LTE: Carrier Aggregation Tutorial

•  What is component carrier?
•  What is Carrier Aggregation? 
• Types of LTE carrier aggregation 
• Symmetric & Asymmetric CA in LTE
• Types of Component Carriers
• Carrier Aggregation – Cell Configuration 
• Concept of Cross-Carrier Resource Scheduling
• Carrier Aggregation Activation/Deactivation

What is component carrier?

Aggregation means ‘grouping’ and carrier is ‘bearer’ so component carrier means Two or more component carriers can be aggregated to   support wider transmission bandwidths up to 100 MHz.

What is Carrier Aggregation?

• It is of the most distinct features of 4G LTE-Advanced. Carrier aggregation allows expansion of effective bandwidth delivered to a user terminal through concurrent utilization of radio resources across multiple carriers. Multiple component carriers are aggregated to form a larger overall transmission bandwidth.
• CA supports wider bandwidths. Using CA, a spectrum of up to 5 Component Carriers (CC) can be aggregated up to a joint bandwidth of 100MHz. Rel–10 defines 5 possible deployment scenarios:

Scenario 1

• Two co-located and overlaid (Fig 1.1) (lay another surface to cover) cells which provide nearly the same coverage and where both layers (cell) allocate different bandwidth parts within the same frequency band.
• CA is possible between both layers.

Scenario 2

• Two co-located and overlaid (Fig 1.1) cells, in which only one layer provides sufficient coverage, while the other layer has smaller coverage due to higher path loss as both layers allocate bandwidth within different frequency bands.
• CA is possible between both layers.

Scenario 3

• Two co-located cells (Fig 1.1) in which the antennas of the second cell are moved towards the cell boundaries of the first cell in order to enhance cell edge performance. The basic coverage is given by the first cell, while the second cell might not provide full coverage. Again, both cells have bandwidth in different frequency bands.
• CA is possible in regions where the coverage of both cells overlaps.

Scenario 4/5

• Cell One provides basic macro coverage, while Cell Two has remote radio heads (RRH) (Scenario 4) or repeaters (Scenario 5) which provide additional capacity at hotspots within the coverage area of Cell One. Both cells transmit in different frequency bands.
• CA is possible from the RRH or repeater cells to the underlying macro Cell One.

All defined CA scenarios and their coverage patterns need to be reflected in planning and optimization solutions.

(Fig 1.1 )Cells (cell 1, cell 2 and cell 3) which provide nearly the same coverage and where all layers (or cells) allocate different bandwidth parts within the same and different frequency band for UE.

• Some of you may be wondering what exactly carrier aggregation is but simply put, CA is a mechanism to increase channel bandwidth, or in other words, achieve higher data rates than standard LTE, as shown in Figure 1.2 Above.

• LTE as a technology supports up to 20MHz channel bandwidth, but with CA, the same can be enhanced to 100MHz as maximum five such channels (called component carriers), up to 20 MHz each, can be combined.

• 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 and maximum aggregated bandwidth is 100 MHz.

• In FDD, The number of aggregated carriers can be different in DL and UL, see figure 1.3. 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.

• Carrier aggregation is applicable to both FDD and TDD. in the case of TDD, the uplink-downlink sub frame configuration must be the same for all component carrier.

• For TDD the number of CCs as well as the bandwidths of each CC will normally be the same for DL and UL.

• Fig 1.3 illustrates two example of carrier aggregation-the first based upon 5 component carriers, and the second based upon 2 component carrier. The bandwidth of each component carrier is scenario dependent and does not have to be the maximum channel bandwidth of 20MHz.

• All component carriers belong to the same eNodeB and are synchronized on the air-interface. This means that a single set of timing advance command are used for all component carriers.

Types of LTE carrier aggregation

There are a number of ways in which LTE carriers can be aggregated.

• Intra-band, Contiguous
• Intra-band ,non Contiguous
• Inter-band ,non-contiguous

                                                       |<–N*300–>|

Figure 1.4. 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).

• The Intra band scenario provide benefits in term of implementation effort. A single transceiver  can transmit  and receive multiple RF carrier when they are positioned within the same operating band.

• The inter-band scenario provides benefit in term of spectrum availability. An operator’s spectrum is likely to the distributed across multiple operating band rather than located within single band.

• Intraband Contiguous CA: when multiple CCs are adjacent to each other within the same band.

• Intraband Non-Contiguous CA: when multiple CCs within the same band are used in a non-contiguous manner (have a gap or gap between CC.)

• Interband Non-Contiguous CA: when multiple CCs are separated along the frequency band.

