LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth SANDEEP KUMAR JINDAL Senior Engineering Project Manager

www.aricent.com

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth LTE Advanced promised peak data rate of 1 GBPS for downlink and 500 MBPS for uplink.

Such data rates cannot be achieved through the current 20 MHz bandwidth. CA is a solution that was proposed by 3GPP, which not only makes it possible to achieve high data rates mentioned above, but is also backward compatible with previous releases such as Rel 8 / 9.

CA aggregates multiple component carriers to achieve a large transmission bandwidth.

CA enables Communication Equipment Providers (CEPs) and Communication Service Providers (CSPs) to not only deliver high bandwidths but also helps them maintain backward compatibility with previous releases. CA, thus, enables the use of spectrum bandwidth from different parts of frequency space -- irrespective of their size -- and also provides the ability to manage control channel interference between high-power macrocell and low-power small cell transmissions.

This whitepaper defines CA and delves into its benefits and how it can be leveraged by Communication Equipment Providers (CEPs) and Communication Service Providers (CSPs).

The whitepaper also discusses the impact of CA on design and implementation of User Equipment (UE) modem protocol stack.

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

1

What is CA CA was introduced as a feature of LTE Advanced in Release-10 of 3GPP Specifications.

LTE Advanced uses CA of multiple Component Carriers (CCs) to achieve high-bandwidth transmission (and hence high data rate). Release 8 LTE carriers have a maximum bandwidth of 20 MHz. LTE Advanced provides a bandwidth of upto 100 MHz by supporting aggregation of upto five 20 MHz CCs. CA is leveraged in LTE Advanced to increase bandwidth and, thereby, increase bit rate. To maintain backward compatibility with Release 8 and Release 9 UEs, aggregation is based on

A terminal may simultaneously receive or transmit on one or

Release 8/Release 9 carriers.

multiple CCs depending on its capabilities like CA support, band combinations support, cross carrier support etc.

Each aggregated carrier is referred to as a as a CC, which can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a

Types of Carrier Aggregation (CA)

maximum of five CCs can be aggregated, hence the

CA is allowed between the CCs from same or different bands. The

maximum aggregated bandwidth can be 100 MHz. In FDD the

CCs can be adjacent to each other in frequency domain or not. CA

number of aggregated carriers can be different in DL and UL.

is also allowed with different CCs in the uplink and downlink. As per

However, the number of UL component carriers is always

the combination, following CA types are defined:

equal to or lower than the number of DL component carriers.

Individual CCs 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. For UE, each CC appears as a separate cell. UE selects one of the available cells during cell search procedure and that cell is called Primary Cell (PCell). So PCell is the one which is selected by the UE during cell search and used for RRC connection establishment. Security and mobility procedures happen only on PCell. Once connection is established, network can assign additional cells/CCs as additional resources to the UE. These cells are called Secondary / Serving cells (SCell) and they are selected by the network based on the UE capability and the position/location of the

Intra-Band Contiguous: The CA using the contiguous CCs

UE. SCells serve the UE simultaneously along with PCell.

within the same operating frequency band (as defined for LTE) is called intra-band contiguous carrier aggregation. This might not

The PCell can never be deactivated. There is only one PCell

always be possible, due to operator frequency allocation scenarios.

per mobile device. SCells are activated /deactivated by MAC layer and get assigned to the mobile device by higher

Intra-Band non-Contiguous: The CA using the CCs from the

layers.There can be more than one SCell per mobile device.

same operating frequency band, but having gap(s), in between is called Intra-Band non-Contiguous CA.

The CCs corresponding to the PCell are referred to as the Primary Component Carriers (PCC) and the CCs correspond-

Inter-Band non-Contiguous: The CA having the CCs belonging

ing to an SCell are referred to as Secondary Component

to different operating frequency bands is called Inter-Band

Carriers (SCCs).

non-Contiguous CA.

