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14G Americas HSPA+LTE Carrier Aggregation – June 2012
TABL E OF CONTENTS
Executive Summary ...................................................................................................................................... 2
1. Introduction .............................................................................................................................................. 3
1.1 Scope .................................................................................................................................................. 3
1.2 HSPA+ and LTE network deployment projections .............................................................................. 3
2. Multicarrier and multi-radio network evolution ......................................................................................... 6
2.1 Spectrum and deployment aspects ..................................................................................................... 6
2.2 HSPA evolution from multiple carriers to multicarrier .......................................................................... 8
2.3 LTE Evolution from multiple carriers to carrier aggregation .............................................................. 13
2.4 HSPA and LTE interworking .............................................................................................................. 16
2.5 HSPA+LTE carrier aggregation ......................................................................................................... 18
3. Benefits and use cases of HSPA+LTE aggregation .............................................................................. 19
3.1 Benefits of HSPA+LTE aggregation .................................................................................................. 19
3.2 Example USE cases for HSPA+LTE Aggregation ............................................................................ 20
4. HSPA+LTE aggregation system architecture considerations ................................................................ 21
4.1 Service or core network level split/merger ........................................................................................ 23
4.2 HSPA RAN level split/merger ............................................................................................................ 25
4.3 LTE RAN level split/merger ............................................................................................................... 28
5. Practical implementation aspects of HSPA+LTE aggregation ............................................................... 30
5.1 Base station Radio implementation aspects ..................................................................................... 30
5.2 Device Radio implementation aspects .............................................................................................. 31
5.3 Implementation aspects other than radio processing ........................................................................ 33
6. Conclusion ............................................................................................................................................. 34
Abbreviations .............................................................................................................................................. 35
Acknowledgements ..................................................................................................................................... 37
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24G Americas HSPA+LTE Carrier Aggregation – June 2012
EXECUTIVE SUMMARY
LTE networks are being rolled out at an increasing speed, while at the same time the existing HSPA
networks are expanded and upgraded with the more advanced HSPA+ features in order to cater to the
ever-increasing appetite for wireless data. Due to the major investments in the HSPA+ infrastructure and
the vast and rapidly increasing HSPA+ based mobile broadband device penetration the two networks canbe foreseen to coexist in parallel for years to come.
The evolution of both HSPA+ and LTE standards has introduced aggregation of carriers for higher data
rates, better load balancing and increased spectrum utilization, and since the dawn of LTE, the standard
support for radio level interworking for HSPA and LTE radios has been included. A natural continuation of
such development is to tighten the interworking even further and introduce similar aggregation of carriers
between the two radio access technologies.
The HSPA+LTE aggregation allows for transmitting data to one user simultaneously using both the HSPA
and the LTE radios for maximal utilization of the available spectrum and the deployed equipment. This is
considered beneficial especially in the environment where the spectrum that needs to be shared between
the two radio access technologies is not abundant, and the deployed HSPA and LTE capacities and userdata rates suffer from spectrum crunch. One example of such deployment is the 900 MHz for HSPA and
the 800 MHz for LTE which are both seen attractive bands for building the coverage due to the low
frequency but also suffer from very limited spectrum availability. With aggregation of the two bands it is
possible to provide the high data rates expected from the LTE services while at the same time maintain
coverage for the HSPA devices.
The same gain mechanisms that have been seen beneficial for Multicarrier HSDPA as well as LTE
Carrier Aggregation can be benefited from by aggregating HSPA with LTE. At low or medium load,
HSPA+LTE aggregation is able to take advantage of the unused resources leading to significant data rate
increases both at the cell edge and the cell center for the carrier aggregation capable devices. In addition,
the carrier aggregation enables fast (millisecond level) load balancing across the carriers thus improving
the data rates of all users.
A number of possible network architectures can be foreseen for HSPA+LTE aggregation, and are briefly
touched upon in this white paper. Most promising architecture options are seen with co-located multiradio
base stations with the base station (NodeB + eNodeB) acting as the data aggregation point, and
simultaneously maintaining the existing network architecture for the devices connecting to the network
with one radio system at a time only. This architecture can utilize some of the already deployed RF
hardware in the base station, while new baseband functionality managing the data flow is required. On
the device side, receiver radio architectures capable of multiband carrier aggregation should be suitable
also for HSPA+LTE aggregation.
While Dual-Cell HSDPA is already in commercial operation, and higher levels of HSPA carrier
aggregation as well as LTE carrier aggregation are part of 3GPP specifications existing today,HSPA+LTE aggregation is currently not standardized. Although conceptually straightforward and building
on already standardized concepts, HSPA+LTE aggregation is a major feature, with a standardization
effort comparable to that of LTE carrier aggregation.
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34G Americas HSPA+LTE Carrier Aggregation – June 2012
1. INTRODUCTION
1.1 SCOPE
This white paper will highlight some of the key aspects and architecture options available for aggregating
HSPA and LTE carriers. Individually within both the HSPA and the LTE evolution, downlink and uplinkcarrier aggregation as well as co-site and inter-site aggregation have been considered. For both radio
access technologies, the co-site aggregation has been standardized first and inter-site aggregation is
currently being worked on.
The HSPA+LTE aggregation is a potential 3GPP Release 12 topic, and at the time of writing, there is yet
no official commitment or ongoing work related to any Release 12 items ongoing in the standards body.
Hence the contents of this white paper can be seen more as visionary and exploratory than describing a
feature already existing, or being worked on in the standard.
1.2 HSPA+ AND LTE NETWORK DEPLOYMENT PROJECTIONS
HSPA, HSPA+ AND LTE DEPLOYMENTS AS OF JUNE 2012
HSPA: 473 commercial networks in 180 countries
HSPA+: 227 commercial networks in 109 countries
LTE: 91 commercial networks in 47 countries
LTE: 335 operator Commitments worldwide
LTE: Over 130 commercial networks expected by year end 2012
4G AMERICAS WHITE PAPER “THE EVOLUTION OF HSPA” – OCTOBER 2011
Whereas, LTE has tremendous momentum in the marketplace and it is clearly the next generation
OFDMA based technology of choice for operators gaining new spectrum, HSPA will continue to be a
leader in mobile broadband subscriptions for the next five to ten years. Some forecasts put HSPA at over
3.5 billion subscribers by the end of 2016, almost five times as many LTE subscribers predicted. Clearly,
operators with HSPA and LTE infrastructure and users with HSPA and LTE multi-mode devices will be
commonplace. With the continued deployment of LTE throughout the world, and the existing ubiquitous
coverage of HSPA in the world, HSPA+ will continue to be enhanced through the 3GPP standards
process to provide a seamless solution for operators as they upgrade their networks.
High-Speed Packet Access (HSPA) systems are now commonplace across Latin America and operators
are looking to get full benefit from this technology as it evolves to HSPA+. Although the future is LTE, the
region is a good example of how 3G networks can take the customer all the way to the cusp of that new
era. Progress made since 3GPP Release 7 has allowed HSPA+ to benefit from the techniques used in
the elaboration of LTE to ensure that both support smart phones, tablets and PCs as user needs grow.
The 4G Americas white paper “The Evolution of HSPA” predicts that future enhancements in 3GPP
Release 11 should allow HSPA+ to deliver up to 336 Mbps. Talking about the paper, Erasmo Rojas, of
4G Americas says, “HSPA+, with its continuously evolving and growing ecosystem, is becoming
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44G Americas HSPA+LTE Carrier Aggregation – June 2012
ubiquitous in major cities throughout Latin America and is setting the stage for future deployments of LTE
in 2012 and beyond as operators gain access to new spectrum assets”.
HSPA+ IN EMERGING MARKETS
Emerging markets operators will hold off on LTE deployments in favor of upgrading their 3G networks to
HSPA+. This technology provides spectral efficiency and headline data speeds similar to current
implementations of LTE for the price of a software upgrade in most cases. Short-term, the HSPA+
ecosystem will be better developed than that of LTE, especially in handsets, meaning there will be plenty
of lower-cost devices better suited to price-sensitive emerging markets consumers.
(RCR Wireless, 2012 Predictions: Emerging markets operators will invest more in HSPA+ than LTE,
Posted on 19 January 2012 by Wally Swain, SVP of Emerging Markets, Yankee Group.)
