Uplink Reference Signal Carrier Phase

Description

Uplink Reference Signal Carrier Phase (UL-RSCP) is a measurement performed by the network, specifically by one or more base stations (gNBs) or Location Measurement Units (LMUs), on uplink reference signals transmitted by a User Equipment (UE). It quantifies the phase of the received carrier signal, which is a critical parameter for carrier-phase-based positioning methods. The measurement works by the UE transmitting a known uplink reference signal, such as a Sounding Reference Signal (SRS) configured for positioning. Multiple geographically separated network reception points then capture this signal. Each receiver measures the absolute carrier phase of the incoming signal at its antenna. Because the transmitted signal has a known frequency, the measured phase at each point is directly related to the propagation distance (range) between the UE and that receiver, modulo the wavelength of the carrier. The key to positioning lies in analyzing the phase differences between signals received at different sites or the phase change over time at a single site. Sophisticated algorithms, often involving ambiguity resolution (determining the integer number of wavelengths in the path), are used to translate these phase measurements into precise range estimates. These range estimates are then combined using multilateration techniques to compute the UE's location. The architecture involves coordination between the UE, the serving gNB, and potentially multiple neighboring gNBs or LMUs, managed by location servers like the Location Management Function (LMF). The UL-RSCP measurement is typically reported with high resolution and is used in conjunction with other measurements like UL-RTOA to achieve centimeter-level accuracy in supported scenarios.

Purpose & Motivation

UL-RSCP was introduced to meet the demanding accuracy requirements for new vertical applications and regulatory mandates, such as enhanced 911 (E911) Phase 2 and commercial services like asset tracking, drone navigation, and augmented reality. Traditional positioning methods like Observed Time Difference of Arrival (OTDOA) or Assisted GNSS (A-GNSS) can struggle in challenging radio environments like deep indoors or urban canyons. Carrier phase positioning, a technique long used in high-precision GNSS systems, offers a significant leap in accuracy by utilizing the signal's phase, which provides a much finer granularity of measurement compared to time-based methods. UL-RSCP enables this technique for cellular networks, addressing the limitation of previous uplink positioning which primarily relied on time-of-arrival measurements with lower precision. Its creation in Rel-18 was motivated by the need for network-based positioning solutions that do not rely on GNSS availability and can provide continuous, reliable, and highly accurate location information.