• With non-contiguous CA, data transmission occurs over multiple separated carriers across a large frequency range. As a result, the radio channel characteristics, such as propagation path loss and geometry (G-) factor. 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 1.5

Symmetric & Asymmetric CA in LTE

• symmetric carrier aggregation is defined as the case where there are equal number of component carriers for the downlink and uplink. In TDD downlink and uplink is always symmetric but in FDD ,the downlink and uplink can either be symmetric or not.

• In asymmetric case, the number of aggregated carrier can be different in DL/UL. However number of UL component carrier is always equal to or less than the number of DL component carriers. the individual component carrier can also be of different bandwidth.

Types of Component Carriers

• Primary Cell ( P-Cell )
• Secondary/Serving Cell ( S-Cell )

 Primary Cell ( P-Cell ):

• The Primary cell is the Cell which is selected by the UE during cell search and used for RRC connection establishment . The measurement and mobility procedures are based on P-Cell. The P-Cell can never be de-activated .There is only one P-Cell per mobile device.

 Secondary/Serving Cell ( S-Cell ):

• The Secondary /Serving cells are those cells which are selected by the Network based on the UE capability and the position/Location of the UE which can serve the UE simultaneously along with the Primary   Cell.

• The Secondary cells are Activated /De-activated by MAC Layer and get assigned to the mobile device by higher layers. There can be more than one S-Cell per mobile device.

• The SCCs are added and removed as required ,while the PCC is only changed at handover.

Carrier Aggregation – Cell Configuration

•  Primary Serving Cell(PCell)
• Secondary Serving Cell(SCell)

• Each components carrier corresponds to a serving cell. 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. one of the serving cells is designed the primary cell, while the rest are known as secondary cells.

• The primary cell is the most important, and manages the CA configuration. the number of serving cells that can be configured depends on the aggregation capability of the UE.

Primary Serving Cell(PCell)

• The RRC Connection is only handled by 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.

• Random access procedure is performed over PCell.
• PDCCH/PDSCH/PUCCH/PUSCH can be transmitted.
• Measurements and mobility procedure are based on PCell
• Cannot be deactivated.

 Secondary Serving Cell(SCell)

• Secondary Serving Cell is configured after connection establishment to provide additional radio resources.

•  RACH procedure is not allowed in a secondary cell.
•  PDCCH/PDSCH/PUSCH/can be transmitted(not PUCCH)
•  MAC-layer based activation/deactivation is supported for SCell for UE battery saving.
•  Can be cross-scheduled.

Concept of Cross-Carrier Resource Scheduling

• Same- carrier scheduling
• Cross-Carrier scheduling

Same- carrier scheduling:

•   Resources are scheduled on the same carrier as the grant is received.
•   Separate PDCCH for each component carrier.
•   Reusing LTE Release 8/9 PDCCH structure and DCI formats.

Cross-Carrier scheduling: Cross carrier is scheduled is only used to schedule resources on SCC without PDCCH. The CIF(Carrier indicator field) on PDCCH indicate on which carrier the scheduled resource is located.

•     Common PDCCH for multiple CCs.
•     Reusing LTE Release 8/9 PDCCH structure.
•     New 3-bit Carrier indicator field (CIF) added to Release 8 DCI.
•     PCell shall be responsible for cross carrier scheduling of the secondary but no voice versa.
•     PCell cannot to cross-scheduled; it is always scheduled through its own PDCCH.

Figure 1.9 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.

Carrier Aggregation Activation/Deactivation

It is a two step process . In the first step, RRC layer configures the SCells for the CA capable UEs. In the second step MAC layer Activates/De-activates the SCells for the UEs.

•        Step 1 – Configuration of Secondary Cells
•        Step 2 – Activation/De-activation of Secondary Cells

There is a Mac Control element of 1 Byte defined which is a bit map of the configured SCells.For Activation of a SCell the corresponding bit has to be set to 1 for activation. For De-activation both explicit as well as implicit mechanisms are provided in the specifications. MAC Control elements can be used for fast activation and deactivation of secondary cells after they have been configured by RRC layer. The primary cell is always activated and cannot be deactivated Carrier Aggregation scheduling Functions

SCell Activation is done via MAC Control Element whereas the deactivation mechanism is either by using MAC control element or by the expiry of the SCellDeactivationTimer.

CA Bandwidth Class

CA Bandwidth Class	Aggregated Transmission Bandwidth configuration	Number of contiguous Componenet Carriers
A	NRB,agg ≤ 100	1
B	25 < NRB,agg ≤ 100	2
C	100 < NRB,agg ≤ 200	2
D	200 < NRB,agg ≤ 300	3
E	300 < NRB,agg ≤ 400	4
F	400 < NRB,agg ≤ 500	5
I	700 < NRB,agg ≤ 800	8

Intra-band contiguous channel spacing

For Intra-band contiguous we have to take in to account the nominal channel spacing. For intra-band contiguous carrier aggregation with two or more component carriers, the nominal channel spacing between two adjacent E-UTRA component carriers is defined as the following:

Intra-band contiguous channel spacing

For Intra-band contiguous we have to take in to account the nominal channel spacing. For intra-band contiguous carrier aggregation with two or more component carriers, the nominal channel spacing between two adjacent E-UTRA component carriers is defined as the following:
where BW Channel(1) and BWChannel(2) are the channel bandwidths of the two respective E-UTRA component carriers.