LTE IN THE US (JUNE 2012)
AT&T's 4G LTE network was live in 41 markets as of June 21, 2012 and covered 74 million POPS. The
carrier expects to cover 150 million POPS by year end 2012 and complete its LTE network by the end of2013. If you're an AT&T customer in a city or town that doesn't have LTE yet, your 4G network is HSPA+.
T-Mobile will launch 4G LTE in 2013 in 1700/2100 MHz spectrum. T-Mobile’s nationwide HSPA+network covers 220 million people in 230 markets as of June 2012. They plan to launch 4G HSPA+service in the 1900 MHz band in a large number of markets by the end of the year.
Verizon’s LTE network was available in 304 cities as of June 21, 2012, covering more than 200 millionAmericans; coverage is expected to surpass its existing 3G footprint by end-2013. More than 260 millioncustomers in 400 markets will be able to access 4G LTE by the end of the year.
Sprint customers in Baltimore, Kansas City, Dallas, San Antonio, Houston and Atlanta are slated to
receive 4G LTE service by mid-2012. The company hopes to cover 123 million POPS with LTE by the end of
2012, and 250 million by the end of 2013.
Clearwire is planning to launch LTE in TDD spectrum in 31 cities in the first half of 2013.
HSPA+ AND LTE GROWTH
There are a number of reports that support the growth for HSPA+ and LTE network deployments,
including research from ABI Research, which predicts that there will be 80 million super-fast LTE mobile
broadband lines across the world by 2013. The preferred frequencies for 4G LTE broadband services are
the 700 MHz and 2.6 GHz bands, although the 1.8 GHz and 2.5 GHz bands have been utilized in
countries such as Poland and Singapore. In the UK, network operators will have the opportunity to deploy
LTE networks using the 800 MHz and 2.6 GHz spectrum bands, but British consumers still face a lengthywait for access to the technology.1
In the Americas, LTE was first deployed in the Americas at 700 MHz and followed soon by the AWS
spectrum band 1700/2100 MHz. In Latin America, HSPA+ and LTE is deployed in the 700, 1700/2100,
1900 and 2600 MHz spectrum bands. It is possible that the AWS 1700/2100 MHz spectrum band will be
a common LTE spectrum band in North, Central and South America.
1 ABI Research predicts LTE mobile broadband lines will hit 80m by 2013. ABI Research, 24 October, 2011.
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54G Americas HSPA+LTE Carrier Aggregation – June 2012
According to research from In-Stat, LTE mobile broadband technology is set for a surge in growth over
the next four years. Between now and 2015, the number of people signed up for the next-generation
service will increase by 3,400 percent, the market intelligence firm claimed.2 In-Stat attributed this rapid
rise in subscriptions to consumer desire to connect to the Internet while on the move at any time of day
and using a variety of devices, such as smartphones and tablet PCs. More than half of all infrastructure
rollouts from network operators are now based on LTE, the organization revealed, sparking a decline in
2G usage from 2012 onwards.3
As LTE is gaining traction throughout the industry, it is also getting an increasingly larger chunk of
network operator budgets. A recent report from IHS iSuppli indicates that spending on LTE infrastructure
worldwide is set to more than triple from $8.7 billion in 2012 to $24.3 billion in 2013 4. IHS iSuppli further
stated that there will be about 200 LTE networks operating commercially or being deployed around the
world by next year, about 40 more than were in place in 2010.
Research from In-Stat claims that tablets will have the highest 3G/4G attach rate among all cellular-
enabled portable and computing devices with 78 percent of tablets shipping with a 3G/4G modem by
2015. The research firm suggests that this trend represents an opportunity for mobile operators to move
beyond the maturing handset market and into connecting emerging wireless device markets, like e-
readers and tablets. A senior analyst at In-Stat predicts that by 2015, 65 percent of e-readers worldwidewill ship with an embedded 3G/4G modem.”5 The research firm also notes that approximately 16 million
portable and computing devices shipped with 3G/4G cellular connectivity in 2010 and that over 50
percent of all 3G/4G tablets in 2015 will have LTE WAN connectivity.
Finally, 4G Americas research shows 473 HSPA operators, of which 227 have deployed HSPA+. As of
June 2012, there were 91 commercial deployments of LTE in 47 countries, with 335 total operator
commitments to the technology.
2 LTE mobile broadband set for 3,400% growth by 2015. (In-Stat, June 2011)3 LTE mobile broadband set for 3,400% growth by 2015. (In-Stat, June 2011)4 Fresh Research forecasts spending surge (IHS iSuppli, February 2012)5 78% of tablets shipped in 2015 will have 3G/4G modem By eGov Innovation Editors | May 23, 2011
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64G Americas HSPA+LTE Carrier Aggregation – June 2012
2. MULTICARRIER AND MULTI-RADIO NETWORK EVOLUTION
2.1 SPECTRUM AND DEPLOYMENT ASPECTS
Mobile operators are being driven to pursue carrier aggregation techniques by both technology and
operational realities. Ever rising traffic volumes are motivating service providers towards technologies thatexploit spectrum resources in the most efficient and economical manner. Spectrum holdings located
across several frequency bands, and the coexistence of deployments based on diverse access
technologies such as HSPA and LTE over long periods of time also incentive the use of carrier
aggregation techniques.
Broadly speaking, carrier aggregation technologies provide benefits such as the following
Maximize the total peak data rate and throughput performance by combining peak capacities and
throughput performance available at different frequencies
Provide a higher and more consistent quality of service to customers as a result of load-balancing
across frequencies and systems. A customer encountering congestion in one band and one
system can seamlessly access unused capacity available at another frequency or system
Mitigate the relative inefficiencies that may be inherent in wireless deployments in non-contiguous
or narrow (5 MHz or less) channel bandwidths, often spread across different spectrum bands
The universe of potential frequencies that could potentially exploit carrier aggregation techniques is large.
Most obviously, these include frequencies being used for IMT systems today. In the future, this should
expand to include spectrum being contemplated for IMT-Advanced systems, as well as spectrum that
may be “re-farmed” from GSM use toward more advanced technologies or other spectrum unlocked or
“re-farmed” for WWAN usage. In the former category are spectrum bands common across many
countries such as “digital dividend” spectrum (700 or 800 MHz depending on the ITU Region) and 2500
(also known as the 2600) MHz bands, as well as AWS (1700/2100 MHz) in the Americas (ITU Region 2).
GSM spectrum that may be repurposed includes widely deployed bands such as the cellular and SMRbands (at 800-850 MHz) and 1900 MHz in the Americas, and 900 and 1800 MHz in other areas of the
globe.
Currently deployed spectrum bands differ widely in terms of contiguous bandwidth and in channelization
schemes. Further, service providers hold much more paired than unpaired bandwidth. Compounding
matters, bands allocated to mobile broadband are diversifying. All of these factors conspire to present
growing challenges to equipment vendors and device OEMs developing multi-frequency (and increasingly
multimode products). Mobile handheld devices present especially keen issues related to battery size
limitations, screen size, weigh, and constrained interiors into which more and more RF components must
be squeezed to accommodate increasing numbers of frequencies. .
HSPA systems can only be deployed with carriers with nominal bandwidth of 5 MHz or multiples thereof(up to 40 MHz with 8 aggregated 5 MHz carriers) and only in paired mode, while LTE technology is
specified for deployment in both unpaired and paired channels, and in a wide range of different channel
widths from 1.4 MHz through 3, 5, 10 and 15 MHz options up to the maximum channel width of 20 MHz
today, to 40 and even 100 MHz (as contemplated in LTE-Advanced with up to 5 aggregated 20 MHz
carriers).
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74G Americas HSPA+LTE Carrier Aggregation – June 2012
The efficiency of LTE is greater in the wider channel widths (2 x 10 MHz and greater). And 5 MHz HSPA
channels combined with existing HSPA carrier aggregation and other technological advances can deliver
performance that is competitive with today’s LTE systems. HSPA systems will continue to be maintained
for many years, as momentum behind LTE technology continually ramps up. Consequently, service
providers will increasing be operating mixes of HSPA and LTE networks, across multiple bands. This
reality suggests that operators and vendors should seriously consider the potential gains that may be had
by combining the performance of existing systems via techniques such as HSPA+LTE carrier aggregation
techniques.
