What are angle of arrival and angle of departure

In many fields such as wireless communication, radar detection, and indoor positioning, the propagation direction of the signal is crucial. Angle of Arrival (AoA) and Angle of Departure (AoD) are key parameters for describing the direction of signal propagation, providing important basis for applications such as positioning, tracking, and signal processing. The following will introduce the concepts, principles, and applications of angle of arrival and angle of departure in detail.

Concept and principle of angle of arrival (AoA)

The angle of arrival refers to the angle between the signal propagation direction and the reference direction (such as the normal direction of the receiving device) when the signal reaches the receiving device. Its core principle is based on the fact that when the signal propagates in space, due to different propagation paths, it will generate information such as phase difference and amplitude difference when it reaches the antenna or sensor at different positions on the receiving device. By analyzing this information, the angle of arrival of the signal can be calculated.

In engineering measurement, phase difference measurement based on array antenna is a common method to achieve angle of arrival estimation. Taking the angle of arrival positioning technology in the Bluetooth 5.1 core specification as an example, the transmitter sends a special data packet through a single antenna, and the receiver uses an antenna array to receive the signal. Since the distances from each antenna to the transmitter are different, there will be phase differences when the signal reaches different antennas. By calculating these phase differences through algorithms and combining them with the distance between the antennas, the angle of arrival of the signal can be calculated. Phased array radar systems also use similar principles. By measuring the arrival angle differences of the received signals of each array element in real time, millimeter-level resolution of the target direction can be achieved.

The application scenarios of arrival angle are wide. In the field of wireless communications, it can be used for base station positioning, mobile device positioning, etc. For example, in a real-time positioning system (RTLS), by installing a battery-powered low-power Bluetooth transmitter (tag) on the asset, the current location of the asset and the received signal strength are sent to a Bluetooth receiver at a fixed location. Combined with the arrival angle information, the location of the asset can be accurately located. In terms of indoor navigation, a Bluetooth transmitter is installed at a fixed location in the facility, and the transmitter and the received signal strength from each known location are located through a mobile phone application. Combined with the arrival angle information, the current location of the mobile phone can be calculated using trilateration.

Concept and principle of departure angle (AoD)

The departure angle refers to the angle between the signal propagation direction and the reference direction (such as the normal direction of the transmitting device) when the signal is sent from the transmitting device. Its principle is similar to that of the angle of arrival, and it also calculates the angle based on information such as the phase difference generated when the signal propagates in space.

Taking the departure angle positioning technology in the Bluetooth 5.1 core specification as an example, the transmitter is equipped with a switchable antenna array to periodically change the direction of signal transmission. The receiving device (usually a smartphone, etc.) determines the departure angle of the signal by analyzing the phase change of the signal and combining information such as the antenna distance. In the indoor positioning system, the transmitters deployed at various fixed locations throughout the facility will transmit signals to the receiving device, and the receiving device can accurately determine its position in the building based on the departure angle of the signal.

The application scenarios of the departure angle are also unique. In industries such as logistics and warehousing, the departure angle technology can be used to achieve accurate management and navigation of items. For example, in a warehouse, by installing transmitters on shelves and using the departure angle technology, staff can quickly find the location of the required items. At the same time, the departure angle technology can also be used for some dynamic and decentralized consumer applications, such as the search for personal items by smart terminals.

Challenges and solutions faced by the arrival angle and departure angle

The arrival angle and departure angle technologies also face some challenges in the application process. Multipath effect is one of the common problems. During the propagation process, the signal may be reflected and scattered multiple times, resulting in multiple components of the signal reaching the receiving device, which increases the complexity of angle estimation. In addition, noise interference will also affect the accuracy of angle estimation.

A series of solutions can be adopted to address these challenges. For multipath effect, high-resolution algorithms such as MUSIC (Multiple Signal Classification) algorithm and ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm can be used. The MUSIC algorithm estimates the arrival angle through eigenvalue decomposition based on the orthogonality of the signal subspace and the noise subspace; the ESPRIT algorithm uses the rotational invariance of the array structure for angle estimation. These algorithms can distinguish the arrival angles of different paths and improve the accuracy of angle estimation. For noise interference, its impact can be reduced by optimizing the antenna array design and improving the anti-interference ability of the signal processing algorithm. For example, the use of adaptive beamforming technology can dynamically adjust the weights of the antenna array according to the characteristics of the signal and noise, enhance the reception of the signal, and suppress noise interference.

As important parameters describing the direction of signal propagation, the angle of arrival and angle of departure play a key role in wireless communications, radar detection, indoor positioning, etc. By deeply understanding their concepts, principles, and applications, and taking effective solutions to address the challenges faced, we can better utilize these technologies and promote the development and progress of related fields.