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

2

Non-Cross Carrier Scheduling The scheduling commands are sent to the UE from the PDCCH channel of the same CC where data gets transmitted /received on PDSCH/PUSCH channels. UE is require to listen to all the PDCCH on all the configured CCs

!!

PDCCH

!!

#1

#2

CC#1

CC#2

Symmetric Aggregation: If the number of CCs in both the uplink and downlink are the same then it is called symmetric CA.

PDSCH/ PUSCH

!

!

CC#1

Asymmetric Aggregation: If the number of CCs in downlink is more than that of uplink then it is said to be asymmetric CA.

CC#2

LTE UE Categories Independent from the LTE Advanced technology components, new UE

Note: Currently only downlink-heavy asymmetries are

categories 6, 7 and 8 are added into LTE Release 10

UE Category

Maximum DL Throughput (Bits)

Maximum DL Throughput (Bits)

Support for 64 QAM in UL

Maximum Number of Supported Layers for Spatial Multiplexing in DL

Category 1

10296

5160

No

1

Category 2

51024

25456

No

2

Category 3

102048

51024

No

2

Category 4

150752

51024

No

2

schedule resources on multiple CCs by using the new carrier

Category 5

299552

75376

Yes

4

indicator field (CFI),

Category 6

301504

51024

No

2 or 4

Cross Carrier Scheduling

Category 7

301504

102048

No

2 or 4

Category 8

2998560

1497760

Yes

8

supported, the number of uplink CCs configured for a terminal is always equal to or smaller than the number of configured downlink CCs.

Cross-Carrier Scheduling & Non-Cross Carrier Scheduling Each CC may use PDCCH to schedule resources for an individual UE that receives multiple carriers in downlink. This scheduling method is backward compatible to LTE Release 8.

Additionally and optionally cross carrier scheduling was introduced. This method uses the common PDCCH in order to

The scheduling commands are sent to the UE from the PDCCH channel of a CC different from the CC where the actual data gets transmitted on PDSCH/PUSCH channels. Cross- carrier scheduling is used to schedule resources on SCC without PDCCH. Note: PCell cannot be cross-scheduled, it is always scheduled through its own PDCCH

Categories 6 and 7 support peak data rate of 300 Mbps and both support MIMO 2x2 and/or 4x4. Category 8 is the highest category, which supports 8x8 MIMO and a peak data rate of 3 Gbps. Uplink category 8 leads to 1.5 Gbps data rate. Note: UE category significantly exceeds the IMT Advanced requirements which provide a peak data rate of up to 1Gbps.

Search Spaces PDCCH

#1

#2

CC#1

PDSCH/ PUSCH

CC#1

Search Spaces #3

#4

CC#3

CC#2

CC#3

CC#4

LTE-Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

3

A. UE CA Bandwidth Classes

In R10 three CA configurations are defined as below

New UE bandwidth classes applicable to CA are specified below.

Type of CA CA and duplex type Configuration CA Bandwidth Class

Aggregated Transmission Bandwidth Configuration

Max No of CC

Nominal Guard Band BWGB

A

NRB,agg

100

1

0.05BWChannel(1)

B

NRB,agg

100

2

FFS

C

100 <NRB,agg

D

200 <NRB,agg

E

[300] <NRB,agg

F

[400] <NRB,agg

0.05 max

200

2

[300]

FFS

FFS

[400]

FFS

FFS

[500]

FFS

FFS

(BWChannel1(1)BW(Channel(2)

Bandwidth classes are defined in terms of number of resource blocks with the aggregated transmission bandwidth and the maximum number of CCs supported. Six UE bandwidth classes

Maximum aggregated bandwidth (MHZ)

Max Number of CC

Intra-band contiguous FDD

CA_IC

40

2

Intra-band contiguous TDD

CA_40C

40

2

Inter-band FDD

CA_1A_5A

20

1+1

In R11, a large number of additional CA configurations are defined, as shown below.

