Carrier Aggregation in LTE-Advanced

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Carrier Aggregation in LTE-Advanced

  1. 1. Carrier AggregationCARRIER AGGREGATION IN LTE-ADVANCED NIDHI ARORA ASST CONSULTANT, TCS Technical White Paper V1.0 March, 2013 1
  2. 2. Carrier AggregationABSTRACTIn order to achieve up to 1Gbps peak data rate in future IMT-Advanced mobile systems, CarrierAggregation concept has been introduced by the 3GPP in its new LTE-Advanced standards (3GPPRelease 10 onwards). Carrier Aggregation is aimed to support very high data rate transmissions overwide frequency bandwidths (e.g., up to 100MHz). This paper gives an overview of Carrier Aggregation,including its types, cell configurations, its need and benefits. This paper also discusses in brief themajor functional changes required in PHY, MAC and RRC for Carrier Aggregation.CONTENTS1. WHAT IS CARRIER AGGREGATION? ............................................................................................................. 32. DRIVING FORCE FOR CARRIER AGGREGATION ............................................................................................... 33. COMPONENT CARRIER/AGGREGATED CARRIER .............................................................................................. 34. TYPES OF CARRIER AGGREGATIONS ............................................................................................................ 35. SYMMETRIC VS ASYMMETRIC CARRIER AGGREGATION .................................................................................... 56. TYPES OF COMPONENT CARRIERS ............................................................................................................... 57. CARRIER AGGREGATION – CELL CONFIGURATION.......................................................................................... 58. RESOURCE SCHEDULING ........................................................................................................................... 69. FUNCTIONAL CHANGES IN PHY, MAC AND RRC FOR CARRIER AGGREGATION .................................................. 710. BENEFITS OF CARRIER AGGREGATION ......................................................................................................... 711. TECHNICAL CHALLENGES IN SUPPORTING CARRIER AGGREGATION ................................................................... 812. CONCLUSION .......................................................................................................................................... 813. REFERENCES ........................................................................................................................................... 8 2
  3. 3. Carrier Aggregation1. WHAT IS CARRIER AGGREGATION?Carrier Aggregation, as the name suggests, is aggregating multiple carriers of same or varyingbandwidths so as to increase the overall transmission bandwidth. `Carrier Aggregation (also called channel aggregation) is one of the most distinct features of 4G LTE-Advanced. Using Carrier Aggregation, it is possible to attain very high data rates by extending thetransmission bandwidth (up to 100MHz) by aggregating up to five LTE carriers. These carriers may bein contiguous elements of the spectrum, or they may even be non-contiguous (intra-band or inter-band). Carrier aggregation facilitates efficient use of fragmented spectrum. And can be used for bothFDD and TDD.2. DRIVING FORCE FOR CARRIER AGGREGATIONLTE-Advanced offers considerably higher data rates than the initial releases of LTE. While thespectrum usage efficiency has been improved, this alone cannot provide the required data rates thatare being headlined for 4G LTE Advanced.To achieve these very high data rates (1Gbps in downlink and 500Mbps in uplink) it is necessary toincrease the transmission bandwidth over those that can be supported by a single carrier or channel(maximum 20MHz in 3GPP Release 8/9). The method introduced by the 3GPP to achieve this is termedCarrier Aggregation, CA. Using LTE-Advanced carrier aggregation, it is possible to utilize more thanone carrier and in this way increase the overall transmission bandwidth. IMT Advanced sets the upperlimit at 100MHz, but with an expectation of 40MHz being used for minimum performance. For thefuture it is possible the top limit of 100MHz could be extended.3. COMPONENT CARRIER/AGGREGATED CARRIEREach aggregated carrier is referred to as a component carrier, CC. The component carrier can haveany of the bandwidths supported in LTE Release 8/9 (i.e. 1.4MHz, 3, 5, 10, 15 or 20MHz) and amaximum of five component carriers can be aggregated, hence the maximum aggregated bandwidthis 100MHz.The spacing between centre frequencies of two contiguously aggregated component carriers is amultiple of 300kHz so as to be compatible with the 100kHz frequency raster of LTE Release 8/9 and atthe same time preserve orthogonality of the subcarriers, which have 15kHz spacing. For non-contiguous cases, the CCs are separated by one, or more, frequency gap(s).4. TYPES OF CARRIER AGGREGATIONSThere are a number of ways in which LTE carriers can be aggregated:4.1 Intra-bandThis form of carrier aggregation uses component carriers within the same operating band (as definedfor LTE). There are two main variants for this type of carrier aggregation: 3
