of
UL transmission slots in a frame of the 3.5 GHz TDD band. By
comparison, for the 3.5 GHz TDD band using 64 transmit and
64 receive antennas, a similar DL coverage performance can be
achieved to that of 1.8 GHz, owing to the beamforming gain
provided by massive MIMOs and by the DL interference
margin difference. Explicitly, since massive MIMOs also
reduce the inter-cell interference, they reduce the DL
interference margin. However, the UL coverage is poorer
compared to the DL of 3.5 GHz, even when massive MIMOs
are employed, because the UL power spectral density of the 3.5
GHz TDD band is lower than that of the 1.8 GHz FDD band at
the same maximum device transmission power. This is partly
due to having less UL slots in a TDD-frame than in an FDD-
frame, which means that more frequency resources per slot
should be allocated for a given UL throughput of say 1 Mbps.
Therefore, how to improve the UL coverage is indeed an
important issue for 5G deployments.
5G spectrum duplexing and DL/UL asymmetry
Duplexing is another key factor affecting the performance
of 5G networks in terms of their wide-area coverage. 5G-NR
supports multiple duplex modes, including static TDD, FDD
and flexible duplexing. In 3GPP, the same frame structures and
resource allocation mechanisms are invoked for both FDD and
TDD. It is expected that early 5G deployments are very likely
to start from the new TDD spectral bands (e.g. 3.5 GHz).
Therefore in the following we will discuss both static and
dynamic 5G TDD networks.
For static TDD, the UL/DL traffic ratio is usually decided
by the statistical UL/DL traffic load ratio among multiple
operators in a specific country or region. As discussed in [9],
the DL traffic constitutes a large portion of the entire tele-
traffic. With the popularity of video streaming increasing, it is
forecast that the proportion of DL content will grow even
further in the future, hence it is natural that more resources
should be allocated to the DL. Therefore, a smaller proportion
of the resources is left for the UL, which will further affect the
UL coverage performance. On the other hand, for LTE FDD
band, the same bandwidth is allocated to both the UL and DL,
which means that the UL spectrum is under-utilized and will be
even severe in the future.
Hence dynamic TDD mechanisms have been specified
from 3GPP Release 12 onwards, especially for the hotspots
where the TDD DL/UL ratio can be adapted based on the
actual traffic. However, it has not yet been deployed in
practical systems due to its severe inter-carrier and intra-carrier
interference.
5G deployment challenges
In the following, we will discuss a few challenging issues
that have to be considered in 5G deployments, particularly for
the TDD mode and in higher frequency bands.
A. 5G Band selection: Wideband spectrum availability vs
coverage. The availability of the bands below 3 GHz remains
limited for 5G-NR in the near-future and the lower bands fail to
support high data-rates due to their limited bandwidth. On the
other hand, the wider NR bands above 3 GHz experience
increased propagation losses, leading to a limited coverage.
Therefore, independent usage of the spectrum below and above
3 GHz fails to strike a compelling trade-off between a high data
rate and large coverage.
B. TDD DL/UL ratio: Spectrum utilization efficiency vs
DL/UL coverage balance. As discussed, the NR TDD
operation is usually configured for a limited number of UL
transmission slots (e.g., DL:UL=4:1) in a frame due to the
heavy DL traffic load, even though more slots should be
allocated to the UL for improving the UL coverage. This can
increase the UL data-rates, when the bandwidth cannot be
further increased due to the maximum transmission power
constraint. While the DL spectral efficiency is usually higher
than that of the UL, having more UL slots would further reduce
the spectral utilization efficiency. Therefore, there is a clear
trade-off between the UL coverage and spectral utilization
efficiency.
C. TDD DL/UL switching period: Transmission
efficiency vs latency. For the TDD operation, frequent DL/UL
switching is required for low-latency DL and UL transmission.
However, a certain guard period is needed at each DL/UL
switching point (e.g. 130 us is used in TD-LTE networks) for
avoiding serious blocking of the UL receiver due to the strong
DL interference emanating from other cells. Frequent DL/UL
switching would lead to a high idle-time (14.3% vs 2.8% for 1
ms and 5 ms switch period), which is undesirable in efficient
eMBB services.