Type of CA CA and duplex type Configuration

Intra-band contiguous FDD

Maximum aggregated bandwidth (MHZ)

Max Number of CC

CA_IC

40

2

CA_7C

40

2

CA_38C

40

2

CA_40C

40

2

configurations with associated bandwidth combination sets. For

CA_41C

40

2

inter-band CA, a CA configuration is a combination of operating

CA_1A_5A

20

1+1

are foreseen, whereas only three are fully specified up to now. R10 and R11. Only 2 CCs are supported till now.

CA Configurations The requirements for CA in the specification are defined for CA

Intra-band contiguous TDD

bands, each supporting a CA bandwidth class. For intra-band contiguous CA, a CA configuration is a single operating band supporting a CA bandwidth class.

CA configuration indicates a combination of E-UTRA operating bands and CA bandwidth classes, for example the configuration CA_40C indicates intra-band contiguous CA on E-UTRA operating

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

20

1+1

CA_3A_7A

30

1+1

CA_4A_12A

20

1+1

CA_4A_13A

30

1+1

on operating band 1 with bandwidth class A and operating band 5

CA_4A_17A

20

1+1

with bandwidth class B.

CA_4A_29A

20

1+1

CA_5A_12A

20

1+1

CA_5A_17A

20

1+1

CA_7A_20A

30

1+1

band 40 and CA bandwidth class C, CA_1A_1A, indicates intra-band non-contiguous CA on band 1 with one CC on each side of the intra-band gap. Finally, CA_1A_5B indicates inter-band CA,

Inter-band FDD

Intra-band non-contiguous FDD

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

CA_8A_20A

20

1+1

CA_11A_18A

25

1+1

CA_25A_25A

20

1+1

4

Note 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.

Impact of CA on Design and Implementation of UEs Introduction of carrier aggregation impacted mainly RRC, MAC and the physical layer protocols. While RRC layer impacts are

Why CA

reasonable, there are almost no changes in PDCP/RLC for CA

1. The primary reason for introducing CA in LTE Advanced is the requirement of the IMT Advanced specifications to meet a 1Gbps downlink (DL) peak data rate. In LTE Release 8, the peak data rate that can be reached is around 300 Mbps even with all the best features utilized. So, methods that can boost the peak DL data rate

except supporting large buffers for higher categories of UEs. There are significant changes at MAC and Phy In order to keep Release 8/Release 9 compatibility the protocol changes have been kept to a minimum. Basically each component carrier is treated as a Release 8 carrier.

were studied and CA was proposed.

In LTE Release 8, bandwidth supported was from 20 MHz till 1.4 MHz. CA is a method by which multiple carriers/channels can be aggregated to realize a large bandwidth for achieving higher peak data rates. Thus instead of defining new continuous channel bandwidths to meet the IMT Advanced peak data rate requirements, CA was proposed for smooth interoperability with legacy Release 8 and Release 9 devices.

2. Another reason was the flexibility CA provides to operators in choosing the bandwidth and band of different carrier components.

In CA, carrier components with different sizes and different bands can be combined. Many operators have already obtained different bandwidths

in

different

bands

for

existing

technologies

(2G/3G/LTE), which can thus be reused for CA. So CA gives the flexibility to the operators that plan to reframe 2G and 3G spectrum and use LTE Advanced technology.

3. There exists inter-cell interference in heterogeneous network environment, for example where small cells are deployed inside Macro cell region for better spectrum efficiency.

One of the

problems in deploying small cells with macrocells is the interference management especially for control channels like PDCCH. To avoid this problem, CA cross-carrier scheduling feature can be used effectively to manage the situation. The control channels of the macro and picocells can be kept in different CCs while the data transmission can intelligently use the combined CA capability of multiple carriers.

Impact of CA on design and implementation at each layer is described below

1.NAS There is no impact on NAS protocols. However, changes at OAM to configure the support of CA and other CA-related functionality to the lower layers.

