Carrier Generation in 3GPP LTE WiMAX

IEEE Communications Magazine • August 201060 0163-6804/10/$25.00 © 2010 IEEE

INTRODUCTIONOne of the key requirements for InternationalMobile Telecommunications Advanced (IMT-ADV) as set by the International Telecommuni-cation Union — Radiocommunication Sector(ITU-R) [1] is support for variable bandwidthswith encouragement to support up to 100 MHz.The IMT-ADV technologies such as Third Gen-eration Partnership Project (3GPP) Long TermEvolution-Advanced (LTE-A) may also bedeployed using a mix of different spectrumbands, which requires systems to have the capa-bility to aggregate radio resources across non-contiguous bands to provide the highest datarates expected of these systems.

Based on these observations, both 3GPP LTE(evolved Universal Mobile TelecommunicationsSystem [UMTS] terrestrial radio access network[E-UTRAN]) and WiMAX (IEEE 802.16m)standards evolution have identified carrier aggre-gation (CA) as one of the major features fortheir roadmap to meet the IMT-ADV require-ments for fourth-generation [4G] wireless net-work deployments.

Note that the basic concept of bandwidth

aggregation has been used in 3G systems. In3GPP2 one of the key elements of the 1xEvolu-tion-Data Optimized Revision B (1xEV-DOREV B) system was the aggregation of multiple1.25 MHz carriers. Similarly, in high-speed pack-et access (HSPA) evolution there are options toaggregate up to four downlink (DL) and up totwo uplink (UL) carriers, of 5 MHz each. Inboth cases the carriers are assumed to be con-tiguous, in the same band and of the same band-width.

In 3GPP LTE Release 8/9 various carrierbandwidths of 1.4, 3, 5, 10, 15, and 20 MHz aresupported to meet different spectrum anddeployment plans. To meet the IMT-ADVrequirements for 100 MHz system bandwidth,3GPP LTE Release 10 has introduced CA asone of the main features of LTE-A to scale thesystem bandwidth beyond 20 MHz. In the CAmode of operation, two or more component car-riers (CCs) of the same or different bandwidthsare aggregated to support effectively wider trans-mission bandwidths in the DL or UL betweenthe E-UTRAN NodeB (eNB) and the userequipment (UE). With CA being defined inRelease 10 [2], system bandwidths of up to 100MHz can be supported by simultaneously aggre-gating up to five CCs of 20 MHz, subject tospectrum availability and the UE’s capability.

To allow smooth network migration andupgrades, it is essential to ensure backward com-patibility of LTE-A design so that both LTERelease 8/9 and LTE Release 10 UE can be sup-ported in the same carrier deployed by Release10 eNBs. In CA defined for Release 10, this isascertained since each component carrier (CC)is compatible with LTE Release 8/9 [3] and hasone of the bandwidths defined in Release 8/9.This would also allow reuse of Release 8/9 RFdesigns and implementation at the eNB and UE.

This article provides an overview of the CAframework in LTE-A. The next section describessome spectrum scenarios and deployment mod-els where CA is expected to be used. We thenshow the overall impact of CA across differentprotocol layers followed by a section thataddresses CC management concepts including

ABSTRACT

Carrier aggregation is one of the most dis-tinct features of 4G systems including LTE-Advanced, which is being standardized in 3GPPas part of LTE Release 10. This feature allowsscalable expansion of effective bandwidth deliv-ered to a user terminal through concurrent uti-lization of radio resources across multiplecarriers. These carriers may be of differentbandwidths, and may be in the same or differentbands to provide maximum flexibility in utilizingthe scarce radio spectrum available to operators.Support for this feature requires enhancementto the LTE Release 8/9 PHY, MAC, and RRClayers while ensuring that LTE Release 10 main-tains backward compatibility to LTE Release8/9. This article provides an overview of carrieraggregation use cases and the framework, andtheir impact on LTE Release 8/9 protocol layers.

WIMAX/LTE UPDATE

Mikio Iwamura, NTT DOCOMO, INC.

Kamran Etemad, Intel Corporation

Mo-Han Fong, Research In Motion Ltd.

Ravi Nory and Robert Love, Motorola

Carrier Aggregation Framework in 3GPP LTE-Advanced

IWAMURA LAYOUT 7/20/10 9:12 AM Page 60

IEEE Communications Magazine • August 2010 61

CC/serving cell configuration and power savingoperations. We then focus on control channeldesign issues to support CA, and show mobilityand interference management aspects. The finalsection provides some concluding remarks.

DEPLOYMENT SCENARIOS ANDUSAGE MODELS FOR CA

Given the scarcity of spectrum for mobile broad-band access and the fact that LTE utilizes widercarrier bandwidths such as 10 and 20 MHz, com-pared to 1.25 and 5 MHz used in 3G systems,special design consideration is needed to enableCA to meet the challenging spectrum anddeployment scenarios.