PCC – Primary Component Carrier (PCell)
SCC – Secondary Component Carrier (SCell)For example: 2CA band 40C

If we are planning to set a PCell of bandwidth 20 MHz(BW1) and a SCell of bandwidth 20 MHz(BW2) then the nominal spacing will be calculated as follows,

 

Nominal Channel Spacing = FLOOR.MATH((20+20-0.1*ABS(20-20))/0.6)*0.3

= FLOOR.MATH((40+0.1*0)/0.6)*0.3
= FLOOR.MATH(40/0.6)*0.3
= FLOOR.MATH(66.6666667)*0.3
Nominal Channel Spacing = (66)*0.3 => 19.8 MHz

 

NOTE: (You can put the line “FLOOR.MATH((20+20-0.1*ABS(20-20))/0.6)*0.3” directly into Excel and it should give you 19.8).

​

Which means for Intra-band contiguous the Channel Spacing should be a multiple of 300 KHz and equal to (or) less than Nominal Channel Spacing,

Channel Spacing < Nominal Channel Spacing

Channel Spacing = ((BW1+BW2)/2).

If we take the PCC EARFCN=39000, Frequency=2335, Bandwidth=20 MHz then the SCC is calculated as,

SCC EARFCN = PCC EARFCN + 19.8 * 10
= 39000 + 198
= 39198

 

SCC EARFCN=39198, Frequency=2354.8, Bandwidth=20 MHzThe Nominal Channel Spacing is 19.8 MHz (2335 – 2354.8) which is less than or equal to Channel Spacing of 19.8 MHz. Only when this condition is satisfied it is treated as Intra-band contiguous.

For intra-band non-contiguous carrier aggregation the channel spacing between two E-UTRA component carriers in different sub-blocks shall be larger than the nominal channel spacing. (ie) The combination is considered as 40A-40A,
Channel Spacing > Nominal Channel Spacing

Scell Addition-CA

E-UTRAN uses IE sCellToAddModList in RRCConnectionReconfiguration message to add the SCell.

At the time of SCell addition, the eNodeB sends the following information to the UE via RRCConnectionReconfiguration message, a few of which are optional.

SCellIndex: This is used to identify/address the SCell that is being configured/reconfigured/released. If the received SCellIndex is part of the UE’s configuration already, then the UE considers the procedure as SCell Modification. Else if the received SCellIndex is not part of the UE’s configuration, then the UE should consider the procedure as SCell Addition. Based on the UE’s capability, at most four SCells can be configured but the SCellIndex can take values from 1 to 7.

cellIdentification: This is mandatory at the time of SCell Addition and shouldn’t be present at the time SCell Modification. cellIdentification consists of Physical Cell Identity and Downlink Carrier Frequency (EARFCN).

radioResourceConfigCommonSCell: This is a big structure of IEs which is mandatory at the time of SCell Addition and shouldn’t be present if the concerned SCell is being Modified.

When adding a new SCell, radioResourceConfigCommonSCell is used for sending all required system information of the SCell i.e. while in RRC_CONNECTED mode, UEs need not acquire broadcasted system information directly from the SCells.

radioResourceConfigCommonSCell contains downlink configurations such as Downlink Bandwidth, Number of Antenna Ports, List of MBSFN subframe Configurations, PHICH Configuration, PDSCH common configuration, TDD configuration (in case of TDD) etc..

In case of SCells with configured uplink (UL CA), radioResourceConfigCommonSCell contains uplink information such as Uplink Bandwidth, Uplink Carrier Frequency, Uplink Power Control (Common) Information, common information of physical channels, SRS common information etc…

radioResourceConfigDedicatedSCell: This is another big structure of IEs which is mandatory at the time of SCell Addition and is optional otherwise. It contains UE specific (dedicated) configurations for SCell.

radioResourceConfigDedicatedSCell contains downlink dedicated configurations such as information related to Transmission Mode for the SCell, Cross Carrier Scheduling configuration, SCell CSI reference signal information, SCell PDSCH dedicated configuration etc…

E-UTRAN might configure the UE with uplink dedicated configuration as well. This is mandatory in case of UL CA where as it is optional for DL CA. It contains information such as Uplink Transmission Mode for SCell, Uplink dedicated Power Control information, CQI Reporting configuration for SCell, SCell’s dedicated SRS configuration etc…

See an example RRCConnectionReconfiguration message for SCell Addition below. 

