Carrier Aggregation explained

Carrier aggregation is used in LTE-Advanced in order to increase the bandwidth, and thereby increase the bitrates. Since it is important to keep backward compatibility with R8 and R9 mobiles the aggregation is of R8/R9 carriers. Carrier aggregation can be used for both FDD and TDD, see figure 1 for an example where FDD is used.

Fig. 01

Figure 1. Carrier Aggregation – FDD The R10 UE can be allocated DL and UL resources on the aggregated resource, the R8/R9 UEs can be allocated resources on any ONE of the Component Carriers (CC). The CCs can be of different bandwidths, and the allocation DL and UL can be the same or different.

Each aggregated carrier is referred to as a component carrier, CC. The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers can be aggregated, hence the maximum aggregated bandwidth is 100 MHz. In FDD the number of aggregated carriers can be different in DL and UL, see figure 1. However, the number of UL component carriers is always equal to or lower than the number of DL component carriers. The individual component carriers can also be of different bandwidths. When TDD is used the number of CCs and the bandwidth of each CC are the same for DL and UL.
The easiest way to arrange aggregation would be to use contiguous component carriers within the same operating frequency band (as defined for LTE), so called intra-band contiguous. This might not always be possible, due to operator frequency allocation scenarios. For non-contiguous allocation it could either be intra-band, i.e. the component carriers belong to the same operating frequency band, but have a gap, or gaps, in between, or it could be inter-band, in which case the component carriers belong to different operating frequency bands, see figure 2.

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Figure 2. Carrier Aggregation; Intra-band and inter-band aggregation alternatives. The spacing between two CCs is Nx300 kHz, N=integer. For non-contiguous cases the CCs are separated by one, or more, frequency gap(s).

For practical reasons CA is initially specified for only a few operating bands. In R10 three CA bands are defined. For intra-band contiguous CA, R8 operating band 1 (FDD) is defined as CA band CA_1 and band 40 (TDD) are defined as CA_40. For inter-band non-contiguous CA, R8 operating bands 1 and 5 (FDD) are defined as one CA band named CA_1-5. More CA bands will be defined in later releases.
When carrier aggregation is used there are a number of serving cells, one for each component carrier. The coverage of the serving cells may differ – both due to component carrier frequencies but also from power planning – which is useful for heterogeneous network planning. The RRC connection is only handled by one cell, the Primary serving cell, served by the Primary component carrier (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 component carriers are all referred to as Secondary component carriers (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, useful for heterogeneous cell-planning.

Different component carriers can be planned to provide different coverage, i.e. different cell size. In the case of inter-band carrier aggregation the component carriers will experience different pathloss, which increases with increasing frequency. In the example shown in figure 3 carrier aggregation on all three component carriers can only be used for the black UE, the white UE is not within the coverage area of the red component carrier. Note that for UEs using the same set of CCs, they can have different PCC.
Introduction of carrier aggregation influences mainly MAC and the physical layer protocol, but also some new RRC messages are introduced. In order to keep R8/R9 compatibility the protocol changes will be kept to a minimum. Basically each component carrier is treated as an R8 carrier. However some information is necessary, such as new RRC messages in order to handle SCC and MAC must be able to handle scheduling on a number of CCs. Major changes on the physical layer are for example that signaling information about scheduling on CCs as well as HARQ ACK/NACK per CC must be carried. 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 may be 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.

Further reading

• TR 36.808 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier Aggregation; Base Station (BS) radio transmission and reception
• TR 36.814 Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA physical layer aspects
• TR 36.815 Further Advancements for E-UTRA; LTE-Advanced feasibility studies in RAN WG4
• TR 36.823 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier Aggregation Enhancements; UE and BS radio transmission and reception
• TR 36.912 Feasibility study for Further Advancements for E-UTRA (LTE-Advanced)
• TR 36.913 Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced)
• TS 36.211 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation
• TS 36.212 Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding
• TS 36.213 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures
• TS 36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2

More, Further reading

from the Work Programme Manager;
You are going deep here...The links below take you to the detailed work plan and a series of auto-generated HTML pages for each detailed 3GPP feature.
Release 10

• Carrier Aggregation for LTE UID_460007 (test ongoing)

Release 11
Inter-band Carrier Aggregation

• LTE Advanced Carrier Aggregation of Band 3 and Band 7 UID_480023
• LTE Advanced Carrier Aggregation of Band 4 and Band 17 UID_500017
• LTE Advanced Carrier Aggregation of Band 4 and Band 13 UID_500018
• LTE Advanced Carrier Aggregation of Band 4 and Band 12 UID_510022
• LTE Advanced Carrier Aggregation of Band 5 and Band 12 UID_510023

Intra-band Carrier Aggregation

• LTE Carrier Aggregation Enhancements UID_510030
• LTE Advanced Carrier Aggregation in Band 38 UID_520015
• LTE Advanced Carrier Aggregation in Band 41 UID_520016
• LTE Advanced Carrier Aggregation in Band 7 UID_530028

Release 12
Inter-band Carrier Aggregation

• LTE Advanced Carrier Aggregation of Band 3 and Band 5 with 2UL UID_550010
• LTE Advanced Carrier Aggregation of Band 3 and Band 8 UID_550018
• LTE Advanced Inter-band Carrier Aggregation of Band 2 and Band 4 UID_560017

Intra-band Carrier Aggregation

• LTE Advanced Intra-band Non-Contiguous Carrier Aggregation in Band 25 UID_530029
• LTE Advanced Intra-band Non-Contiguous Carrier Aggregation in Band 3 UID_550011
• LTE Advanced Intra-band Non-Contiguous Carrier Aggregation in Band 4 UID_560016
• LTE Advanced Intra-band Contiguous Carrier Aggregation in Band 1 UID_560015