There is some commonality of frequencies within ITU regions, and to some extent between regions. In
general, commonality exists to greater extent between ITU Regions 1 and 3 than between these two
regions and Region 2 (Americas). One exception is the 2500/2600 MHz band, which is on a path to
achieve widespread global use with the increasing adoption of the ITU Option 1 (2x70 MHz paired
spectrum and a mid-band of 50 MHz unpaired spectrum).
In summary, there are a number of factors heightening the importance of carrier aggregation
developments, including the potential benefits of HSPA/LTE carrier aggregation. These include
Overlapping deployments of HSPA and LTE that will persist through the end of the decade, if notbeyond.
The need to maintain and enhance existing networks, both for service continuity to the installed
base of device as well as to maximize returns on investment
Varying technology features (bandwidth flexibility or limitations, channelization scheme, duplex
options)
Spectrum scheduled to be auctioned, as well as additional spectrum being pursued globally (i.e.,
post WRC-12) or regionally, which should be deployed in the most optimal way given existing
network investments, capabilities, and limitations.
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84G Americas HSPA+LTE Carrier Aggregation – June 2012
2.2 HSPA EVOLUTION FROM MULTIPLE CARRIERS TO MULTICARRIER
Traditionally in the HSPA technology evolution the possibility for aggregating two 5 MHz HSDPA cells
together has been called Dual-Cell HSDPA (or DC-HSDPA), and further evolution where more than two
cells can be aggregated has been dubbed N-cell or N-carrier HSDPA, where the number N refers to the
number of 5 MHz HSDPA carriers aggregated together. Commonly the multiple HSDPA carrieraggregation options are often referred to as Multicarrier HSDPA.
Figure 1: HSPA multi carrier evolutio n in 3GPP standard releases
The concept of multiple carriers operation for HSPA was first introduced in Rel-8 as Dual-Cell HSDPA,
with the scope of increasing coverage for high data rates in deployments where multiple carriers are
available. DC-HSDPA operation is applied to two adjacent 5 MHz carriers and by scheduling HSDPA
transmissions on both carriers simultaneously, allows doubling the peak data rate from a single HSPA+
carrier’s 21 Mbps to 42 Mbps with 64QAM when MIMO is not used. DC-HSDPA users can be scheduled
on either of the two carriers, and either carrier can be configured as the primary serving cell, thereby
benefiting an efficient load balancing between carriers. The two HS-DSCH transport blocks are processedindependently, including the HARQ retransmissions. Rel-9 further extended the DC-HSDPA operation to
be possible simultaneously with MIMO. 3GPP specifications define DC-HSDPA requirements for all the
same frequency bands that have been defined for single carrier operation.
Combining multiple carriers to Multicarrier HSDPA for a UE is performed only on the MAC-hs in the Node
B, and there is a single RLC and PDCP layer just as with the single carrier operation, and practically the
only difference in the RNC user plane when comparing to single carrier HSDPA is higher user throughput.
At the MAC-hs layer in the Node B, each aggregated carrier has its own independent Hybrid Automatic
Repeat reQuest (HARQ) entity. From a UE perspective, characteristics of each carrier procedures are
unchanged with respect to basic single carrier HSDPA operation. Figure 2 shows the multicarrier
mapping on downlink.
Rel‐
7ASN.1
Freeze
2007 2008 2009 2010 2011 2012 2013
Rel‐
8ASN.1
Freeze
Rel‐
9ASN.1
Freeze
Rel‐
10ASN.1
Freeze
Rel‐
11ASN.1
Freeze
HSPA+ DC‐HSDPA 4C‐HSDPA 8C‐HSDPADB DC‐HSDPA
DC‐HSUPA …
Rel‐7 Rel‐8 Rel‐9 Rel‐10 Rel‐11 Rel‐12
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94G Americas HSPA+LTE Carrier Aggregation – June 2012
Figure 2: PDCP-RLC-MAC-PHY layer mapping on Downlink6
To allow multiple carriers operation in deployment scenarios when adjacent bands are not available,
Dual-Band DC-HSDPA was introduced in Rel-9. The primary and secondary serving carriers reside in
different bands, and the uplink transmission can be configured in either one of the two bands. The
introduction of DB DC-HSDPA can be regarded as taking the evolution from multiple single carrier cell
systems to multicarrier systems to include also the aggregation of non-contiguous spectrum bands. The
capability of scheduling transmissions over multiple carriers of different bands provides an efficient
utilization of the spectrum resources resulting in a substantial increase in cell capacity. In Rel-9 only three
band combinations were allowed, and other band combinations have been added at a later stage while
retaining the same functionalities as in Rel-9 specifications and can be implemented in Rel-9 networks
and devices in a release independent manner. Currently defined band-combinations DB DC-HSDPA are
listed in Table 1.
Table 1: 3GPP-defined Dual-Band Dual-Cell HSDPA band combinations
Dual Band DC-HSDPAConfiguration
Band A Band B 3GPP release
1 I (2100 MHz) VIII (900 MHz) Rel-9
2 II (1900 MHz) IV (1.7/2.1 GHz) Rel-93 I (2100 MHz) V (850 MHz) Rel-94 I (2100 MHz) XI (1500 MHz) Rel-105 II (1900 MHz) V (850 MHz) Rel-10
6 Figure adapted from 3GPP TS36.300 “Evolved Universal Terrestrial Radio Access (E-UTRA) andEvolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description”, V10.7.0
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104G Americas HSPA+LTE Carrier Aggregation – June 2012
Similar to DC-HSDPA, multiple carriers operation was introduced in the uplink in Rel-9 as DC-HSUPA,
and the peak data rate doubled to 23 Mbps with 16QAM. DC-HSUPA users transmit two E-DCH transport
blocks, one on each uplink carrier, and each transmission is done independently according to the
principles used for the non-serving cells. The two carriers belong to the same sector of a serving NodeB,
and the serving NodeB can activate/deactivate the secondary carrier dynamically. DC-HSUPA can only
be used with DC-HSDPA because control signaling for the secondary UL carrier is carried over the
secondary DL carrier. DC-HSDPA can instead be activated regardless if the uplink uses single or dual
carrier(s). 3GPP specifications define DC-HSUPA requirements for all the same frequency bands that
have been defined for single carrier operation.
Figure 3: Aggregating more and more carriers to increase the total transmit bandwidt h
Driven by an increasing demand for high data rates, multicarrier operation in the DL has evolved with theintroduction of 4 carriers and 8 carriers in Rel-10 and Rel-11, respectively. The additional flexibility
provided by the larger number of carriers improves the load balancing through the dynamic configuration
of the serving cell of each multicarrier user. As is the case with DC-HSDPA, also with 4C-HSDPA and 8C-
HSDPA, all secondary carriers can be dynamically activated/deactivated by the serving NodeB through
HS-SCCH orders. Depending on the type of traffic, the deactivation of all carriers in a frequency band can
be useful for UE power savings.
With the 4C-HSDPA feature, four HSDPA transmissions can be scheduled simultaneously over four
carriers that do not need to be adjacent and can reside on different bands, featuring a peak data rate of
168 Mbps when configured with 2x2 MIMO and 64QAM. Similar to DC-HSDPA, each transmission is
done independently and all secondary serving carriers can be activated/deactivated in a dynamic fashion
by the serving NodeB. The uplink signaling, as in DC-HSDPA, is carried over a single carrier, and thefeedback channel has been redesigned to include the information for all four DL transmissions. The band
combinations for 4C-HSDPA include up to two frequency bands, and up to three carriers can be
scheduled in the same band. All supported band combinations up to Release 10 require configuring
adjacent carriers within each aggregated band to facilitate the UE receiver implementation. As for DB DC-
HSDPA, other band combinations can be added at a later stage. Currently defined band-combinations for
4C-HSDPA where carriers on a band are adjacent to each other are listed in Table 2.
