Carrier Aggregation.docx

Carrier Aggregation Carrier aggregation is used in LTE-Advanced to increase the bandwidth, and thereby increase the bitrate. Since it is important to keep backward compatibility with R8 and R9 UEs the aggregation is based on R8/R9 carriers. Carrier aggregation can be used for both FDD and TDD, see below fig for an example where FDD is used.

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 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. For TDD the number of CCs as well as the bandwidths of each CC will normally be 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,

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 CA is initially specified for only a few combinations of E-UTRA operating bands and number of CCs. To specify different CA combinations some new definitions are used:  

Aggregated Transmission Bandwidth Configuration (ATBC): total number of aggregated physical resource blocks (PRB). CA bandwidth class: indicates a combination of maximum ATBC and maximum number of CCs. In R10 and R11 three classes are defined:

Class A: ATBC ≤ 100, maximum number of CC = 1 Class B: ATBC ≤ 100, maximum number of CC = 2 Class C: 100 < ATBC ≤ 200, maximum number of CC = 2 

CA configuration: indicates a combination of E-UTRA operating band(s) and CA bandwidth classes, to exemplify the configuration CA_1C indicates intra-band contiguous CA on E-UTRA operating band 1 and CA bandwidth class C, CA_1A_1A, indicates intra-band non-contiguous CA on 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

Intra-band contiguous TDD

CA_40C

40

2

Inter-band FDD

CA_1A_5A

20

1+1

CA configurations defined for R10 In R11 many additional CA configurations are defined, see table 2. The maximum aggregated bandwidth is still 40 MHz and maximum number of CC 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. Type of CA and duplex type Intra-band contiguous FDD

Intra-band contiguous TDD

Inter-band FDD

CA configuration

Maximum aggregated bandwidth (MHz)

Max number of CC

CA_1C

40

2

CA_7C

40

2

CA_38C

40

2

CA_40C

40

2

CA_41C

40

2

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

Intra-band noncontiguous FDD

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

CA_25A_25A

20

1+1

CA configurations defined in R11 In later releases more configurations will be added. For example, in R12 configurations for UL interband CA configuration will be introduced. 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, for example due to that CCs on different frequency bands will experience different path loss, see figure 3. 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.

Carrier Aggregation; Primary and Secondary serving cells. Each component carrier corresponds to a serving cell. The different serving cells may have different coverage. 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 path loss, 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, can have different PCC. Introduction of carrier aggregation influences mainly MAC and the physical layer protocol, but also some new RRC messages are introduced. 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 changes are required, such as new RRC messages to handle SCC, and MAC must be able to handle scheduling on several CCs. Major changes on the physical layer are for example that signaling information about scheduling on CCs must be provided DL as well as HARQ ACK/NACK per CC must be delivered UL and DL,

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.

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 of for example remote radio heads (RRH) is of importance. From R11 it will be possible to handle CA with CCs requiring different timing advance (TA), for example combining CC from eNB with CC from RRH, see figure 6.

LTE Advanced offers considerably higher data rates than even the initial releases of LTE. While the spectrum usage efficiency has been improved, this alone cannot provide the required data rates that are being headlined for 4G LTE Advanced.

To achieve these very high data rates it is necessary to increase the transmission bandwidths over those that can be supported by a single carrier or channel. The method being proposed is termed carrier aggregation, CA, or sometimes channel aggregation. Using LTE Advanced carrier aggregation, it is possible to utilize more than one carrier and in this way, increase the overall transmission bandwidth. These channels or carriers may be in contiguous elements of the spectrum, or they may be in different bands. Spectrum availability is a key issue for 4G LTE. In many areas, only small bands are available, often as small as 10 MHz thus carrier aggregation, over more than one band is contained within the specification, although it does present some technical challenges. Carrier aggregation is supported by both formats of LTE, namely the FDD and TDD variants. This ensures that both FDD LTE and TDD LTE can meet the high data throughput requirements placed upon them.

LTE carrier aggregation basics The target figures for data throughput in the downlink is 1 Gbps for 4G LTE Advanced. Even with the improvements in spectral efficiency it is not possible to provide the required headline data throughput rates within the maximum 20 MHz channel. The only way to achieve the higher data rates is to increase the overall bandwidth used. IMT Advanced sets the upper limit at 100 MHz, but with an expectation of 40 MHz being used for minimum performance. For the future, it is possible the top limit of 100 MHz could be extended. It is well understood that spectrum is a valuable commodity, and it takes time to re-assign it from one use to another in view - the cost of forcing users to move is huge as new equipment needs to be bought. Accordingly, as sections of the spectrum fall out of use, they can be re-assigned. This leads to significant levels of fragmentation. To an LTE terminal, each component carrier appears as an LTE carrier, while an LTE-Advanced terminal can exploit the total aggregated bandwidth.