Scell Activation-CA

MAC CE - SCell Activation/Deactivation

 

The purpose and the mechanism of NR SCell Activation/Deactivation is same as LTE SCell Activation/Deactivation. It is used to activate or deactivate data transmission for SCell while it is in Carrier Aggregation as illustrated below. That is, setting up carrier aggregation is done by RRC process but once RRC job is done, the real swich on/off of data transmission is done by this MAC CE.

The LCID for MAC CE is defined as below. As in LTE Rel 13 or later, there are two types of SCell Activation / Deactivation MAC CE depending on how many SCells are in the aggregated cells.

The way to activate or deactivate a SCell is to set 1 or 0 in the each field of the following data structure. If the aggregated cell has less than 8 SCells in it, it uses the data structure Figure 6.1.3.10-1 and if it has more than 7 SCells it uses the data structure Figure 6.1.3.10-2.

MAC CE - Activation/Deactivation MAC Control Element

 

Refer to 36.321 6.1.3.8 Activation/Deactivation MAC Control Element if you want to have formal description. The field structure is as shown below.

 

< 36.321 Rel 10,11,12 - Figure 6.1.3.8-1: Activation/Deactivation MAC control element >

 

From Rel 13, UE Category 17 is added. Based Category 17, we can achieve 32 CC Carrier Aggregation and 25 Gbps. To support 32 CC (31 SCC), a new Activation/Deactivation MAC CE is added as shown below.

 

< 36.321 Rel 13 - Figure 6.1.3.8-2: Activation/Deactivation MAC control element of four octets >

 

Now the question is to figure out what does C1, C2, C3 etc mean ? Does it automatically mean C1 = SCC1, C2 = SCC2 etc ?

No.. the mapping between C1, C2,..,C7 and SCC1,SCC2,...,SCC7 is defined in RRC Connection Reconfig message. One example is as follows : (In example, sCellIndex-r10 for SCC1 (the first SCC) is set to be 2. It means C2 field in MAC CE is mapped to SCC1. Therefore, in this case 00000100 means that the first SCC shall be activated. This mean that absolution bit position in MAC CE does not specify whether it is for SCC1 or SCC2 etc. The meaning of each bit in MAC is indicated by RRC message. It means what is important is the matching between sCellIndex-r10 and MAC CE bit position. I hope following illustration would sound clearer to you.  

 

ollowing is an example of RRC Connection Reconfiguration message to add SCC1.

 

rrcConnectionReconfiguration-r8

    radioResourceConfigDedicated

        physicalConfigDedicated

            pucch-ConfigDedicated-v1020

                pucch-Format-r10: channelSelection-r10 (1)

                    channelSelection-r10

                        n1PUCCH-AN-CS-r10: setup (1)

                            setup

                                n1PUCCH-AN-CS-List-r10: 2 items

                            Item 0

                                        N1PUCCH-AN-CS-r10: 4 items

                                            Item 0

                                                N1PUCCH-AN-CS-r10 item: 361

                                            Item 1

                                                N1PUCCH-AN-CS-r10 item: 362

                                            Item 2

                                                N1PUCCH-AN-CS-r10 item: 363

                                            Item 3

                                                N1PUCCH-AN-CS-r10 item: 364

                                    Item 1

                                        N1PUCCH-AN-CS-r10: 4 items

                                            Item 0

                                                N1PUCCH-AN-CS-r10 item: 365

                                            Item 1

                                                N1PUCCH-AN-CS-r10 item: 366

                                            Item 2

                                                N1PUCCH-AN-CS-r10 item: 367

                                            Item 3

                                                N1PUCCH-AN-CS-r10 item: 368

    nonCriticalExtension

        nonCriticalExtension

            nonCriticalExtension

                sCellToAddModList-r10: 1 item

                    Item 0

            SCellToAddMod-r10

                            sCellIndex-r10: 2

                            cellIdentification-r10

                                physCellId-r10: 1

                                dl-CarrierFreq-r10: 3100

                            radioResourceConfigCommonSCell-r10

                                nonUL-Configuration-r10

                                    dl-Bandwidth-r10: n50 (3)

                                    antennaInfoCommon-r10

                                        antennaPortsCount: an1 (0)

                                    phich-Config-r10

                                        phich-Duration: normal (0)

                                        phich-Resource: one (2)

                                    pdsch-ConfigCommon-r10

                                        referenceSignalPower: 18dBm

                                        p-b: 0

                            radioResourceConfigDedicatedSCell-r10

                                physicalConfigDedicatedSCell-r10

                                    nonUL-Configuration-r10

                                        antennaInfo-r10

                                            transmissionMode-r10: tm1 (0)

                                            ue-TransmitAntennaSelection: release (0)