  4. 4. Carrier Aggregation Contiguous:In intra-band contiguous CA, the carriers are adjacent to each other.The aggregated channel can be considered by the terminal as a single enlarged channel from the RFviewpoint. In this instance, only one transceiver is required within the terminal or UE, whereas moreare required where the channels are not adjacent. f Band A Figure1. Intra-band Contiguous Carrier Aggregation Non-contiguous:The component carriers belong to the same operating frequency band, but have a gap, or gaps, inbetween.Non-contiguous intra-band carrier aggregation is somewhat more complicated than the contiguousone. In this case, since the multi-carrier signal cannot be treated as a single signal, multipletransceivers are required. This adds significant complexity, particularly to the UE where space, powerand cost are prime considerations. f Band A Figure2. Intra-band Non-contiguous Carrier Aggregation4.2 Inter-band non-contiguousThis form of carrier aggregation uses different operating bands. It will be of particular use because ofthe fragmentation of bands - some of which are only 10MHz wide. For the UE it requires the use ofmultiple transceivers within the single item, with the usual impact on cost, performance and power. Inaddition to this there are also additional complexities resulting from the requirements to reduce inter-modulation and cross modulation from the two transceivers. f Band A Band B Figure3. Inter-band Non-contiguous Carrier Aggregation 4
  5. 5. Carrier Aggregation5. SYMMETRIC VS ASYMMETRIC CARRIER AGGREGATION5.1 Symmetric Carrier AggregationSymmetric carrier aggregation is defined as the case where there are equal number of componentcarriers for the downlink and uplink.In TDD, the downlink and uplink are always symmetric. In FDD, the downlink and uplink can either besymmetric or asymmetric.5.2 Asymmetric Carrier AggregationIn asymmetric case, the number of aggregated carriers can be different in DL and UL. However, thenumber of UL component carriers is always equal to or less than the number of DL componentcarriers. The individual component carriers can also be of different bandwidths. f DL f UL Figure4. Asymmetric Carrier Aggregation6. TYPES OF COMPONENT CARRIERSComponent carriers are classified under two categories: Primary component carrier (PCC): This is the main carrier in any group. There will be a primary downlink carrier and an associated uplink primary component carrier. If DL PCC changes, UL PCC also changes. The association between the DL PCC and the corresponding UL PCC is cell specific. The information is signaled to the terminal or user equipment as part of the overall signaling between the terminal and the base station. Secondary component carrier (SCC): There may be one or more secondary component carriers.The SCCs are added and removed as required, while the PCC is only changed at handover.7. CARRIER AGGREGATION – CELL CONFIGURATIONEach component 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 forheterogeneous network planning. 5
  6. 6. Carrier AggregationOne of the serving cells is designated 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 servingcells 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 procedures 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.8. RESOURCE SCHEDULINGResources can be allocated to a UE in two ways: 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 scheduling is only used to schedule resources on SCC without PDCCH. The CIF (Carrier Indicator Field) on PDCCH indicates on which carrier the scheduled resource is located. - Common PDCCH for multiple CCs. - Reusing LTE Release 8/9 PDCCH structure - New 3-bit Carrier Indication Field (CIF) added to Release 8 DCI. - PCell shall be responsible for cross-carrier scheduling of the secondary but not vice-versa. - PCell cannot be cross-scheduled; it is always scheduled through its own PDCCH. Control Control Region Data Region Data Region Region SCC1 PCC SCC2 Same-Carrier Scheduling Cross-Carrier Scheduling Figure5. Same-Carrier Scheduling vs Cross-Carrier Scheduling 6