2. RRC UE Capability During LTE Registration procedure, UE reports CA capability in UE Capability Information Message.

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

5

To configure CA, network send SCell configuration to only

sends RRCConnectionReconfiguration message having SCellToRe-

those UEs which are at least release 10 compatible and

leaseList.

support CA. The CA-related information sent by the UE is summarized below:

Note: SCells are added or deleted through RRC signaling whereas activation/deactivation of SCell is done at MAC layer.

• UE category – CA support is implied by UE categories 6, 7, and 8. However it does not indicate the support for a particular

UE does not read SI of SCell. RRC Connection Reconfiguration carries

CA configuration, which is signalled separately.

all the mandatory information for Scell, required to access/configure the cell like SCell BW, Antenna information, PHICH configuration,

• Supported band combinations – Indicates the specific

PDSCH/PUSCH configuration, SRS configuration, uplink Power

frequency band and channel bandwidth configurations that

Control information, PUSCH/PRACH configuration, SCell CQI report-

the UE support for CA.

ing configuration etc.

• Cross-carrier scheduling support – Indicates that the UE

RRC Connection Reconfiguration also carrier Cross-carrier schedul-

support cross-carrier scheduling.

ing configuration for the SCell which indicates, if scheduling for the referenced SCell is handled by that SCell or by another cell.

• Simultaneous PUCCH and PUSCH transmission support – For CA-capable UEs, this implies that the UE can simultane-

Measurement Events

ously support PUCCH and PUSCH transmission on different

One new measurement event ‘Event A6’is introduced for CA. As

CCs.

indicated in the UE capability section, event A6 occurs when a neighboring cell’s strength becomes better than SCell’s strength by

• Multi-cluster PUSCH within a CC support – Indicates

an offset.

baseband (non-band-specific) support for multi-cluster PUSCH transmission within CCs.

Handover Handover processing for LTE in Release 10 is largely the same as

• Non-contiguous uplink resource allocation within a CC

Releases 8 and 9, except that clarifications are made to refer to PCell

support – Indicates UE support for non-contiguous uplink

in the measurement-related RRC signaling messages. Handover for

resource allocations within CCs.

SCell is also possible while keeping the same PCell through the event A6.

• Measurement Reporting Event A6 support – Indicates that the UE support measurement reporting at the trigger of Even

3. PDCP Impact

6, which occurs when a neighbour cell becomes stronger than

There is no impact on PDCP protocol

a serving SCell by an offset.

4. RLC Impact • Periodic SRS transmission on all CCs support – Indicates

There is not much impact on RLC protocol. Only change at RLC layer

that the UE can transmit periodic SRSs on all SCells.

is to provide higher data rates by having a larger buffer size

• SCell addition within the Handover to EUTRA procedure

5. MAC Impact

support – Indicates that the UE can support E-UTRAN inbound

Introduction of CA mainly influences MAC and the physical layer

inter-radio access technology (IRAT) handover directly into CA

protocol. MAC must be able to handle scheduling on a number of

mode.

CCs. The MAC layer plays the role of multiplexing entity for the aggregated CCs. Each MAC entity will provide to its corresponding CC its

SCell Addition, Deletion

own Physical Layer (PHY) entity, providing resource mapping, data

The CA additional SCells cannot be activated immediately at

modulation, HARQ, and channel coding.

the time of RRC establishment. Thus, there is no provision in

Following are the design considerations/impact at MAC layer:

the RRC Connection Setup procedure for SCells. They are added and removed from the set of serving cells through the

SCell Activation and Deactivation

RRC Connection Reconfiguration procedure. In the connected

A new Mac Control (activation/deactivation) element of 1 Byte is

mode, in order to add SCell or modify SCell, network sends

defined which is a bit map of the configured SCells. For activation of

RRCConnectionReconfiguration message having SCellToAdd-

an SCell the corresponding bit has to be set to 1 for activation. For

ModList IE to add/modify SCells. To release SCell network

deactivation both explicit as well as implicit mechanisms are provided in

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

6

the specification. Note: Configuration of SCell is done through RRC signalling as described in the RCC impact section

Scheduling Request There is no major change in the working of this scheduling request functionality except that scheduling request can also be sent in

Cross-Carrier Scheduling

UCI format 3.

