What is carrier aggregation in 4G LTE Advanced networks?

4G LTE networks use various technologies to increase the download and upload speeds for our mobile phones. Carrier Aggregation or CA is one of the most fundamental techniques used in 4G networks. Carrier Aggregation was initially introduced in LTE-Advanced networks, and today it is part of LTE-Advanced, LTE-Advanced Pro and 5G New Radio (NR) networks.

Carrier Aggregation (CA) in 4G LTE networks is a technique that allows them to combine multiple frequency carriers into one to offer larger carrier bandwidths and higher data rates to end-users. LTE-Advanced Pro networks can achieve a maximum carrier bandwidth of 640 MHz by combining 32 carriers.

Why is Carrier Aggregation used in LTE networks?

Carrier Aggregation offers flexibility to a mobile network operator by allowing them to utilise the existing frequency carriers better. It allows the operators to combine multiple frequency carriers (or RF carriers) into one to serve user devices.

4G LTE networks can provide a maximum download speed of up to 3 Gbps with LTE-Advanced Pro and up to 1 Gbps with LTE-Advanced networks. Based on the difference between bit rate and bandwidth, we know that higher bandwidths enable higher bit rates which is what Carrier Aggregation is all about. Carrier Aggregation allows a mobile network to combine multiple frequency carriers to increase the overall bandwidth, which in turn increases the achievable bit rates. Before introducing Carrier Aggregation in 4G LTE networks, the only way to increase the bandwidth was to use channels of higher bandwidths.

The basic concept of Carrier Aggregation in LTE

The original LTE technology was introduced in 2009 as part of 3GPP Release 8. At the time of the original launch, LTE networks did not have Carrier Aggregation but they supported flexible frequency carrier bandwidths. LTE networks support the bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. Carrier Aggregation was backwards compatible in terms of the carriers it supported, which allowed the older LTE user devices (as per 3GPP release 8 and 9) to support CA once it was enabled in the network.

3GPP Release 10 specified the LTE-Advanced enhancement, which was expected to provide peak downlink data rates of up to 1 Gbps and peak uplink data rates of up to 500 Mbps. Carrier Aggregation was defined by 3GPP (Third Generation Partnership Project) as part of the LTE-Advanced specifications. LTE-Advanced networks employ the same carrier bandwidths as the original LTE networks but add Carrier Aggregation to offer bigger overall bandwidths to user devices by combining multiple carriers. The individual RF carriers are called Component Carriers (CC), and they support carrier aggregation in both uplink and downlink. A straightforward example of carrier aggregation is two channels of 20 MHz each that can be combined to make an overall bandwidth of 40 MHz. Component Carriers, however, do not have to be 20 MHz, and other available bandwidths can also be used.

Carrier Aggregation is supported in both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) modes of LTE. For LTE TDD, Carrier Aggregation requires that the uplink and downlink radio subframe configurations be the same for all component carriers (CC).

I have written a dedicated post on FDD and TDD modes of LTE and 5G that can help you understand the concept of radio subframes in LTE TDD.

Carrier aggregation in 4G LTE Advanced and LTE Advanced Pro networks

In LTE-Advanced, as specified by 3GPP release 10, a 4G LTE network can combine up to five (5) radio frequency carriers. With five carriers, LTE-Advanced networks can achieve a maximum bandwidth of 20 MHz x 5 = 100 MHZ through carrier aggregation.

LTE (3GPP Release 8)LTE-Advanced (3GPP Release 10)LTE-Advanced Pro (3GPP Release 13)
No support for Carrier AggregationAggregation of up to five (5) carriersAggregation of up to thirty-two (32) carriers
Maximum total bandwidth = 20 MHzMaximum bandwidth with CA = 100 MHzMaximum bandwidth with CA = 640 MHz
—Table showing carrier aggregation in LTE Advanced and LTE Advanced Pro networks—

LTE-Advanced Pro networks, as specified by 3GPP release 13, support carrier aggregation of up to thirty-two (32) carriers. Through carrier aggregation, LTE-Advanced Pro networks can achieve a maximum carrier bandwidth of 20 MHz x 32 = 640 MHz.

Since LTE-Advanced Pro networks also support the unlicensed frequency spectrum, combining 32 carriers can give a mobile operator additional options for increasing the overall carrier bandwidth and data rates. Both LTE-Advanced and LTE-Advanced Pro networks use higher-order modulation (256 QAM) and spatial multiplexing (Multiple Input Multiple Output – MIMO) to efficiently utilise the higher bandwidth made possible by carrier aggregation. MIMO and Carrier Aggregation are different technologies, but they work together to improve the overall data rates in LTE networks.

Types or scenarios of Carrier Aggregation in LTE

In LTE, there are several frequency bands that mobile operators in various countries can deploy. Within those bands, some sub-bands or carriers are assigned to the radio units within the base station to transmit and receive the mobile signals. Carrier aggregation can be implemented by combining carriers within the same frequency band or different frequency bands.

There are three carrier-aggregation scenarios or types defined by 3GPP: intra-band contiguous that uses adjacent RF carriers within a single band, intra-band non-contiguous that uses non-adjacent carriers within a single band, and inter-band non-contiguous that uses carriers across separate bands.

1 – Intra-band contiguous

The first scenario for carrier aggregation uses two adjacent RF carriers (Component Carriers or CC) within the same frequency band, as shown in the diagram below. These two carries are next to each other, which is why they are called contiguous.

Intra-band contiguous carrier aggregation

2 – Intra-band non-contiguous

The second scenario for carrier aggregation uses two non-adjacent RF carriers (Component Carriers) within a single frequency band. The carriers are intra-band as they exist in the same frequency band, but since they are not next to each other, they are non-contiguous.

Intra-band non-contiguous carrier aggregation

3 – Inter-band non-contiguous

The third carrier aggregation scenario is called inter-band non-contiguous which makes it possible to combine two RF carriers (Component Carriers) that are within two separate frequency bands.

Inter-band non-contiguous carrier aggregation

Cell categorisation in Carrier Aggregation

Since Carrier Aggregation (CA) involves combining multiple frequency carriers, it goes without saying that multiple cells are involved in enabling CA. Each RF carrier (Component Carrier) equates to a serving cell that can be Primary or Secondary. Primary Serving Cell is one with which the user device (UE) establishes an initial connection. Each connection has a single Primary Serving Cell with a Primary Component Carrier. The Secondary Serving Cell provides additional radio resources, and there can be multiple secondary serving cells.

Conclusion

Carrier aggregation is a technique that allows 4G LTE-Advanced and LTE-Advanced Pro networks to combine multiple frequency carriers into one and use the combined bandwidth to serve individual user devices. This way, the end-user devices can get much broader carrier bandwidths, resulting in higher data rates. This applies to 4G LTE networks with FDD and TDD duplex schemes. As a simple example, if a mobile operator has two 5 MHz carriers in a geographical location, then with carrier aggregation, they can combine these two carriers to achieve a total bandwidth of 10 MHz to double the available capacity and data rates.

Here are some helpful downloads

Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:

Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.

Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.

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