D. Site planning: seamless coverage vs deployment
investment. For early 5G-NR deployment, co-site installation
with the existing LTE networks would be cost-effective and
convenient. However, due to the higher propagation loss above
3 GHz, one has to introduce denser cells and new sites.
Otherwise, 5G-NR cannot attain the same seamless UL
coverage as that of LTE.
To circumvent this challenge, a new LTE/NR frequency
sharing concept was accepted by 3GPP, which will be
elaborated on below.
NR/LTE Frequency Sharing: Addressing
Deployment Challenges
The concept of LTE/NR frequency sharing is to exploit the
spare resources in the existing LTE frequency band for 5G-NR
operation as a complement of the new 5G wideband spectrum.
For example, as shown in Figure 3, the C-band (frequency
ranges of 3-5 GHz) TDD carrier can be paired with the UL part
of a FDD band overlapped with LTE (e.g., 1.8 GHz). In other
words, an UL carrier within the lower frequency FDD band is
coupled with a TDD carrier in the higher frequency band for
NR users. Then a NR user has two UL carriers and one DL
carrier in the same serving cell. By contrast, only one DL
carrier and one UL carrier are invoked for a traditional serving
cell. With the advent of this concept, the cell-edge NR users
can employ either the lower frequency FDD band carrier (UL
part) or the higher frequency TDD band carrier to transmit their
uplink data. In this case, since the UL propagation loss on the
lower frequency band is much lower than that of the higher
frequency TDD band, the coverage performance of NR users
can be substantially extended and a high UL data-rate is
guaranteed even if this user is relatively far from the BS. On
the other hand, the cell-centre users can rely on the higher-
frequency TDD band to take advantage of its higher bandwidth.
Figure 3 LTE/NR uplink spectrum sharing to extend 5G
coverage at higher frequencies (e.g. 3.5 GHz)
Usually, it is not necessary to allocate the low-frequency
FDD band for the DL of NR, since as discussed in Section II,
the DL coverage in the C-band is good. Then the low-
frequency FDD band is employed in NR only for the UL. In
3GPP, the UL-only carrier frequency is referred to as the
supplementary uplink (SUL) frequency from a NR perspective.
Given the concept of LTE/NR frequency sharing, the four
challenging issues described in Section II can be dealt with
appropriately.
Balance between wideband spectrum availability
and coverage quality.
With the advent of LTE/NR frequency sharing, the
spectrum availability vs. coverage trade-off can be well
balanced. In this case, the 5G-NR DL traffic is scheduled on
the higher TDD bands, and a high DL/UL tele-traffic ratio
facilitates the efficient exploitation of the large bandwidth. The
DL coverage quality remains similar to that of LTE with the
aid of massive MIMO and multiple beam scanning (for
example, 3D beam forming [7]). Additionally, the 5G-NR UL
traffic can be supported by either a low-frequency SUL carrier
or by a high-frequency TDD carrier. The cell-edge users rely
on lower-frequency bands for ensuring that their spectral
efficiency can be maintained at the same level as that of LTE,
and their UL scheduling opportunities can be increased
compared to that in the high-frequency TDD-only system.
Consequently, both higher data-rates and large coverage are
achieved.
Balance between spectrum utilization efficiency and
DL/UL coverage
LTE/NR frequency sharing is instrumental in striking a
compelling trade-off between high spectrum exploitation
efficiency and wider DL/UL coverage. For the high-frequency
TDD carrier, the DL/UL time-slot (TS) ratio configuration only
has to take into account the long-term DL/UL traffic statistics
for guaranteeing the DL spectrum exploitation efficiency
(usually 4:1). The cell-edge users and IoT devices may opt for
the SUL carrier philosophy for their UL transmission. In this
case, the high DL/UL TS ratio on the TDD carrier does not
impose any detrimental effects on IoT services. In addition, the
lower propagation loss of the lower band is helpful for
improving the spectrum efficiency. As a result, given a certain
packet size, the requirements imposed on the scheduled
bandwidth, or the UE’s transmit power are reduced on the
lower band compared to that on the higher band.
