4G/LTE - LTE Advanced

 

 

 

Message Flow

 

There are a good news and a bad news on RRC messages for Carrier Aggregation. Which one do you want to go first ? Both come down to the same pair of messages. The rest of this page sets out what each of them carries and when each of them is sent.

Good News first ?

Good News is that basically there would be only two messages you have to dig into. (UE Capability Information and RRC Connection Reconfiguration).

Now goes the Bad News.

Bad News is that the two message are so complicated (contains so much information), it is extremely difficult to understand all the details. At least to me, it seems to take a couple of YEARS to understand all of those details.

The two messages that carry Carrier Aggregation

Carrier aggregation appears in only two messages of an ordinary call, and the division between them is clean. One reports what the UE can do, and the other tells the UE what to do. Everything below is one or the other.

Following is overall steps for LTE adavanced Carrier Aggregation and UE Capability Information and RRC Connection Reconfiguration is the critical steps for Carrier Aggregation configuration.

 

A UE to network sequence running RRC Setup, Authentication, NAS Security, UE Capability Enquiry and Information, RRC Security, then RRC Connection Reconfiguration and Complete, with notes on the two carrier aggregation steps

The two notes on the right mark the only two arrows in the whole sequence that carry carrier aggregation. Everything else runs exactly as it does without it.

  • Capability comes before configuration : UE Capability Enquiry and Information sit above RRC Security, and the Reconfiguration that acts on the answer sits below it.
  • The network asks first : the enquiry is drawn from the network to the UE, so a UE never offers its carrier aggregation capability unprompted.
  • Only two arrows are annotated : UE Capability Information reports the capability, and RRC Connection Reconfiguration configures against it.
  • The rest of the call is ordinary : RRC Setup, Authentication, NAS Security and RRC Security are drawn without comment, because carrier aggregation changes none of them.

Followings is the information carried by UE Capability Information and its IEs for carrier aggregation and antenna configuration. (These are from 3GPP TS 36.331 version 10.4.0 Release 10)

 

36.331 ASN.1 for UE-EUTRA-Capability-v1020-IEs with phyLayerParameters-v1020 and rf-Parameters-v1020 highlighted, and the definitions of PhyLayerParameters-v1020 and RF-Parameters-v1020 below

 

36.331 ASN.1 for BandCombinationParameters-r10, BandParameters-r10, the uplink and downlink CA-MIMO-Parameters, CA-BandwidthClass-r10 and the two MIMO-Capability enumerations

Those two listings still read as 36.331 reads today. UE-EUTRA-Capability-v1020-IEs, RF-Parameters-v1020, BandCombinationParameters-r10, BandParameters-r10, the two CA-MIMO-Parameters structures and both MIMO-Capability enumerations are unchanged in v19.3.0.

One line in the first of them has moved on. The picture shows nonCriticalExtension as an empty SEQUENCE, which was the Release 10 placeholder. 36.331 v19.3.0 points it at UE-EUTRA-Capability-v1060-IEs, and the chain of extensions behind that runs well past Release 10.

CA-BandwidthClass-r10 is worth a second look while it is on screen. The enumeration reads a, b, c, d, e, f and then an extension marker, and 36.331 v19.3.0 still reads exactly that. The table below adds letters that the RRC enumeration never gained.

Following table is used to configure 'CA-BandwidthClass-r10' IE (This table is from 3GPP TS 36.101 version 10.8.0 Release 10)

 

36.101 Table 5.6A-1 in an early release, giving CA bandwidth classes A to F with FFS in the maximum number of CC column for B, D, E and F and square brackets round the bandwidth ranges for D, E and F

An early copy of the table. Four of its rows have not been settled yet, and the square brackets round the bandwidth ranges are the specification's way of saying so.

  • FFS means the row was unfinished : the maximum number of CC column reads FFS for classes B, D, E and F, so only A and C carry a number here.
  • Square brackets mark a value under discussion : the aggregated bandwidth ranges for D, E and F are written as [300], [400] and [500].
  • The table stops at class F : no class I row appears here, and the guard band column reads FFS for every class except A and C.
  • Later releases completed the table : Release 13 gives B two carriers, D three, E four and F five, and adds a class I for eight.
  • The RRC enumeration did not follow : CA-BandwidthClass-r10 still lists only a to f, so class I is reached through the extension marker rather than a named value.

