Indoor positioning technology: AOA and AOD
In view of the problem that satellite positioning cannot be used in indoor environments, there are many different indoor positioning technologies on the market. Common indoor wireless positioning technologies include: Wi-Fi, Bluetooth, infrared, ultra-wideband, RFID, ZigBee, etc. 

Bluetooth is a short-range and low-power wireless transmission technology. After installing an appropriate Bluetooth LAN access point indoors, configure the network into a multi-user-based basic network connection mode, and ensure that the Bluetooth LAN access point is always this micro-connection mode. The main device of the network, so that the user's location information can be obtained by detecting the signal strength.

AOA and AOD ranging principle
Two new and more precise positioning methods are proposed in the Bluetooth 5.1 protocol. One is the angle of arrival (AOA) and the other is the angle of departure (AOD). AOA positioning: That is, the Bluetooth receiver has multiple antennas, and the distance between the transmitting antenna and each receiving antenna is different, so the transmitted signal has a receiving time difference at each receiving antenna, and the phase difference can be calculated.

AOD positioning: It sends a signal through the antenna array through the Bluetooth positioning beacon, and the receiving device receives the signal through a single antenna, and calculates the sending direction of the signal by decoding the received signal. Through this triangulation distance measurement, high-precision real-time indoor positioning can be achieved. It is worth mentioning that, regardless of AOA or AOD positioning, it is necessary to ensure that the environment between the transmitter and receiver is open and unobstructed. When there are obvious obstacles between the two, the signal strength of Bluetooth will decrease.
It will inevitably lead to a very significant drop in the accuracy of positioning.

This direction finding function uses in-phase and quadrature sampling to measure the receiving phase of the antenna. For example, in the method of measuring the angle of arrival, the signal process is to transmit the test data sequentially through all the antennas in the array according to the pre-designed antenna sequence.

The data is transmitted to the Bluetooth protocol stack inside the device through the controller interface of the host, and the algorithm operation in the protocol stack is used to determine which direction one device is in the other device. where on the other device.

Throughout the application, the real-time positioning system device used to track the device will call several Bluetooth receivers installed in fixed locations throughout the scene. This device is generally referred to as a locator. The locator is connected to the data center of the entire system, ie centralized on the server. Put the Bluetooth transmitter on the object that needs to track the position, the transmitter will send positioning signals to the surrounding at a certain period, and the positioning system will roughly calculate the position of each transmitter through each signal. The user can query the specific Bluetooth positioning beacon by accessing the application program of the mobile phone and other application terminals, and the application program can calculate the real-time location of the inquired beacon based on the signal quality received in real time, and display it.

Compared with other indoor positioning technologies, the positioning accuracy of AOA and AOD positioning is more accurate, and using Bluetooth technology, it can be directly adapted to the user's mobile phone, which is more flexible and convenient to use, and is suitable for indoor navigation and other applications.

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