SPECTRUM SCENARIOSIn general, CA may be used in three differentspectrum scenarios as follows.

Intraband Contiguous CA — This is where acontiguous bandwidth wider than 20 MHz isused for CA (Fig. 1a). Although this may be aless likely scenario given frequency allocationstoday, it can be common when new spectrumbands like 3.5 GHz are allocated in the future invarious parts of the world. The spacing betweencenter frequencies of contiguously aggregatedCCs is a multiple of 300 kHz to be compatiblewith the 100 kHz frequency raster of Release 8/9and preserving orthogonality of the subcarrierswith 15 kHz spacing.

Intraband Non-Contiguous CA — This iswhere multiple CCs belonging to the same bandare used in a non-contiguous manner (Fig. 1b).This scenario can be expected in countries wherespectrum allocation is non-contiguous within asingle band, when the middle carriers are loadedwith other users, or when network sharing isconsidered.

Interband Non-Contiguous CA — This iswhere multiple CCs belonging to different bands(e.g., 2 GHz and 800 MHz are aggregated; Fig.1c). With this type of aggregation, mobilityrobustness can potentially be improved byexploiting different radio propagation character-istics of different bands. This form of CA mayalso require additional complexity in the radiofrequency (RF) front-end of UE.

In LTE Release 10, for the UL the focus ison intraband CA, due to difficulties in defining

RF requirements for simultaneous transmissionon multiple CCs with large frequency separation,considering realistic device linearity. For the DL,however, both intra- and interband cases areconsidered in Release 10, while specific RFrequirements are being developed.

DEPLOYMENT SCENARIOSWith CA, various network deployments are pos-sible [4]. Figure 2 shows some of the deploymentscenarios in which CA may be used. Note that inFig. 2 only two CCs, denoted CC1 and CC2, areassumed. In practice, a larger number of CCscan be considered as well as deployments withmixed scenarios.

One of the most typical scenarios envisaged iswhere eNB antennas are collocated and have thesame beam directions/patterns for the CCs, pro-viding nearly the same coverage on all CCs (Fig.2a). This would be a typical scenario when theCCs are of the same band or different bands butwith little frequency separation.

In practice, spectrum allocation for an opera-tor is often dispersed across different bands,having large frequency separation. In such casescoverage for a CC (of a higher frequency) maybe smaller than that of another CC (Fig. 2b).Even in the same band CCs may be deployed ateNBs with different transmit power levels toprovide different coverage footprints for inter-cell interference management purposes. In eithercase CA allows higher user throughput at placeswhere coverage of CCs overlap.

In some deployments the eNB antennas fordifferent CCs can have different beam direc-tions/patterns. This may be due to different sec-torization schemes (e.g., three-sector carrier andsix-sector carrier), or in a configuration wherethe direction of antenna beams are intentionallyshifted across carriers to improve throughput atsector boundaries (Fig. 2c). CA would be possi-ble where coverage overlaps for the CCs of thesame eNB.

As seen in Fig. 2b and 2c, a benefit of CA isthat coverage does not have to be ascertainedfor each CC, as long as one of the CCs providessufficient coverage to ensure service continuity.This provides operators with additional deploy-ment scenarios such as CA deployments shownin Fig. 2d, where a CC of a regular eNB pro-vides macrocell coverage, whereas remote radiohead (RRH) cells are placed at traffic hotspotsto provide extended throughput by another CC.RRH cells are connected via optical fibers to theeNB, thereby allowing the aggregation of CCs

Figure 1. Carrier aggregation types: a) intraband contiguous; b) intraband non-contiguous; c) interbandnon-contiguous.

f

Component carrier

Band A (a)

f

Component carrier

Band A (b)

f

Component carrier

Band A (c)

Band B

One of the most

typical scenarios

envisaged is where

eNB antennas are

collocated and have

the same beam

directions/patterns

for the CCs, provid-

ing nearly the same

coverage on all CCs.

This would be a

typical scenario

when the CCs are of

the same band or

different bands but

with little frequency

separation.


IEEE Communications Magazine • August 201062

between the macrocell and RRH cell based onthe same CA framework for collocated cells.Such deployment allows the operator to effi-ciently improve system throughput by using low-cost RRH equipment.

When CA is applied to existing networkdeployments, limitations also need to be consid-ered. One such limitation is the existence of fre-quency selective repeaters that boost certainCCs only. When such repeaters are present, thepropagation delay across boosted and non-boost-ed CCs may be different, thus requiring separatetransmission timing control for the UL. For sim-plicity, in LTE Release 10 the focus is on scenar-ios not requiring separate management oftransmission timings and synchronization statusper CC for the UL. Hence, scenarios withrepeaters and scenarios with RRH as shown inFig. 2d, which may also require separate ULtiming control, are not considered for Release10 for the UL. These scenarios may be consid-ered in a later release when interband CAbecomes realistic for the UL considering trafficgrowth, spectrum allocation, and device imple-mentation feasibility. However, scenarios withRRH and repeaters have already been consid-ered for the DL in Release 10.