Cross-Carrier scheduling is an optional feature for the UE introduced in Release 10, UE indicates its support through the RRC

Downlink CQI reporting

signaling during the UE capability transfer procedure. Cross-carri-

The mobile device shall now report CQI for all the CCs where

er scheduling is used to schedule resources on an SCell without

PDSCH data gets transmitted. The CQI can either be reported on

PDCCH. A carrier indication field (3 bits) is added to the DCI

PUCCH channel or PUSCH channel. Now, CQI gets reported per

formats providing the index of the CC for which the scheduling

CC wise.

grant/scheduling assignment is valid. The Carrier indication field is optional in the DCI formats. A higher layer provides this informa-

SRS transmission

tion to the mobile device. For non-cross carrier scheduling, CIF is

Now, it is also possible to configure SRS transmission on SCell as

not present. If cross carrier-scheduling is configured for SCell then

well as on PCell per mobile device by higher layers. The support of

UE is not required to decode the PCFICH on that SCell anymore.

this functionality may not be supported by devices.

Cross Carrier scheduling information contains the starting OFDM symbol of PDSCH for the concerned SCell.

Downlink Ack/Nack for UL In LTE advanced, 4x4 UL MIMO transmission is allowed. This

Uplink HARQ Ack/Nack for DL

MIMO transmission results in Multiple Transport Blocks(TB)

The PUCCH channel is always on the Primary CC and not on all

getting transmitted in the Uplink direction. The PHICH channels

uplink CCs. So, the HARQ Ack/Nack will be sent on this channel if

shall support Ack/Nack support for multiple Transport Blocks.

there is no grant for PUSCH transmission. But there are some

There is one PHICH channel transmitted per TB.

challenges due to CA. PDCCH/PHICH channels are bundled for scheduling grants. It The maximum number of bits to be sent for FDD HARQ Ack/Nack

implies that the same CC will carry Ack/Nack which provided the

can be 10 now instead of 2 previously. This function is not depend-

uplink scheduling grant.

ent upon whether the downlink assignments are cross carrier or channel. Therefore the existing UCI formats like 1, 1a, 1b, 2, 2a and

Uplink Transmit Power Control for PUCCH/PUSCH channels

2b are not sufficient for HARQ Ack/Nack sending. A new UCI

The TPC power control commands for the PUCCH channel is only

format is defined named UCI format 3 which allows sending more

through PCell. The TPC commands for the PUSCH channel will be

uplink Ack/Nack bits. As a special case, up to two CC Ack/Nack

through the serving cell which provides the scheduling grant to the

can be sent using existing 1b format known as PUCCH format 1b

device.

non â&#x20AC;&#x201C;cross carrier if HARQ Ack/Nacks are transmitted on PUCCH

with channel selection. The higher layer configures the format to be used either PUCCH format 1b with channel selection or PUCCH

Synchronisation

format 3. The order of information transmitted using PUCCH

In Release 10, PCell and SCell are synchronized with the same

format 3 is Ack/Nack bits, scheduling request bit and CQI bits.

single Timing Advance(TA). With Release 11, it is possible to handle CA with CCs requiring different TA, for example combining CC from eNB with CC from remote radio heads, So with R11 it is

Simultaneous PUCCH and PUSCHs

possible that the PCell and SCell may have different TA values for

Now, it is possible that a mobile device is capable of transmitting

uplink synchronisation. It implies that a mechanism shall be

PUCCH and PUSCH channels simultaneously. This adds complexi-

defined to compute the SCell TA values as the device only

ty to existing mechanism along with CA. There are now four

transmits on the PRACH channel on Pcell.

