Round Trip Time Estimation Utilizing Cyclic Shift of Uplink Reference Signal
Rajeev Gangula, Tommaso Melodia, Rakesh Mundlamuri, Florian Kaltenberger
TL;DR
The paper addresses the challenge of RTT estimation in 5G NR by exploiting a Zadoff-Chu-based uplink reference signal (URS) and its cyclic-shift property to obtain an absolute UL CIR and hence RTT at the gNB, circumventing the limitations imposed by timing advance. The method introduces URS alongside a protocol-stack enhancement, and validates the approach with over-the-air experiments on an OpenAirInterface testbed, showing reduced overhead and latency compared to prior schemes. Key contributions include a detailed URS/UFRS design, a clear cyclic-shift RTT mechanism, and extensions for larger timing advances and multi-user scenarios. The work demonstrates practical viability for accurate RTT-based positioning and sensing, with stronger performance in low-SNR conditions, suggesting applicability to future 6G positioning and sensing systems.
Abstract
In the context of fifth-generation new radio (5G NR) technology, it is not possible to directly obtain an absolute uplink (UL) channel impulse response (CIR) at the base station (gNB) from a user equipment (UE). The UL CIR obtained through the sounding reference signal (SRS) is always time-shifted by the timing advance (TA) applied at the UE. The TA is crucial for maintaining UL synchronization, and transmitting SRS without applying the TA will result in interference. In this work, we propose a new method to obtain absolute UL CIR from a UE and then use it to estimate the round trip time (RTT) at the gNB. This method requires enhancing the current 5G protocol stack with a new Zadoff-Chu (ZC) based wideband uplink reference signal (URS). Capitalizing on the cyclic shift property of the URS sequence, we can obtain the RTT with a significant reduction in overhead and latency compared to existing schemes. The proposed method is experimentally validated using a real-world testbed based on OpenAirInterface (OAI).