The most efficient deployment depends onvarious factors, such as whether the area in con-sideration is urban, suburban, or rural; whethercommon antennas can be used for the CCs underconsideration; and whether there are hot spotsin the area. A likely scenario is that an existingdeployment on a legacy frequency band (e.g., 2GHz) is used to provide sufficient coverageacross the service area, and new bands like 3.5GHz, to be allocated in the future, are used in amore flexible manner (e.g., to serve traffic in themost cost-effective manner). CA allows for suchflexible use of frequencies in a heterogeneousnetwork, depending on the operator’s needs.

PROTOCOL ARCHITECTUREThe overall protocol structure for LTE-A withCA, as shown in Fig. 3, is the same as that ofRelease 8/9. However, the following enhance-ments are made to support CA.

Similar to LTE Release 8/9, a cell corre-sponds to a DL CC and a UL CC, which arelinked based on the system information (SI)broadcast on the DL CC. When in RRC_CON-

NECTED state, a Release 10 UE can be config-ured with multiple serving cells where each serv-ing cell corresponds to a different DL CC.

From the control plane perspective:• UE only has one radio resource control

(RRC) connection with the network. UEestablishes/re-establishes RRC connectionon a single serving cell, and RRC signalingis used to add, remove, or reconfigure addi-tional serving cells to the UE.

• Radio resource management (RRM) mea-surements performed by UE are enhancedto assist CC management by the eNB.

• Although an LTE-A UE unit can be config-ured with multiple serving cells, the UE isassigned a single cell radio network tempo-rary identifier (C-RNTI), which is used touniquely identify the RRC connection ofthe UE and for scheduling purposes on thephysical downlink control channel(PDCCH) transmitted on any of the acti-vated DL CCs on which the UE monitorsthe PDCCH.

• The medium access control (MAC) sublayeris used for dynamic management of servingcells to be used among the configured setof serving cells of a UE unit, depending ondata activity. Mechanisms under considera-tion include per serving cell discontinuousreception (DRX) control and activation/deactivation control of serving cells. Theseoperations are transparent to the RRC andupper layers.From the data plane perspective:

• When CA is configured, the multicarriernature of the physical layer in the dataplane is only exposed to the MAC sublayer.

• The MAC sublayer performs unicastscheduling and priority handling across allactive serving cells of a UE unit in a waythat is transparent to upper layers. Eachtransport block and its potential hybridautomatic repeat request (HARQ) retrans-missions are mapped to a single servingcell, and there is one independent HARQprocess for each DL or UL CC.

• The Packet Data Convergence Protocol(PDCP) and radio link control (RLC) ofLTE Release 8/9 also apply to CA, andallows the handling of data rates up to 1 Gb/sin the DL and up to 500 Mb/s in the UL.Note that CA is only applicable in

RRC_CONNECTED state. Hence, duringRRC_IDLE state an LTE Release 10 UE unitcamps on a single cell and applies the same pro-cedures, such as cell selection and reselection, asin LTE Release 8/9. The UE also receives pag-ing only on a single cell in RRC_IDLE state, asin Release 8/9.

CARRIER TYPES AND MANAGEMENTIn LTE-A deployments, where multiple cells ofdifferent CCs are deployed at an eNB, all cellsfrom the baseband perspective are configured tobe backward compatible with LTE Release 8/9.Each cell that corresponds to a DL CC and alinked UL CC has a unique E-UTRAN cell glob-al identifier (ECGI) and broadcasts its own cell-specific SI. A cell with a duplex distance the

Figure 2. Carrier aggregation deployment scenarios referred to as: a) scenario1; b) scenario 2; c) scenario 3; d) scenario 4 in 3GPP [4].

(d)

(b)(a)

(c)

CC1 CC2



same as those defined in Release 8/9 is accessi-ble to Release 8/9 UE where each accessible cellrepresents a separate cell to Release 8/9 UE.Hence, Release 10 UE and Release 8/9 UE cancoexist in the same cell. While Release 8/9 UEcan receive and transmit on only one servingcell, Release 10 UE in CA mode can receive andtransmit on multiple serving cells when inRRC_CONNECTED state.

When an LTE-A UE first establishes or re-establishes RRC connection, only one servingcell is configured, which corresponds to the pri-mary serving cell (PCell). Depending on trafficload, quality of service (QoS) requirements orother implementation-specific considerations,the UE can be configured with additional serv-ing cells via dedicated RRC signaling from theeNB. Depending on the UE’s capability anddeployment scenarios, the set of CCs involved inthe configured set of serving cells may be con-tiguous or non-contiguous.

SERVING CELLTo operate in CA mode, LTE-A UE first needsto be configured by the serving eNB with multi-ple serving cells. Upon serving cell addition, theeNB provides the UE by dedicated RRC signal-ing with the SI of that serving cell and assigns ita cell index, which is used to reference to thatcell in subsequent signaling. The set of config-ured serving cells can also be modified via dedi-cated RRC signaling.

