Wideband Operation in Shared Spectrum

Description

Wideband operation is a concept introduced in 3GPP Release 19 for UEs that support shared spectrum channel access, such as in unlicensed bands (e.g., NR-U). It defines operation within a channel larger than 20 MHz, where the available bandwidth is divided into multiple resource block (RB) sets. To distinguish these RB-sets and comply with regulatory requirements for shared spectrum, intra-cell guard bands may be configured between them. These guard bands serve to isolate transmissions and reduce interference, similar to inter-cell guard bands but within a single cell.

The need for wideband operation arises from the availability of wider channels in shared spectrum, particularly in the 5 GHz and 6 GHz bands, where channel bandwidths can be 40 MHz, 80 MHz, or even 160 MHz. Traditional operation in these bands often used a single 20 MHz channel or carrier aggregation of multiple 20 MHz carriers. However, to improve spectral efficiency and reduce overhead, 3GPP introduced the concept of operating a single wide channel with multiple RB-sets. Each RB-set can be used for independent transmissions, and the intra-cell guard bands ensure that the RB-sets do not interfere with each other, especially when different UEs or services use different RB-sets.

From a physical layer perspective, wideband operation affects the design of the OFDM numerology, resource allocation, and channel access procedures. The intra-cell guard bands are configured by the gNB and signaled to the UE via RRC. The UE must be capable of operating over the entire wide channel, including the guard bands, and must handle the associated RF requirements such as spectrum flatness, EVM, and blocking. The 3GPP specifications 38.101 (UE radio transmission and reception) and 38.521 (UE conformance testing) define the RF performance requirements for wideband operation, ensuring that UEs can coexist with other technologies in the shared spectrum.

Wideband operation also impacts the MAC and RACH procedures. For example, the initial access may occur on a specific RB-set, and the UE may be configured to operate on multiple RB-sets. The gNB scheduler must manage resource allocation across RB-sets, taking into account the guard bands. The concept is particularly relevant for NR-U, where listen-before-talk (LBT) procedures are required. Wideband operation allows for more flexible LBT, as each RB-set can be subject to independent LBT, potentially increasing access opportunities.

In summary, wideband operation is a key enhancement for shared spectrum access, enabling efficient use of wide channels while maintaining coexistence and regulatory compliance. It represents an evolution from earlier carrier aggregation-based approaches, offering a more integrated and spectrally efficient solution.

Purpose & Motivation

Wideband operation was introduced to address the limitations of using multiple 20 MHz carriers in shared spectrum. In earlier releases, such as Rel-16 NR-U, operation in unlicensed bands was primarily based on a single 20 MHz channel or carrier aggregation of multiple 20 MHz carriers. While carrier aggregation can achieve wider bandwidths, it introduces overhead in terms of multiple synchronization signals, control channels, and guard bands between carriers. Moreover, regulatory requirements in some regions allow for wider channels (e.g., 80 MHz or 160 MHz) that can be used more efficiently as a single wide channel.

The motivation for wideband operation is to improve spectral efficiency, reduce latency, and simplify the air interface for shared spectrum. By operating a single wide channel with multiple RB-sets, the network can avoid the overhead of multiple carriers and achieve better resource utilization. The intra-cell guard bands allow for flexible partitioning of the wide channel, enabling different RB-sets to be used for different purposes, such as different numerologies or services, while minimizing interference.

Another key driver is the need to support advanced features like enhanced LBT and flexible numerology in shared spectrum. Wideband operation allows for independent LBT per RB-set, which can increase the chances of channel access in congested environments. It also facilitates the coexistence of NR-U with other technologies like Wi-Fi, as the guard bands and RB-set structure can be designed to align with regulatory emission masks. Overall, wideband operation is a step towards more efficient and flexible use of shared spectrum, paving the way for future spectrum sharing paradigms.