Let us now observe the UL user throughputs of various
UL channel allocations in the 3.5 GHz band, the joint 3.5 GHz
and 0.8 GHz bands and joint 3.5 GHz and 1.8 GHz bands seen
in Figure 4. OFDM waveform is adopted for both the LTE DL
as well as for the 5G-NR DL and UL, while the LTE UL
adopts the SC-FDMA (single carrier frequency division
multiple access) waveform based on similar frequency-domain
subcarrier mapping as that of OFDM waveform. The UE’s
maximum total transmission power for all cases is 23 dBm and
the DL/UL TS ratio of the 3.5 GHz TDD system is 4:1. The
channel bandwidths of the 3.5 GHz, 0.8 GHz and 1.8 GHz
scenarios are 100 MHz, 10 MHz and 20 MHz, respectively.
Observe in Figure 4 that the UL throughput of the cell-edge
UEs relying on the SUL is substantially improved compared to
that of the UEs operating without SUL, which is a joint benefit
of the additional bandwidth, of the lower propagation loss and
of the continuous UL resource of the SUL. Additionally, the
UL throughput of UEs relying on the SUL at 0.8 GHz is better
than that of the UEs with SUL at 1.8 GHz at lower throughput,
but it is lower than that of UEs with SUL at 1.8 GHz at higher
throughput. The reason for this trend is that when the UL
throughput is low, the UEs are usually power-limited and the
propagation loss is lower at low frequencies, hence the
throughput of the SUL at 0.8 GHz is better than at 1.8 GHz. By
contrast, when the throughput is high, the uplink transmission
power is not an issue and it is the bandwidth that becomes the
bottleneck, thus the throughput of the SUL at 1.8 GHz within a
20 MHz bandwidth outperforms that at 0.8 GHz with 10 MHz
bandwidth. Therefore, with the advent of the LTE/NR
frequency sharing concept, the spectrum exploitation efficiency
and DL/UL coverage can be beneficially balanced.
Figure 4 UL user throughput comparison
Balance between transmission efficiency and
latency
Low latency is a critical requirement for URLLC services.
In 5G-NR design, a self-contained TDD frame structure [10] is
proposed, where in each sub-frame/slot, both DL and UL can
be included. As indicated, frequent DL/UL switching may help
reduce the UL latency, but it also introduces a non-negligible
overhead, which is inefficient for both of eMBB and URLLC
services in a unified system. Under the LTE/NR frequency
sharing concept, the URLLC devices can be scheduled at the
SUL carrier for the UL data or control messages, which means
that UL resources always exist, whenever an UL message
arrives. Thus, the latency due to the discontinuous UL
resources of the TDD carrier is beneficially reduced and
simultaneously, the overhead caused by the frequent DL/UL
switching on the higher-frequency TDD band can also be
avoided.
Figure 5 shows both the latency and the overhead
comparison of various TDD frame structures. For the “TDD
carrier only” system associated with a 5 ms switch period, the
round trip time (RTT) cannot be tolerated by the URLLC
services, due to the long feedback latency. If a self-contained
TDD frame is applied in the “TDD carrier only” system having
a 1 ms switch period, although the RTT is reduced, the
overhead increases dramatically due to the frequent DL/UL
switch. For the proposed LTE/NR frequency sharing concept,
the SUL can provide timely UL feedback without frequent
DL/UL switching, which hence beneficially reduces the RTT
without any extra overhead. Therefore, the transmission
efficiency and latency become well-balanced.
Balance between seamless coverage and
deployment investment.
Seamless coverage is highly desirable for 5G NR in order
to provide a uniform user experience. Again, it is difficult for
5G NR to achieve seamless coverage in case of co-site
deployment with LTE by only using the frequency band above
3 GHz. With the advent of the LTE/NR frequency sharing, the
5G-NR UL becomes capable of exploiting the precious limited
spectrum resources in the lower frequency bands that the
operators have been using for LTE. Therefore, the NR UL
coverage can be improved to a similar level as that of LTE.
This implies that the seamless NR coverage can be supported
in co-site NR/LTE deployment.
