RF-Chord: Towards Deployable RFID Localization System for Logistics Network
In a typical logistics scenario, packages are discharged from trucks, scanned at an inventory gate, and sorted for warehouse check-in. The RFID-based inventory gate should meet reliability, throughput, and range requirements simultaneously.
Abstract
RFID localization is considered the key enabler of automating the process of inventory tracking and management for high-performance logistics networks. A practical and deployable RFID localization system needs to meet reliability, throughput, and range requirements. This paper presents RF-Chord, the first RFID localization system that simultaneously meets all three requirements. RF-Chord features a multisine-constructed wideband design that can process RF signals with a 200 MHz bandwidth in real-time to facilitate one-shot localization at scale. In addition, multiple SINR enhancement techniques are designed for range extension. On top of that, a kernel-layer near-field localization framework and a multipath-suppression algorithm are proposed to reduce the 99th long-tail errors. Our empirical results show that RF-Chord can localize up to 180 tags 6 m away from a reader within 1 second and with 99th long-tail error of 0.786 m, achieving a 0% miss reading rate and ~0.01% cross-reading rate in the warehouse and fresh food delivery store deployment.
System Overview
RF-Chord embraces any ISM-band reader as the tag activator. An active sniffer reader emits a low-power (-15 dBm) wideband multisine waveform to pick up tag responses over a wide frequency band. The RF frontend and FPGA hardware receive tag responses from 8 antennas and 16 frequencies simultaneously. After one-shot tag inventory, RF-Chord obtains information from both frequency and spatial domains for robust localization in multipath-rich environments. A kernel-layer-based near-field localization algorithm then suppresses the multipath effect and estimates the tag location.
Key Designs
One-shot Wideband with Multisine Wave
RF-Chord constructs a wideband backscatter signal using a multisine waveform. Instead of frequency hopping (which takes seconds per tag), RF-Chord transmits multiple narrowband carriers simultaneously across 200 MHz. Digital channelization on the receiver separates the combined wideband signal into parallel narrowband channels, reducing the data rate by 50x without information loss. This enables one-shot channel measurement across 8 antennas and 16 frequencies in ~5 ms.
Single-tone backscatter
Multisine excitation signals
Analog channelization
Digital channelization
SINR Enhancement for Long Range
To follow FCC regulations, the sniffer's emission power must be below -13.3 dBm — 50 dB weaker than commercial readers. RF-Chord achieves 6 m range through multiple SINR enhancement techniques: (1) narrowband channelization reduces thermal noise by 29 dB; (2) high-resolution digital channelization and low crest factor waveform design handle self-interference; (3) full packet matching exploits integration gain across the entire tag response; (4) accurate clock offset mitigation and channel diversity decoding further boost SNR.
Kernel-Layer Near-Field Localization
Even with 200 MHz bandwidth, multipath remains the primary source of long-tail localization errors (distance resolution of 0.75 m is insufficient for all indoor scenarios). RF-Chord proposes a kernel-layer framework: the kernel function characterizes location estimation from a single channel, and layer functions coherently combine multiple channels into a final estimate. This framework supports choosing different kernel and layer functions for various deployment scenarios — for example, incorporating prior knowledge about the region of interest to enhance the direct path and suppress multipath.
Results
RF-Chord is evaluated at 384 locations with over 20k tag responses. Key results:
- Throughput: Localizes up to 180 tags/s (1000x faster than previous sniffer-based wideband systems)
- Range: Operates at up to 6 m with no obvious throughput or reliability loss
- Reliability: 99th-percentile localization error of 0.786 m
- Deployment: 0% miss-reading rate and ~0.01% cross-reading rate in real-world logistics
Localization Performance
Bandwidth impact
Antenna number impact
Algorithm impact
Increasing bandwidth from 50 MHz to 200 MHz reduces the 99th-percentile error from 2.398 m to 0.786 m. More antennas consistently improve long-tail performance. The multipath-suppression algorithm further reduces long-tail errors by incorporating prior information, while median performance remains stable at ~0.14 m.
Real-World Deployment
RF-Chord is deployed in a warehouse dock door and a fresh food delivery store scanning gate. Over 10,000 tags are attached to various items. RF-Chord achieves 100% tag reading accuracy (0% miss-reading) with only 0.0025%–0.0154% cross-reading rate, significantly outperforming commercial solutions like Impinj xSpan (~6% miss-reading, ~2% cross-reading).
Warehouse dock door
Fresh food delivery store
Hardware
Custom PCB with ADRV9009 RF transceiver and XCKU060 FPGA
Evaluation setup with 8-antenna array
Open Source
We open-sourced RF-Chord's hardware design, evaluation dataset, and example localization scripts:
- Hardware design (Google Drive) — Custom PCB schematics and antenna array design
- Evaluation dataset (Google Drive) — ~20k wideband RFID channel measurements at 384 locations
The dataset is collected in the Geek Lab at Peking University, in an office environment with multiple metal reflectors. The evaluation area is 6 x 3.2 m, divided into 20 cm grids. Five tags are mounted on a guide rail at each measurement location. Example localization scripts (hologram_localization_script.m and DPE.m) are provided to reproduce the basic hologram and direct path enhancement algorithms from the paper.
BibTeX
@inproceedings{liang2023rfchord,
title={{RF-Chord}: Towards Deployable {RFID} Localization System for Logistics Network},
author={Liang, Bo and Wang, Purui and Zhao, Renjie and Guo, Heyu and Zhang, Pengyu and Guo, Junchen and Zhu, Shunmin and Liu, Hongqiang Harry and Zhang, Xinyu and Xu, Chenren},
booktitle={20th USENIX Symposium on Networked Systems Design and Implementation (NSDI 23)},
pages={1783--1799},
year={2023}
}