4 x 5 MHz
20 MHz
8 x 5 MHz
40 MHz
2 x 5 MHz
10 MHz
5 MHz
5 MHz
Single carrier HSDPA
up to Rel‐7
Dual‐Cell
HSDPA,
Rel
‐8
Dual‐Band, Rel‐9
4C‐HSDPA, Rel‐10
Non‐contig. single‐band, Rel‐11
8C‐HSDPA
Rel‐11
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Table 2: 3GPP-defined 4-Carrier HSDPA band com binations with all carriers wi thin a band adjacent to each other
4C-HSDPAConfiguration
Band A Band B Carriercombination
3GPP release
I-3 I (2100 MHz) N/A 3 Rel-10II-3 II (1900 MHz) N/A 3 Rel-11II-4 4 Rel-11I-2 – VIII-1 I (2100 MHz) VIII (900 MHz) 2+1 Rel-10I-3 – VIII-1 3+1 Rel-10I-2 – VIII-2 2+2 Rel-11I-1 – V-2 I (2100 MHz) V (850 MHz) 1+2 Rel-10I-2 – V-1 2+1 Rel-10I-2 – V-2 2+2 Rel-11II-1 – IV-2 II (1900 MHz) IV (1.7/2.1) 1+2 Rel-10II-2 – IV-1 2+1 Rel-10II-2 – IV-2 2+2 Rel-10II-1 – V-2 II (1900 MHz) V (850 MHz) 1+2 Rel-11
3GPP Rel-11 further extended the supported cases for 4C-HSDPA to include single-band non-adjacent
carrier configurations. In these cases all carriers of a 4C-HSDPA configuration reside in the same
frequency band, but in two non-adjacent blocks. The carriers within each block are adjacent to each
other, but there is a gap between the two blocks. Currently defined band-block combinations for the non-
contiguous single-band 4C-HSDPA are listed in Table 3.
Table 3: 3GPP-defined 4-Carrier HSDPA single band non-adjacent carrier combinations
Single-band non-adjacent 4C-HSDPA
Configuration
BandCarrier
combinationGap betweenband blocks
3GPP release
I – 1-5-1I (2100 MHz)
1+1 5 MHz Rel-11I – 1-5-2 1+2 5 MHz Rel-11I – 1-10-3 1+3 10 MHz Rel-11IV – 1-5-1
IV (1.7/2.1 GHz)
1+1 5 MHz Rel-11IV – 1-10-2 1+2 10 MHz Rel-11IV – 2-15-2 2+2 15 MHz Rel-11IV – 2-20-1 2+1 20 MHz Rel-11IV – 2-25-2 2+2 25 MHz Rel-11
The introduction of 8C-HSDPA is a further extension of the multicarrier operation with eight carriers.
Similar to the four carrier feature, in 8C-HSDPA the transmissions are independent. The carriers do notneed to be adjacent and can reside on different frequency bands. The activation/deactivation of the
secondary carriers is done by the serving NodeB through physical layer signaling. The uplink signaling is
carried over a single carrier. The first band combination for 8C-HSDPA to be introduced in 3GPP is 8
adjacent carriers on band I (2100 MHz).
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124G Americas HSPA+LTE Carrier Aggregation – June 2012
Figure 4: HSDPA peak data rate evolution in 3GPP standard releases
14 Mbps
5 MHz
No MIMO
Release 528 Mbps
5 MHz
2x2 MIMO
42 Mbps
10 MHz
No MIMO
84 Mbps
10 MHz
2x2 MIMO
Release 7
Release 8
Release 9
168 Mbps
20 MHz
2x2 MIMO
Release 10
336 Mbps
40 MHz, 2x2 MIMO
20 MHz, 4x4 MIMO
Release 11
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2.3 LTE EVOLUTION FROM MULTIPLE CARRIERS TO CARRIER AGGREGATION
Traditionally in the LTE technology evolution the possibility for aggregating multiple LTE cells together
has been called “LTE Carrier Aggregation” rather than e.g. Multicell or Multicarrier LTE.
LTE Release 8 and 9 supports single carrier operation with variable bandwidth from 1.4 MHz through 3,
5, 10 and 15 MHz up to the maximum of 20 MHz. In order to provide support for operation beyond 20
MHz on downlink and uplink, carrier aggregation was introduced as a part of LTE-Advanced Release-10
(shown in Figure 5).
Release-10 carrier aggregation supports the following features:
Peak data rates of 1 Gbps on downlink and 500 Mbps on uplink.
Up to five carriers can be aggregated, where each carrier is called a “component carrier”.
Each component carrier can have any of the bandwidths supported in LTE Rel-8 (1.4, 3, 5, 10, 15
and 20 MHz). As a result, LTE carrier aggregation can support operation on transmission
bandwidths of up to 100 MHz by aggregating five 20 MHz carriers.
Each component carrier is fully backward compatible to Release-8/9. This backward compatibilityto Release 8/9 allows the technologies developed for LTE Release-8/9 to be fully reused in
Release-10. It also allows the coexistence of Release 8 and 9 UEs together with Release-10
UEs, which is very important for seamless system transition from Release 8 and 9 to Release 10.
A carrier aggregation capable UE can simultaneously receive and transmit in one or multiple
component carriers.
Figure 5: LTE/LTE-A multicarrier evoluti on i n 3GPP standard releases
Carrier aggregation for LTE is performed on the MAC and PHY layers only, and there is a single RLC and
PDCP layer for all aggregated component carriers. At the MAC layer, each component carrier has its own
independent Hybrid Automatic Repeat reQuest (HARQ) entity and physical layer. From a UE perspective,
characteristics of the HARQ procedures for each component carrier are unchanged with respect toRelease-8/9. Figure 6 shows the CC mapping on DL.
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Figure 6: PDCP-RLC-MAC-PHY layer mapping on downlink7
From the higher layer perspective, each component carrier appears as a separate cell with its physical
cell identifier. Therefore, it appears that a carrier aggregation UE is connected to multiple cells. Among
the multiple cells the UE is connected to, one particular cell is denoted as “primary serving cell”, while
other cells (up to four) are denoted as “secondary serving cells”. Primary serving cell plays a unique and
essential role with respect to security, upper layer system information, and some lower layer functions,while secondary serving cells are configured to primarily provide additional resources for UE to transmit
and receive data. Another difference between primary and secondary serving cell is that primary serving
cell can only be changed via RRC (re)configuration, while secondary serving cells, once configured via
RRC signaling, can be activated or deactivated by MAC signaling without additional RRC signaling. This
feature enables very fast activation and deactivation of secondary serving cells.
One salient feature of LTE carrier aggregation is “cross-carrier assignment”, where DL scheduling or UL
grant information of one component carrier can be carried via the PDCCH of another component carrier.
Specifically, a PDCCH on one component carrier can schedule data transmissions on another component
carrier by including a 3-bit Carrier Indicator Field (CIF) in the grant message to indicate the target
component carrier. This is especially useful when secondary serving cell cannot be used to convey
control information reliably. As an example, Figure 7 illustrates the regular DL assignment without cross-
carrier assignment, while Figure 8 shows DL assignment with cross-carrier assignment, where PDCCH of
component carrier 2 is used to schedule not only component carrier 2, but also component carrier 1 and
3.
7 Figure adapted from 3GPP TS36.300 “Evolved Universal Terrestrial Radio Access (E-UTRA) andEvolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description”, V10.7.0
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Figure 7: Regular DL resource assignment Figure 8: DL resource assignment with cross-carrier contro l
While Release-10 air interface allows up to five component carriers, only limited inter-band carrier
aggregation combinations are defined (Table 5), and only intra-band carrier aggregation with contiguous
component carriers for limited bands are defined (Table 4) as of Release-10. More band combinations
are being defined in Release-11 and beyond.
Table 4 : Intra-band contiguous carrier aggregation operating bands8
E-UTRACA Band
E-UTRABand
Uplink (UL) operating band Downlink (DL) operating band DuplexModeBS receive / UE transmi t BS transmi t / UE receive
FUL_low – FUL_high FDL_low – FDL_high
CA_1 1 1920 MHz – 1980 MHz 2110 MHz – 2170 MHz FDD
CA_40 40 2300 MHz – 2400 MHz 2300 MHz – 2400 MHz TDD
Table 5: Inter-band carrier aggregation operating bands8
E-UTRACA Band
E-UTRABand
Uplink (UL) operating band Downlink (DL) operating band DuplexModeBS receive / UE transmi t BS transmi t / UE receive
FUL_low – FUL_high FDL_low – FDL_high
CA_1-51 1920 MHz – 1980 MHz 2110 MHz – 2170 MHz
FDD5 824 MHz – 849 MHz 869 MHz – 894 MHz
8 3GPP TS36.101 “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radiotransmission and reception”, V10.6.0
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2.4 HSPA AND LTE INTERWORKING
During the roll-out of the LTE, it is vital to be able to use the HSPA system to provide coverage outside
the LTE deployment. This is the main reason why the LTE standard includes mechanisms to transfer UEs
from LTE to HSPA and back. The same philosophy was used during the 3G standardization, where
fallback to 2G was included from the start. In addition to the difference in coverage, LTE does not initiallysupport all services that WCDMA supports. The most prominent example is voice, where an LTE solution
is not readily available. Instead, fallback to WCDMA is the initial solution used by many operators.