Mobility improvement
With the advent of LTE/NR frequency sharing, seamless
NR coverage is achieved and the mobility-related user-
experience is also improved. As illustrated in the co-site
deployment example of Figure 6(a), due to the limited UL
Figure 5 Latency comparison of different TDD frame structures
coverage, the radius of 5G C-band cells is much smaller than
that of the LTE 1.8 GHz cells. When a UE moves to the
boundary of the cells, inter-Radio Access Technology (RAT)
handovers will occur. Note that each inter-RAT handover will
impose interruptions in excess of 100 ms, which is much higher
than that of the intra-RAT handover. With the advent of the
LTE/NR spectrum sharing concept, the SUL carrier
beneficially extends the coverage of 5G cells. As shown in
Figure 6(b), with the help of SUL, the coverage range of 5G
cells and LTE cells becomes similar. Then inter-RAT
handovers will occur much less frequently, because handovers
are only encountered when the UE goes beyond the boundary
of the area contiguously covered by multiple 5G-NR cells.
Thus the probability of the inter-RAT handovers is
significantly reduced, consequently the UE’s mobility-related
experience is remarkably improved with the aid of LTE/NR
spectrum sharing mechanisms.
Unified support for IoT and eMBB
LTE/NR frequency sharing also provides a unified support
for diverse IoT and eMBB services, including the following
aspects:
In 5G-NR operation a cell can include both a TDD
carrier and a SUL carrier.
A unified eMBB and IoT TDD DL/UL frame
structure configuration can be used at a high-
frequency TDD carrier. The eMBB-optimized
configuration imposes no detrimental impact on low-
latency IoT devices, because a pair of uplinks are
available for transmission and the tele-traffic of the
low-latency IoT devices can be offloaded to a SUL
carrier. Moreover, the unified eMBB and IoT TDD
DL/UL ratio eliminates the potential network
synchronization or inter-carrier synchronization
problems of multiple operators.
A unified site planning can be arranged for 5G-NR
deployment in harmony with the existing LTE
networks to meet the diverse requirements of both
eMBB and IoT services.
Technical Enablers of NR/LTE Frequency
Sharing
To enable LTE/NR spectrum sharing, the relevant
LTE/NR coexistence mechanisms have been specified in 3GPP
Release 15. In this section, some key mechanisms, including
efficient spectrum sharing management, frequency sensing and
UL frequency selection as well as service-oriented dynamic
scheduling are introduced.
Efficient LTE/NR frequency sharing management
As for the LTE/NR frequency sharing, the specific
resource sharing philosophy is of prime concern [11]. Based on
statistical spectral-activity results of practical LTE networks,
the UL resources in the paired spectrum are typically
underutilized. This offers opportunities for exploiting the idle
LTE UL resources for the UL transmission of 5G-NR.
According to the 3GPP specification ratified for LTE FDD
bands, there is a provision for feedback information in all the
UL sub-frames. Thus, it is important to reserve UL feedback
resources in all sub-frames for legacy LTE UEs for improving
the network’s performance. As illustrated in Figure 7(a),
frequency division multiplexing (FDM) between LTE and NR
is recommended either in a semi-static or in a dynamic manner.
Semi-static sharing is suitable for multiple vendors’
deployment, because it requires no frequent scheduling
information exchange between the LTE and NR equipment,
while dynamic sharing is more suitable for the deployment of
NR and LTE equipment from the same vendor and it typically
achieves a higher spectral efficiency. In addition, the LTE/NR
frequency sharing will cause little burden on inter-operator
cooperation. On one side, almost all the operators who have 5G
NR deployment plan today also have existing LTE network at
low frequency bands and there is no need for inter-operator
cooperation. On the other side, if LTE and 5G-NR belong to
5G C-Band Cell
LTE 1.8 GHz Cell
①②③④⑤⑥ Handover
5G C-Band & 1.8 GHz –SUL Cell
LTE 1.8 GHz Cell
①②Handover
(a) (b)
Figure 6. Seamless coverage by co-site NR/LTE deployment: a) UL only over higher frequency band; b) UL over both C-band and
lower frequency band.
different operators, it is difficult for LTE UL and 5G-NR-SUL
to conduct the dynamic TDM carrier sharing. Static or semi-
static frequency domain reservation for 5G-NR-SUL carrier
can be used, which can relax the tight inter-operator
coordination requirement significantly.
