New Radio Sidelink Carrier Aggregation

Description

NR Sidelink Carrier Aggregation (NR SL CA) is a Rel-19 feature that allows a User Equipment (UE) to simultaneously transmit and receive on multiple NR Sidelink component carriers (CCs). Sidelink communication, defined in 3GPP Release 16 and enhanced in later releases, enables direct communication between UEs without traversing the network infrastructure. NR SL CA extends this by aggregating two or more sidelink carriers, analogous to carrier aggregation in the Uu interface, to achieve wider effective bandwidths and higher data rates.

From an architectural perspective, NR SL CA operates within the PC5 interface, which is the direct radio link between UEs. Each component carrier used for sidelink is configured with its own resource pool, and the UE must support the simultaneous transmission and reception on these carriers. The aggregation can be intra-band contiguous, intra-band non-contiguous, or inter-band, depending on the UE's radio frequency capabilities. The UE's capability information, signaled to the network or peer UE, indicates the supported carrier aggregation combinations, including the number of carriers, bandwidths, and MIMO layers per carrier.

The physical layer aspects of NR SL CA are specified in 3GPP TS 38.101 (UE radio transmission and reception) and TS 38.521 (UE conformance specification). These specifications define the RF requirements for aggregated sidelink operation, such as maximum output power, spectrum emission masks, adjacent channel leakage ratio, and reference sensitivity levels for each carrier and for the aggregation. The UE must manage power sharing across multiple carriers while maintaining signal quality and minimizing interference.

At the MAC layer, NR SL CA introduces enhancements to the sidelink HARQ and scheduling procedures. The UE can transmit transport blocks on multiple carriers, and the receiver can aggregate the data across carriers. The scheduling of sidelink resources can be either network-controlled (Mode 1) or autonomous (Mode 2). In Mode 1, the gNB allocates sidelink resources on each carrier; in Mode 2, the UE selects resources from the configured pools based on sensing and reservation. Carrier aggregation increases the complexity of resource selection and collision avoidance, requiring new algorithms for efficient spectrum utilization.

NR SL CA is particularly beneficial for advanced V2X (Vehicle-to-Everything) use cases, such as cooperative perception, sensor data sharing, and platooning, which demand high throughput and low latency. It also enhances public safety communications by providing higher data rates for video streaming and file sharing among first responders. The feature is expected to evolve in Release 20 with further optimizations for power efficiency and multi-carrier scheduling.

Purpose & Motivation

NR SL CA was introduced to address the growing demand for higher data rates and capacity in sidelink communications, especially for advanced V2X and public safety applications. Prior to Rel-19, sidelink operation was limited to a single component carrier, which constrained the achievable throughput and spectral efficiency. With the proliferation of high-bandwidth applications like cooperative autonomous driving, high-resolution sensor sharing, and real-time video, a single carrier became insufficient.

Carrier aggregation, already proven in the Uu interface (network-to-UE), was a natural evolution for sidelink. By aggregating multiple carriers, NR SL CA enables wider effective bandwidths, which directly translates to higher peak data rates and improved reliability through diversity across carriers. It also allows more flexible spectrum utilization, as operators can deploy sidelink across fragmented spectrum bands.

The feature solves the problem of limited bandwidth in dedicated sidelink spectrum, which is often scarce. By aggregating carriers in licensed or unlicensed bands, it provides a scalable solution to meet the throughput requirements of emerging applications. Additionally, NR SL CA improves link reliability by enabling redundant transmissions across carriers, which is crucial for safety-critical V2X communications.