What Is Carrier Aggregation?

Carrier aggregation is a radio access technique in which two or more separate frequency carriers are combined and used simultaneously by a single mobile device, so that the device sees one wide effective channel built from several narrower ones. Each combined carrier is called a component carrier. The technique was introduced by 3GPP in Release 10, the specification set marketed as LTE-Advanced, to reach wider transmission bandwidths than the 20 MHz maximum of the original LTE design while keeping backward compatibility with devices that understand only a single carrier.

The motivating problem is spectrum fragmentation. Operators rarely hold a single contiguous block wide enough to deliver peak rates, and instead own scattered allocations across bands from 700 MHz to 3.5 GHz and above. Carrier aggregation lets those fragments be used together. In the 3GPP overview of LTE-Advanced, up to five component carriers of 1.4 to 20 MHz each may be aggregated for a total of 100 MHz. In 5G New Radio the limit rises to sixteen component carriers, giving roughly a gigahertz of aggregate bandwidth when millimeter wave carriers are included.

Aggregation Modes and Band Combinations

Three arrangements are defined. Intra-band contiguous aggregation joins adjacent carriers in the same band and is the simplest case for radio frequency design, since a single wideband transceiver chain can cover the whole span. Intra-band non-contiguous aggregation joins separated carriers within one band, and inter-band aggregation combines carriers in different bands entirely. The last two require multiple receive chains, additional filtering, and careful management of intermodulation products that can fall on top of a wanted signal. Because every workable arrangement must be tested and declared, 3GPP maintains an explicit list of permitted band combinations, and a handset supports only those combinations its front end and its declared capability signaling cover.

Scheduling, Control, and the Serving Cell Structure

Aggregation is organized around a primary cell and one or more secondary cells. The primary cell carries the radio resource control connection, handles initial access and re-establishment, and provides the security and mobility anchor, while secondary cells are added, activated, deactivated, and released as traffic demand and channel conditions warrant. Scheduling can be per-carrier, with each component carrier carrying its own control channel, or cross-carrier, with one carrier scheduling transmissions on another. Hybrid ARQ operates independently per carrier, so a retransmission on one does not stall the others, and the receiver reassembles the flows above the medium access control layer. Practical performance analysis, discussed in work on multi-channel access solutions for 5G New Radio, shows that the gain depends heavily on how carriers of different propagation characteristics are paired.

Relationship to Dual Connectivity

Carrier aggregation is often confused with dual connectivity, which also gives a device more than one carrier but does so from two separate network nodes with independent schedulers. Aggregation assumes a single scheduler with tight coordination across carriers, so it requires the carriers to be served from the same baseband unit or from sites with very low latency between them. Dual connectivity tolerates a much looser backhaul and is what makes non-standalone 5G deployment possible, with an LTE master node and an NR secondary node. Measurement practice for the two, as Ericsson has described for network performance analysis, differs accordingly, since throughput must be attributed correctly across nodes and carriers.

Applications

Carrier aggregation is used throughout mobile network deployment and design, including:

• Peak downlink and uplink throughput improvement in LTE-Advanced and 5G NR
• Combining low-band coverage carriers with high-band capacity carriers
• Licensed assisted access and shared spectrum operation
• Fixed wireless access services competing with wired broadband
• Load balancing across carriers in dense urban cells
• Handset front-end module and RF filter design