The ABCs of 5G LTE Router - Carrier Aggregation (CA)
What’s Carrier Aggregation (CA)?
Carrier Aggregation is an important feature of both 5G<E telecom wireless networks and terminals such as 5G SIM routers and smart phones.
Carrier Aggregation (CA) was introduced in LTE Release 10 and it has been enhanced in the later releases. CA is considered to be the most important feature of LTE-A because it offers higher data rates, improves the DL coverage and allows operators with fragmented spectrum to utilize spectrum resources more effectively. And naturally, Carrier Aggregation feature was introduced in the initial version of Release-15 of 3GPP 5G Specifications. 5G New Radio uses carrier aggregation of multiple Component Carriers (CCs) to achieve high-bandwidth transmission (and hence high data rate).
In LTE, you can aggregate a maximum up to five carriers that is one primary component carrier and four secondary component carriers. But in 5G NR supports aggregation of up to 16 components carriers.
Carrier aggregation is designed to support aggregation of a variety of different arrangements of CCs, including CCs of the same or different bandwidths, adjacent or non-adjacent CCs in the same frequency band, including CCs of the same or different numerologies and CCs in different frequency bands. Each CC can take any of the transmission bandwidths, namely (1.4, 3, 5, 10, 15, 20) MHz for LTE, (5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 90, 100) MHz for 5G FR1, and (50, 100, 200, 400) MHz for FR2 respectively. For example, if five 20 MHz CCs are aggregated, the maximum aggregated bandwidth is 100 MHz.
Figure 1. CA with three 20 MHz carriers. UE is LTE Cat. 9 device and is in theory capable of achieving 450 Mbps in the downlink.
Carrier Aggregation Modes
In Carrier Aggregation, the user has several serving cells. The cells may have different coverage areas or the cell size may differ because of different frequency bands or because the cells are located in different base stations. The UE is connected to one Primary Cell (PCell), which handles the RRC connection and security parameters, for example. PCell is only changed at handover procedure. PCell is served by the Primary Component Carrier (PCC) both in UL and in DL. The other serving cells are referred as Secondary Cells (SCells), which are served by Secondary Component Carriers (SCCs). The SCCs are primarily used for bandwidth expansion and can be added or removed by the network, for example to match the UE’s traffic demand. Figure 2 illustrates different CA modes.
Figure 2. Carrier Aggregation modes. Contiguous intra-band, non-contiguous intra-band and non-contiguous inter-band
Contiguous Intra-band aggregation is a CA mode, in which the carriers are adjacent to each other and are located in the same frequency band.
Another CA mode is non-contiguous intra-band aggregation, which means that the carriers are located in the same frequency band, but are not adjacent. Fragmented spectrum can be exploited by operators with wide overall bandwidth, but with limited single wideband spectrum allocation. In this scenario, the UE has to use separate transceivers for each carrier.
The third CA mode is inter-band aggregation, where the CCs are located in different frequency bands, such as 1800 MHz and 2600 MHz. This scenario is useful for operators, but challenging for the UE, as it is necessary to include a transceiver for each carrier and to ensure the effective use of different frequency bands simultaneously.
Carrier Aggregation Deployment Scenarios
Carrier Aggregation can be deployed to the network in several different manners, and Figure 3 illustrates some of the possible CA deployment scenarios.
Figure 3. CA deployment scenarios. Aggregation is possible between the overlapping Frequency 1 and Frequency 2 cells
The aggregated carriers can be co-located and with nearly the same coverage areas as in case A. It is likely that the band is same for the carriers in case A. The coverage areas may also differ due to different frequency as in case B or, for example use of Remote Radio Head (RRH) as in case C.
In case B, the frequency 1 could be 800 MHz and frequency 2 could be 2600 MHz. Case C provides macro cell coverage in the area and RRHs are used in hot spots to increase throughput. The coverage area can also differ because antenna patterns or antenna tilts as in case D, where antenna of frequency 2 is directed to improve the coverage near the cell edges of frequency 1. CA is possible in the overlapping coverage areas and will increase the peak data rate and improve the data rate near cell edge.
Denoting Band combination
CA_X:
Denotes intra band contiguous CA
e.g CA_10(band)
CA_X-X:
Denotes intra band non-contiguous CA
e.g CA_10-10
CA_X-Y:
Denotes inter band non-contiguous CA
e.g CA_10-20
Precondition for CA
UE can be configured CA only when it is capable to support CA. UE informs its capability to the network during registration procedure in "UE capability information" message to network.
- CA Configuration and CA Bandwidth Class
- Aggregated Transmission Bandwidth Configuration (ATBC): total number of aggregated physical resource blocks (PRB).
- CA bandwidth class: indicates a combination of maximum ATBC and maximum number of CCs. In R10 and R11 three classes are defined:
Class A: ATBC ≤ 100, maximum number of CC = 1
Class B: ATBC ≤ 100, maximum number of CC = 2
Class C: 100 < ATBC ≤ 200, maximum number of CC = 2
- CA configuration: indicates a combination of E-UTRA operating band(s) and CA bandwidth class(es), to exemplify the configuration CA_1C indicates intra-band contiguous CA on E-UTRA operating band 1 and CA bandwidth class C, CA_1A_1A, indicates intra-band non-contiguous CA on band 1 with a one CC on each side of the intra-band gap, finally CA_1A-5B indicates inter-band CA, on operating band 1 with bandwidth class A and operating band 5 with bandwidth class B.