In the future, other inter-working scenarios will become interesting. One example is load sharing, where
traffic can be steered to the least loaded RAT, leading to better performance. Here the service aspect
should also be taken into account, so that services that benefit most from the LTE would use LTE, while
less demanding services would use HSPA.
In Idle mode, the RAT is autonomously selected by the UEs, based on broadcast information. The UEs
thus performs cell reselection, based on measurements of the quality of the serving and target RATs. For
Release-8 UEs, there is also the notion of priority: with each RAT, there is also an associated priority. A
typical use-case here is that a RAT is selected if its quality is good enough, and its priority is higher thanthat of the serving RAT. This makes it possible for idle UEs to start in HSPA, and autonomously reselect
LTE when LTE coverage becomes available. As an alternative, each UE may individually be provided
with a dedicated priority which overrides the one in broadcast.
Figure 9: The procedures for moving UEs between HSPA and LTE.
LTEHSPA
Idle
Connected
Idle
URA_PCH
CELL_PCH
CELL_FACH
CELL_DCH
ConnectedConnected
Handover
Reselect
Redirect
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When the UE is in connected mode in LTE, it can be moved to HSPA by means of an inter-RAT handover
or inter-RAT redirection procedures. An inter-RAT handover leads to a very short interrupt in the
communication: in the order of tens of milliseconds. This is achieved through reservation of resources in
the target cell before the serving cell is released. Also, data is forwarded from the serving RAT to the
target RAT. After the inter-RAT handover procedure, the UE ends up in the RRC state CELL_DCH in
WCDMA and the UE immediately proceeds to update the routing area so that it can be reached in the
new RAT. The corresponding procedure can be used to move a UE in CELL_DCH in WCDMA to LTE.
After the inter-RAT redirection procedure the UE ends up in the RRC idle mode and after finding the
target cell proceeds to register to the WCDMA RAT with Cell Update procedure. The inter-RAT redirection
procedure leads to significantly longer outage than the inter-RAT HO procedure: no data transmission is
possible until the routing area update has been performed. The outage is in the order of a few seconds.
When an HSPA UE is in any of the RRC states CELL_FACH, CELL_PCH or URA_PCH, it performs
normal cell reselection. Here, if the UE reselects an LTE cell, the UE enters Idle mode and makes an
access to LTE.
In CELL_FACH, it is also possible to use inter-RAT redirection procedure. If the UE finds a cell where it
was directed, this procedure performs relatively well. However, when the UE fails to find a cell, it needs toestablish a connection to another cell, and this may take some time. In 3GPP Release 11, improvements
to this redirection procedure are being discussed that will minimize the interrupt in case the redirection is
unsuccessful. The transitions are depicted in Figure 9.
The Operation and Maintenance (O&M) and Self Optimizing Network (SON) is another important aspect
of interworking of HSPA and LTE radios. The O&M/SON management principle is shown in Figure 10.
The same framework used to operate joint HSPA and LTE network deployments is naturally applicable
also for the HSPA+LTE aggregating network. Depending on which radio aggregation architecture is
considered, different O&M/SON improvements benefiting from tighter radio integration could be foreseen,
although this aspect of the HSPA+LTE aggregation is not considered further in this paper.
Figure 10: SON umbrella for joint LTE and HSPA deployment
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2.5 HSPA+LTE CARRIER AGGREGATION
The previous sections capture the work done in the HSPA and LTE evolution from single carrier operation
to supporting bandwidths up to 40 MHz with Multicarrier HSPA and 100 MHz with LTE carrier
aggregation, as well as summarize the different HSPA and LTE interworking cases already considered in
the 3GPP standards. Furthermore, the expectation is for the industry to move more towards multi-standard radios in base stations, small cells, baseband hotels with fiber-connected remote radio heads.
When in addition considering a multimode device equipped with both HSPA and LTE receivers and
spectrum-limited deployments where commercial situation requires operating both HSPA and LTE
networks, the desire to be able to aggregate carriers of the two radio access technologies when
transmitting to a single user starts to look like a natural next step to work on.
When thinking of the evolution of multicarrier HSPA as well as LTE carrier aggregation, there seems to be
a common trend in both to
Support both downlink and uplink carrier aggregation
Initially support aggregating carriers of one base station site, and only later on investigate the
possibilities of aggregating cells of multiple base station sites.
Similarly, when considering HSPA+LTE aggregation, one can think of both downlink and uplink as well as
co-site and inter-site aggregation of the two technologies, but for the same reasons leading to being
downlink-centric and emphasizing downlink aggregation, the main focus of the HSPA+LTE aggregation,
at least initially, can be expected to be on intra-site aggregation of downlink carriers.
Figure 11: 3GPP standard evolutio n of LTE carrier aggregation, HSPA carrier aggregation and HSPA+LTE interworking
Simultaneousreception of HSPA + LTE
LTE Carrier
aggregationLTE evolution
HSPA Carr ieraggregation
HSPA evo lution
HSPA + LTEaggregation
Load balancing,Re-selections,
Handov ers, vo ice c ontinuity,co-siting
HSPA
LTE
Rel-5…Rel-9 Rel-7…Rel-11 …and beyond
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3. BENEFITS AND USE CASES OF HSPA+L TE AGGREGA TION
3.1 BENEFITS OF HSPA+LTE AGGREGATION
As discussed in sections 1 and 2, at least the initial focus of the HSPA+LTE aggregation is seen to be on
aggregating co-sited downlink carriers. Respectively, the discussion of the benefits in this section isconsidering co-sited downlink carriers.
HSPA+LTE aggregation utilizes same mechanisms as the intra-RAT carrier aggregation schemes
described in section 2 and is thus expected to bring similar data rate gains:
Data rates of carrier aggregation UEs boosted by utilizing unused resources from overlapping
cell(s) operating on different carrier(s)
Data rates of all UEs improved by fast (TTI level) load balancing
Similar to the intra-RAT carrier aggregation, the gains are highest at low/medium load and they benefit
both the cell edge and the cell center UEs. At high load with multiple active UEs per cell it is possible to
perform load balancing handovers to balance the load and thus aggregation of carriers is less beneficial.However, statistics from today’s mature HSPA networks have shown that due to the burstyness of the
data traffic there is often only one active UE per cell with data in the RAN buffers (even though there
might be several UEs connected to the cell). In such case load balancing handovers are not helpful and
part of the resources remain unused. Also if the data bursts are very short, the load balancing handovers
can be rather inefficient due to the handover delays and overheads. In such scenarios carrier aggregation
clearly outperforms load balancing handovers and HSPA+LTE aggregation simply brings the same
benefits to inter-RAT domain. In uplink the aggregation (both in intra- and inter-RAT domain) is however
less appealing due to UL coverage and UE power consumption limitations.
In addition to the data rate gains, HSPA+LTE aggregation allows more relaxed re-farming strategies for
HSPA spectrum; HSPA+LTE aggregation capable UEs can enjoy improved data rates by utilizing
efficiently both LTE and HSPA spectrum without reducing the data rates of the HSPA UEs.
Figure 12: Average downlink data rate before and after refarming of one HSPA carrier (assuming low-to-medium system
loading, 10MHz LTE and 2x5MHz HSPA before refarming)
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Figure 12 illustrates downlink data rates in a single UE scenario, where before re-farming both HSPA and
LTE have 10MHz bandwidth. By re-farming of one HSPA carrier the data rates of LTE UEs can be
boosted by ~50% but that happens at the cost of ~50% lower HSPA data rates. With HSPA+LTE
aggregation it is possible to postpone the re-farming until HSPA penetration is very low, but at the same
time provide almost 100% higher data rate for the LTE UEs with HSPA+LTE carrier aggregation
capability.
3.2 EXAMPLE USE CASES FOR HSPA+LTE AGGREGATION
Next some example use cases for HSPA+LTE aggregation are given;
1. Aggregat ion of two low bands
The increased penetration of HSPA capable UEs together with the GSM spectral efficiency
improvements have made it possible for many European operators to start deploying HSPA at 900
MHz. At the same time some of the operators are starting to deploy LTE at 800 MHz with rather
limited bandwidth. Both of these bands are appealing due to their better coverage compared to theother available frequency bands (such as the 2100 MHz for HSPA, 2600 MHz for LTE).
