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Carrier Aggregation in LTE

Advanced

By: Aviral Garg

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Technical Challenges

Some Challenges in implementing LTE Advanced (Rel 10)

which are solved using Carrier Aggregation:

1) Larger bandwidth requirement of 100 Mhz. In other wordshigher Data Rate requirement of 1 Gbps in DL.

2) Scattered Bands used by the cellular operators.

3) Backward Compatibility with Rel 8 and Rel 9 mobiles.

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LTE R8 vs R10

LTE Data Rate Requirements:

DL: 100 Mbps

UL: 50 Mbps

R8 Spectrum Flexibility:

LTE 3GPP R8:

LTE R10 (LTE Advanced) Key

Requirements:DL: 1Gbps

UL 500 Mbps

Ch. Bandwidth: 100 Mhz andBackward compatibility

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Carrier Aggregation in LTE Advanced

Note: DC-HSDPA & DC-HSUPA in R9

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Implementation Types

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Introduction

Carrier aggregation can be used for both FDD and TDD.

The component carrier can have a bandwidth of 1.4, 3, 5,

10, 15 or 20 MHz and a maximum of five componentcarriers can be aggregated, hence the maximum

aggregated bandwidth is 100 MHz.

The spacing between two CCs is Nx300 kHz, N=integer. Fornon-contiguous cases the CCs are separated by one, or

more, frequency gap(s).

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Changes into LTE protocols due to CA

1) E-UTRAN Aspects Basically each component carrier is treated as an R8 carrier.

When carrier aggregation is used there are a number of

serving cells (namely PCell and SCell), one for each component

carrier. The coverage of the serving cells may differboth due to

component carrier frequencies but also from power planning

which is useful for heterogeneous network planning.

The SCCs are added and removed as required, while the PCC isonly changed at handover.

The RRC connection is only handled by one cell, the Primary

serving cell, served by the Primary component carrier (DL and

UL PCC).

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E-UTRAN Aspects (Contd.)

Each PCell is equipped with one physical downlink control

channel (PDCCH) and one physical uplink control channel

(PUCCH). An SCell could be equipped with a PDCCH or not,

depending on UE capabilities. An SCell never has a PUCCH.

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2) MAC Layer Aspects

CA simply adds additional shared channels (SCHs) per CC.

Below shown is R8 stack.

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After Carrier Aggregation:

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3) Physical Layer Aspects:

Downlink Channel Quality

Downlink channel quality, per LTE Release 8 and 9, is

assessed at the UE and reported via the channel state

information (CSI) Information Element (IE). In Release 10, the

existence of multiple CCs means that CQI must be evaluated

and reported for each CC individually when CA is active.

CQIas well as downlink HARQ ACK/NACK indicators and

other informationis reported to the base station via the

uplink control information (UCI) IE. Since there is exactly one

PUCCH (on the PCell) regardless of the number of CCs, UCI

for each CC must be reported via the same PUCCH.

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Downlink Channel Quality (Contd.)

Thus there is a need to distinguish the CC to which a givenUCI pertains. This is accomplished through the carrier

indicator field (CIF), which is header on the UCI.

Since it is possible to require the UE to report CQIperiodically, and since UEs do not necessarily support

simultaneous transmission of PUCCH and PUSCH, CQI also

could be reported on the PUSCH, if the PUSCH happens to

be active at the time of a periodic reporting instance.

In the context of CA, this means that CQI could be

transmitted on an SCell if an SCell uplink burst is ongoing

while a PCell burst is not.

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Uplink Channel Quality

Uplink channel quality, again per LTE Release 8 and 9, isassessed at the base station via sounding reference symbols

(SRS) transmitted by the UE.

CA implies that channel sounding could be required onmultiple CCs. Release 10 introduces enhancements to

permit the base station to request periodic SRS

transmission on SCells in addition to PCells, though this

function is optional at the UE.

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Timing and Synchronization

The PCell and the SCell(s) are deemed to be transmitted bythe same base station.

The electrical path length between the base station and theUE therefore is deemed to be the same for all carriers.

This is the case regardless of frequency band. Thus, there isa single timing advance value applied to all uplinktransmissions, regardless of whether they occur on the

PCell or an SCell.

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Cross-carrier Scheduling

SCells might or might not be equipped with a PDCCH. In the

former case, the UE assumes that the scheduling

information is carried by the SCellsPDCCH. In the latter

case, scheduling information for the SCell must be

delivered via another cells PDCCH.

In Release 10, this is referred to as cross-carrier scheduling.

Cross Carrier Scheduling does not apply to PCell.

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Cross-carrier Scheduling (Contd.)

As with other functionality described above, the carrier

responsible for the delivering scheduling information in

the context of cross-carrier scheduling is indicated by the

CIF.

Cross-carrier scheduling support is optional for the UE.

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Handover

Handover processing for LTE in Release 10 is largely the same asReleases 8 and 9, except that clarifications are made to refer toPCell in the measurement-related RRC signaling messages.

Release 10 does introduce one new measurement event: Event

A6. As indicated above, Event A6 occurs when a neighboringcells strength becomes better than an SCellsstrength by anoffset. In the case of intra-band SCells, this event is less useful,as the strength of the PCell and the SCells usually is very similar.However, with inter-band serving cells, the strength of a

neighboring PCell could be significantly different from a servingSCell.

Depending on network conditionssuch as traffic loaddistributionit could be advantageous to execute a handover to

the cell identified by Event A6.

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Conclusion

Achieving the requirements set out in IMT-Advanced is

even more challenging given the fragmentation of the

available cellular radio spectrum, particularly in North

America.

Release 10LTE-Advanced delivers IMT-Advanced within

practical spectrum constraints. It does this through the

carrier aggregation feature. Carrier aggregation permits an

LTE base station to group several distinct channels into one

logical channel, thereby enabling very high peak traffic

channel data rates.

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References

Some material taken from Dr. HmimysSMU EETS 8315

course slides.

http://lteworld.org/blog/carrier-aggregation-lte-advanced

http://www.3gpp.org/Carrier-Aggregation-explained http://www.radio-

electronics.com/info/cellulartelecomms/lte-long-term-

evolution/4g-lte-advanced-carrier-channel-aggregation.php

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