Beamforming is used in various applications, including sensor arrays, radar, biomedicine, WiFi, and wireless communications. It is an active approach that combines antenna array elements to direct signals at specific angles and create constructive interference. In contrast, other signals are subjected to destructive interference to maximize signal transmission and/or reception. Beamforming can improve the spatial selectivity and efficiency of transmitters and receivers.
In the case of transmitters, beamforming combined with precoding is used in communications applications to affect maximization of the received signal to specific receivers and antennas while reducing the interference to all other receivers and receiving antennas (for a discussion of precoding, see: “What is precoding and what are the benefits?”). Beamforming is a well-established technique adapted to and extended for use in 5G communications systems.
In the case of sonar, for example, beamforming is used to send pulses of underwater sound from an array where the pulse from each projector in the array is sent at slightly different times (the projector closest to the ship being targeted is last), so that every pulse hits the target ship at the same time, producing the effect of a single strong pulse from a single powerful projector. The same technique can be carried out in the air using loudspeakers or in radar/radio using antennas. Receiver beamforming can also be used with microphones (sound waves) or radar antennas.
Beamforming in wireless communications systems is not new. Legacy LTE uses what’s called single-user multiple-input multiple-output (SU-MIMO) antenna systems to improve performance. In SU-MIMO, both the base station and user equipment have multiple antennas. Multiple data streams are sent simultaneously to the user using the same time and frequency resources, doubling (2×2 MIMO) or quadrupling (4×4 MIMO) the peak data rate available to a single user.
Beamforming is important when implementing SU-MIMO and is critically important when implementing MU-MIMO and mMIMO. In 5G installations, SU-MIMO is replaced by MU-MIMO (multi-user MIMO) and massive MIMO (mMIMO). While SU-MIMO increases the data available to a single user, MU-MIMO and mMIMO (massive MIMO) send multiple data streams to multiple users, increasing the cell site’s total capacity well as the amount of data available to individual users.
mMIMO technology is designed to provide appropriate quality of service (QoS) to large numbers of wireless receivers in high-mobility environments. The base stations have “massive” antenna arrays that can serve many users simultaneously, using the same time and frequency resources. An mMIMO base station array for the 2GHz band can include 200 dual-polarized elements in a half-wavelength-spaced array measuring about 1.5 meters by 0.75 meters.
Multiple antennas are used in beamforming to control the direction of a wave-front by controlling the magnitude and phase of individual antenna signals in the mMIMO array. The individual antennas in an mMIMO array are spaced at least ½ wavelength apart. With analog beamforming, the signal phases of individual antenna elements are adjusted in the RF domain; analog phase shifters are used to steer the antenna array’s signal.
Analog beamforming is the most straightforward technique, with a single-phase adjusted in the analog domain. With analog beamforming, the single RF transceiver’s output is split into several paths, corresponding to the array’s number of antennal. Each signal is put through a phase shifter and amplified before getting to the antenna element. This is the simplest and most cost-effective beamforming implementation since it uses minimal hardware and software overhead.
Analog beamforming significantly improves the coverage of a cell site. But analog beamforming is still limited in performance since only one beam can be formed per set of antenna elements. Digital beamforming, which will be the next article’s topic, enables the optimization in the digital domain of multiple signals for each antenna and provides a further increase in performance and flexibility, with corresponding increases in complexity and cost.
Testing of beamforming performance for 5G is different. In contrast with LTE systems, 5G coverage is not cell site-based; it is beam-based. 5G has no cell-specific reference signal (CRS). 5G uses a synchronization signal (SS) and channel state information (CSI) signal. As a result, new definitions have been defined for quantifying user equipment performance and 5G systems in general.
In exchange for increased complexity and cost, beamforming delivers several advantages, including:
- Boosting beam power in specific directions to support the farthest subscribers
- Reducing beam power for close-in subscribers and reducing interference issues for subscribers nearest the transmitter
- Increased carrier-to-noise (C/N) ratio of the signal resulting in a signal that is more robust in noisy and attenuating channel environments
- Supports immunity against fading and interference
The next article in this series will discuss the capabilities, benefits, and limitations of digital beamforming and hybrid analog/digital beamforming designs.