RF aspects of carrier aggregation There are several ways in which LTE carriers can be aggregated:

Types of LTE carrier aggregation 

Intra-band: This form of carrier aggregation uses a single band. There are two main formats for this type of carrier aggregation: o

Contiguous: The Intra-band contiguous carrier aggregation is the easiest form of LTE carrier aggregation to implement. Here the carriers are adjacent to each other.

Contiguous aggregation of two uplink component carriers

The aggregated channel can be considered by the terminal as a single enlarged channel from the RF viewpoint. In this instance, only one transceiver is required within the terminal or UE, whereas more are required where the channels are not adjacent. However, as the RF bandwidth increases it is necessary to ensure that the UE is able to operate over such a wide bandwidth without a reduction in performance. Although the performance requirements are the same for the base station, the space, power consumption, and cost requirements are considerably less stringent, allowing greater flexibility in the design. Additionally, for the base station, multi-carrier operation, even if non-aggregated, is already a requirement in many instances, requiring little or no change to the RF elements of the design. Software upgrades would naturally be required to cater for the additional capability.

o



Non-contiguous: Non-contiguous intra-band carrier aggregation is somewhat more complicated than the instance where adjacent carriers are used. No longer can the multi-carrier signal be treated as a single signal and therefore two transceivers are required. This adds significant complexity, particularly to the UE where space, power and cost are prime considerations. Inter-band non-contiguous: This form of carrier aggregation uses different bands. It will be of use because of the fragmentation of bands - some of which are only 10 MHz wide. For the UE, it requires the use of multiple transceivers within the single item, with the usual impact on cost, performance and power. In addition to this there are also additional complexities resulting from the requirements to reduce intermodulation and cross modulation from the two transceivers

The current standards allow for up to five 20 MHz carriers to be aggregated, although in practice two or three is likely to be the practical limit. These aggregated carriers can be transmitted in parallel to or from the same terminal, thereby enabling a much higher throughput to be obtained.

Carrier aggregation bandwidths When aggregating carriers for an LTE signal, there are several definitions required for the bandwidth of the combined channels. As there as several bandwidths that need to be described, it is necessary to define them to reduce confusion.

LTE Carrier Aggregation Bandwidth Definitions for Intra-Band Case

LTE carrier aggregation bandwidth classes There is a total of six different carrier aggregation, CA bandwidth classes which are being defined.

Carrier Aggregation Bandwidth Class

Aggregated Transmission BW Configuration

Number of component carriers

A

≤100

1

B

≤100

2

C

100 - 200

2

NB: classes D, E, & F are in the study phase.

LTE aggregated carriers When carriers are aggregated, each carrier is referred to as a component carrier. There are two categories:  

Primary component carrier: This is the main carrier in any group. There will be a primary downlink carrier and an associated uplink primary component carrier. Secondary component carrier: There may be one or more secondary component carriers.

There is no definition of which carrier should be used as a primary component carrier - different terminals may use different carriers. The configuration of the primary component carrier is terminal specific and will be determined according to the loading on the various carriers as well as other relevant parameters. In addition to this the association between the downlink primary carrier and the corresponding uplink primary component carrier is cell specific. Again, there are no definitions of how this must be organized. The information is signaled to the terminal of user equipment as part of the overall signaling between the terminal and the base station.

Carrier aggregation cross carrier scheduling When LTE carrier aggregation is used, it is necessary to be able to schedule the data across the carriers and to inform the terminal of the DCI rates for the different component carriers. This information may be implicit, or it may be explicit dependent upon whether cross carrier scheduling is used.

Enabling of the cross-carrier scheduling is achieved individually via the RRC signaling on a per component carrier basis or a per terminal basis. When no cross-carrier scheduling is arranged, the downlink scheduling assignments achieved on a per carrier basis, i.e. they are valid for the component carrier on which they were transmitted. For the uplink, an association is created between one downlink component carrier and an uplink component carrier. In this way when uplink grants are sent the terminal or UE will know to which uplink component carrier they apply. Where cross carrier scheduling is active, the PDSCH on the downlink or the PUSCH on the uplink is transmitted on an associate component carrier other than the PDCCH, the carrier indicator in the PDCCH provides the information about the component carrier used for the PDSCH or PUSCH. It is necessary to be able to indicate to which component carrier in any aggregation scheme a grant relates. To facilitate this, component carriers are numbered. The primary component carrier is numbered zero, for all instances, and the different secondary component carriers are assigned a unique number through the UE specific RRC signaling. This means that even if the terminal or user equipment and the base station, Enodeb may have different understandings of the component carrier numbering during reconfiguration, transmissions on the primary component carrier can be scheduled.