3GPP TSG RAN WG1 Meeting #54
`Jeju, Korea, August 18 – 22, 2008
`
`Source:
`Title:
`Agenda Item:
`Document for:
`
`
`Panasonic
`Support of UL/DL asymmetric carrier aggregation
`12 Study Item on LTE-Advanced
`Discussion and Decision
`
`R1-082999
`
`Introduction
`1.
`Support of wider bandwidth is one of important enhancements for LTE Advanced. Carrier aggregation has been
`proposed in order to support backward compatibility to LTE UEs and higher peak throughput to LTE-A UEs
`simultaneously[1]-[3]. However, the required peak rate for DL and that for UL are different. In this document,
`we discuss how the carrier aggregation works in case DL allocated frequency band is wider than UL allocated
`frequency band. We propose to have asymmetric combination of carrier aggregation for the efficient usage.
`2. Asymmetric carrier aggregation in DL and UL
`Figure 1 shows 2 alternatives for allocating 40MHz in DL and 20MHz in UL system frequency. In the figure
`1(a), there are 2 independent pairs of DL/UL component carriers (i.e. symmetric carrier aggregation). In the
`figure 1(b), there are only one component carriers in UL and 2 component carriers in DL (i.e. asymmetric carrier
`aggregation). Comparing these two alternatives, to use UL 20 MHz band as one component carrier has
`advantages of higher peak rate, lower PAPR, reduced control channel overhead and more user diversity gain.
`Therefore, to have asymmetric carrier aggregation is benefitical. So we focus on supporting asymmetric carrier
`aggregation in LTE-A.
`
`Component carriers
`
`--------
`
`PDCCH
`
`PDSCH
`
`~
`
`~
`
`PDCCH
`
`PDSCH
`
`SCH
`
`/BCH
`
`f
`
`/
`
`/
`
`20MHz
`
`/
`
`PUCCH
`
`PUSCH
`
`PUCCH
`
`SCH
`
`/BCH
`
`20MHz
`
`f
`
`/
`
`/
`
`'-
`
`'-
`
`/
`
`--------
`
`PDCCH
`
`PDSCH
`
`SCH
`
`/BCH
`
`-
`
`Component carriers
`
`20MHz
`
`/
`
`PUSCH
`
`PUCCH
`PUCCH
`
`PUSCH
`
`PUCCH
`
`PDCCH
`
`PDSCH
`
`SCH
`
`/BCH
`
`-
`
`20MHz
`
`/
`
`PUCCH
`
`Downlink
`
`'-
`
`'-
`
`Uplink
`
`20MHz
`
`20MHz
`
`(a)
`
`(b)
`
`
`Figure 1 Asymmetric carrier aggregation and symmetric carrier aggregation (DL: 40MHz, UL: 20MHz)
`3. Discussions on backward compatibility to LTE UE
`Needless to say, the symmetric carrier aggregation supports LTE UE because there is no difference between one
`pair of DL/UL band and that of Release 8 from UE perspective.
`Regarding asymmetric carrier aggregation, backward compatibility aspects to LTE UE on some physical
`channels are discussed below.
`1. PCFICH
`No problem since PCFICH can be sent via each component carriers. Note that, the value of PCFICH on
`each component carriers could be different.
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`2. PDCCH
`
`No problem since PDCCH to each LTE UE can be sent via each component can-iers where LTE UE camps.
`
`3. PHICH
`
`No problem from the view point of the backward compatibility although some PRICH resources would not
`be used depending on the allocation of con-esponding UL grants in each component can-ier.
`
`4. PDSCH
`
`No problem since PDSCH to each LTE UE can be sent with PDCCH (DL assignment) via each component
`can-iers where LTE UE camps.
`
`5. PRACH
`
`No problem from the view point of the backward compatibility although there are still FFS in release 8.
`
`If "PUCCH-resource-size" is explicitly signaled in release 8, eNB can allocate the same physical
`frequency/time resources and the different sequence numbers to each DL component ca1rier via D-BCH.
`eNB can distinguish in which component can-ier the RACH response should be sent based on the used
`sequence number.
`
`If "PUCCH-resource-size" is not explicitly signaled in release 8 and UE de1-ives the frequency resources
`for PRACH by the offset values for D-ACK and maximum number of CCEs, eNB has to manage the
`possible collision of PUSCH and PRACH. Thus there might be some resti-ictions to the eNB's scheduler.
`
`6. PUCCH
`
`No problems for the CQis, P-ACKs and SRis since the resources are explicitly signaled to each UE. Also
`no problem for D-ACKs since D-BCH in each DL component can-ier can indicate different offset values for
`D-ACK resources as shown in Figure 2.
`
`Cyclic Shilt index (0 to 11)
`
`-
`
`~14-i•IIIIQ~
`•
`resource 1
`
`1 m.s subframe
`
`Different offs.et value.$ are indicated by
`D-BCH of each component carrier
`
`Figure 2 An example of PUCCH allocations for supporting asymmetric carrier aggregation
`
`7. PUSCH
`
`No problem from the view point of the backward compatibility since PUSCH can be assigned by PDCCH
`(UL grant) via each component can-iers where LTE UE camps.
`
`When UL FH is configured, there would be some restrictions to the eNB's scheduler based on the signaling
`of the "PUCCH-resource-size" so that the hopped PUSCH doesn't collide with D-ACKs.
`
`•
`
`•
`
`If"PUCCH-resource-size" is explicitly signaled in release 8, this collision can be avoided by signalling
`appropriate value of "PUCCH-resource-size" in each component can-ier.
`
`If"PUCCH-resource-size" is not explicitly signaled (i.e. UE derives the frequency resources for FH by
`the offset values for D-ACK and number of CCE) in release 8, eNB scheduler should manage this
`possible collision of PUSCH and D-ACK by e.g. not to assign such FH resources to the UEs.
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`4. Conclusion
`In this paper, we discussed the support of asymmetric carrier aggregation. Based on the considerations here, it
`would be possible that LTE UE can coexist in the system with asymmetric carrier aggregation. So we propose to
`support asymmetric carrier aggregation in LTE-A system to handle asymmetric DL/UL traffic effectively.
`In addition, we found that it is preferable to signal the “PUCCH-resource-size” explicitly in release 8 at least in
`FDD system to ensure the extensibility of the LTE spec.
`References
`[1] R1-082468, Ericsson, “Carrier aggregation in LTE-Advanced”
`[2] R1-082448, Huawei, “Carrier aggregation in Advanced E-UTRA”
`[3] R1-082575, NTT DOCOMO, “Proposals for LTE-Advanced Technologies”
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