# Carrier Aggregation

## Summary


Carrier Aggregation (CA) is a feature that allows a mobile device to send and receive packets from multiple carriers (aka frequencies) at the same time. Carrier aggregation as a feature is available in Wi-Fi, 3G HSPA+, LTE-Advanced and 5G networks. The many carriers are controlled by a single access point (in Wi-Fi) or base station (in cellular systems).

The figure shows an example of CA in LTE. LTE has a maximum carrier bandwidth of 20 MHz. Via CA, five carriers are combined to achieve an aggregated bandwidth of 100 MHz. CA brings higher data rate, lower latency, better user experience, and better use of network resources. 

CA is possible in downlink and uplink, symmetric or asymmetric. Carriers can be aggregated across FDD and TDD bands, licensed or unlicensed bands.

## Discussion

### Could you explain intra-band versus inter-band CA?

There are broadly two types of CA: 

    + **Intra-Band CA**: The carriers belong to the same band. Carriers can be contiguous (next to each other) or non-contiguous. CA with contiguous carriers is easier to implement. Operators most likely don't have large contiguous spectrum. Hence, non-contiguous CA is a useful feature.
    + **Inter-Band CA**: The carriers belong to different bands. This is more complex to implement than the intra-band contiguous type. Because cellular operators have licenses to different bands, this has led to the standardization of many inter-band CA combinations to cater to industry needs.

### What technologies use carrier aggregation?

In HSPA+, 8 DL and 2 UL carriers can be aggregated. HSDPA Multiflow enables aggregation across cells controlled from the same RNC. 

In LTE-Advanced, up to five carriers can be aggregated for a maximum bandwidth of 100 MHz. Each carrier can use a different channel bandwidth. In LTE-Advanced Pro (Rel-13), up to 32 carriers can be aggregated for a maximum bandwidth of 640 MHz. Carriers from unlicensed bands could be aggregated. 

In 5G, intra-band and inter-band CA is possible within FR1, within FR2, and across FR1 and FR2. Up to 16 carriers can be aggregated. Maximum aggregated bandwidth is limited to 1 GHz. Each aggregated carrier can be of a different numerology. 

In both LTE and 5G, Supplemental Downlink (SDL) carriers provide additional throughput. This is nothing but CA. On the contrary, Supplemental Uplink (SUL) is not CA. In general, a UE can be assigned more downlink than uplink carriers. 

Wi-Fi's channel bonding is intra-band and contiguous. OFDM symbols from adjacent channels are combined to yield a single channel. Hence, this isn't CA as used in HSPA+/LTE/5G. However, Wi-Fi 7 introduced multi-link operation in which a device can aggregate channels from 2.4/5/6 GHz bands.  


### What are the possible deployment scenarios of CA?

The figure show five possible deployment scenarios with CA using carriers F1 and F2 (assume F2 > F1): 

    + **Scenario 1**: F1 and F2 are in the same band and have similar coverage with co-located cells. This scenario provides higher capacity and more bandwidth to the UE.
    + **Scenario 2**: F2 is in a higher band and hence has smaller coverage. F1 provides coverage while F2 improves the throughput.
    + **Scenario 3**: Cells are co-located but F2 is directed to cover the holes in F1. F1 provides coverage and mobility while F2 improves the throughput.
    + **Scenario 4**: F1 provides macro cell coverage while F2 Remote Radio Heads (RRHs) improve throughput at hot spots. F1 and F2 can be non-contiguous carriers in the same band or inter-band carriers.
    + **Scenario 5**: Similar to scenario 2, but frequency-selective repeaters are used to improve F2 coverage.The above scenarios suggest that the benefits of CA are many: higher throughput, better cell coverage, consistent user experience, higher energy efficiency, and higher ROI for operators.  


### What's the basic terminology used in LTE and 5G NR carrier aggregation?

Each aggregated carrier is called a Component Carrier (CC). One of them is called Primary Component Carrier (PCC) and the rest are called Secondary Component Carriers (SCCs). The respective cells are called PCell and SCells. Both PCell and SCell are considered as serving cells. There's one HARQ entity per serving cell. 

PCC is essential since cell search/selection, system information acquisition and initial random access happen on it. PCC is UE-specific; that is, different UEs can have different carriers as their PCC. In the uplink, only the PCC carries PUCCH (unless two PUCCH groups are configured). Uplink SCCs provide only PUSCH. In the downlink, PCC carries both PDSCH and PDCCH. Downlink SCCs carry PDSCH and optionally PDCCH. 

CA can be used together with Dual Connectivity (DC). A UE may be connected to a Master Cell Group (MCG) and a Secondary Cell Group (SCG). CA can happen in each cell group.  


### How are CA band combinations named?

Figure shows examples of LTE CA. *CA\_1C* operates in band 1 with 101-200 number of aggregated resource blocks for CA class C. The maximum aggregated channel bandwidth is 40 MHz from two CCs. Moreover, *CA\_1C* has two **Bandwidth Combination Sets (BCSs)**. Each BCS allows for a certain combination of channel bandwidth in MHz per CC: (15 + 15, 20 + 20) for BCS0 and (5/10/15 + 20, 20 + 5/10/15/20) for BCS1. 

*CA\_25A-25A* indicates non-contiguous intra-band CA, hence "25A" is repeated. *CA\_1A-5A* indicates inter-band CA involving bands 1 and 5, both applicable to CA class A. 