                                                release: NULL

                                        pdsch-ConfigDedicated-r10

                                            p-a: dB0 (4)

                                    ul-Configuration-r10

                                        cqi-ReportConfigSCell-r10

                                            nomPDSCH-RS-EPRE-Offset-r10: 0dB (0)

                                            cqi-ReportPeriodicSCell-r10: release 

PCell Vs SCell:
- PCell always have both Uplink(UL) and Downlink(DL). Scell always have DL (While activated) but may or may not have UL.
- PCell is always activated whereas SCell has to be activated or deactivated using MAC-CE.
- UE does not required to acquire System Information and decode Paging from SCell.
- For Scell SI is passed to UE while adding the Scell. 
- When an Scell is added using RRC Connection Reconfiguration Message it remains in the deactivated state till it is activated using MAC-CE.
- If Scell activation/deactivation MAC-CE is received on Subframe n the Scell is activated/deactivated on Subframe n+24 or n+34.(TS 36.133 Section 7.7.2)
- When sCellDeactivationTimer expires then Scell is deactivated.
- Once Scell is deactivated 
         - PDCCH on Scell and PDCCH for Scell is not monitored. 
         - PUSCH is not transmitted and PDSCH is not received. 
         - The SRS is not transmitted.
         - The CQI/PMI/RI for Scell is not reported.


Activation/Deactivation MAC-CE:
- The MAC-CE can activate and deactivate Scell(s) which is already configured using RRC Connection Reconfiguration Meassage.
- Control Element is identified by a MAC PDU subheader with LCID.
.

Values of LCID for DL-SCH

Index	LCID values
11011	Activation/Deactivation

- fixed size and consists of a single octet containing seven C-fields and one R-field.

Activation/Deactivation MAC control element

- The Ci field is set to "1" to indicate that the SCell with SCellIndex i shall be activated.

- The Ci field is set to "0" to indicate thatthe SCell with SCellIndex i shall be deactivated.

- R: Reserved bit, set to “0”.
- 

Pcell and Scell Concepts:
- Pcell can be changed using RRC Connection Reconfiguration With MobilityControlInfo i.e. Handover.
- Scell can be changed using RRC Connection Reconfiguration message.
- During Radio Link Failure, the Scell is release first before initiating RRC Connection Re-establishment procedure.
- On receiving Handover Command i.e. RRC Connection Reconfiguration With MobilityControlInfo, UE deactivates the Scell, if configured.
- TTI Bundling is not supported when configured with one or more Scell with Configured Uplink.
- The RSRP and RSRQ measurement for Pcell shall follow time domain measurement resource restriction in accordance with measSubframePatternPCell, if configured.

Procedure	Pcell	Scell
Radio Link Monitoring	Y	N
System Information Acquisition	Y	N
Random Access Procedure	Y	N
Time Alignment	Y	N
SPS	Y	N
Connected Mode DRX	Y	Y
Paging	Y	N
NAS Mobility Information	Y	N
Data Transmission	Y	Y
S-measure Criteria	Y	N
Power Control	Y	Y
Link Adaptation	Y	Y
Pathloss Meaurement	Y	Y= when pathloss reference=Scell N= when pathloss reference=Pcell

Channel	Pcell	Scell
PDCCH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PDCCH when CCS but eNodeB might be transmitting PDCCH.)
PDSCH	Y	Y
PUCCH	Y	N
PUSCH	Y	Y
PRACH	Y	N
SRS	Y	Y
PBCH	Y	N - UE does not look at PBCH But eNodeB might be transmitting it.
PSS/SSS	Y	N - UE does not look at PSS/SSS but to get timing information eNodeB might be transmitting PSS/SSS.
PCFICH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PCFICH when CCS but eNodeB might be transmitting PCFICH.)
PHICH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PHICH when CCS but eNodeB might be transmitting PHICH.)

What are new for the new DCI format : Format 2B and 2C ? I created a comparative table for you to see the difference between these new format and the existing field.

 

Format 2 (Rel 10)	Format 2A (Rel 10)	Format 2B (Rel 10)	Format 2C (Rel 10)
Carrier indicator	Carrier indicator	Carrier indicator	Carrier indicator
RA header	RA header	RA header	RA header
Resource block assignment	Resource block assignment	Resource block assignment	Resource block assignment
TPC command for PUCCH	TPC command for PUCCH	TPC command for PUCCH	TPC command for PUCCH
Downlink Assignment Index(TDD only)	Downlink Assignment Index(TDD only)	Downlink Assignment Index(TDD only)	Downlink Assignment Index(TDD only)
HARQ process number	HARQ process number	HARQ process number	HARQ process number
		Scrambling identity	Antenna ports Scrambling identity Number of Layers
		SRS request(TDD Only)	SRS request(TDD Only)
TB to CW flag	TB to CW flag	TB to CW flag	TB to CW flag
MCS for TB1	MCS for TB1	MCS for TB1	MCS for TB1
NDI for TB1	NDI for TB1	NDI for TB1	NDI for TB1
RV for TB1	RV for TB1	RV for TB1	RV for TB1
MCS for TB2	MCS for TB2	MCS for TB2	MCS for TB2
NDI for TB2	NDI for TB2	NDI for TB2	NDI for TB2
RV for TB2	RV for TB2	RV for TB2	RV for TB2
Precoding information	Precoding information	Precoding information	Precoding information

 

The totally new DCI introduced in Release 10, Format 4, carries the information as shown below.