The spectrum available at 800 MHz is however very limited and thus may not provide the data rates
consumers are expecting from a 4G service. Furthermore it will be also difficult to re-farm the HSPA
from 900 MHz band without sacrificing the HSPA coverage.
HSPA+LTE aggregation can help to boost LTE data rates to the level of expectations also in the
areas where only LTE at 800 MHz is available, and therefore motivating adaptation of LTE capable
devices, while still maintaining the coverage for HSPA services.
2. Limited LTE spectrum
Some operators have access only to rather limited amount of spectrum to be used for LTE thus
making it difficult to provide high data rate LTE services.
One example of such case is a North American carrier announcing plans to transfer part of its’
HSPA+ services to PCS band (1900 MHz) thus freeing-up spectrum for future LTE deployments in
AWS band (1700/2100 MHz). In this case aggregation of HSPA+LTE would provide significant data
rate boost compared to the HSPA+ services.
3. Intra-band aggregation
Even though refarming of HSPA spectrum for LTE is not that relevant to most of the operators
currently, in longer term also this will become a relevant use case (e.g. on 2100 MHz in Europe or onthe above mentioned PCS band in US).
As described in the previous section, the aggregation of HSPA and LTE enables more relaxed
refarming by providing the needed additional capacity for LTE capable UEs while still maintaining the
HSPA data rates.
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4. HSPA+L TE AGGREGA TION SYSTEM ARCHITECTURE CONSIDERATIONS
The most critical architectural design choice for HSPA+LTE aggregation is the choice of the network
element where the single data stream is split to two independent downlink data streams to be send over
the two carriers; one going via HSPA radio interface and the other one via LTE radio interface. If uplink
aggregation is desired to be supported, this network element will serve also as merger point where theuplink data received from the two data streams is merged back to a single data stream.
Figure 13 shows the main elements of the existing system architecture for E-UTRAN and UTRAN. There
are five different levels of network elements where the data split/merge of HSPA+LTE aggregation might
potentially take place; at Services, Core Network, LTE eNodeB, HSPA RNC, or HSPA NodeB level.
Figure 13: Potential split /merger points of HSPA + LTE aggregation shown on top of cu rrent network archit ecture
In the next sections each of these potential split/merge points are analyzed in more detail. It is worth to
notice that no changes to the existing system architecture or protocols regarding the operation (HSPA
only or LTE only operation) are envisioned. Thus, the following architecture considerations focus purely
on the HSPA+LTE carrier aggregation operation. Table 6 summarizes some aspects of the different
architecture options.
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Table 6: A high level summary of the different architecture approachses for HSPA+LTE aggregation
Split/MergerPoint RAN aspects UE aspects Other aspects
Service or CoreNetwork
‐ Minimal impact on RAN‐ Simultaneous UL on both
RATs required‐ Impact on battery
‐ Challenges in optimizingusage of resources
HSPA RNC
‐ No/minimal changes atNodeB
‐ Changes at RNC andeNB to support newinterface between eNBand RNC
‐ Deep reordering ofpackets
‐ Simultaneous UL on bothRATs may be required
‐ RNC based schedulingslower than base stationbased one
‐ Cannot benefit from thefaster setups over LTE
‐ All traffic go through 3GCN
HSPA NodeB
‐ Most changes limited tobase station
‐ No impact on corenetwork
‐ No or limited impact on
higher layers
‐ Changes to radiointerface needed totransport signaling andsetup of radio interfaces
‐ UL range reduction ifsimultaneous HSPA/LTEUL
‐ For HSPA UL only case,HARQ timing may be anissue
‐ Very good performancedue to fast schedulingand shallow reordering
‐ Fast load balancing‐ Cannot benefit from the
faster setups over LTE‐ All traffic go through 3G
CN
LTE eNB
‐ Most changes limited to
base station‐ No impact on core
network‐ No or limited impact on
higher layers‐ Changes to radio
interface needed totransport of signaling andsetup of radio interfaces
‐ UL range reduction ifsimultaneous HSPA/LTEUL
‐ Very good performancedue to fast schedulingand shallow reordering
‐ Fast load balancing
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4.1 SERVICE OR CORE NETWORK LEVEL SPLIT/MERGER
While carrier aggregation is usually seen as a RAN functionality, in theory the data path split/merge point
could be located also above RAN either in services or core network level, as illustrated in Figure 14 and
Figure 15.
Figure 14: HSPA+LTE aggregation w ith sp lit/merger at service level
Having the split/merger at service level or CN level has the following advantage:
It can be introduced with a minimum impact on the RAN. In principle, no changes are
required to the user plane processing; however to make it feasible in practice (i.e. to mitigate
the disadvantages listed below), some changes in the UE will be required, and the networkside of the RRC layers need to be aware of the dual-radio operation.
Figure 15: HSPA+LTE aggregation wi th spl it/merger at core network level
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Both alternatives would however require pure dual-radio with simultaneous dual-transmission which leads
(at least) to the following further requirements and drawbacks:
o Double security, mobility context and CN protocol layers
o UE total maximum TX power management and handling SAR requirements (e.g., UE TX
power reduced by 3 dB in both systems)
o Significant impact to UE battery life due to needing to operate multiple power amplifiers
simultaneously
o UE RF implementation issues such as inter-modulation, or interference to own receiver, due
to two simultaneous transmissions
o Challenging to optimise usage of HSPA or LTE resources, leading to that the full capacity
gain cannot be achieved.
In addition it appears that the RAN control plane processing could not stay agnostic to dual-radio user
plane due to tight interworking of the HSPA and LTE. At least some level of coordination of the two RRC
protocol layers would be inevitable due to access and mobility management.
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4.2 HSPA RAN LEVEL SPLIT/MERGER
Another alternative is to place the data split/merger point in HSPA RAN in which case the data needs to
be divided between LTE and HSPA radio either in RNC or in NodeB. In this architecture HSPA would be
the controlling RAT which decides how much data is to be transmitted over HSPA or LTE.
The data split/merger at RNC level would suggest a new interface to be defined between RNC and LTE
eNodeB, as shown in Figure 16.
The advantages with having the aggregation point in the RNC are:
No changes in the node B, and relatively small changes are required in the RNC and eNode B,
No changes to the physical layer: the required standardization changes are limited to higher layers.
The disadvantages are:
The RNC scheduler will lead to suboptimum performance
Deep reordering required in the UE, due to potentially very different delays in the two RATs
Simultaneous uplink transmission on LTE and HSPA may be required.
Figure 16: HSPA+LTE aggregation w ith split/merger at RNC
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Introducing the data split/merger at NodeB level (Figure 17) would limit most of the changes to base
station level which could be seen more desirable especially in the context of multi-radio base stations.
Figure 17: HSPA+LTE aggregation w ith split/merger at NodeB
While an aggregation in the RNC has the same main disadvantages as the CN/service layer solution, an
aggregation in the node B has the possibility to avoid several of the drawbacks. Providing that the node B
– eNode B interface has low enough latency, the following advantages may be achievable:
Very good performance, due to fast scheduling and shallow reordering
Possibility to use either HSPA uplink or LTE uplink or both for fast feedback
No impact on core network nodes, and very limited if any impact to the existing RNC functionalities
No or very limited impact to higher layers
Both HSPA and LTE uplink could be used simultaneously for fast feedback. This approach has the
advantage of requiring little or no layer 1 change. The drawback is that UL range is reduced due to
simultaneous transmission on both links. A single uplink may be desirable to avoid UL range reduction,
though the approach has the following disadvantages:
Standardization changes required to lower layers, in particular for the uplink layer 1 control signalling.
This is true irrespective of which RAT is used for uplink transmission.
LTE data would have to be routed via RNC which would violate the flat architecture design principleof LTE and lead to increased RNC load (the LTE UEs could be however still served using the existing
LTE architecture without routing their traffic via RNC).
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Here, the most appealing alternative would be to reuse the PDCP, RLC and MAC-d protocols from HSPA.