PRIMARY AND SECONDARY CCS

Among the configured set of serving cells forUE in RRC_CONNECTED state, one of themis designated as the PCell in which the corre-sponding DL CC is designated as the DL prima-ry CC (PCC), and the corresponding UL CC isdesignated as the UL PCC. The UE may be con-figured with one or more additional serving cells,called secondary serving cells (SCells). The DLand UL CCs corresponding to an SCell arecalled the DL and UL secondary CCs (SCCs),respectively. For each SCell, the usage of a ULSCC by the UE in addition to the DL SCC isconfigurable by the eNB.

The PCell designation is UE-specific and canbe different for different UEs served by thesame eNB. In other words, a cell at an eNB maybe the PCell for one UE and an SCell for anoth-er UE (Fig. 4).

The PCell has the following distinct charac-teristics:• It provides the non-access stratum (NAS)

mobility information (e.g., tracking areaidentifier [TAI] used to determine the needfor location registration) consistent withRelease 8/9. Whether the PCell also pro-vides the security key input is being dis-cussed in 3GPP.

• UE only needs to perform radio link moni-toring on the DL of the PCell. RRC re-establishment is triggered when the UE

Figure 3. Protocol layers for CA in the control and data planes.

User equipmentE-UTRAN NodeBMobility management entityNon-access stratumRadio resource controlPacket Data Convergence ProtocolRadio link controlMedium access controlPhysical layerHybrid automatic repeat requestDL shared channelComponent carrier

UE: eNB:MME:NAS: RRC:PDCP:

RLC:MAC: PHY: HARQ:

DL-SCH: CC:

HARQ HARQ

DL-SCHon CC1

DL-SCHon CCx

Logicalchannels

Transportchannels

Control plane

Data plane

NAS NAS

MMEeNBUE

RRC RRC

PDCP PDCP

RLC RLC

MAC

PHY PHY PHY PHY

MAC

eNB UE

PDCP PDCP

RLC RLC

MAC

PHY PHY PHY PHY

MAC

Multiplexing UE1

Unicast scheduling/ priority handling

Both Release 10 UE

and Release 8/9 UE

can coexist in the

same cell. While

Release 8/9 UEs can

receive and transmit

on only one serving

cell, Release 10 UE in

CA mode can receive

and transmit on

multiple serving cells

when in RRC_CON-

NECTED state.



experiences radio link failure (RLF) on theDL of the PCell, but not when the UEexperiences RLF on an SCell.

• The PCell of UE cannot be deactivated.• Use of semi-persistent scheduling (SPS) in

DL or UL, which was defined in LTERelease 8 to reduce PDCCH usage, is limit-ed to PCell only. This is appropriate givenSPS is primarily targeted toward voice overIP (VoIP) traffic, which does not requireaggregation of resources across multipleCCs.

• UE transmits the physical uplink controlchannel (PUCCH) on the PCell to the eNB.

• The UE only performs random access pro-cedure on the PCell. Random access failureon the PCell triggers RRC re-establishmentby the UE.The DL and UL PCCs should therefore be

robust, and are typically chosen such that theyprovide the most ubiquitous coverage and/orbest overall signal quality. The PCell of UE ini-tially corresponds to the cell with which the UEestablishes or re-establishes the RRC connec-tion. As UE moves within the geographical areaserved by an eNB, the PCell may be changed tocorrespond to the cell with the best signal quali-ty. For example, in the scenario shown in Fig.2b, as the UE moves away from the eNB, thePCC may be changed from CC2 to CC1. PCellchange can also be performed based on otherconsiderations by the eNB such as load balanc-ing. PCell change is performed using the hand-over procedure, which involves security keychange, RLC/PDCP re-establishment, and a ran-dom access procedure. Considering the poten-tially high frequency of PCell changes needed incertain deployments (e.g., the scenario shown inFig. 2c), the need for more optimized proce-dures that can avoid security key change andRLC/PDCP re-establishment is under study.

LINKAGE OF DL AND UL CCSIn LTE Release 8/9 each DL carrier is linked toa single UL carrier based on SI broadcast signal-ing, and the UE always deals with one pair ofDL/UL carriers. In LTE-A, however, UE config-ured with CA may need to interact with eNBson multiple and sometimes unequal numbers of

DL and UL CCs when the usage of UL SCCs ofsome of the SCells is not configured. Therefore,proper linkage between the DL and UL CCsneeds to be defined for the following purposes:• The DL CC that transmits the random

access response to a random access pream-ble transmitted on a UL CC should bedefined so that the UE knows from whichDL CC it should expect to receive the ran-dom access response. This is also needed toavoid ambiguity between random accesspreambles sent by different UE on differentUL CCs. Cell-specific linkage as defined inSI conforming to Release 8/9 is applied forthis purpose. For LTE Release 10, UE onlymaintains one UL timing advance for allthe configured UL CCs.

