According to the 3rd Generation Partnership Project (3GPP), carrier aggregation is used in LTE-Advanced in order to increase the bandwidth, and thereby increase the bitrates. Because of this, it’s important to understand what is LTE carrier aggregation testing. Because it is important to keep backward compatibility with R8 and R9 mobiles the aggregation is of R8/R9 carriers. Carrier aggregation can be used for both FDD and TDD.
“Each aggregated carrier is referred to as a component carrier, CC. The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers can be aggregated, hence the maximum aggregated bandwidth is 100 MHz. In FDD the number of aggregated carriers can be different in DL and UL. However, the number of UL component carriers is always equal to or lower than the number of DL component carriers. The individual component carriers can also be of different bandwidths. When TDD is used the number of CCs and the bandwidth of each CC are the same for DL and UL.”
Using contiguous component carriers within the same operating frequency band (as defined for LTE), so called intra-band contiguous is the easiest way to arrange aggregation. However, due to operator frequency allocation scenarios, this might not always be possible. For non-contiguous allocation it could either be intra-band, known as the component carriers, belong to the same operating frequency band, but have gaps in between, or it could be inter-band, in which case the component carriers belong to different operating frequency bands.
“For practical reasons CA is initially specified for only a few operating bands. In R10 three CA bands are defined. For intra-band contiguous CA, R8 operating band 1 (FDD) is defined as CA band CA_1 and band 40 (TDD) are defined as CA_40. For inter-band non-contiguous CA, R8 operating bands 1 and 5 (FDD) are defined as one CA band named CA_1-5. More CA bands will be defined in later releases. 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 – both due to component carrier frequencies but also from power planning – which is useful for heterogeneous network planning. 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. The SCCs are added and removed as required, while the PCC is only changed at handover.”
3GPP goes on to explain that different component carriers can be planned to provide different coverage or a different cell size. When it comes to inter-band carrier aggregation, the component carriers will experience different pathloss, which increases with increasing frequency.
“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 information is necessary, 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 as well as HARQ ACK/NACK per CC must be carried.” [1]
Companies such as Spirent, Agilent, Anritsu, Rohde & Schwarz and others provide testing equipment necessary to test LTE carrier aggregation.
[1] 3GPP, Carrier Aggregation Explained, http://www.3gpp.org/Carrier-Aggregation-explained