 

Format 4

Carrier indicator (0 or 3 bits)

Resource block assignment

TPC command for scheduled PUSCH (2 bits)

Cyclic shift for DM RS and OCC index (3 bits)

Downlink Assignment Index (DAI) (2 bits)

CSI request (1 or 2 bits)

SRS request (2 bits)

Resource allocation type (1 bit)

MCS and RV for TB1 (5 bits)

NDI for TB1 (1 bit)

MCS and RV for TB2 (5 bits)

NDI for TB2 (1 bit)

Precoding information and number of layers

 

< DCI 1 Examples > ----------------------------------------------------------------------------------------

 

Example 1 > DCI Format 1 - 20 Mhz, Value = 0x0FC00005DC40

 

Field	Value	Value (Binary)
CarrierIndicator	0 (Dec)	000
RA header	RAType0	0
Resource block assignment	1111110000000000000000000	1111110000000000000000000
MCS	23 (Dec)	10111
HARQ process number	3 (Dec)	011
NDI	1	1
RV	0 (Dec)	00
TPC	01	01

 

Example 2 > DCI Format 1 - 20 Mhz, Value = 0x2FC00005DC00

 

Field	Value	Value (Binary)
CarrierIndicator	1 (Dec)	001
RA header	RAType0	0
Resource block assignment	1111110000000000000000000	1111110000000000000000000
MCS	23 (Dec)	10111
HARQ process number	3 (Dec)	011
NDI	1	1
RV	0 (Dec)	00
TPC	0 (Dec)	00

 

 

< DCI 2A Examples > ----------------------------------------------------------------------------------------

 

Example 1 > DCI Format 2A - 20 Mhz, Value = 0x7E000010BCBC

 

Field	Value	Value (Binary)
RA header	RAType0	0
Resource block assignment	1111110000000000000000000	1111110000000000000000000
TPC command for PUCCH	01	01
HARQ process number	0 (Dec)	000
TB to CW flag	0	0
MCS for TB1	23 (Dec)	10111
NDI for TB1	1	0
RV for TB1	0 (Dec)	00
MCS for TB2	23 (Dec)	10111
NDI for TB2	1	1
RV for TB2	0 (Dec)	00

 

 

 

< DCI 2C Examples > ----------------------------------------------------------------------------------------

 

Example 1 > DCI Format 2C - 10 Mhz, Value = 0x7FFFDC3435C0

 

Field	Value	Value (Binary)
RA header	RAType0	0
Resource block assignment	11111111111111111	11111111111111111
TPC command for PUCCH	01	01
HARQ process number	6 (Dec)	110
Ports-SCID-Number of Layers	0	000
MCS for TB1	26 (Dec)	11010
NDI for TB1	0	0
RV for TB1	0 (Dec)	00
MCS for TB2	26 (Dec)	11010
NDI for TB2	1	1
RV for TB2	3 (Dec)	11

 

 

Cross Carrier Scheduling(CCS):
- Downlink Scheduling or Uplink Grant information of One Component Carrier(CC) can be carried by the PDCCH of another Component Carrier(CC).
- 3 bit CIF field indicates target CC.
- Pcell shall always be scheduled by Pcell only.
- Scell can be cross scheduled by Pcell or by other Scell.
- UE indicates whether it supports CCS or not.
- Cross Carrier Scheduling is not applicable for PDCCH order. It is transmitted on Pcell.
- CCS is applicable for aperiodic SRS transmission.

- The cif-Presence-r10 in physicalConfigDedicated indicates whether CIF will be present in PDCCH of Pcell.

- The RadioResourceConfigDedicatedSCell-r10.PhysicalConfigDedicatedSCell-r10.CrossCarrierSchedulingConfig-r10 indicates CCS status of Scell.

- cif-Presence indicates whether carrier indicator field is present (value TRUE) or not (value FALSE) in PDCCH DCI formats.

- pdsch-Start indicates the starting OFDM symbol of PDSCH for the concerned SCell. Values 1, 2, 3 are applicable when dl-Bandwidth for the concerned SCell is greater than 10 resource blocks, values 2, 3, 4 are applicable when dl-Bandwidth for the concerned SCell is less than or equal to 10 resource blocks.

