Slide 1

LTE-A Carrier Aggregation
Dale Little
The problem
• Increase data rates over current LTE speeds
– LTE Rel8 Peak DL: 100Mbps UL: 50Mbps
– LTE Rel10: Peak DL: 1 Gbps UL: 500 Mbps
• Overcome spectrum fragmentation issues
• Maintain compatibility with 3GPP Rel8/9
The solution
Carrier Aggregation: Combining various
contiguous and non-contiguous bands of
spectrum into a single logical channel.
Design Principles
• Backward Compatibility
• Minimal Protocol Impact
• Limited Control Procedure Change
CA Implementation
• CA permits the LTE radio interface to be
configured with up to five component carriers
(CC) of any bandwidth (1.4, 3, 5, 10, 15, 20
• Each CC is equivalent to a Rel8/9 carrier.
• Three types of carrier aggregation are defined:
inter-band aggregation, contiguous intra-band
aggregation, and non-contiguous intra-band
CA Implementation
• UE connects to a primary cell (PCell) and one or more
secondary cells (SCells)
– PCell: Primary cell where the UE establishes the RRC
connection and where PUCCH is used
– SCell: Secondary cell(s) that the UE could be monitoring for
DL assignment and using to transmit UL data
• UE establishes an RRC connection only to the PCell and
will only transmit PUCCH (uplink control information)
to PCell.
• The SCell(s) transmit PDCCH and PDSCH (downlink
control and data) and receives PUSCH (uplink data).
CA Implementation
• In terms of network architecture, the main
layers impacted by CA are the RRC, MAC and
PHY layers.
• The core network, PDCP, and RLC are not
impacted. From the perspective of the user
plane, the aggregated carrier is just like any
other single bearer.
Impact of CA on the RRC layer
Important changes to RRC include:
• UE capability transfer procedure
• Measurement events
• RRC connection reconfiguration
• Handover and RRC connection
reestablishment procedures
UE capability transfer procedure
Carrier aggregation requires the addition of new Information Elements. These enable
the communication of the UE’s carrier aggregation capability and include:
• UE category
– Defines the performance standards to which a UE will operate. Enables eNB to effectively
communicate with UEs by knowing their performance levels.
Supported Band Combination
– Indicates which band(s) and bandwidth class(es) support CA.
Cross-Carrier Scheduling
– Defines whether the UE supports cross-carrier scheduling operation.
Simultaneous PUCCH and PUSCH transmission
– Indicates if the UE supports simultaneous transmission of PUCCH and PUSCH across any UL CCs
which the UE can aggregate.
Multi-cluster PUSCH
– Indicates if the UE supports PUSCH transmissions over non-contiguous resource blocks across
any UL CCs which the UE can aggregate.
Event A6 support
– Indicates that the UE supports enabling and triggering measurement event A6 related to a SCell.
SCell addition within the Handover to E-UTRAN
– Indicates that the UE can support an E-UTRAN handover directly into CA mode.
Measurement events
• Measurement procedures and capabilities are
used by the network to manage network
resources and perform different mobility
procedures. Measurement event A6 is
introduced as an optional UE capability in Rel10
to enable the addition and removal of SCells. In
Rel10, Events A3 and A5 are specific to PCells.
RRC connection reconfiguration
• RRC connection reconfiguration handles the
addition or removal of SCells.
• RRC connection reconfiguration can only add
an SCell after Access Stratum (AS) security has
been activated.
• SCell Deactivation Timer information is
signaled to the mobile. It indicates how many
frames of inactivity on an SCell should cause
the UE to remove that SCell.
Handover and RRC connection
reestablishment procedures
• CA does not directly impact handover and RRC
connection reestablishment procedures.
However, all SCells are released by the UE
upon RRC reestablishment due to Radio Link
failure or handover. Rel10 allows for direct EUTRAN handover in CA mode if the UE
supports it.
Impact of CA on the MAC Layer
• SCell activation and deactivation
– RRC connection reconfiguration is used to add or remove
SCells. Once a cell is added, it must be activated by the
MAC layer. With Rel10, one formerly reserved value for
the Logical Channel ID (LCID) is added to the list of valid
values used for MAC control element activation and
deactivation. LCID is used by the UE to activate or
deactivate the reception of SCells. PCells cannot be
• MAC scheduling over multiple carriers
– From the user plane perspective, the main change made to
the MAC to support carrier aggregation the enabling of
scheduling on multiple CCs. Each CC has a unique HARQ
entity and independent HARQ processes.
Impact of CA on the PHY Layer
• PUCCH is only transmitted on PCells. To
provide information about other carriers, the
Carrier Indicator Field (CIF) is provided in the
Uplink Control Information (UCI) header.
• PDCCH may optionally not be transmitted (on
the SCell) if cross-carrier scheduling is
enabled. In this case the Downlink Control
Information (DCI) header includes a CIF that
identifies the intended carrier.
Channel Quality
• DL channel quality is measured by the UE and
reported to the eNB in the Uplink Control
Information (UCI). The UCI header includes a
Carrier Indicator Field (CIF) that indicates which
CC is being referenced. This is used in the case
when cross-carrier scheduling is enabled.
• UL channel quality is measured by the eNB using
Sounding Reference Symbols (SRS) transmitted by
the UE. An optional capability of Rel10 allows the
UE to transmit SRS on SCells as well as PCells.
Cross-Carrier Scheduling
• SCells may optionally not use the PDCCH if a feature
called cross-carrier scheduling is supported by both the
UE and the network.
• Cross-carrier scheduling: Scheduling information for an
SCell is transmitted over PDCCH of the PCell or another
• By scheduling on the PCell, the SCell traffic “pipes” are
reserved for user data only, which minimizes SCell
control channel overhead. It also enables coordinated
scheduling of data across multiple carriers, which in
turn enables efficient network planning.
Timing Advance
• Timing advance is a method in which eNB
requests that the UE adjusts its UL timing
(relative to DL time) in order to mitigate the
effect of propagation delay. In CA, only a
single timing advance value that applies to all
carriers is used by the E-UTRAN.
• CA has been specified by 3GPP as a means for
addressing the wireless industry’s requirement
for greater spectrum utilization and faster data
• CA allows the eNB to group several different
channels into one logical channel. By enabling
RRC connections with multiple cells at low
protocol layers, CA creates wide-band bearers for
delivery of higher data rates and allows LTE-A to
meet ITU specifications for 4G.
• D. Gerstenberger, et al., “Overview of 3GPP LTEAdvanced Carrier Aggregation for 4G Wireless
Communications,” IEEE Communications
Magazine, February 2012, pp. 122-130.
• Spirent Communications, “LTE Advanced - Carrier
Aggregation. Introduction and Implications for
Mobile Device Testing,” White Paper, June 2013.
• I. Poole, “LTE CA: Carrier Aggregation Tutorial,”

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