5G Carrier Aggregation explained

Understanding carriers: capacity and coverage

Each frequency layer, known as a carrier, has specific characteristics in terms of coverage (the range around the antenna where signals can still be received) and capacity (bandwidth, data rates, throughput).  

Simply put, carriers in the higher frequency ranges typically provide greater capacity, while carriers in the lower range provide wider or deeper coverage. The coverage is impacted by two effects. Firstly, lower frequencies translate into larger wavelengths, which propagate better, and reach deeper indoors. Secondly, lower frequency bands are typically used in FDD (Frequency Division Duplex) mode, which has higher uplink signal strength compared to TDD (Time Division Duplex) mode, which is almost exclusively used for cellular allocations above 2.5 GHz. The mid-band frequencies, for example, the 3.5 GHz band (or n78 in 3GPP terminology), are known to provide a good combination of capacity and coverage.  

In most markets worldwide, national regulators have awarded spectrum licences to MNOs in the form of carriers in allocated frequency bands. Several operators may have access to carriers in a specific band and there can be several carriers per operator in a band. Consequently, operators may have access to low bandwidth carriers in the low-frequency bands (e.g., as little as 10 MHz of bandwidth in some allocations below 1 GHz). On mid to high frequency bands, such as n78, allocations are likely to be larger, reaching 100 MHz or even more per operator. To get the best 5G performance out of the available spectrum assets, Carrier Aggregation is the key.  

At the cell edge, Carrier Aggregation brings another benefit to the user. To understand that one needs to recap that coverage is typically limited by uplink, not by downlink. That means that in downlink the cell could reach further than in uplink. But for a stable connection one needs both, downlink and uplink. And this is where Carrier Aggregation comes in: it allows the use of downlink for all aggregated cells, while securing the connection stability through a common uplink on the lower aggregated carrier. Especially when aggregating low and mid bands, Carrier Aggregation does not only boost data rates, but also the range in which the user will benefit from it.  It is key to turning the cumulative spectrum bandwidth of all these allocations into higher data rates for end users.

The evolution of Carrier Aggregation from 3G HSPA+, to LTE-Advanced and 5G SA

Carrier Aggregation is an established technology in mobile networks. It was first used in the evolution of 3G to HSPA+, at a time when individual carriers were limited to 5 MHz bandwidth.  

When LTE was superseded by LTE-Advanced, Carrier Aggregation was one of the most important features, enabling data rates that demonstrated the real potential of the mobile internet. In LTE, the maximum carrier bandwidth is limited to 20 MHz and aggregating multiple 20 MHz carriers paved the way towards Gigabit-LTE.  

In 5G, Carrier Aggregation is supported for low and mid-band frequencies below 7 GHz (Frequency Range 1 or FR1) and for high band millimetre wave frequencies above 24 GHz (Frequency Range 2 or FR2). The maximum bandwidth of an individual 5G carrier is 100 MHz. On the low bands, the carrier bandwidth is typically dictated by earlier regulatory decisions reflecting the demands of previous network generations at that point of time.  

The precise spectrum bands available to operators vary depending on market or region. In most markets, more spectrum will be made available for 5G by repurposing frequency bands originally used for 2G, 3G or 4G, or by introducing new frequency bands to meet the increasing demand for bandwidth as the data volumes served by the mobile network keep growing.

Carrier Aggregation in 5G Networks

To accelerate rollout, the initial commercial 5G networks relied on the LTE infrastructure in radio access and the core network, referred to as 5G non-Standalone (5G NSA). 5G NSA allows MNOs to increase the bandwidth available to end users by bundling 4G and 5G carriers through 4G-5G Dual Connectivity. This allowed 5G users to benefit from existing LTE Carrier Aggregation capabilities with up to two 5G carriers aggregated alongside LTE carriers.  

To unlock the full capability of 5G, including ultra-low latency, reliability, and efficiency, MNOs are introducing 5G Standalone (5G SA) with a dedicated 5G Core and highly efficient 5G air interface, without dependency on existing LTE networks.    

The number of specified 5G SA Carrier Aggregation band combinations is increasing with each quarterly revision of the 3GPP specifications, enabling further options to achieve multi-Gigabit 5G data rates. These specifications are important to ensure interoperability between user equipment, such as smartphones, and networks. Between 2021 to 2022, the focus of the specifications for Carrier Aggregation in 5G SA in FR1 shifted from two component carriers to three and four component carriers. As the number of 5G frequency bands exceeds that of LTE-A, the possible Carrier Aggregation band combinations are greatly expanded, offering increased deployment options and flexibility for MNOs in different markets.

Commercial smartphones supporting three component 5G Carrier Aggregation on FR1 are already available and supported by commercially deployed Nokia 5G RAN network software, including combinations of FDD low-bands and TDD mid-bands, optimising both throughput and coverage.   Nokia has demonstrated four component 5G Carrier Aggregation with modem vendors MediaTek and Qualcomm at MWC22 Barcelona. The Nokia four component carrier Carrier Aggregation technology is ready for commercial deployment, providing operators with more flexibility than the three-component carrier equivalent.