- schedulingCellId Indicates which cell signals the downlink allocations and uplink grants, if applicable, for the concerned SCell.(When Scell cross scheduled  other Scell.)

- The other-r10.schedulingCellId-r10 and cif-Presence-r10 of that cell should be consistent.

Who Schedule Each Component Carriers ?

 

When you have multiple Carriers in Carrier Aggregation, you naturally have a question. That is, who (which carrier) will be schedule resource allocation for each sub carriers ?

There are two types of method we can think of as illustrated below. In one case (Own Scheduling), each component carrier schedules for its own carrier. In the other case (Cross Carrier Scheduling), Primary Compnent Cell (or any specified serving cell) schedules the resource for all the component carriers.

Then you would have another question. How UE knows whether eNB is doing "cross carrier scheduling" or "non cross carrier scheduling" ?

This information is informed to UE via Higher Layer Signaling (RRC Message) as shown below.

 

 

 

< Own (Non Cross Carrier) Scheduling >

 

If Network (eNB) decided to do Own(Non-Cross carrier) Scheduling, it notifies UE using RRC message as shown below.

 

rrcConnectionReconfiguration

    rrc-TransactionIdentifier: 0

    criticalExtensions: c1 (0)

        c1: rrcConnectionReconfiguration-r8 (0)

            rrcConnectionReconfiguration-r8

                radioResourceConfigDedicated

                    physicalConfigDedicated

                nonCriticalExtension

                    lateNonCriticalExtension: <MISSING>

                    nonCriticalExtension

                        nonCriticalExtension

                            sCellToAddModList-r10: 1 item

                                Item 0

                                    SCellToAddMod-r10

                                        sCellIndex-r10: 1

                                        radioResourceConfigDedicatedSCell-r10

                                            physicalConfigDedicatedSCell-r10

                                                nonUL-Configuration-r10

                                                    crossCarrierSchedulingConfig-r10

                                                        schedulingCellInfo-r10: own-r10 (0)

                                                            own-r10

                                                                .... ..0. cif-Presence-r10: False

 

 

< Cross Carrier Scheduling >

 

If Network (eNB) decided to do Cross carrier Scheduling, it notifies UE using RRC message as shown below.

 

rrcConnectionReconfiguration

    rrc-TransactionIdentifier: 0

    criticalExtensions: c1 (0)

        c1: rrcConnectionReconfiguration-r8 (0)

            rrcConnectionReconfiguration-r8

                radioResourceConfigDedicated

                    physicalConfigDedicated

                nonCriticalExtension

                    lateNonCriticalExtension: <MISSING>

                    nonCriticalExtension

                        nonCriticalExtension

                            sCellToAddModList-r10: 1 item

                                Item 0

                                    SCellToAddMod-r10

                                        sCellIndex-r10: 1

                                        radioResourceConfigDedicatedSCell-r10

                                            physicalConfigDedicatedSCell-r10

                                                nonUL-Configuration-r10

                                                    crossCarrierSchedulingConfig-r10

                                                        schedulingCellInfo-r10: other-r10 (1)

                                                            other-r10

                                                                schedulingCellId-r10: 0

                                                                pdsch-Start-r10: 3

 

 

Carrier Aggregation and Measurement Events:

- Definition of Serving Cell Measurement is Modified.

- For Event A1 and Event A2 The Carrier Frequency in Measurement Object indicates whether this event is for Pcell or any Scell. 

- The eNodeB shall configure separate A1/A2 events for each serving cell.

- Event A3 - Neighbor becomes offset better than Pcell.

- Event A5 - Pcell becomes worse than theshold1 and neighbour becomes better than threshold2.

        - For Event A3 and Event A5 the frequency mentioned in the associated measObjectEUTRA indicates neighbours.

        - For Event A3 and Event A5 the Scell become neighbouring cell.

- Event B2 - Pcell becomes worse than theshold1 and inter RAT neighbour becomes better than threshold2.

- Event A6 - Intra Frequency Neighbour becomes offset better than Scell.

- No change in the definition of Event A4 and Event B1.

Carrier Aggregation and Periodic Measurement:

- If (Purpose == reportStrongestCells && reportAmount > 1)

      UE initiates a first MR immediately after the quantity to be reported becomes available for the Pcell.

- If (Purpose == reportStrongestCells && reportAmount == 1)

      UE initiates a first MR immediately after the quantity to be reported becomes available for the Pcell and for the strongest cell among the applicable cells.

- If (Purpose == reportStrongestCellsForSON)

      UE initiates a first MR when it has determined the strongest cells on the associated frequency.

Carrier Aggregation and Measurement Gap:

- UE shall be able to carry out Measurement on any serving frequency without measurement gap i.e. intra-frequency measurement for any serving frequency.