With this approach, the impact is limited to MAC in LTE. As the control plane and the data routing to
higher layers in this architecture would be managed by UTRAN, the HSPA uplink might be seen as most
natural choice in case that single UL is desired. In this case, also the LTE feedback (HARQ
acknowledgements and CQI) would have to be transmitted via HSPA uplink which can be challengingdue to the shorter (1ms) TTI of LTE. This approach is depicted in Figure 18. Alternatively the uplink
control and data could be mapped on the LTE. This approach might be more attractive from the control
signalling perspective, but would require additional user plane modifications.
Figure 18: Single uplink with data split/merger at NodeB. Most of the protocols are used as is: the main impact is seen in
LTE MAC
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4.3 LTE RAN LEVEL SPLIT/MERGER
In this alternative data is divided between LTE and HSPA radio in eNodeB (as illustrated in Figure 19),
and LTE would be the controlling RAT.
Even though eNodeB is in control of how the data is divided between HSPA and LTE radios, RNC can be
expected to stay in control of overall HSPA resources. This is possible e.g. by introducing additional
signaling over Iub interface enabling RNC to provide limits how much HSPA resources NodeB is allowed
to provide for HSPA+LTE carrier aggregation UEs at a given time. If there is no congestion, also more
resources can be temporarily allocated for HSPA+LTE aggregation. In general controlling the aggregation
in base station level enables fast (TTI level) load balancing between HSPA and LTE, equivalent to the
intra-RAT load balancing available to multicarrier deployments with Multicarrier HSDPA and LTE Carrier
Aggregation.
Figure 19: HSPA+LTE aggregation w ith split/merger at eNodeB
Since in HSPA the PDCP and RLC layers are located in RNC, the data split should take place at/below
LTE RLC layer. Using the LTE MAC could potentially lead to a more optimized performance and flexibility,
but it would require rather dramatic modifications in the LTE MAC implementation, particularly in the UE,
as major parts of the MAC-ehs would have to be ported to the LTE MAC to support HSDPA L1. If the data
split is however performed in the RLC-MAC interface both the LTE and HSPA MAC (and L1) can be kept
intact (if so desired) making this the most appealing alternative from the implementation complexity point
of view.
As a consequence of the proposed architecture choices described above, the RLC, PDCP, and RRCprotocol layers of HSPA side would not be used for HSPA+LTE aggregation, instead only the LTE RLC,
PDCP, and RRC would be utilized. Similarly, as S1 interface is terminated in LTE eNodeB, the GPRS
packet core protocols are not utilized, but core network functions are provided by EPC.
As mentioned in Section 4.2, both HSPA and LTE uplink can be used simultaneously for fast feedback to
avoid any layer 1 change, but this approach comes with a drawback of UL range reduction. Alternatively,
if a single UL is used to maintain the UL range, the HSPA feedback (CQI, HARQ status) would have to be
delivered via LTE UL, as illustrated in Figure 20. The tight delay budget for delivering such feedback
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implies that NodeB and eNodeB should either be co-located or integrated into one multi-radio BTS. This
is however not that strict requirement as co-location is in any case desirable to maximize the overlapping
of the cell coverage areas, as well as to minimize the site costs. This approach is also well aligned with
the LTE-Advanced carrier aggregation frame work where single UL can provide feedback for multiple DL
carriers.
Figure 20: Single upl ink wi th data split/merger at eNodeB
As a summary, the solution with the split in the eNode B shares many of benefits of having the split in the
node B:
o No impacts at core network or services level, and only minor impacts on RNC
o Data split at BTS level enables fast load balancing
o Single uplink via LTE UL possible thus maximizing the uplink range
In addition, using the eNode B as the aggregation point also means that
o The LTE data flow would not have to travel via RNC
o Allows to utilize the existing LTE CA framework
Quite naturally, the solution also has these disadvantages:
Standardization changes required to lower layers, in particular for the uplink layer 1 control signalling,
if a single uplink is used for feedback of both HSPA and LTE.
Since the RNC maintains the overall responsibility for the HSPA resources, the eNodeB cannot
control all the resources in the node B, leading to somewhat degraded performance gains.
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5. PRACTICAL IMPLEMENTATION ASPECTS OF HSPA+LTE A GGREGATION
5.1 BASE STATION RADIO IMPLEMENTATION ASPECTS
Figure 21 shows a very simplified block-diagram of a base station capable of transmitting on two
frequency bands with two transmit antennas common to the bands. The same transmit chain can inprinciple be able to transmit either LTE, HSPA, or even both LTE and HSPA carriers simultaneously on
separate carrier frequencies within the bandwidth of the transmitter, e.g. a 10 MHz transmitter could
support one 10 MHz LTE carrier or two adjacent 5 MHz HSPA carriers, and a 40 MHz transmitter could
support two adjacent 20 MHz carriers or 8 adjacent 5 MHz HSPA carriers, or even one 20 MHz LTE
carrier and next to it 4 adjacent 5 MHz HSPA carriers.
Figure 21: A simplifi ed block d iagram of a dual-band Tx diversity/MIMO base station transmit chain
As a concrete example, we can consider a co-sited HSPA and LTE deployment with one or several HSPA
carriers on PCS band and one LTE carrier on AWS band. Extending such deployment to support
HSPA+LTE aggregation would be directly able to utilize the RF hardware already in place. This can be
generalized to say that an existing co-sited deployment of HSPA and LTE RATs can be extended to
support HSPA+LTE aggregation without any new requirements to the already deployed RF hardware.
Note that new baseband functionality needs to be introduced.
The architectures with data split point in the base station (HSPA Node B or LTE eNode B) would be
easiest to implement with a multi-standard radio base station, where both RATs are served by the same
physical entity. If the uplink is only limited to one or the other RAT, then a fast feedback loop would be
required from one RAT to the other to get the uplink channel state information and HARQ ACK/NACK
feedback across to the other RAT. These architectures assume either a high-speed, low latency interface
between two base stations, or more advantageously one multi-standard radio base station.
P AFilter DAC
Filter DAC
Band A
Filter DAC
Filter DAC
B a s e s t a t i o n t r a n s m i t
B B p r o c e s s i n g
Filter
Filter
Filter
Filter
P A
P A
P A
Upconversion
Upconversion
Band B
Filter
Filter
Filter
Filter
Signal flow through base station transmit processing
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5.2 DEVICE RADIO IMPLEMENTATION ASPECTS
Figure 22 shows a very simplified block-diagram of a UE capable of receiving simultaneously on two
frequency bands with two receive antennas common to the bands. If the two receivers are configurable to
operate in a Dual Band Multicarrier HSDPA configuration or Dual Band LTE Carrier Aggregation
configuration then the two receivers are required to receive data at the same time as in the figure.
Similarly, the UE of Figure 22 could represent a dual-mode UE capable of receiving LTE on band A and
HSPA on band B but not vice versa, and if they UE is to be able to aggregate the data received on the
two radio technologies, then the same logical architecture capable of dual-band LTE or HSPA
aggregation can be used also in aggregating HSPA and LTE. One could say that the receiver capable of
aggregating carriers on two bands is as complex regardless of whether it is aggregating LTE and HSPA
carriers on both bands, or LTE carriers on one band and HSPA carriers on the other band.
Figure 22: A simplifi ed block d iagram of a dual-band carrier aggregating Rx diversit y/MIMO UE receiver chain
A device supporting intra-band carrier aggregation could use a single wide band receiver rather than two
narrow band ones, e.g. a Dual Cell HSDPA UE can be expected to have one 10 MHz receiver rather than
two 5 MHz receivers. If the aggregated carriers can be non-adjacent, then there may be a need to go to
the architecture similar to that used in inter-band carrier aggregation.
A m pFilter ADC
Filter ADC
Band A
Filter ADC
Filter ADC
Filter
Filter
Filter
Filter
A m p
A m p
A m
p
A m p
A m p
A m p
A m
p
Downconversion
Downconversion
Band B
U E
r e c e i v e r
B B p r o c e s s i n g
Signal flow through UE receiver processing
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Figure 23 shows a very simplified block-diagram of a UE capable of transmitting on two frequency bands,
but not simultaneously. Such dual-band (or multi-band) transmitter architectures are expected to be more
commonplace than those capable of using multiple transmitter chains simultaneously, e.g. a 10 MHz
transmitter could be used for aggregating two adjacent 5 MHz HSUPA carriers for Dual Cell HSUPA
configuration, or be able to transmit on one 10 MHz LTE carrier. It however is to be noted that this
architecture is not readily able to lend itself to transmitting on multiple frequency bands – a full dual-
transmit chain architecture would be required for that.