  7. 7. Carrier Aggregation9. FUNCTIONAL CHANGES IN PHY, MAC AND RRC FOR CARRIER AGGREGATIONIntroduction of carrier aggregation influences mainly MAC and the physical layer protocol, but alsosome new RRC messages have been introduced. RLC and PDCP do not have any functional changes forcarrier aggregation.In order to keep LTE Release 8/9 compatibility the protocol changes have been kept to a minimum.Basically each component carrier is treated as a Release 8/9 carrier. However some information isnecessary, such as new RRC messages in order to handle SCC(s).The reconfiguration, addition and removal of SCells are performed by RRC. When adding a new SCell,dedicated RRC signaling is used for sending all required system information of the SCell.Activation/deactivation of the SCell is done in MAC and handled by control element indication on SCC.MAC must be able to handle scheduling on a number of CCs. The MAC layer divides the data betweendifferent CCs and separate HARQ processes for each CC. Also the data aggregation happens in MAClayer.In case of carrier aggregation, the multi-carrier nature of the PHY layer is only exposed to the MAClayer for which one HARQ entity is required per serving cell. In both uplink and downlink, there is oneindependent HARQ entity per serving cell and one transport block is generated per TTI per serving cellin the absence of spatial multiplexing. Each transport block and its potential HARQ retransmissions aremapped to a single serving cell.Independent configuration of downlink and uplink transmission modes is handled in the MAC and PHYlayers.Major changes on the physical layer are for example that signaling information about scheduling onCCs as well as HARQ ACK/NACK per CC must be carried. A new Physical Uplink Control Channel(PUCCH) format3 signaling is introduced to support downlink carrier aggregation. Also a new type ofDL reference signal for TM9 has been introduced.10. BENEFITS OF CARRIER AGGREGATION Help attaining very high data rates by increasing the overall transmission bandwidth. Flexible bandwidth combination allowing new LTE bandwidths even up to 100MHz Each component carrier is fully backward compatible to Release 8/9. This backward compatibility to Release 8/9 allows the technologies developed for LTE Release 8/9 to be fully reused in Release 10. It also allows the co-existence of Release 8/9 UEs together with Release 10 UEs, which is very important for seamless system transition from Release 8/9 to Release 10. Facilitates efficient use of fragmented spectrum. Facilitates efficient interference management for control channels in heterogeneous networks. Signaling extensions to Release 10 are transparent to Release 8/9 UEs The versatility of carrier aggregation makes for much easier network deployment, because the second component carrier can be used either to boost the data rates close to the eNodeB, to 7
  8. 8. Carrier Aggregation eliminate weak coverage at the cell edge, or to serve hot spots where peak rates are very important.11. TECHNICAL CHALLENGES IN SUPPORTING CARRIER AGGREGATIONFollowing are some technical challenges mainly for PHY/RF, but definitely there would be lot ofchallenges in higher layer implementation as well (e.g., MAC for L1 scheduling). Handling multiple simultaneous receiver and transmitter chains Challenging radio environment in terms of intermodulation and cross-modulation within the UE device. Simultaneous transmit or receive with mandatory MIMO support add significantly to the challenge of antenna design.12. CONCLUSIONCarrier Aggregation - the ability to combine multiple carriers scattered across the spectrum - has beenidentified as one of the most crucial aspects in the evolution towards LTE-Advanced, and has alsobeen recognized as presenting a major challenge to the design of user equipment and base stations.13. REFERENCES[1] 3GPP TS 36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved UniversalTerrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 10)[2] 3GPP TS 36.331 Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control;Protocol specification (Release 10)[3] 3GPP TS 36.321 Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control(MAC) protocol specification (Release 10)[4] 3GPP TS 36.213 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures(Release 10)[5] Jeanette Wannstrom, “Carrier Aggregation explained”, for 3GPP (Submission May 2012)[6] 4G LTE Carrier Aggregation :: Radio-Electronics.com[7] Martha Zemede, “LTE-Advanced Design and Test Challenges – Carrier Aggregation”, AgilentTechnologies, Webcast Aug 2012. 8

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