Aerial Measurements from Outdoor 2.4GHz 802.15.4 Network
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In March 2019, we performed experimental measurements in an outdoor aerial testbed near our university. In our experiments, IoT nodes running the 802.15.4 protocol were placed on the ground, broadcasting messages at 500ms intervals. The UAS was flown over the network to collect data. We conducted nine total experimental runs divided evenly across three locations. Each run comprised thirteen altitudes, spanning roughly one hour of flight time, for a total of nine hours of flights. We used six Digi WRL-15126 XBee3 RF 2.4GHz transceivers utilizing 802.15.4. The advertised outdoor range for each node is 1200m at a power of 8dBm and a receiver sensitivity of -103dBm. We selected the XBee3 2.4GHz (as opposed to 900MHz or 868MHz) radio. Four XBee radios served as IoT transmitters, mounted on a SparkFun XBee Explorer with a USB-to-Serial converter, and controlled by a SparkFun Teensy LC. An external battery supplied power to the Teensy LCs through its own USB-to-Serial converter and forwarded power to the XBee. The nodes were configured to broadcast packets, 23 bytes each, every 500ms. The packet contained unique device and sequence identifiers and a randomly generated floating point number to simulate sensor data. For each trial, transmitters were placed in a line approximately 11m apart with no obstruction within the 15cm vicinity. XBee placement was randomized for each trial, and GPS coordinates were recorded manually from repeated readings using a smartphone GPS. The UAS was a DJI Matrice 100, which communicates with a remote control at 5.725 - 5.825 GHz (outside our monitoring frequency of 2.4GHz). We equipped the UAS with two XBees set to receive only. They were mounted to the bottom of the UAS with the \textit{horizontal receiver} parallel to the ground and the \textit{vertical receiver} perpendicular to the ground. Packets and their associated received signal strength indicator (RSSI) reported by the XBee radios were stored via a USB-attached on-board Raspberry Pi2 Model B. The location of the UAS for the duration of each trial was recorded from the Matrice 100 GPS connected via UART to the Pi, sampling at a rate of 50Hz. The UAS was flown in a straight line approximately over the transmitters at an average speed of about 2.2m/s (5mph). The flight path of each trial and altitude varied, as the flights were manually executed under varying wind conditions. Each flight reached a total horizontal distance of 250-300m in both directions from the nearest transmitter (depending on local topography). For each trial, we flew at 13 altitudes (in relation to the lowest transmitter): 30ft, 40ft, 50ft, 60ft, 70ft, 80ft, 90ft, 100ft, 150ft, 200ft, 250ft, 300ft, and 400ft
2019年3月，我们于校园周边的室外空中试验场开展了实验测量工作。本次实验中，搭载802.15.4协议的物联网（IoT）节点被部署于地面，以500毫秒的间隔广播数据包。无人机系统（UAS）在该网络上空飞行以采集数据。我们共开展9组实验，将其均匀分配至3个试验点位。每组实验涵盖13个飞行高度，单次飞行时长约1小时，总飞行时长共计9小时。本次实验使用6台Digi WRL-15126型XBee3射频2.4GHz收发器，其通信协议为802.15.4。该节点标称的室外通信距离为1200米，对应发射功率8dBm、接收灵敏度-103dBm。我们选用了XBee3 2.4GHz频段无线电（而非900MHz或868MHz频段）。4台XBee无线电设备作为物联网发射节点，被安装于搭载USB转串口转换器的SparkFun XBee Explorer拓展板上，并由SparkFun Teensy LC开发板控制。外置电池通过自身的USB转串口转换器为Teensy LC供电，并为XBee设备提供电力。所有节点被配置为每500毫秒广播一个23字节的数据包。数据包包含唯一的设备标识与序列标识，以及一个用于模拟传感器数据的随机生成浮点数。每组实验中，发射节点被部署为一条直线，节点间距约11米，且节点15厘米范围内无遮挡物。每组实验的XBee设备部署位置均随机选取，GPS坐标通过智能手机GPS多次读取后手动记录。本次实验使用的无人机系统为DJI Matrice 100，其与遥控器的通信频段为5.725~5.825GHz（处于本次实验监测的2.4GHz频段之外）。我们为该无人机系统加装了2台仅支持接收功能的XBee设备。这两台设备被安装于无人机底部，其中**水平接收器**与地面平行，**垂直接收器**与地面垂直。XBee无线电设备上报的数据包及其对应的接收信号强度指示器（RSSI）数据，通过挂载于机载的树莓派2 Model B（Raspberry Pi2 Model B）的USB接口进行存储。每组实验期间，无人机系统的位置通过DJI Matrice 100的GPS模块记录，该模块通过通用异步收发传输器（UART）与树莓派连接，采样频率为50Hz。无人机系统沿直线飞行，大致覆盖发射节点区域，平均飞行速度约为2.2米/秒（5英里/小时）。由于飞行过程为手动操控且受不同风力条件影响，每组实验的飞行路径与飞行高度均存在差异。单次飞行在距离最近发射节点的两个方向上的总水平飞行距离为250~300米（具体数值取决于当地地形）。每组实验我们共设置13个飞行高度（以最低发射节点为基准）：30英尺、40英尺、50英尺、60英尺、70英尺、80英尺、90英尺、100英尺、150英尺、200英尺、250英尺、300英尺及400英尺。