• When a UL grant is sent on the PDCCHwithout inclusion of a carrier indicator field(CIF) (used for cross-carrier scheduling;details are discussed later), a linkagebetween UL and DL CCs allows identifica-tion of the UL CC for which the grantapplies. The linkage is cell-specific and isdefined based on the SI broadcast signaling.

• The DL CC on which the UE measures theDL path loss for UL power control of a ULCC needs to be defined. Several optionscurrently being considered in 3GPP includeusing the linked DL CC indicated in the SIas the reference for path loss measurement,using DL PCC as the reference for pathloss measurement, and so on.Figure 5 shows the concept of flexible linkage

between DL and UL CCs. Note that DL CCsmay be linked to UL CCs with duplex distancesdifferent than those defined in Release 8/9. Inthis case such DL/UL CCs may not be accessibleby Release 8/9 UE.

SERVING CELL DRX CONTROL ANDACTIVATION/DEACTIVATION CONTROL

While CA provides higher data rates and lowerlatency to UE, the UE’s battery consumptionremains an important aspect that needs to beconsidered while operating in CA mode. Twomechanisms are therefore being considered inthe MAC sublayer to provide efficient batterypower saving: serving cell DRX control and acti-vation/deactivation control.

Serving Cell DRX Control — This is similar toDRX in Release 8/9 with enhancements to sup-port independent per serving cell control ofDRX. In Release 8/9 when DRX is applied, theUE monitors the PDCCH only during the activetime, which is defined as the sum of periodic on-duration and additional subframes where poten-tial HARQ retransmissions may take place forinitial transmissions. The periodicity and length ofon-duration are configurable by RRC. The DRXinactivity timer can be configured so that the UEimplicitly enters DRX operation when no dataare scheduled for the timer duration. In addition,a MAC control element (CE) can be used by theeNB to push the UE immediately into DRX oper-ation. The detailed control for DRX in CA modeis yet to be clarified, such as how the DRX inac-tivity timer is applied to individual serving cells.

Figure 4. PCell/SCell configuration for different UE served by the same eNB.

PCC_2 PCC_3SCC1_3

SCC2_3SCC1_2

Band 1

PCC_1

UE1 UE2 UE3

Carrier 1 Carrier 2 Carrier 3

Band 2



Activation/Deactivation Control — This is todynamically activate and deactivate the DL ofSCells to be used, depending on such elementsas the buffered data amount, required QoS, andcarrier loading. In this approach the DL of aconfigured SCell of UE may be in either activat-ed or deactivated state. The current assumptionis that no explicit activation or deactivation isneeded for the UL of SCells, as the UE shouldbe able to transmit on the physical uplink sharedchannel (PUSCH) of any SCells when scheduledon the corresponding PDCCH. In the DL theUE only receives the physical downlink sharedchannel (PDSCH) and the correspondingPDCCH (depending on whether cross-carrierscheduling is configured) on activated SCells. Ondeactivated SCells, the UE only performs RRMmeasurements to save battery power. The activa-tion/deactivation mechanism is based on a com-bination of a MAC CE and deactivation timers.The MAC CE sent from an eNB to UE carries abitmap where bit positions may map to config-ured serving cells according to their index. Usingthe bitmap, the DL of one or more SCells canbe activated or deactivated independently withinone MAC CE. Implicit deactivation timers aretypically used to provision for error cases whereUE fails to receive the MAC CE with deactiva-tion commands from the eNB.

Which approach to take is still under study in3GPP. Note that even if activation/deactivationcontrol is adopted, common DRX operation isstill applicable to the PCell and all configuredand activated SCells. This means that the PCelland all configured and activated SCells have thesame active time for PDCCH monitoring andPDSCH reception.

CONTROL CHANNEL DESIGN FOR CAThe overall control channel design of Release8/9 has been maintained in Release 10. Howev-er, some special considerations and updates areneeded to support CA operation. This sectionhighlights some considerations for DL and ULcontrol channels based on the latest decisionswithin 3GPP.

DL CONTROL SIGNALINGDL physical layer (PHY) control signaling forCA is based on extending the existing LTERelease 8/9 control channel structure to supportmultiple CCs. Signaling is designed to be flexibleenough to support asymmetric CA configura-tions where a different number of UL and DLCCs are aggregated. The bandwidth of each ofthe aggregated CCs can also be different.

Signaling design is based on a frameworkwhere, for each scheduled UE, dedicated datatransmissions are sent using one transport block(or two transport blocks with spatial multiplex-ing) and one HARQ entity per scheduled CC.The UE may be scheduled over multiple CCssimultaneously. To maintain similarity with LTERelease 8/9, each transport block is mappedcompletely within a single CC.