- UE may required measurement gap to perform inter-frequency or inter-RAT measurement.

Typical CA Call Flow:

Scenario:::

Antennas of the cells are co-located and the cells are overlaid with small frequency separation; component carriers in the same band. Almost same coverage is provided due to similar path loss. Mobility is supported on either carrier. Higher data rates are achievable throughout the cell by CA.

Antennas of the cells are collocated and the cells are overlaid with large frequency separation; component carriers in different bands. Different coverage is provided on different carriers; and the higher frequency carriers do show smaller coverage due to larger path loss. Mobility is supported on the carrier in the lower frequency band providing sufficient coverage. Higher data rates and throughput are ensured by the carrier in the higher frequency band

Antennas of the cells are collocated; but the antenna directions are different in order to fill the coverage hole at the cell boundary; component carriers are in different bands. Coverage holes exist for cells in the higher frequency band due to the larger path loss. CA is supported in areas with overlapping coverage. Mobility management is typically not performed on the cells in the higher frequency band. Antennas oriented to the cell edge to achieve improved cell edge data rates and throughput.

Scenario for Scell-CA

At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage with the target PCell. Some of the possible scenarios (considering only one SCell) are given below.

― SCell is already configured in the source PCell and the SCell is left modified/ unmodified i.e., same SCell is used even in the target PCell.

― SCell is already configured in the source PCell and the SCell is left released during handover. i.e., release of the existing SCell during handover

― SCell1 is configured in the source PCell and during the handover, SCell1 released and a new SCell (SCell2) is configured for use in the target PCell i.e., change of SCell during handover.

― SCell is not already configured in the source PCell and an SCell is configured during handover for use in the target PCell.

― SCell is not already configured in the source PCell and an SCell is added during handover for use in the target PCell.

― SCell is already configured in the source PCell and UE receives handover command to handover to configured SCell i.e., Handover to SCell. In this case, RRCConnectionReconfiguration message contains SCell Release and also mobilityControlInfo containing carrierFreq and phyCellId which are same as that of SCell.

― SCell is already configured in the source PCell and UE receives handover command to handover to configured SCell and a new SCell is added whose frequency is same as that of source PCell i.e., Swapping PCell with SCell and vice versa. In this case, RRCConnectionReconfiguration message contains SCell Release (SCell1) and also mobilityControlInfo containing carrierFreq and phyCellId which are same as that of SCell1, SCell Addition whose carrierFreq and phyCellId are same as that of source PCell.

SCell can also be added during Handover from another RAT (e.g. GERAN or UTRAN) to E-UTRAN.

---

Q.How carrier aggregation is communicated to the device ? A carrier indication field ( 3 bits ) is added to the DCI formats providing the index of the component carrier for which the scheduling grant / scheduling assignment is valid. The Carrier indication field is optional in the DCI formats. The higher layer provides this information to the mobile device.  Q.How the Uplink HARQ Ack/Nack function works for DL in carrier aggregation? Q.How the SRs function works in carrier aggregation? Q.How the Downlink CQI reporting function works in Carrier aggregation? Q.How the SRS function works in carrier aggregation ? Q.How the Downlink Ack/Nack function works for UL in carrier aggregation? Q.How TPC function works for uplink PUCCH/PUSCH channels? Q.How synchronisation function works ?

• Support for carrier aggregation feature requires enhancement to the 3GPP LTE Release 8 & 9 physical, MAC, and RRC protocol layers. To an LTE Release 8 terminal, each component carrier will appear as an LTE carrier, while an LTE-Advanced terminal can use the total aggregated bandwidth.

 Q.How does the eNodeB scheduler allocate the resource blocks on the secondary cell SCell  from the primary cell PCell ? A: By using Carrier Indicator Field on PCell to convey allocation on SCell. Q.How does the PCell communicate with the SCell ?   What is the physical connection between the PCell and SCell ? A: At this time, PCell and SCell are essentially different carrier frequencies covering the same geographic area (e.g., a given 120 degree cell).  So, the same eNodeB controls both and there is no “connection” between the PCell and the SCell.  When CoMP is implemented, two eNodeBs controlling two cells will communicate via regular X2. Q.For Carrier Aggregation to work, a second 3-cell eNB is deployed (most likely co-located) but I am very unclear as of how these two eNBs (PCell and SCell) will communicate and how the RBs are assigned.  Would appreciated detailed information on this incl White Papers, etc. A.  Such info is proprietary and implementation-specific.  Conceptually, the MAC scheduler at the eNodeB distributes data between the cells (e.g., 1 transport block on 1 cell).  For the uplink, the eNodeB allocates resources on multiple UL cells and the UE’s MAC layer distributes bits between the cells.  One of the best references on LTE-Advanced is attached.