possible cases, namely: In R11, it is possible to command the mobile device to initiate 1. Single Carrier with no Simultaneous PUCCH and PUSCH

PRACH transmission on any S-Cell using PDCCH channel of the

2. Single Carrier with Simultaneous PUCCH and PUSCH

PCell by sending PDCCH order for the SCell. Now, there is a

3. Multiple Carrier with no Simultaneous PUCCH and PUSCH

concept of Timing Advance Group (TAG). Each and every SCell

4. Multiple Carrier with Simultaneous PUCCH and PUSCH

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

8

will belong to a TAG, allocated by higher layers.

The mobile device will initiate PRACH on the SCell based upon the PDCCH order received on the PCell. The SCell will compute the TA for the mobile device for the SCell.

The TA for the SCell will be communicated to the device using

complexity. Impact of CA on design and implementation of protocol stack is a challenge for user equipment.

Aricent has good understanding and capabilities on CA and also provides femto/pico eNodeB software enablers which supports CA. Aricent provides end-to-end support for Modem Stack development and maintenance for CA.

MAC layer control element along with TAG.

Aricent Offering

About the Author Sandeep Kumar Jindal is a Senior Project Manager

Aricent provides end-to-end support for Modem Stack

and has 14 years of experience in the wireless

development and maintenance. Aricent has deep domain

communication protocols domain. He has signifi-

expertise in the CA space and can provides outsourcing for CA

cant knowledge on UE designing, developing,

in the following areas

functional testing and conformance testing of various protocols at different layers of LTE, GSM,

Maintenance Bug Fixing

GPRS and 3G in NAS and AS.

• Management and Delivery of Incoming Defects • Support Verification New Feature Development

References

• Delivery of Features / Enhancements • Planned Optimizations and Performance Improvements

1. 3GPP. Carrier Aggregation explained. http://www.3gpp.org/technolo-

System Integration and System Testing

gies/keywords-acronyms/101-carrier-aggregation-explained.

• Build, Patch , Hot fix & Release management

2. 3GPP TR 36.912, Technical Specification Group Radio Access

• Continuous Integration & Environment Automation

Network; Feasibility study for further advancements for E-UTRA (LTEAd-

• Smoke Test & Integration Test

vanced),

HW Customization

3. 3GPP TS 36.331, Technical Specification Group Radio Access

• New Platform Bring-Up

Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio

• Support and Integrate with new RF Engines / Customiza

Resource Control (RRC); Protocol specification

tions

4. 3GPP TS 36.300, Technical Specification Group Radio Access

Customer Support

Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and

• Provide On-site / Off-Site Technical Support for Board

Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall

Bring-up, Verification and Type Approval

description; Stage 2

• Plan and Deliver Project Specific Customizations

5. 3GPP TR 36.913, Technical Specification Group Radio Access

Other Activities

Network; Requirements for further advancements for Evolved Universal

• Project Planning

Terrestrial Radio Access (E-UTRA) LTE-Advanced

• Status Reporting and Alignment with Customer

6. GPP TS 36.211, Technical Specification Group Radio Access Network;

• Project Operation and Monitoring

Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels

• SW Correction Propagations

and Modulation

• Technical Workshops

7. 3GPP TS 36.212, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and

Aricent has implemented CA on network side for eNodeB and

channel coding

has its own femto/pico eNodeB enabling software which

8. 3GPP TS 36.213, Technical Specification Group Radio Access

supports CA.

Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical

Conclusion

layer procedures 9. 3GPP TS 36.321, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium

CA is one of the most crucial features of LTE Advanced. The

Access Control (MAC) protocol specification

peak data rate is improved according to the number of aggregate carriers (up to five), with a related impact on the UE

LTE Advanced: Implementing Carrier Aggregation (CA) for Maximizing Bandwidth

9

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