As CA is applied to UE only when they arein RRC_CONNECTED state, physical channelsand physical signals such as physical broadcastchannel (PBCH) and primary/secondary-syn-

chronization signals (P/S-SCH) that provide ini-tial access signaling are not modified comparedto LTE Release 8/9. Per CC physical control for-mat indicator channel (PCFICH) signaling isprovided to independently vary the controlregion size of each CC. Transmission mode,which determines PDSCH transmission type(MIMO, beamforming, etc.), can also be config-ured independently for each CC.

PDCCH is used to schedule UL grants or DLresource assignments corresponding to PUSCHor PDSCH transmissions, respectively, on eachCC. To maintain similarity with LTE Release8/9, each PDCCH message is mapped complete-ly within a single CC and various physical layertransmission aspects such as convolutional cod-ing and control channel element (CCE) basedresource element mapping are reused. A Release10 UE supporting CA is expected to receivemultiple separate PDCCH messages, with eachmessage corresponding to a UL grant or a DLassignment for each scheduled CC.

As shown in Fig. 6, a PDCCH message on agiven CC can schedule resources on the sameCC (same-carrier scheduling) or on a differentCC (cross-carrier scheduling). Cross-carrierscheduling is supported for both DL resourceassignments and UL grants. UE can be config-ured for cross-carrier scheduling using RRC sig-naling. When UE is configured with cross-carrierscheduling, a 3-bit CIF is appended to the pay-load of PDCCH messages to identify the CCthat corresponds to the resource grant/assign-ment contained in the PDCCH messages.

Same-carrier scheduling is suitable for sce-narios where the UE can reliably receivePDCCH on all aggregated CCs. Cross-carrierscheduling is suitable when the UE is either notconfigured to receive the PDCCH or cannot reli-ably receive it on some CCs. For example, cross-carrier scheduling can be used in heterogeneousnetwork deployments where the interferencelevel varies between CCs and the UE can onlyreceive the PDCCH reliably on a subset of theCCs. Cross-carrier scheduling can also be benefi-cial in scenarios where a wide bandwidth CC(e.g., a 20 MHz LTE Release 8 carrier) is aggre-gated with a narrow bandwidth CC (e.g., a 1.25MHz carrier obtained from re-mining legacy 2Gspectrum). In such scenarios it is more efficientto schedule the PDSCH/PUSCH for the narrowbandwidth CC using cross-carrier schedulingfrom the wide bandwidth CC, since the narrowbandwidth CC has limited control channelresources.

The physical HARQ indicator channel(PHICH) is used in the DL to signal HARQ

Figure 5. Flexible DL/UL CC linkage.

Carrier 1 (DL CC) DL spectrum Carrier 2 (DL CC) Carrier 3 (DL CC)

Carrier 1 (UL CC) UL spectrum Carrier 2 (UL CC)



feedback corresponding to UL PUSCH trans-missions on multiple CCs. Various physical layerPHICH transmission aspects such as orthogonalcode design, modulation, scrambling sequence,mapping to resource elements, and PHICHresource dimensioning are reused from LTERelease 8/9. The PHICH is transmitted in thesame DL CC used to transmit the UL grant.

While the basic framework for DL controlsignaling with CA has already been agreed, sev-eral detailed standardization aspects are stillopen [5], which include:• Design for multiple CC-specific UE search

spaces (i.e., candidate resources within theDL control region where the UE performsblind decoding of PDCCH) within a CC inorder to support signaling of multiple cross-carrier scheduling PDCCH messages on theCC

• Defining linkage between UL PUSCHtransmissions on multiple CCs and PHICHresources on the corresponding DL CC

• Other details related to configuration/recon-figuration of cross-carrier scheduling

UL CONTROL SIGNALINGUL control information (UCI) with CA compris-es HARQ feedback and channel state informa-tion (CSI) feedback for up to five DL CCs, andscheduling request (SR) information. CSIincludes CQI as well as precoding matrix indica-tor (PMI) and rank indicator (RI) for MIMOtransmission control. UCI can be multiplexedwithin PUSCH transmissions of a particular CCor transmitted separately on a physical uplinkcontrol channel (PUCCH) on the PCC. In agiven subframe UCI can only be multiplexedwithin one PUSCH transmission. As previouslydescribed, the PUCCH is transmitted by eachUE only on its UL PCC, as transmissions onmultiple CCs is not desirable from the link-levelperformance, intermodulation, and battery con-sumption perspectives.

Several standardization aspects of UL controlsignaling such as details on HARQ feedback(e.g., acknowledgment [ACK]/negative ACK[NACK]) for up to five DL CCs and the relatedPUCCH formats needed to support it are stillbeing discussed at the time of this writing [5].

While DL and UL control signaling designfor CA in Release 10 can support aggregation ofup to five CCs, some early proposals for specify-ing RF requirements and UE capabilities for

LTE Release 10 [6] suggest that RF require-ments and UE capabilities should first be definedfor an aggregation of two CCs (both intra- andinterband for DL, intraband only for UL) withmaximum aggregated bandwidth limited to 40MHz.

