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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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