Followings is RRC Connection Reconfiguration message and its IEs for carrier aggregation and antenna configuration. (These are from 3GPP TS 36.331 version 10.4.0 Release 10)

 

36.331 ASN.1 for RRCConnectionReconfiguration-v1020-IEs with sCellToAddModList-r10 highlighted, and SCellToAddModList-r10 and SCellToAddMod-r10 below it

 

36.331 ASN.1 for PhysicalConfigDedicatedSCell-r10 with antennaInfo-r10 highlighted, and AntennaInfoDedicated-r10 with its transmission mode enumeration below it

Two of the listings in those two pictures have since changed, and both changes are one line each. In AntennaInfoDedicated-r10 the first spare of the transmission mode enumeration is now tm10-v1130, so the list runs tm1 to tm9-v1020, then tm10-v1130, then spare6 down to spare1. In RRCConnectionReconfiguration-v1020-IEs the nonCriticalExtension now points at RRCConnectionReconfiguration-v1130-IEs rather than being an empty SEQUENCE.

SCellToAddMod-r10 has grown further than either. The picture shows four fields and stops. 36.331 v19.3.0 closes them with an extension marker and adds four groups after it: dl-CarrierFreq-v1090, antennaInfoDedicatedSCell-v10i0, srs-SwitchFromServCellIndex-r14, and sCellState-r15, an ENUMERATED of activated and dormant.

That last field matters to both sequence diagrams below. A secondary cell added with sCellState-r15 set to activated is already on when the reconfiguration completes, so the Activation MAC CE drawn at step (7) has nothing left to do.

  • Capability first, configuration second : the call flow puts UE Capability Information above RRC Connection Reconfiguration, and nothing else in the call is touched.
  • The capability listings are still current : 36.331 v19.3.0 defines RF-Parameters-v1020 and the band parameter structures exactly as the pictures show them.
  • Two transmission modes were added after the screenshot : tm10-v1130 took the first spare of the AntennaInfoDedicated-r10 enumeration.
  • SCellToAddMod-r10 gained four extension groups : the last of them, sCellState-r15, can add a secondary cell already activated.
  • The bandwidth class table on this page is an early one : four of its rows read FFS, and the version in Release 13 fills all of them in.

Closer view on Carrier Aggregation Establishment - 2CC

Following sequence diagram shows the process of establishing Carrier Aggregation (2 Carrier) with more details. If you think it is too complicated, just focus on step (1), (6), (7), (8) first.

(1) is the command (RRC message) to tel the UE to configure the Radio Stack (PHY, MAC) to establish the aggregated communication (Carrier Aggregration). You need to look into every details of RRC Connection Configuration message to fully understand this step.

(2) is the HARQ ACK from UE saying 'I got a PDSCH (carrying RRC Connection Reconfiguration).

(3) is the step where both UE and Network performs the necessary setup for Carrier Aggregation.

(4) is the step where UE send SR saying 'I need a physical resource to send some data (PUSCH carrying RRC Connection Reconfiguration Complete message in this case)'.

(5) is the step where Network allocate resource in response to step (4).

(6) is the step where UE reporting to network 'I am done with the setup and the setup is successful'. At this step, the setup has been established only at RRC layer and MAC layer for the second carrier is not yet activated.

(7) is the step where Network send a commond to UE saying 'Now activate MAC layer for the second carrier as well'.

(8) indicate the status where MAC/PHY for both carrier are fully activated.

 

A sequence diagram over eNB PCC, eNB SCC, UE PCC and UE SCC, steps 1 to 8 from RRC Connection Reconfiguration to PDSCH on both carriers, with three throughput snapshots on the right

Three throughput snapshots on the right, taken at the three red marks on the UE lifeline. The rise does not arrive with the reconfiguration. It arrives two steps later.

  • Four lifelines, not two : eNB PCC and eNB SCC on the left, UE PCC and UE SCC on the right, and the UE SCC line stays idle until step (8).
  • Blue is RRC, green is everything else : steps (1) and (6) are solid blue, step (7) is a dashed blue MAC CE, and every other arrow is green.
  • Both reconfiguration steps run on the PCC : the command and the completion touch the eNB PCC lifeline only.
  • The throughput moves at the last snapshot : the first two show one carrier, and only the snapshot after step (8) shows the second one carrying data.

The four steps the paragraph above tells a first-time reader to watch are the four that change state. Step (1) delivers the configuration, step (6) confirms the UE applied it, step (7) turns the secondary cell on, and step (8) is the first subframe where both carriers deliver data.

The steps in between are there because of what the messages are. An RRC message needs a transport block to carry it, so step (1) draws a HARQ acknowledgement at step (2). The completion travels the other way and needs an uplink grant, so the UE asks for one with a scheduling request at step (4) and receives it at step (5).

