Carrier aggregation explained

Carrier Aggregation explained
Author: Jeanette Wannstrom, for 3GPP, (Submission, June 2013)
Carrier aggregation is used in LTE-Advanced in order to increase the bandwidth,and thereby increase the bitrate.
Since it is importantto keep backward compatibilitywith R8 and R9 UEs the aggregation is based on R8/R9 carriers.
Carrier aggregation can be used for both FDD and TDD, see figure 1 for an example where FDD is used.
Figure 1. Carrier Aggregation (FDD); The LTE-Advanced UE can be allocated DL and UL resources on the
aggregated resource consisting of two or more Component Carriers (CC), the R8/R9 UEs can be allocated
resources on any ONE of the CCs. The CCs can be of different bandwidths.
Each aggregated carrier is referred to as a componentcarrier,CC.The componentcarrier can have a bandwidth of
1.4, 3, 5, 10, 15 or 20 MHz and a maximum offive componentcarriers can be aggregated,hence the maximum
aggregated bandwidth is 100 MHz. In FDD the number of aggregated carriers can be differentin DL and UL, see
figure 1. However, the number ofUL componentcarriers is always equal to or lower than the number ofDL
componentcarriers.The individual componentcarriers can also be ofdifferent bandwidths.For TDD the number of
CCs as well as the bandwidths ofeach CC will normallybe the same for DL and UL.
The easiestwayto arrange aggregation would be to use contiguous componentcarriers within the same operating
frequency band (as defined for LTE), so called intra-band contiguous.This mightnotalways be possible,due to

operator frequency allocation scenarios.For non-contiguous allocation itcould either be intra-band,i.e. the
componentcarriers belong to the same operating frequencyband,buthave a gap, or gaps,in between,or it could be
inter-band,in which case the componentcarriers belong to differentoperating frequencybands,see figure 2.
Figure 2. Carrier Aggregation; Intra-band and inter-band aggregation alternatives. The spacing between the
centre frequencies of two contiguous CCs is Nx300 kHz, N=integer. For non-contiguous cases the CCs are
separated by one, or more, frequency gap(s).
For practical reasons CAis initially specified for only a few com binations ofE-UTRA operating bands and number of
CCs.To specify different CA combinations some new definitions are used:
 Aggregated Transmission Bandwidth Configuration (ATBC): total number ofaggregated physical resource
blocks (PRB).
 CA bandwidth class:indicates a combination ofmaximum ATBC and maximum number ofCCs.In R10 and
R11 three classes are defined:
Class A: ATBC ≤ 100, maximum number ofCC = 1
Class B:ATBC ≤ 100, maximum number ofCC = 2
Class C:100 < ATBC ≤ 200,maximum number ofCC = 2
 CA configuration:indicates a combination ofE-UTRA operating band(s) and CAbandwidth class(es),to
exemplify the configuration CA_1C indicates intra-band contiguous CAon E-UTRA operating band 1 and CA
bandwidth class C,CA_1A_1A, indicates intra-band non-contiguous CAon band 1 with a one CC on each
side of the intra-band gap,finally CA_1A-5B indicates inter-band CA,on operating band 1 with bandwidth
class A and operating band 5 with bandwidth class B.
In R10 three CA configurations are defined,see table 1.

Type of CA and
duplex type
CA configuration
Maximum aggregated
bandwidth (MHz)
Max number of CC
Intra-band contiguous
FDD
CA_1C 40 2
TDD
CA_40C 40 2
Inter-band
FDD
CA_1A_5A 20 1 + 1
Table 1. CA configurations defined for R10
In R11 a large number ofadditional CA configurations are defined,see table 2.The maximum aggregated bandwidth
is still 40 MHz and maximum number ofCC is 2. Note also that for both R10 and R11 any UL CC will have the same
bandwidth as the corresponding DL CC.Also for inter-band CA there will only be ONE UL CC, i.e. no UL CA.
++ Updated table of Rel-11 Bands can be found in Section 7 of the "Carrier Aggregation for LTE" document ...here ++
Type of CA and
duplex type
CA configuration
Maximum aggregated
bandwidth (MHz)
Max number of CC
FDD
CA_1C 40 2
CA_7C 40 2
TDD
CA_38C 40 2
CA_40C 40 2
CA_41C 40 2
Inter-band
FDD
CA_1A_5A 20 1 + 1
CA_1A_18A 35 1 + 1
CA_1A_19A 35 1 + 1
CA_1A_21A 35 1 + 1
CA_2A_17A 20 1 + 1
CA_2A_29A 20 1 + 1
CA_3A_5A 30 1 + 1
CA_3A_7A 40 1 + 1

