Aerial Measurements from Outdoor 2.4GHz 802.15.4 Network

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，我们于本校周边的室外空中试验场开展了实验测量工作。在本次实验中，运行802.15.4协议的物联网（Internet of Things, IoT）节点被部署于地面，以500毫秒的间隔广播报文。我们操控无人机系统（Unmanned Aerial System, UAS）飞越该测试网络以采集数据。本实验共开展9组实验运行，均匀分布于3个测试点位。每组运行包含13个高度档位，单次飞行时长约1小时，总飞行时长累计达9小时。我们使用了6台采用802.15.4协议的Digi WRL-15126 XBee3射频2.4GHz收发器，该收发器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射频模块（而非900MHz或868MHz频段版本），其中4台XBee射频模块作为物联网发射节点，搭载于SparkFun XBee Explorer拓展板，该拓展板搭配USB转串口转换器，并由SparkFun Teensy LC开发板进行控制。外置电池通过自带的USB转串口转换器为Teensy LC供电，并为XBee模块转发供电。所有节点均配置为每500毫秒广播一次报文，单条报文长度为23字节，报文中包含唯一的设备与序列标识符，以及用于模拟传感器数据的随机生成浮点数。每组实验中，发射节点以约11米的间距呈直线部署，节点周边15厘米范围内无遮挡物；每组实验的XBee节点部署位置均为随机选取，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大疆（DJI）经纬Matrice 100，其遥控器通信频段为5.725~5.825GHz，处于本次实验的2.4GHz监测频段之外。我们为该无人机搭载了2台仅开启接收模式的XBee模块，将其安装于无人机底部，其中水平接收机与地面平行，垂直接收机与地面垂直。XBee射频模块上报的报文及其关联的接收信号强度指示器（Received Signal Strength Indicator, RSSI）数据，通过USB连接的机载树莓派2型号B（Raspberry Pi2 Model B）进行存储。每组实验期间，无人机的位置数据通过UART接口连接至树莓派的Matrice 100 GPS模块采集，采样率为50Hz。无人机以约2.2米/秒（5英里/小时）的平均速度，沿直线飞越发射节点群。每组实验的飞行路径与飞行高度均存在差异，因飞行过程为手动操控，且受到不同风速条件的影响。单次飞行在距最近发射节点的双向水平距离均为250~300米，具体距离取决于当地地形。每组实验中，我们分别在13个相对最低发射节点的高度档位飞行：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。