Читать книгу Advanced Antenna Array Engineering for 6G and Beyond Wireless Communications - Richard W. Ziolkowski - Страница 21

There are a number of ways to create steerable antenna beams in an analog manner. These include the use of circuit-type beamformers, reflectors, lenses, and phased arrays. These and other more advanced methods will be presented in later chapters. We review some of the basic concepts here.

The most common analog beamforming antennas are phased arrays. The original technology dates back to the mid‐twentieth century. It remained primarily as a military technology until the 5G era. In a phased array, the same signal is fed to each antenna element. The amplitudes of the elements are weighted according to the desired shape of the beam, i.e., the shape of the radiated pattern, and then phase‐shifters are used to steer the beam emitted by the array into the desired direction. In order to save cost, the current commercial 5G mm‐wave systems employ phased arrays to conduct analog beamforming. Both the base station and the user equipment (UE) use a number of fixed weight settings, or sets of phase‐shifting values, to produce different beams pointed in specific directions. There are only two beams pointed in the same direction at any given point of time, each for the horizontal and vertical polarizations, respectively. Consequently, current base stations steer their beams sequentially in different directions to provide the desired coverage. The system capacity could be significantly improved by introducing multiple beams. However, it remains a major technological challenge to provide sufficient flexibility to achieve multiple beam directions with an analog beamforming system.

Phased arrays are inherently suited for producing single beams. Because one signal is fed to all of its elements, a phased array constitutes only one antenna port per beam. The beam is steered to follow the intended user by controlling the values of its phase shifters. Some sacrifices have to be made to produce individually steerable multiple beams with a phased array. They include partitioning the array aperture for different beams; and, hence, this limits the overall performance of each generated beam. In the following subsections, we present two multiple analog beamforming techniques that are currently popular for cellular systems: Butler matrices, and Luneburg lenses.