MOBILITY AND INTERFERENCEMANAGEMENT WITH CA

In order to assist the eNB in managing CCs, theeNB can configure UE to perform RRM mea-surements on some or all of the configured CCs.A UE can also be configured to perform RRMmeasurements on non-configured CCs. The sig-nal quality measurement report from the UE canbe used by the serving eNB, along with other fac-tors such as loading and interference conditions,for example, to decide whether a configured CCshould be removed, a non-configured CC shouldbe added to the configured set of CCs for theUE, or whether PCC change or inter-eNB han-dover should be performed. Inter-eNB handoveris supported while CA is ongoing, in which casethe source eNB indicates to the target eNB thePCell to be selected at the target, to be backwardcompatible. However, as is possible with Release8/9, the target eNB may override this selection, ifthe security key is provided as part of the han-dover preparation procedure for the cell to beselected as the PCell. SCells are selected by thetarget eNB, although details of information to betransferred from the source eNB to assist thisselection are yet to be specified.

In a heterogeneous network environment,various techniques are being considered to miti-gate interference [3], one of which is to use CA.Figure 7 shows an example scenario where twoCCs are available and a pico eNB is deployedwithin a macro eNB’s coverage. The pico eNBuses CC1 as the PCC and CC2 as the SCC,whereas the macro eNB uses CC2 as the PCCand CC1 as the SCC. Provided that the macroand pico eNBs are frame synchronized, cross-carrier scheduling can be applied to some of theUE under CA so that PDSCH resources on theSCC can be allocated through the PDCCH onthe PCC. The PDCCH on the SCC can still beused where possible (e.g., at low transmissionpower) so that strong interference to thePDCCH on the PCC can be avoided. Whileother techniques to mitigate interference arealso under consideration (e.g., time splitting ofresources between macro and pico eNBs on thesame CC), CA can be a viable technique in sucha heterogeneous environment. In these scenariosRRM measurement reports and CQI feedbackplay a critical role in helping the RRC andscheduler at the eNB to manage the often severeintercell interference.

CONCLUSIONSIn this article we provide an overview of CA in3GPP LTE-A, with particular focus on the LTERelease 10 features being developed in 3GPP1

as well as expected deployment scenarios andusage models of CA. Combined with other fea-tures defined in LTE Release 10, such as higher

Figure 6. Scheduling options using PDCCH.

CC1

CC2

PDCCH messages

Same-carrier scheduling

Resource assignments (PDSCH)

CC1

CC2

PDCCH messages

Cross-carrier scheduling

Resource assignments(PDSCH)

1 As LTE Release 10 stan-dard specifications arestill under developmentwithin 3GPP, some of thefeatures described in thisarticle may change.

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order MIMO, CA provides a powerful means toboost the peak user throughput in LTE Release10 and to meet the IMT-Advanced requirementsset by the ITU-R. CA allows aggregation of CCsdispersed across different bands as well as CCshaving different bandwidths. CA also allowsaggregation of cells having different coverage,thereby enabling flexible network deploymentsaccording to traffic demands. By exploitingcross-carrier scheduling, efficient interferencemanagement is possible in heterogeneous net-work deployments, thereby improving systemcapacity. Moreover, each CC is backwards com-patible with LTE Release 8/9, allowing smoothupgrade and migration of LTE networks towardsLTE-Advanced. Further evolution of CA isexpected in future releases of LTE to includemore advanced features such as inter-band CAfor the UL and separate timing control for dif-ferent UL CCs, to support additional deploy-ment scenarios.

REFERENCE[1] ITU-R M.2134 Rep., “Requirements Related to Technical

Performance for IMT-Advanced Radio Interface(s),” Nov.2008.

[2] 3GPP RP-091440, “Work Item Description: CarrierAggregation for LTE,” Dec. 2009.

[3] 3GPP TR 36.814, “Evolved Universal Terrestrial RadioAccess (E-UTRA); Further Advancements for E-UTRAPhysical Layer Aspects,” Version 9.0.0, Mar. 2010.

[4] 3GPP TS 36.300, “Evolved Universal Terrestrial RadioAccess (E-UTRA); Stage 2 Description,” Version 9.3.0,Apr. 2010.

[5] 3GPP R1-102601, “Final Report of 3GPP TSG RAN WG1#60bis v1.0.0,” Beijing, China, Apr. 12–16, 2010.

[6] 3GPP R1-103348, “UE Categories for Rel.10,” NTTDOCOMO, AT&T, TeliaSonera, Orange, Deutsche Tele-com, Telecom Italia, 3GPP TSG RAN WG1 #61, Montre-al, Canada, May 10–14, 2010.