Figure 23: A simpli fied block d iagram of a dual-band UE transmitted chain
Figure 24 shows a very simplified block-diagram of a UE capable of transmitting on two frequency bands
simultaneously, i.e. capable of uplink carrier aggregation on two bands. Such dual-band (or multi-band)
transmitter able to lend itself to transmitting on multiple frequency bands and thus would be able to
support also HSPA+LTE aggregation in the uplink.
Figure 24: A simplified block diagram of a uplink dual-band carrier aggregating UE transmitted chain
P AFilter
DAC
B B p r o c e s s i n g
Filter
upconversion
Band A
Filter
Band B
Signal flow through UE transmit processing
P AFilter Filter
DACFilter
upconversion
Band A
Band B
Signal flow through UE transmit processing
P AFilter Filter
DACFilter
upconversion
P AFilter Filter
B B p r o c e s s i n g
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5.3 IMPLEMENTATION ASPECTS OTHER THAN RADIO PROCESSING
Different architecture options discussed in section 4 set different requirements for data processing and
node interfacing in the network. The architectures where the base station acts as the data managing
entity the base station needs to be able to have a fast interface between the LTE and the HSPA
processing in order to be able to split the downlink data flow over the two radios. Similarly in the uplinkdirection the base station needs to either route uplink data between the two processing entities (dual
uplink), or forward the fast feedback from one side to the other (single uplink). At least for some of the
options discussed in chapter 4, the RRC layers of the two RATs, LTE RRC in the base station and HSPA
RRC in the RNC, need to be aware of each other to some extent in order for the master RAT to be able to
assign the UE with the resources of the other RAT. This implies some modifications to both eNodeB and
RNC RRC layers, and introducing means for negotiating the resources assignable.
The radio network centric architectures can be expected to avoid the need for the core network to be
aware of the new UE type – again analogous to aggregating carriers of one RAT being visible to the core
network only by increased user data rates. The data split/aggregation in the core network obviously
means that the core would need to be able to support such functionality and the LTE core would need to
be able to interface with HSPA RAN or the HSPA core with the LTE RAN, and also indicate the radio thatthere is a new type of connection taking place.
Similarly to the changes in the network side, the device needs a higher degree of integration between
MAC and RRC layers of the two RATs than when aggregating HSPA or LTE carriers. The actual data
processing requirement would not differ from that of aggregating carriers within one RAT, but it would
require two different types of protocol stacks to be able to run simultaneously and interface at the layer
where the data aggregation/split is taking place.
Operating both LTE and HSPA receivers simultaneously, can expected to increase device battery
consumption, although this can be assumed to be no different to aggregating carriers in multiple bands
within one RAT. Architectures with UE transmitting in the uplink on both LTE and HSPA simultaneously
can be expected to have a more significant device battery consumption impact than that of the two RATreception, but again, the added power consumption can be expected to be comparable to what the
aggregation of two uplink carriers on different frequency bands of one RAT would result with.
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6. CONCLUSION
HSPA+ and LTE are the overwhelming mobile broadband technologies of choice for operators throughout
the world. The evolution of both HSPA+ and LTE standards has introduced aggregation of carriers for
higher data rates, better load balancing and increased spectrum utilization, and since the dawn of LTE,
the standard support for radio level interworking for HSPA and LTE radios has been included. A naturalcontinuation of such development is to tighten the interworking even further and introduce similar
aggregation of carriers between the two radio access technologies.
The same gain mechanisms that have been seen beneficial for Multicarrier HSDPA as well as LTE
Carrier Aggregation can be benefited from by aggregating HSPA with LTE. At low or medium load,
HSPA+LTE aggregation is able to take advantage of the unused resources leading to significant data rate
increases both at the cell edge and the cell center for the carrier aggregation capable devices. In addition,
the carrier aggregation enables fast (millisecond level) load balancing across the carriers thus improving
the data rates of all users.
A number of possible network architectures can be foreseen for HSPA+LTE aggregation, and are briefly
touched upon in this white paper. Most promising architecture options are seen with co-located multiradiobase stations with the base station (NodeB + eNodeB) acting as the data aggregation point, and
simultaneously maintaining the existing network architecture for the devices connecting to the network
with one radio system at a time only. This architecture can utilize some of the already deployed RF
hardware in the base station, whereas new baseband functionality managing the data flow will be
needed. On the device side receiver radio architectures capable of multiband carrier aggregation should
be suitable also for aggregated HSPA+LTE.
While Dual-Cell HSDPA is already in commercial operation, and higher levels of HSPA carrier
aggregation as well as LTE carrier aggregation are part of 3GPP specifications existing today,
HSPA+LTE aggregation is currently not standardized. Although conceptually straightforward and building
on already standardized concepts, HSPA+LTE aggregation is a major feature, with a standardization
effort comparable to that of LTE carrier aggregation.
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ABBREVIATIONS
3GPP 3rd Generation Partnership Project
4C-HSDPA 4-Carrier HSDPA
8C-HSDPA 8-Carrier HSDPA
A/N ACK/NACKACK Acknowledgement
ADC Analog to Digital Conversion
ASN.1 Abstract Syntax Notation One
AWS Advanced Wireless Spectrum
BB Base Band
BTS Base Transceiver Station
CA Carrier Aggregation
CC Component Carrier
CDMA Code Division Multiple Access
CIF Carrier Indicator Field
CN Core Network
CQI Channel Quality IndicationDAC Digital to Analog Conversion
DB Dual Band
DC-HSDPA Dual Cell HSDPA
DC-HSUPA Dual Cell HSUPA
DCH Dedicated Channel
DL Downlink
E-DCH Enhanced DCH
E-UTRAN Evolved UTRAN
EPC Evolved Packet Core
FACH Forward Access Cannel
GPRS General Packet Radio Service
Gbps Gigabits per second
GSM Global System for Mobile Communications
HS-SCCH High Speed Shared Control Channel
HARQ Hybrid Automatic Repeat reQuest
HSDPA High Speed Downlink Packet Access
HSPA High Speed Packet Access
HSUPA High Speed Uplink Packet Access
ID Identity
ITU International Telecommunication Union
L1 Layer one
LTE Long Term Evolution
LTE-A LTE AdvancedMAC Medium Access Control
MAC-d MAC dedicated
MAC-hs MAC high speed
MAC-ehs MAC enhanced high speed
Mbps Megabits per second
MHz Megahertz
MIMO Multiple Input Multiple Output
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NACK Negative ACK
O&M Operation and Maintenance
OEM Original Equipment Manufacturer
PA Power Amplifier
PCC Primary Component Carrier
PCell Primary Serving Cell
PCH Paging Channel
PCS Personal Communications Service
PDCCH Physical Downlink Control Channel
PDSCH Physical Downlink Shared Channel
PHY Physical [layer]
PDCP Packet Data Convergence Protocol
QAM Quadrature Amplitude Modulation
RAT Radio Access Technology
Rel Release
RF Radio Frequency
RLC Radio Link Control
RNC Radio Network ControllerROHC Robust Header Compression
RRC Radio Resource Control
SCC Secondary Component Carrier
SCell Secondary Serving Cell
SIB System Information Block
SMR Specialized Mobile Radio
SON Self Optimizing Network
TTI Transmission Time Interval
TX Transmit
UE User Equipment
UL Uplink
UMTS Universal Mobile Telecommunications SystemURA UMTS Routing Area
US United States
UTRAN UMTS Terrestrial Radio Access Network
WCDMA Wideband CDMA
WRC World Radio Congress
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ACKNOWLEDGEMENTS
The mission of 4G Americas is to promote, facilitate and advocate for the deployment and adoption of the
3GPP family of technologies throughout the Americas. 4G Americas' Board of Governor members include
Alcatel-Lucent, América Móvil, AT&T, Cable & Wireless, CommScope, Entel, Ericsson, Gemalto, HP,
Huawei, Nokia Siemens Networks, Openwave, Powerwave, Qualcomm, Research In Motion (RIM),Rogers, T-Mobile USA and Telefónica.
4G Americas would like to recognize the project leadership and important contributions of Karri Ranta-
aho of Nokia Siemens Networks (NSN), as well as representatives from the other member companies on
4G Americas’ Board of Governors who participated in the development of this white paper.