Figure 7 LTE-NR FDM sharing: semi-static or dynamic
In order to make full use of the spectral resources, it is
expected that the LTE and NR UEs are scheduled in orthogonal
frequency resources without any extra overhead at the
boundaries between the frequency resources allocated to LTE
and NR. Accordingly, as shown in Figure 7(b), the subcarrier
spacing (SCS) of NR SUL can be configured in the same way
as in LTE. The NR SUL scheduling granularity is designed to
be aligned with the physical resource block (PRB) boundary of
LTE, otherwise wasteful guard bands would be needed. In
Figure 7(b), the abbreviations represent the: physical uplink
control channel (PUCCH), physical random access channel
(PRACH), and physical uplink shared channel (PUSCH).
In NR, different SCSs are specified for different frequency
ranges, while only 15 kHz SCS is defined in LTE. In order to
coexist with LTE, the SCS of the SUL carrier is recommended
to be 15 kHz, which is likely to be different from that of the
new TDD band for NR, e.g. 30 kHz SCS for 3.5 GHz TDD
band. As a consequence of different SCSs on the SUL and on
the TDD carrier, the parameters, including the lengths of
orthogonal frequency division multiplexing (OFDM) symbols
and slots on the two carriers are different. 3GPP Release 15
defined the corresponding mechanisms for supporting efficient
scheduling, and the feedback for UL and DL.
Additionally, for the LTE UL carrier, there is a half-SCS
(7.5 kHz) shift of the subcarriers to reduce the impact of the
direct-current leakage to the discrete Fourier transform-spread-
OFDM (DFT-S-OFDM) waveform. Hence a 7.5 kHz shift is
also required for the SUL bands. Otherwise, the subcarriers of
LTE and NR would not be orthogonal [12]. Additionally, the
LTE frequency bands will be re-farmed for NR in the future. In
this case, the 7.5 kHz shift should also be introduced for the
LTE re-farmed bands in order to support its coexistence with
the narrow band internet of things (NB-IoT) and enhanced
MTC.
The UE implementation design of the SUL and TDD UL
transmission is another important issue. A potential prototype
design is portrayed in Figure 8. To facilitate the prompt UL
carrier switching, the 7.5 kHz subcarrier shift of the SUL
carrier can be more beneficially carried out in the digital
domain. This is because if the frequency shift is implemented
in the RF domain, much longer retuning time would be
imposed between the LTE UL and NR SUL [14].
Figure 8 Joint LTE-NR UE architecture [14].
Single uplink transmission
Another challenge for LTE/NR uplink frequency sharing
is the deleterious interference. Simultaneous UL transmissions
on the 1.8 GHz SUL band and the 3.5 GHz TDD band will
impose serious in-device inter-modulation interference, which
may degrade the 1.8 GHz DL reception quality. 3GPP Release
15 has specified that LTE/NR uplink sharing is only allowed to
select a single UL carrier to transmit at any instant in a UE.
Additionally, prompt carrier switching between an SUL and
TDD carrier is supported, if an SRS is needed at a TDD carrier
for the specific cell-edge UEs, which are scheduled on the SUL
carrier. The standard UE architecture design has already
supported individual RF chains for the SUL band and TDD
band, which supports prompt UL carrier switching, hence it is
very convenient for scheduling.
Frequency sensing and UL frequency selection
For a 5G NR system having a combined TDD carrier and
SUL carrier, frequency sensing is required for the UL
frequency selection and random access [11].
For initial access, it is best for cell-edge users to transmit
the random access preamble on the SUL carrier, while the cell-
center users may be better served by selecting the higher-
frequency TDD carrier for random access. Therefore, during
the initial access, each UE compares its DL reference signal
received power (RSRP) measurement on the TDD carrier to the
RSRP threshold configured by the network to select the UL
carrier for random access. If the RSRP is lower than the
threshold, the UE is classed as a cell-edge UE and will request
random access on the SUL carrier, while if the RSRP is higher
than the threshold, the UE is treated as a cell-center UE and
will select the TDD carrier for random access.
Service-oriented dynamic scheduling
5G NR provides a unified air interface for the flexible
support of various services. Additionally, to support the various
services by appropriate system configurations, scheduling and
resource allocation relying on Quality of Service (QoS)
awareness is supported. 3GPP Release 15 defines three slice
types for the so-called 5G new core, including eMBB, URLLC
and mMTC. Each slice type is configured to meet a specified
set of QoS parameters. The QoS of each slice type can be
passed down from the core network to the radio access network.
