Implementation of “phase shifting”
Due to the direct correlation between the “phase” of each signal and the emission direction and stacking strength of the signal, the “phase shifting” function is a very important functional module in phased array systems. In modern phased array systems, the phase shifting function is usually achieved by a phase shifter circuit.
As the name suggests, a phase shifter is a circuit that realizes the phase change of a signal. Through signal delay, signal superposition, and other methods, the input signal undergoes a phase shift, thereby changing the phase of the input signal.
Generally, there are two types of circuit implementation: passive phase shifting and active phase shifting. The common circuit structures and characteristics of the two phase shifting methods are as follows.
Table: Architecture and characteristics of different phase shifters
Classification of phased array systems
In the classification of phased array systems, there are mainly two types: passive phased array and active phased array.
Figure: Passive phased array system and active phased array system architecture
Both systems can realize antenna array of directional receiving and transmitting. In terms of implementation, the array of passive phased array system is partially realized by passive antenna+phase shifter, and the reception and transmission of signals are realized by central receiver and transmitter. In active phased array radar, each radiator is equipped with an independent active receiving/transmitting component.
In the active phased array system, the radar system is more stable because the power source is placed in front of the antenna array element. And because there are T/R components on each channel, even if a small number of T/R components are damaged, the overall performance will not be significantly affected. Due to the independent operation of each channel, the unit components of the active phased array system can also be grouped to achieve characteristics such as simultaneous tracking of multiple targets.
Although passive phased array systems only have one transmitting and receiving component, which is relatively simple to implement and relatively lower in cost, active phased array systems are flexible and reliable in application, and are more widely used in radar and wireless communication.
Active phased array system architecture
The main function of phased array system implementation is to achieve phase shifting. According to the position of the phase shifter in the system, active phased array systems can be divided into the following three architectures. Respectively:
RF phase shifting architecture
Local oscillator phase shifting architecture
Digital Phase Shift Architecture
The implementation methods and advantages and disadvantages of the three architectures are compared as follows.
Table: System architecture of active phased array
Among the above architectures, the RF phase shifting architecture is currently the most widely used implementation architecture.
Millimeter wave+phased array: complementary advantages and disadvantages, complementing each other
The above discusses two major technologies, millimeter wave and phased array, respectively. Although the two are independent technologies, they are often combined in use to complement each other and achieve complementary advantages:
The characteristic of millimeter wave technology is its large bandwidth, but its path loss is large and the propagation distance is short. By utilizing the beam focusing function of phased array technology, millimeter waves can be directionally transmitted and the transmission distance can be increased.
The advantage of phased array systems is that they can achieve directional signal transmission, but due to the need for dozens or even hundreds of arrays, the circuit area increases. The advantage of small area in millimeter wave circuits can be used to achieve large-scale arrays.
Therefore, the combination of “millimeter wave phased array” complements each other and is invincible in some specific application fields.
Application of Millimeter Wave Phased Array System
5G mobile phone
Millimeter wave phased array technology is not far from us, and many 5G mobile phones are already equipped with this technology.
In October 2020, Apple added millimeter wave support to the North American version of the iPhone 12. The iPhone 12 adopts Qualcomm’s millimeter wave solution, deploying four antenna millimeter wave arrays on the top and side of the phone to achieve millimeter wave signal transmission and reception.
According to data provided by Apple, the iPhone 12 equipped with millimeter wave technology can achieve a peak downlink rate of up to 4Gbps.
Image: iPhone 12 phone equipped with Qualcomm millimeter wave phased array solution (US version)
Vehicle mounted millimeter wave radar
The working principle of on-board millimeter wave radar is to emit millimeter wave electromagnetic wave signals to the detected object and receive reflected waves from the target. By calculating the time difference between the transmission and reception signals, the measured object can be detected.
Figure: Working principle of typical on-board radar
In terms of implementation mode, the vehicle mounted millimeter wave radar also needs to use the millimeter wave phased array technology and the direction of multiple antenna arrays to achieve the accurate shaping of millimeter wave signals and the accurate detection of objects.
The following diagram is one of the implementation schemes for a 24GHz vehicular millimeter wave radar. In the receiving channel, a 4-channel phased array is used for design.
Figure: 24GHz vehicular millimeter wave phased array radar system
satellite communication
Satellite communication is currently a hot topic in wireless communication research, especially in the field of low orbit satellites. Due to its low latency and large bandwidth characteristics, it can be used as a good blind patch for cellular communication.
Although satellite communication has the advantage of not being limited by geographical location, its implementation is not easy. Even for low orbit satellites, their distance from the Earth is in the order of 1000 kilometers, which is basically equivalent to the distance from Beijing to Shanghai. The transmission distance of ordinary ground based cellular base stations is only a few kilometers. It is not easy to establish a signal connection directly within the distance range from the ground to the satellite. It requires high transmission power or a highly directional transmission system.
In addition, the rapid operation of satellites also poses challenges for ground to air connectivity. The time for a low orbit satellite to orbit the Earth is only about 100 minutes. If calculated at a visual angle of 60 degrees, each satellite has only 17 minutes of time within the viewing range. And the satellite is still flying rapidly at a speed of 30000 kilometers per hour. This requires ground stations to have fast scanning characteristics of signal beams.
The beam directionality of the millimeter wave phased array system and the fast scanning characteristics of electronic phase control can be demonstrated in satellite communication. In SpaceX’s star chain system, a phased array system operating at millimeter waves is used.
Figure: Ground stations and low orbit satellite systems used in the star chain system
The Star Link system refers to its ground station as the Starlink Dish, with a diameter of 58.9 centimeters and a appearance similar to a disk. In the disk, 1280 antenna array elements are densely arranged. A millimeter wave phased array system with high pointing and fast scanning is achieved through phase-shifting control and RF transceiver circuits connected to the lower layer, completing satellite connections that move rapidly at speeds of 30000 kilometers per hour beyond 550 kilometers.