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LTE Carrier Aggregation

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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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114G Americas HSPA+LTE Carrier Aggregation – June 2012

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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134G Americas HSPA+LTE Carrier Aggregation – June 2012

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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304G Americas HSPA+LTE Carrier Aggregation – June 2012

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.

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Signal flow through base station transmit processing

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314G Americas HSPA+LTE Carrier Aggregation – June 2012

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.

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Signal flow through UE receiver processing

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324G Americas HSPA+LTE Carrier Aggregation – June 2012

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

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334G Americas HSPA+LTE Carrier Aggregation – June 2012

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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344G Americas HSPA+LTE Carrier Aggregation – June 2012

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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364G Americas HSPA+LTE Carrier Aggregation – June 2012

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.