In 5G NR, CA bandwidth classes are defined separately for FR1 and FR2. We note a few CA NR band combinations:

    + Intra-band Contiguous in FR1: *CA\_n78B* (20 + 50)
    + Intra-band Non-Contiguous in FR1: *CA\_n77(3A)* (3 CCs, 2 BCSs, 1 combination per BCS)
    + Inter-band in FR1: *CA\_n1A-n77A*, *CA\_n1A-n77(3A)* (2 non-contiguous CCs in n77)
    + Inter-band in FR1 and FR2: *CA\_n66-n77-n260* (3 bands), *CA\_n1-n3-n8-n77-n257* (5 bands)Band combinations can be browsed online: LTE CA, NR FR1, NR FR2 and NR FR1-FR2


### Which LTE or 5G NR protocol layers are affected by CA?

The main impact of CA is on MAC and PHY. RLC needs to have larger buffers to handle higher data rates but otherwise RLC is unaffected by CA. RRC signalling is modified. For example, once the UE is connected on the PCell, SCells are set up via RRC signalling. 

MAC distributes the RLC PDUs among the many CCs. Each CC handles one transport block per TTI. Each CC is associated with it's own HARQ entity, though all CCs are handled by a single MAC entity. 


### What are some additional features of LTE and 5G NR CA?

With **cross-carrier scheduling**, the UE monitors a single PDCCH (for example, on the PCC). This will contain the scheduling information for all the CCs. 

Feedback signalling such as HARQ acknowledgements of all CCs are sent on the uplink of the PCell. This suits the asymmetric design of CA in downlink and uplink. However, this may overload the single uplink carrier carrying the signalling. It's therefore possible to configure **two PUCCH groups**. The cell carrying the signalling in the second group may be called Primary Second Cell (PSCell). 

In Rel-16, **unaligned frame boundary** and **Tx switching** are allowed. This provides the UE more transmission opportunities and utilize effectively both its Tx chains. In Rel-17, the PUCCH carrier can be **dynamically selected** to reduce latency. Moreover, HARQ retransmissions can be on a different carrier. 

In LTE Rel-15 and 5G NR Rel-16, **enhanced utilization of Carrier Aggregation (euCA)** was introduced. This allows the UE to perform early measurements so that SCCs can be activated quickly when RRC connection is resumed from an inactive state. 

## Milestones

2009

HSPA+ Rel-8 is released. This introduces a feature called **multicarrier** (aka dual-carrier). Two adjacent 5 GHz carriers can be combined to offer a UE higher data rates. Rel-9 (2010) allows non-adjacent carriers to be combined this way. It also brings dual-carrier feature to uplink. This expands to four DL carriers in Rel-10 (2011) and eight DL carriers in Rel-11 (2012). With eight carriers, a UE is able to obtain about 168 Mbps on 40 MHz of aggregated bandwidth, and 672 Mbps when combined with 4x4 MIMO. 

2011

Rel-10 of LTE-Advanced comes out. This is the first LTE release to introduce CA (intra-band contiguous and inter-band). This standardizes 5 x 20 MHz CA, though in practice it's limited to only two carriers. Rel-11 (2012) of LTE-Advanced introduces support for intra-band non-contiguous CA. Rel-13 (2016) of LTE-Advanced Pro allows 32 CCs that can accommodate unlicensed bands as well. Overall, CA in LTE along with other enhancements (256QAM and 4x4 MIMO) enables *Gigabit LTE*. 

Mar  
2015

Rel-12 of LTE-Advanced reaches functional freeze. This release includes the capability to aggregate carriers across FDD (paired spectrum) and TDD (unpaired spectrum) bands. This benefits operators who have licenses to both FDD and TDD bands. In fact, operators demonstrated FDD+TDD CA at the MWC 2014. 

2021

3GPP starts to focus on the specifications for 5G Standalone CA in FR1 with more than two CCs. Meanwhile, Ericsson claims to support CA in both 5G SA and NSA, with DL 8CC and UL 2CC in mmWave band, 2CC in mid-band, and 3CC in low-band. Qualcomm supports CA via its Snapdragon X65 5G modem. 

Mar  
2022

Rel-17 reaches stage 3 functional freeze. In 5G NR, **PUCCH Carrier Switching** is possible whereby the PUCCH carrier can be dynamically selected to reduce latency. HARQ retransmissions can use a different carrier from the original transmission. 

May  
2022

Nokia and Optus achieve 3 CC CA over a 5G SA network. This uses the bands 2100 MHz (FDD), 2300 MHz (TDD) and 3500 MHz (TDD). They state that their Samsung S22 customers will soon be able to experience this in commercial service. In August, Nokia and BT Networks demonstrate 4 CC CA over a 5G SA network. Four carriers combined are in the bands 2.1 GHz, 2.6 GHz, 3.4 GHz and 3.6 GHz. 

Oct  
2022

It's seen that most LTE networks use 2-band CA combinations. The two most popular combinations are *CA\_3A-7A* (1800 MHz and 2600 MHz) and *CA\_3A-20A* (1800 MHz and 800 MHz), achieving an aggregated bandwidth of 40 MHz. The most popular 3-band combination is *CA\_3A-7A-20A*, achieving an aggregated bandwidth of 60 MHz.