Then, based on the QoS requirements, the BS can perform
either QoS-prioritized scheduling or service-oriented
scheduling. Such a service-oriented scheduling mechanism can
work together with the UL carrier selection in the above-
mentioned LTE/NR uplink sharing. For example, the URLLC
service can automatically select the SUL carrier from the outset
without the need for comparing the RSRP to the appropriately
configured threshold.
Independent configuration of non-SUL and SUL
To support a pair of UL carriers in a serving cell, various
specific configurations are needed. In the standardization, some
of the parameters, such as the random access related
configurations, data transmission bandwidth, transmission
power settings, downlink to uplink scheduling timing etc. are
configured for the SUL and non-SUL (TDD carrier)
independently. Given these carefully specified configurations,
the SUL and non-SUL can seamlessly work together for
improving the system performance.
Standardization of NR/LTE Frequency Sharing
3GPP standardization progress on LTE/NR
coexistence
On December 21st, 2017, the first version of non-
standalone (NSA) 5G was declared to be frozen and the
LTE/NR coexistence is one of the important features on the
completed list. The completed technology components include
the spectrum to be used for standalone NR and for the non-
standalone LTE/NR dual-connectivity mode, as well as for
HARQ feedback, power control, UL scheduling mechanisms
and so on. In the following, we will mainly discuss the
LTE/NR coexistence band combinations specified in 3GPP
Release 15.
LTE NR coexistence band combination definition
As shown in Table 2, 3GPP Release 15 has defined a
number of bands for SUL and for the corresponding SUL and
TDD band combinations conceived for NR standalone and
non-standalone deployment, respectively [13].
In the “5G NR New bands” column of Table 2, typical
examples of the frequency bands specified for the NR
operation are given. The frequency bands include the C-band
frequencies spanning from 3.3 GHz to 5 GHz, and the
millimeter wave band having frequencies around 26 GHz and
38 GHz. The SUL bands spanning from 700 MHz to 2 GHz are
also specified, as shown in the “5G NR New bands” column of
Table 2. As described above, when SUL is used, there are two
UL carriers in a serving cell. Then the frequency band
combinations for the two UL carriers in a serving cell are
defined in the column of “5G NR Band combinations”. In order
to make the band combination definitions more clearly, some
examples are given in the following. Taking SUL_n78-n80 of
Table 2 as an example, in a serving cell, the non-SUL carrier is
on band n78 and the SUL carrier is on band n80. Another
example is DC_1-SUL_n78-n84, where DC means that the
dual-connectivity-aided UE is configured with both LTE and
NR. The LTE cell is on LTE band 1 and the NR cell is on band
n78 with an additional SUL carrier on band n84. Since the NR
SUL band n84 overlaps with LTE band 1, the LTE UL carrier
and the NR SUL carrier share the same frequency resources.
5G NR New bands 5G NR Band combinations
Standalone Non-standalone
Band number Frequency Duplex Band number Band number
n77 3.3 - 4.2 GHz TDD SUL_n78-n80 DC_1-SUL_n78-n84
n78 3.3 - 3.8 GHz TDD SUL_n78-n81 DC_3-SUL_n78-n80
n79 4.4 - 5.0 GHz TDD SUL_n78-n82 DC_3-SUL_n78-n82
n80 1710 - 1785 MHz SUL SUL_n78-n83 DC_8-SUL_n78-n81
n81 880 - 915 MHz SUL SUL_n78-n84 DC_20-SUL_n78-82
n82 832 - 862 MHz SUL * For example: SUL_n78-
n80 means band combination
of NR band n78 and band
n80(SUL) for NR operation
DC_28-SUL_n78-n83
n83 703 - 748 MHz SUL * For example: DC_1-SUL_n78-n84
means LTE-NR dual connectivity
between LTE Band 1, and NR bands
n81 (SUL) and n78 including NR-LTE
coexistence with UL sharing.
n84 1920-1980 MHz SUL
n257 26.5 29.5 GHz TDD
n258 24.25 27.5 GHz TDD
n260 37 40 GHz TDD
Table 2 5G NR new bands and band combination definitions
Summary and Future Work
This paper introduced an innovative spectrum exploitation
mechanism, namely the LTE/NR spectrum sharing philosophy,
for efficient 5G deployment in order to serve both eMBB and
IoT applications. This solution eminently balances the various
conflicting requirements, such as DL/UL traffic asymmetry,
DL/UL coverage imbalance, transmission efficiency versus
latency, etc. The proposed spectrum sharing between LTE and
NR also allows operators to retain their LTE investment
without “re-farming” the LTE band to NR, given that the
spared LTE UL resources can be used as a 5G NR SUL carrier
paired with a wideband TDD carrier above 3 GHz.
