A phased array is an array of antennas that can have their beam path steered without physically moving the antennas. This is accomplished by changing the phase angle of the beam using a computerized system. The phase angle can be continuously changed from one angle to the next, enabling the signal to sweep an area and look for a response. This response can be a reflection from a plane, reception from a satellite, or analysis of weather patterns. In order to achieve maximum signal gain, phased arrays are made of many smaller antennas since a smaller emitting area correlates to higher power concentration in the beam. As a result, phased arrays are often made of hundreds or thousands of tiny antennas. These small antennas make phased arrays most popular in the higher end RF frequencies as well as the microwave region and the near infrared optical regions since the antennas required to operate in those areas are already very small.
In recent years, phased arrays have started to enter the satcom services sector. OneWeb and SpaceX’s Starlink both planned to use phased array antennas in their user terminals. The MESSENGER spacecraft mission used phased arrays to communicate with Earth ground stations.
For FSO applications, optical phased arrays are used for both transmitting and receiving. For transmitting, a laser source is split into many separate beam paths (each with their own phase controllers), is recombined in the far field, and is controlled by adding relative phase shifts between each beam path. For receiving, the received light is phase shifted and amplitude weighted on a pixel array to create a reception beam cone similar to using a handheld telescope to look at specific parts of the night sky. By adjusting the phase shifts on the pixel arrays, the direction that the signal is received from can be changed.
In 2019, phased arrays were proposed for implementation in high throughput satcom links for their multi-antenna robustness and flexibility over single feed per beam (SFPB) systems. The primary challenges encountered in implementing phased arrays were the complexities of the hardware, the high accuracy required to steer the beams, and the potential losses incurred from transmitting and receiving. Photonics was proposed as a solution particularly for the hardware challenge.