3rd Generation Partnership Project (3GPP) requires LTE-Advanced networks to provide a downlink peak data rate of 1 Gbit/s. However, radio spectrum resources are so scarce that in most cases an operator owns only non-adjacent chunks of the spectrum. Due to the limited bandwidth of a single chunk of the spectrum, the 1 Gbit/s data rate requirement is hard to meet.
To deal with this situation, 3GPP TR 36.913 of Release 10 introduced carrier aggregation (CA) to LTE-Advanced networks, allowing aggregation of contiguous or non-contiguous carriers. CA achieves wider bandwidths (a maximum of 100 MHz) and higher spectral efficiency (especially in spectrum refarming scenarios).
During CA, upper-layer data streams are mapped to individual component carriers (CCs) at the Media Access Control (MAC) layer in LTE-Advanced networks. An eNodeB constructs one (two or more in the case of spatial multiplexing) transport block (TB) in each transmission time interval (TTI) for each CC. Each CC uses its own hybrid automatic repeat request (HARQ) entities and link adaptation mechanism. Therefore, the LTE-Advanced system can inherit single-carrier-based physical layer designs from the LTE system.
why we use the Carrier Aggregation :
Maximized Resource Utilization
A CA-capable UE (referred to as CA UE in this document) can use idle resource blocks (RBs) on up to five CCs to maximize utilization of resources.
Efficient Utilization of Non-contiguous Spectrum Chunks
With CA, an operator's non-contiguous spectrum chunks can be aggregated for efficient utilization.
Better User Experience
With CA enabled, a single UE can reach higher uplink and downlink peak data rates. On a network that serves a number of UEs, CA UEs with activated secondary serving cells (SCells) can use idle resources in their SCells and achieve increased throughput if the network is not overloaded.
Introduction of carrier aggregation influences mainly MAC and the physical layer protocol, but also some new RRC messages are introduced. In order to keep R8/R9 compatibility the protocol changes will be kept to a minimum. Basically each component carrier is treated as an R8 carrier. However some changes are required, such as new RRC messages in order to handle SCC, and MAC must be able to handle scheduling on a number of CCs. Major changes on the physical layer are for example that signaling information about scheduling on CCs must be provided DL as well as HARQ ACK/NACK per CC must be delivered UL and DL,as shown in the below figure:
The uplink and downlink air-interface protocol stack with CA enabled has the following characteristics:
A single radio bearer has only one Packet Data Convergence Protocol (PDCP) entity and one Radio Link Control (RLC) entity. In addition, the number of CCs at the physical layer is invisible to the RLC layer.
User-plane data scheduling at the MAC layer is performed separately for individual CCs.
Each CC has an independent set of transport channels and separate HARQ entities and retransmission processes.
When carrier aggregation is used there are a number of serving cells, one for each component carrier. The coverage of the serving cells may differ, for example due to that CCs on different frequency bands will experience different pathloss, see figure 3. The RRC connection is only handled by one cell, 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. The other component carriers are all referred to as Secondary component carriers (DL and UL SCC), serving the Secondary serving cells, see figure 3. The SCCs are added and removed as required, while the PCC is only changed at handover.
there are a lot of scenarios that we can use the CA in it:
1- intra-eNodeB CO Coverage
2- intra-eNodeB different coverage carriers
3- Intra-eNodeB carriers (one for macro coverage; another for edge coverage)
4-Intra-eNodeB carriers (one provided by the site; another provided by RRHs)
5-Intra-eNodeB carriers (one provided only by the site; another provided by the site and a repeater)
