Contiguous Partial Sensing

Description

Contiguous Partial Sensing (CPS) is a foundational mechanism within the 3GPP sidelink (SL) framework, designed for resource allocation in Mode 2 (distributed scheduling). In Mode 2, UEs autonomously select their transmission resources from a predefined resource pool without centralized scheduling from the network. CPS provides a method for a UE to perform this selection by monitoring, or 'sensing', radio resources. Unlike full sensing, which requires monitoring all subframes or slots within a sensing window, CPS mandates sensing only a contiguous block of resources. This contiguous block is defined relative to the potential transmission opportunity the UE is evaluating.

The operational principle involves the UE selecting a candidate single-subframe resource (SSR) or multi-subframe resource within a selection window. For each such candidate, the UE must sense a specific set of contiguous subframes immediately preceding the candidate resource. The exact location and length of this sensing window are defined by parameters such as the sensing window length (e.g., 1000 subframes in LTE) and the specific contiguous subset required for evaluation. During this sensing period, the UE measures received signal strength (e.g., S-RSSI), decodes Sidelink Control Information (SCI) from other UEs to identify reserved resources, and builds a picture of channel occupancy.

Based on the data collected during the contiguous partial sensing window, the UE applies exclusion rules. It excludes from its candidate set any resources that are indicated as reserved by other UEs' SCI and any resources where the measured signal strength exceeds a configured threshold, indicating high interference or occupancy. From the remaining set of eligible resources, the UE randomly selects one for its transmission. This process balances the need for reliable, collision-avoiding resource selection with the imperative to conserve UE battery life and processing capability. CPS is a key enabler for scalable and efficient direct communication, particularly for vehicular (V2X) and IoT applications where devices may have limited power.

Purpose & Motivation

CPS was introduced to address the significant energy consumption and processing burden associated with full sensing in autonomous sidelink communication. Early sidelink and D2D proposals required UEs to sense the entire resource pool over an extended window (e.g., 1000 ms) prior to every transmission selection. This continuous monitoring of all subframes was prohibitively power-intensive, especially for battery-powered devices like smartphones, wearables, or sensors, and limited the practicality of long-lived direct communication sessions.

The creation of CPS was motivated by the need to enable feasible Mode 2 operation for a wider range of use cases, including the demanding requirements of Vehicle-to-Everything (V2X) communication. V2X scenarios require low-latency, high-reliability packet exchanges, but vehicular UEs, while often connected to a power source, also benefit from reduced complexity. CPS provides a standardized, predictable method for resource selection that significantly reduces the sensing duty cycle. By sensing only a contiguous portion of the timeline, the UE's receiver can be active for a shorter period, leading to direct power savings. This made sidelink communication a more viable technology for power-constrained public safety devices and IoT endpoints, expanding the ecosystem beyond high-power applications.

Key Features

• Reduces UE power consumption by sensing only a contiguous subset of the resource pool
• Defines a deterministic sensing window relative to each candidate transmission resource
• Enables autonomous resource selection (Mode 2) for sidelink communication
• Utilizes Sidelink Control Information (SCI) decoding to identify reserved resources
• Applies S-RSSI measurements and threshold-based exclusion to avoid congested resources
• Provides a foundation for more advanced partial sensing techniques in later releases

Evolution Across Releases

Defining Specifications