Reference Signal Carrier Phase Difference

Description

Reference Signal Carrier Phase Difference (RSCPD) is a specialized measurement introduced in 3GPP Release 18. Unlike RSCP, which measures an absolute phase, RSCPD quantifies the difference in carrier phase between two distinct reference signals received by the same device. These signals can originate from different transmission points (e.g., two different gNBs or TRPs), from different antenna panels of the same base station, or be the same reference signal type received at two different times. The measurement process involves independently calculating the RSCP for each target signal and then computing their difference, typically modulo 2π to handle phase wrapping.

Architecturally, RSCPD measurement is a higher-layer function built upon the foundational RSCP measurement capability. It is implemented within the UE's physical layer measurement engine and the associated Layer 3 (RRC) reporting protocols. The network configures the measurement via RRC or MAC control elements, specifying the pair of reference signals (identified by their resource element patterns, cell IDs, or timing instances) between which the phase difference must be calculated. This configuration is more complex than for RSCP, as it defines a relationship between two entities. The key components are the dual phase-tracking receivers (or time-multiplexed measurements on a single receiver) and the arithmetic logic unit that performs the subtraction, often including sophisticated algorithms to handle cyclic ambiguity.

In the 5G-Advanced and future 6G network context, RSCPD's role is transformative for precision sensing and relative positioning. By measuring a phase difference, common sources of error that affect both measurements equally—such as imperfections in the UE's local oscillator or certain atmospheric delays—are effectively canceled out. This makes RSCPD an exceptionally stable and accurate metric. Its primary application is in network-based sensing, where the phase difference of signals reflected off an object can be used to estimate the object's velocity and minute changes in position. It also enhances cooperative positioning between devices (device-to-device positioning) by providing a direct measure of their relative phase offset, which correlates directly with their relative distance and orientation, enabling sub-centimeter relative localization for swarm robotics or vehicular platoons.

Purpose & Motivation

RSCPD was created to address the limitations of absolute phase measurements (RSCP) in emerging 5G-Advanced and 6G use cases, particularly joint communication and sensing (JCAS) and ultra-precise relative positioning. While RSCP is excellent for determining an absolute position relative to a network, its accuracy is limited by the stability of the device's own internal clock. Any drift or jitter in the UE's oscillator directly corrupts the RSCP measurement. RSCPD elegantly solves this by being a differential measurement, where the common clock error subtracts out.

The historical context is the 3GPP's exploration of 6G capabilities, where sensing the physical environment becomes a native network function. Traditional radar systems use phase difference measurements (e.g., interferometry) for precise velocity and micro-Doppler analysis. RSCPD brings this radar-grade technique into the cellular domain. Prior to Rel-18, networks could infer similar information by comparing separately reported RSCP values from a UE, but this was inefficient, less accurate due to reporting latency and quantization, and not standardized. RSCPD provides a direct, optimized, and standardized measurement primitive for this purpose.

Furthermore, RSCPD supports the evolution towards extremely dense networks and integrated access and backhaul (IAB). In such environments, knowing the precise relative phase alignment between multiple transmission/reception points is vital for coherent joint transmission. RSCPD measurements from a UE can help the network calibrate and align its own distributed transmitters, solving a key challenge in distributed MIMO and cell-free massive MIMO architectures. It was motivated by the need for a standardized, UE-assisted method to achieve and maintain this ultra-tight phase synchronization across the network.