What is carrier aggregation in 4G LTE networks?

4G LTE (Long Term Evolution) and 5G NR (New Radio) mobile networks employ a wide range of network technologies to improve data rates for end-user devices. The enhancements are implemented at a network level and require updates to the devices so that they can benefit from the enhancements. However, these network improvements are backwards compatible so that customers with older devices can continue to use the services without any disruptions. Carrier aggregation is one of the most crucial enhancements introduced in 4G LTE networks to increase the overall carrier bandwidth. It improves the achievable data rates by allocating bigger carrier bandwidths to user devices. Let’s break this terminology down into ‘carrier’ and ‘aggregation’ in the interest of simplicity. Carrier refers to the frequency channel serving a user device, and ‘aggregation’ indicates that multiple frequency channels or carriers are combined to serve a single user device. In a nutshell:

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 which result in higher data rates. This applies to 4G LTE networks with both 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.

In this post, we will look at carrier aggregation from a 4G perspective. 5G networks also use carrier aggregation, but the terminologies differ slightly. In 5G NR networks, the main serving carrier is called the primary cell, and the other carriers that are part of the aggregation are called secondary cells.

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

The original LTE networks launched as part of 3GPP release 8 in 2009 did not support carrier aggregation. They, however, supported flexible carrier bandwidths to give mobile operators some choice in selecting their carriers. LTE networks employed bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. Later, as part of 3GPP release 10 that specified the LTE-Advanced networks, carrier aggregation was first introduced. LTE-Advanced networks used the same frequency carriers as the original LTE networks to ensure backwards compatibility. However, LTE-Advanced enabled mobile networks to combine multiple carriers into one. As a result, multiple channels could join forces to deliver a much bigger overall carrier bandwidth to user devices. With LTE-Advanced, mobile networks can combine up to five (5) carriers. That way, LTE-Advanced networks can achieve a maximum bandwidth of 20 MHz x 5 = 100 MHZ through carrier aggregation.

The LTE-Advanced Pro networks, as per 3GPP release 13, took carrier aggregation to another level. LTE-Advanced Pro networks support carrier aggregation of up to thirty-two (32) carriers. 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. LTE-Advanced Pro networks can achieve a maximum carrier bandwidth of 640 MHz. Both LTE-Advanced and LTE-Advanced Pro networks use higher-order modulation (256 QAM) and spatial multiplexing (Multiple Input Multiple Output – MIMO), which efficiently utilises the higher bandwidth made possible by carrier aggregation. Have a look at this post for more about MIMO for LTE.

TechnologyNumber of carriersMaximum carrier bandwidth
LTENo support for carrier aggregation20 MHz
LTE-AdvancedUp to 5 carriers100 MHz with carrier aggregation
LTE-Advanced ProUp to 32 carriers 640 MHz with carrier aggregation
Carrier aggregation in 4G LTE networks

What are intra-band,inter-band, contiguous and non-contiguous carrier aggregations?

For carrier aggregation to work, we need at least two carriers that can be combined to achieve a bigger overall bandwidth. This can be done in at least three different ways, as explained below. In LTE, there are several frequency bands that mobile operators in various countries can deploy. Within those bands, there are some sub-bands of a certain range which are called carriers. These 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 as well as different frequency bands. Let us have a look below at the three possible permutations of carrier aggregation.

Intra-band contiguous carrier aggregation

The first way to achieve carrier aggregation is by using two carriers within the same frequency band, as shown in the diagram below. These two carries are next to each other hence the term contiguous.

Intra-band contiguous carrier aggregation

Intra-band non-contiguous carrier aggregation

The second way to implement carrier aggregation is by using two non-adjacent carriers in the same frequency band. Since these carriers are in the same frequency band, the implementation is called intra-band but a non-contiguous one.

Intra-band non-contiguous carrier aggregation

Inter-band non-contiguous carrier aggregation

It is also possible to implement carrier aggregation so that the two carriers are not in the same frequency band. So essentially, we take two carriers from two different frequency bands and combine them. Of course, it goes without saying that they are non-contiguous.

Inter-band non-contiguous carrier aggregation

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