[Paper Review] Joint Frequency-and-Phase Modulation for Backscatter-Tag Assisted Vehicular Positioning

Kaifeng Han, Seung‐Woo Ko+3 more|arXiv (Cornell University)|May 17, 2019

Energy Harvesting in Wireless Networks11 references4 citations

TL;DR

This paper proposes a joint frequency-and-phase modulation (JFPM) waveform for backscatter-tag assisted vehicular positioning, enabling accurate relative distance and angle estimation between vehicles and roadside-mounted tags despite Doppler shifts. By exploiting maximum degrees of freedom in the backscatter channel, the method achieves sub-meter positioning accuracy (down to 0.1 m) even at high speeds, with a 0.05 m error difference between low- and high-mobility waveforms due to resolution trade-offs.

ABSTRACT

Autonomous driving (auto-driving) has been becoming a killer technology for next generation vehicles, whereas some fatal accidents grow concerns about its safety. A fundamental function for safer auto-driving is to recognize the vehicles' locations, termed vehicular positioning. The state-of-the-art vehicular positioning is to rely on anchors that are stationary objects whose locations are known, i.e. satellites for GPS and base stations for cellular positioning. It is important for reliable positioning to install anchors densely, helping find enough anchors nearby. For the deployment to be cost-effective, there are some trials to use backscatter tags as alternative anchors by deploying them on a road surface, but its gain is limited by several reasons such as short contact time and difficulties in maintenance. Instead, we propose a new backscatter-tag assisted vehicular positioning system where tags are deployed along a roadside, which enables the extension of contact duration and facilitates the maintenance. On the other hand, there is a location mismatch between the vehicle and the tag, calling for developing a new backscatter transmission to estimate their relative position. To this end, we design a novel waveform called joint frequency-and-phase modulation (JFPM) for backscatter-tag assisted vehicular positioning where a transmit frequency is modulated for the distance estimation assuming that the relevant signal is clearly differentiable from the others while the phase modulation helps the differentiation. The JFPM waveform leads to exploiting the maximum Degree-of-Freedoms (DoFs) of backscatter channel in which multiple-access and broadcasting channels coexist, leading to more accurate positioning verified by extensive simulations.

Motivation & Objective

• To address the safety limitations of autonomous driving by improving vehicular positioning accuracy.
• To overcome the short contact time and maintenance challenges of on-road backscatter tags.
• To enable accurate relative positioning between high-speed vehicles and roadside-mounted backscatter tags.
• To design a low-cost, scalable backscatter-based positioning system using passive tags and minimal reader hardware.
• To achieve high positioning accuracy by maximizing channel degrees of freedom through novel waveform design.

Proposed method

• Designs a joint frequency-and-phase modulation (JFPM) waveform that modulates the incident RF wave's frequency for distance estimation and phase for signal differentiation.
• Deploys backscatter tags along the roadside instead of on the road surface to extend contact duration and improve durability.
• Uses two waveform types: Type I for low mobility (no Doppler) and Type II for high mobility with Doppler shift mitigation via separate processing of odd and even sweeps.
• Applies 2D Fourier transforms to beat signals from odd and even sweeps, then computes peak coordinate differences to cancel Doppler-induced phase shifts.
• Employs a minimum of two antennas at the reader to estimate relative angles, avoiding costly array processing.
• Derives signal models for both line-of-sight and non-line-of-sight conditions, incorporating Doppler shifts and phase modulation effects.

Experimental results

Research questions

• RQ1 Can a backscatter-tag system achieve reliable vehicular positioning with extended contact time and reduced maintenance costs compared to on-road tag deployment?
• RQ2 How can relative distance and angle between a high-speed vehicle and a roadside backscatter tag be accurately estimated despite Doppler shifts?
• RQ3 What waveform design maximizes the degrees of freedom in a backscatter channel to support multiple-access and broadcasting gains simultaneously?
• RQ4 How does the trade-off between resolution and Doppler mitigation affect positioning accuracy in high-mobility scenarios?
• RQ5 What is the minimum number of reader antennas required to achieve accurate relative positioning with the proposed JFPM waveform?

Key findings

• The proposed JFPM system achieves a positioning error as low as 0.1 meters, meeting the 3GPP vehicular positioning requirement.
• Positioning error decreases with increasing number of reader antennas, demonstrating MAC and BC gains from multiple-antenna diversity.
• The Type II waveform, designed for high mobility, shows a 0.05 m higher error than Type I due to half-resolution processing, but maintains accuracy up to 40 m/s.
• At a vehicle speed of 15 m/s, the performance of the Doppler-mitigated Type II waveform surpasses that of the non-mitigated Type I, indicating a performance crossover point.
• The system requires more than twice the number of sweeps than antennas (L > 2N) to ensure signal differentiation in the Type II waveform, as per the revised design criterion.