BIOGRAPHIESMIKIO IWAMURA ([email protected]) received hisB.S. and M.S. degrees in electrical engineering from Sci-ence University of Tokyo in 1996 and 1998, respectively,and received his Ph.D. degree in telecommunications fromthe Centre for Telecommunications Research at King’s Col-lege, London, United Kingdom, in 2006. Joining NTTDOCOMO in 1998, he has worked on WCDMA/HSPA tech-nology assessment, planning, and development. Since2005 he has been a major contributor in developing radioaccess protocol standards for LTE/LTE-Advanced in 3GPP,especially in the control plane. He is now a manager of theLTE/LTE-Advanced standardization group at NTT DOCOMO.He has numerous patents and publications, and has con-tributed to a book on UMTS modeling, planning, and auto-mated optimization.

KAMRAN ETEMAD ([email protected]) received hisB.S. degree in electronic engineering from Sharif Universityof Technology, and M.S. and Ph.D. degrees in electricalengineering from the University of Maryland. Currently, heis the director of technology standards at Intel Corporationwhere he leads various technical and strategic initiativesrelated to 3GPP/LTE and IEEE802.16/WiMAX standardsdevelopment. Prior to Intel he held senior technical andmanagement positions with Sprint-Nextel as an executivetechnology consultant, WFI as vice president of AdvancedTechnology, and Hughes Network Systems as a SeniorMember of Technical Staff. His current focus is on advancedwireless technology development for the next generationof mobile broadband networks. Among his key areas ofresearch are radio network architecture/protocols designfor multilayer and multicarrier networks, enhanced mobili-ty, location, and multicast/broadcast services. Prior to hisinvolvement in 3GPP and WiMAX he made many contribu-tions to the development of cdma2000 technology in3GPP2. He has numerous publications and patents in these

areas, including two books: CDMA2000 Evolution andWiMAX Technology and Network Evolution.

MO-HAN FONG ([email protected]) received her Ph.D. andM.A.Sc. degrees in electrical engineering from the Universi-ty of Victoria, British Columbia, Canada. She received herB.Eng. degree in electrical engineering from Nanyang Tech-nological University, Singapore. She is currently a seniorstandards manager, Global Standards, at Research inMotion Ltd. (RIM), where she is responsible for radioaccess standards development with specific focus on 3GPPLTE/LTE-Advanced. Prior to RIM she was a senior managerin the Wireless Technology Laboratories, Nortel Networks,where she led the research, technology assessment, andstandards development of 3G and 4G technologies includ-ing 3GPP UMTS/HSPA/LTE, 3GPP2 1xEV-DO/1xEV-DV/UMB,IEEE 802.16e/802.16m, and WiMAX. Her areas of researchinclude advanced wireless access technologies in the physi-cal, MAC, and upper protocol layers, and next-generationbroadband wireless systems and networks. She is a majorcontributor to 3G and 4G wireless standards fora, includ-ing 3GPP, 3GPP2, and IEEE 802.16/WiMAX. She has pub-lished numerous scientific papers and has over 200 patentsgranted or pending.

RAVIKIRAN NORY [S’00, M‘02] received his B.S. degree in elec-tronics and communication engineering from JawaharlalNehru Technological University in 2000 and his M.S.degree in electrical engineering from Virginia Tech in 2002.He is currently a senior staff research engineer at Motorolaand has been participating in standardization efforts for3GPP LTE since 2006. Prior to this he worked at Motorolaon location technologies such as Assisted-GPS (A-GPS) andEnhanced-Observed Time Difference (E-OTD). His currentfocus is on physical and MAC layer signaling design forwireless communication and modeling of wireless networkperformance.

ROBERT LOVE [S’82, M‘84] graduated from the University ofFlorida, Gainesville in 1982 and received his M.S. degreefrom the same university in 1984. From 1984 to 1986 heworked at Texas Instruments on communication and radarsystems, and from 1986 to 1990 he worked at E-systemson signal processing and communication systems. He iscurrently a Fellow of the Technical Staff at Motorola, andsince 1990 has worked at Motorola in the wireless infra-structure and handset groups on products and researchrelated to wireless communication systems such as GSM,PDC, CDMA, WCDMA, 1xEVDO/V, HSPA, 802.16e, LTE, andcurrently on LTE Advanced and 802.11. He has numerouspatents and publications and most recently contributed tothe book LTE — The UMTS Long Term Evolution: FromTheory to Practice. His research interests include digitalcommunications, signal processing, and wireless networkperformance. He has also been attending and contributingto 3GPP standards (primarily RAN WG1) since 1999.

Figure 7. Interference management in heterogeneous network deployment. Diag-onal arrows: cross-carrier scheduling; diagonal lines: interference mitigation.

Low power PDCCH PDCCH

PDSCH

SCC CC1 PCC

PCC CC2 SCC

Macro eNB

Macro eNB

Pico eNB

Pico eNB PDCCH

PDSCH

PDCCH CC1 CC2

Cross-carrier scheduling

Interference mitigation

PDSCH

PDCCH

PDSCH

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