As to future work, firstly, it is expected that more
spectrum combinations can be introduced. For example, the
SUL carrier can be paired with the DL-only band in order to
form an independent cell. Another promising technique of
LTE-NR coexistence is to combine the SUL carrier with the
mmWave band in order to improve both the UL coverage and
the mobility, whilst simultaneously reducing the number of
mmWave base stations required for providing seamless
coverage. In this case the SUL receiver and the mmWave
transceiver may be deployed at non-collocated base stations.
There are several challenges for the non-collocated scenario,
such as the provision of power control, uplink synchronization,
uplink access point switching, etc. Other evolving scenarios
may include multiple SUL carriers being paired with higher
frequency bands within the same cell. The strategies of traffic
and user allocation among multiple SUL and UL carriers also
have to be studied. The evolution of LTE/NR frequency
sharing can also aim for supporting the IoT services at a low
latency in a large coverage area, in addition to supporting
eMBB operation.
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... However, the deployment of services on such high frequencies has to be studied with attention, especially because of several open challenges. In accordance to that, Wan et al. [75], Al-Khatib et al. [76], and Shah et al. [77] briefly discuss spectrum sharing towards 5G, presenting the idea to reuse existing LTE spectrum together with new frequency bands used by 5G New Radio (NR). ...
... The authors also suggest new research directions in investigating the integration of multiple spectrum sharing techniques to address the highly heterogeneous nature of 5G networks. Finally, there are several observed and yet very important challenges related to LTE/NR uplink (UL) sharing which is expected to benefit IoT applications [75]. In particular, this approach generates trade-offs between spectrum availability and coverage, spectral efficiency and downlink (DL)/UL coverage balance, transmission efficiency and latency, and seamless coverage and deployment investment [75]. ...
... Finally, there are several observed and yet very important challenges related to LTE/NR uplink (UL) sharing which is expected to benefit IoT applications [75]. In particular, this approach generates trade-offs between spectrum availability and coverage, spectral efficiency and downlink (DL)/UL coverage balance, transmission efficiency and latency, and seamless coverage and deployment investment [75]. Due to their relevance for incorporating IoT into FCNs, they are elaborated within Section VII. ...
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IMT vision-framework and overall objectives of the future development of IMT for 2020 and beyond
  • Itu-Rm Recommendation
Recommendation ITU-RM.2083, "IMT vision-framework and overall objectives of the future development of IMT for 2020 and beyond," Sep. 2015.
Minimum requirements related to technical performance for IMT-2020 radio interface(s)
  • Itu-R M Report
Report ITU-R M.2040, "Minimum requirements related to technical performance for IMT-2020 radio interface(s)". Nov, 2017.
IMT traffic estimates for the years 2020 to
  • Itu-R M Report
Report ITU-R M.2370-0, "IMT traffic estimates for the years 2020 to 2030". July, 2015.
R1-1706905 Overview of NR UL for LTE-NR coexistence
  • Hisilicon Huawei
Huawei, HiSilicon, "R1-1706905 Overview of NR UL for LTE-NR coexistence", 3GPP TSG-RAN WG4-NR Meeting #2, June, 2017. https://portal.3gpp.org/ngppapp/CreateTdoc.aspx?mode=view&c ontributionId=783916
R1-1706906 Consideration on subcarrier mapping for LTE-NR coexistence
  • Hisilicon Huawei
Huawei, HiSilicon, "R1-1706906 Consideration on subcarrier mapping for LTE-NR coexistence", 3GPP TSG-RAN WG1 Meeting #89, May, 2017. https://portal.3gpp.org/ngppapp/CreateTdoc.aspx?mode=view&c ontributionId=783917
User Equipment (UE) radio transmission and reception
3GPP, "User Equipment (UE) radio transmission and reception", TS 36.101 V15.0.0, September. 2017.