Wireless Communication Basics
Which is the most appropriate main reason that beamforming with array antennas of many elements is used in 5G millimeter-wave base stations and terminals?
- ABecause the wavelength is short, many elements fit in a small area, and the high gain of a sharp beam makes up for the large propagation loss
- BBecause multipath never occurs in millimeter-wave bands, the beam direction never needs to change once set
- CBecause increasing the number of antenna elements reduces the propagation loss along the path itself
Show answer and explanation
Answer: A. Because the wavelength is short, many elements fit in a small area, and the high gain of a sharp beam makes up for the large propagation loss
Antenna elements are generally spaced about half a wavelength apart, so in millimeter-wave bands with wavelengths of around 1 cm, several tens of elements fit within a few centimeters square. The more elements, the sharper the beam and the higher the gain in its direction, which makes up for the large propagation loss of millimeter-wave bands. Because the beam is narrow, however, its direction must be searched for again and tracked as the terminal moves or the human body blocks it. Even in bands below 6 GHz, massive MIMO with many elements at the base station is used to communicate with many terminals at once through spatial multiplexing.
- A. Correct. Many small elements give high gain that makes up for the loss.
- B. Multipath from reflections and other causes also occurs in millimeter-wave bands, and the beam must track terminal motion and blockage.
- C. The loss along the path is set by distance, frequency and obstacles, not by the number of elements. What more elements raise is the gain in the beam direction.