LTE-A Carrier Aggregation Dale Little EETS8316 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 MHz). • 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 aggregation. 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 deactivated. • 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 SCell. • 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. Conclusion • CA has been specified by 3GPP as a means for addressing the wireless industry’s requirement for greater spectrum utilization and faster data delivery. • 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. References • 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,” Radio-Electronics.com.