Intra-band non-contiguous carrier aggregation

Description

Intra-band non-contiguous carrier aggregation is a technique introduced in 3GPP Release 10 as part of LTE-Advanced. It enables the aggregation of multiple component carriers (CCs) that are within the same operating band but are not adjacent in frequency. This means there is a frequency gap between the carriers, which can be used by other systems or simply be unused spectrum. For example, an operator might have two 10 MHz carriers in the same band separated by 5 MHz of spectrum used by another operator. Intra-band non-contiguous CA allows these two carriers to be aggregated, effectively providing a 20 MHz channel to the user. This is specified in 3GPP TS 36.101 and TS 36.104 for LTE, and later in TS 38.101 and TS 38.104 for NR.

The key challenge of intra-band non-contiguous CA is that it requires more complex RF front-end design compared to contiguous aggregation. Since the carriers are not adjacent, the UE must be able to transmit and receive on multiple separated frequencies simultaneously, which may require multiple transceivers or a wideband transceiver with sufficient linearity. This can increase power consumption and cost. However, it offers greater flexibility in utilizing fragmented spectrum, which is common in real-world deployments.

In terms of operation, the aggregated carriers are treated as separate cells (PCell and SCells) from the MAC layer perspective, but they share the same band. The base station can schedule data independently on each carrier, and the UE aggregates the data streams. Control signaling is typically on the PCell, while SCells can be activated/deactivated dynamically to save power. The UE indicates its support for specific intra-band non-contiguous CA configurations through its capability signaling.

Intra-band non-contiguous CA is particularly useful in scenarios where an operator has non-contiguous spectrum holdings within a band, such as in refarmed bands or in bands with fragmented allocations. It allows them to achieve higher peak rates without acquiring new contiguous spectrum. In 5G NR, this feature is supported in both FR1 and FR2, with additional complexity due to flexible numerology and wider bandwidths. The 3GPP specifications define numerous band combinations and UE capability sets to ensure interoperability.

The benefits include increased peak throughput, better spectrum utilization, and improved user experience. However, it also introduces challenges such as intermodulation interference and the need for advanced filtering. Despite these challenges, intra-band non-contiguous CA is widely deployed in modern networks, especially in bands with fragmented spectrum. It is a key enabler for achieving the high data rates expected in 4G and 5G.

Purpose & Motivation

Intra-band non-contiguous carrier aggregation was created to address the reality of fragmented spectrum allocations. In many cases, operators do not have large contiguous blocks of spectrum within a band; instead, they have multiple smaller blocks separated by gaps. Before this feature, such fragmented spectrum could not be aggregated, limiting the achievable peak data rates. Intra-band non-contiguous CA solves this by allowing the aggregation of non-adjacent carriers within the same band, thereby enabling higher throughput without requiring new contiguous spectrum. This is particularly important for operators who want to maximize the use of their existing spectrum assets.

The motivation also included the need for flexibility and efficiency. By aggregating non-contiguous carriers, operators can dynamically allocate resources across multiple carriers, improving load balancing and spectral efficiency. It also allows for better coexistence with other systems that may occupy the gaps between carriers. For example, in a band shared with other technologies, an operator can aggregate its carriers while avoiding interference with others.

Historically, intra-band non-contiguous CA was defined in Release 10 alongside contiguous and inter-band CA. It was initially specified for LTE, and later extended to 5G NR in Release 15. The wording change in Release 15 reflects the inclusion of NR, ensuring the term covers both radio interfaces. This evolution mirrors the broader trend in 3GPP to support multiple radio access technologies under a unified framework. Today, intra-band non-contiguous CA is a standard feature in LTE-Advanced Pro and 5G networks, enabling operators to deliver gigabit speeds even with fragmented spectrum.