  • Configuration and activation are separate : step (1) configures the secondary cell and step (7) activates it, and a MAC CE does the second job.
  • Release 15 can merge them : sCellState-r15 set to activated leaves step (7) with nothing to do.
  • The middle steps are transport, not carrier aggregation : the HARQ acknowledgement, the scheduling request and the grant would appear for any RRC exchange.
  • Throughput follows activation, not configuration : the drawing puts the rise after step (8), two steps past the reconfiguration.

Closer view on Carrier Aggregation Establishment - 3CC

Following sequence diagram shows an example for establishing the 3CC (Component Carrier) Carrier Aggregation. In 3CC case, there can be two different ways to establish the CA. One is to add SCC1 and SCC2 one by one and the other one is to add SCC1 and SCC2 at the same time. This sequence shows the case where we add SCC1/SCC2 at the same time.

In this case, overal sequence is exactly same as the 2CC case that we saw above.

(1) is the command (RRC message) to tel the UE to configure the Radio Stack (PHY, MAC) to establish the aggregated communication (Carrier Aggregration). You need to look into every details of RRC Connection Configuration message to fully understand this step.

(2) is the HARQ ACK from UE saying 'I got a PDSCH (carrying RRC Connection Reconfiguration).

(3) is the step where both UE and Network performs the necessary setup for Carrier Aggregation.

(4) is the step where UE send SR saying 'I need a physical resource to send some data (PUSCH carrying RRC Connection Reconfiguration Complete message in this case)'.

(5) is the step where Network allocate resource in response to step (4).

(6) is the step where UE reporting to network 'I am done with the setup and the setup is successful'. At this step, the setup has been established only at RRC layer and MAC layer for the second carrier is not yet activated.

(7) is the step where Network send a commond to UE saying 'Now activate MAC layer for the second carrier as well'.

(8) indicate the status where MAC/PHY for both carrier are fully activated.

The same sequence with a second secondary carrier, where step 1 reads RRC Connection Reconfiguration adding SCC1 and SCC2 in one message

The same eight steps with one more carrier. Only the first arrow changes its label, and it changes by naming a second secondary cell rather than by adding a step.

  • One message adds both : step (1) is labelled RRC Connection Reconfiguration with Add SCC1 and SCC2, which is the second of the two ways the paragraph above describes.
  • The step count does not grow : the sequence still runs (1) to (8), which is what the paragraph above means by the same sequence.
  • Six lifelines now : three on the network side and three on the UE side, with both secondary UE lines idle until step (8).
  • One MAC CE covers both : step (7) is still a single dashed arrow, because the Activation MAC CE carries one bit per secondary cell.

The field behind that first arrow is a list, which is why one message is enough for both. 36.331 defines sCellToAddModList-r10 as a SEQUENCE OF SCellToAddMod-r10 with a size of 1 to maxSCell-r10, and it defines maxSCell-r10 as 4. Up to four secondary cells therefore fit in one reconfiguration.

The other way the paragraph above mentions, adding SCC1 and SCC2 one at a time, uses the same field twice. Each reconfiguration carries a list of one, and the second one adds to what the first configured rather than replacing it.

Release 13 raised the ceiling without changing the shape of the message. maxSCell-r13 is 31 and SCellIndex-r13 runs from 1 to 31, where SCellIndex-r10 runs from 1 to 7. It is still one list in one reconfiguration, only a longer one.

  • The list is what allows one message : sCellToAddModList-r10 is a SEQUENCE OF, so several secondary cells travel together.
  • Four at a time in Release 10 : maxSCell-r10 is 4, which bounds a five carrier aggregate.
  • Thirty-one in Release 13 : maxSCell-r13 is 31 and SCellIndex-r13 widens to match.
  • Adding one at a time uses the same field : each reconfiguration then carries a list of one, and the cells accumulate.

Reference

One specification defines both messages and every field named above, and a second defines the bandwidth class table. The third entry is the page on this site that carries the same material with decoded examples beside it.

  • [1] 36.331 : 3GPP - E-UTRA; Radio Resource Control, v19.3.0. UE-EUTRA-Capability-v1020-IEs, RRCConnectionReconfiguration-v1020-IEs, SCellToAddMod-r10 and the maxSCell constants are quoted above.
  • [2] 36.101 : 3GPP - E-UTRA; UE radio transmission and reception. Table 5.6A-1 is the bandwidth class table shown above.
  • [3] LTE Advanced RRC : the same two messages on this site, with decoded captures and the band combination tables.