[论文解读] 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 Networks参考文献 11被引用 4

一句话总结

本文提出一种联合频移与相位调制（JFPM）波形，用于背散射标签辅助的车载定位，即使在多普勒频移存在的情况下，也能实现车辆与路边安装标签之间精确的相对距离和角度估计。通过充分利用背散射信道中的最大自由度，该方法在高速情况下仍可实现亚米级定位精度（最低达0.1米），且由于分辨率权衡，低速与高速波形之间的误差差异仅为0.05米。

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.

研究动机与目标

• 通过提升车载定位精度，解决自动驾驶的安全局限性。
• 克服路面上背散射标签接触时间短和维护困难的问题。
• 实现在高速车辆与路边安装的背散射标签之间精确的相对定位。
• 设计一种低成本、可扩展的基于背散射的定位系统，采用无源标签和最少的读取器硬件。
• 通过新颖的波形设计最大化信道自由度，实现高精度定位。

提出的方法

• 设计一种联合频移与相位调制（JFPM）波形，通过调制入射射频波的频率实现距离估计，通过调制相位实现信号区分。
• 将背散射标签部署在路边而非路面，以延长接触时间并提高耐用性。
• 采用两种波形类型：I型用于低速（无多普勒）场景，II型用于高速场景，通过分别处理奇数和偶数扫描实现多普勒频移抑制。
• 对奇数和偶数扫描的拍频信号应用二维傅里叶变换，然后计算峰值坐标差值，以抵消多普勒引起的相位偏移。
• 在读取器上使用最少两个天线来估计相对角度，避免昂贵的阵列处理。
• 推导了视 Line-of-Sight 和非视 Line-of-Sight 条件下的信号模型，包含多普勒频移和相位调制的影响。

实验结果

研究问题

• RQ1 与路面部署相比，背散射标签系统能否通过延长接触时间并降低维护成本，实现可靠的车载定位？
• RQ2 在存在多普勒频移的情况下，如何精确估计高速车辆与路边背散射标签之间的相对距离和角度？
• RQ3 何种波形设计能最大化背散射信道中的自由度，以同时支持多址接入和广播增益？
• RQ4 在高速移动场景中，分辨率与多普勒抑制之间的权衡如何影响定位精度？
• RQ5 在所提出的JFPM波形下，实现精确相对定位所需的最少读取器天线数量是多少？

主要发现

• 所提出的JFPM系统定位误差最低可达0.1米，满足3GPP车载定位要求。
• 随着读取器天线数量的增加，定位误差减小，表明多天线分集带来了多址接入（MAC）和广播（BC）增益。
• 专为高速设计的II型波形相比I型波形误差高出0.05米，这是由于采用半分辨率处理，但在40 m/s速度下仍保持高精度。
• 在15 m/s车速下，经多普勒抑制的II型波形性能优于未抑制的I型波形，表明存在性能交叉点。
• 根据修订的设计准则，为确保II型波形中信号可区分，所需扫描数必须超过天线数的两倍（L > 2N）。