CA_3A_8A 30 1 + 1
CA_3A_20A 30 1 + 1
CA_4A_5A 20 1 + 1
CA_4A_7A 30 1 + 1
CA_4A_12A 20 1 + 1
CA_4A_13A 30 1 + 1
CA_4A_17A 20 1 + 1
CA_4A_29A 20 1 + 1
CA_5A_12A 20 1 + 1
CA_5A_17A 20 1 + 1
CA_7A_20A 30 1 + 1
CA_8A_20A 20 1 + 1
CA_11A_18A 25 1 + 1
Intra-band non-
contiguous
FDD
CA_25A_25A 20 1 + 1
Table 2. CA configurations defined in R11
In later releases more configurations will be added.For example,in R12 configurations for UL inter-band CA
configuration will be introduced.
++ A table ofRel-12 Bands can be found in Section 8 of the "Carrier Aggregation for LTE" document ...here ++
When carrier aggregation is used there are a number ofserving cells,one for each componentcarrier.The coverage
of the serving cells may differ, for example due to that CCs on differentfrequency bands will experience different
pathloss,see figure 3.The RRC connection is only handled by one cell, the Primary serving cell, served by the
Primary componentcarrier (DL and UL PCC). It is also on the DL PCC that the UE receives NAS information,such as
security parameters.In idle mode the UE listens to system information on the DL PCC. On the UL PCC PUCCH is
sent.The other componentcarriers are all referred to as Secondary componentcarriers (DL and UL SCC), serving
the Secondary serving cells,see figure 3. The SCCs are added and removed as required,while the PCC is only
changed at handover.

Figure 3. Carrier Aggregation; Primary and Secondary serving cells.
Each component carrier corresponds to a serving cell. The different serving cells may have different
coverage.
Different componentcarriers can be planned to provide different coverage, i.e. different cell size. In the case of inter-
band carrier aggregation the componentcarriers will experience differentpathloss,which increases with increasing
frequency. In the example shown in figure 3 carrier aggregation on all three componentcarriers can only be used for
the black UE, the white UE is not within the coverage area of the red componentcarrier.Note that for UEs using the
same setofCCs,can have different PCC.
Introduction of carrier aggregation influences mainlyMAC and the physical layer protocol, but also some new RRC
messages are introduced.In order to keep R8/R9 compatibilitythe protocol changes will be keptto a minimum.
Basicallyeach componentcarrier is treated as an R8 carrier. However some changes are required,such as new
RRC messages in order to handle SCC,and MAC mustbe able to handle scheduling on a number ofCCs.Major
changes on the physical layer are for example that signaling information aboutscheduling on CCs mustbe provided
DL as well as HARQ ACK/NACK per CC mustbe delivered UL and DL, see figure 4.

Figure 4. LTE protocols for the radio interface, with main changes due to introduction of CA.
Regarding scheduling there are two main alternatives for CA, either resources are scheduled on the same carrier as
the grant is received, or so called cross-carrier scheduling maybe used,see figure 5.

Figure 5. CA scheduling (FDD); Cross- carrier scheduling is only used to schedule resources on SCC without
PDCCH. The CIF (Carrier Indicator Field) on PDCCH (represented by the red area) indicates on which carrier
the scheduled resource is located.
For heterogeneous network planning the use offor example remote radio heads (RRH) is ofimportance.From R11 it
will be possible to handle CA with CCs requiring differenttiming advance (TA), for example combining CC from eNB
with CC from RRH,see figure 6.
Figure 6. In R11 it will be possible to supportserving cells with differentTiming Advance (TA). Serving cells with the

same TA belongs to the same TA Group (TAG).

Carrier aggregation explained

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Carrier aggregation explained