Hybrid beamforming: the new main force of future 5G construction

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Hybrid beamforming: the new main force of future 5G construction [Copy link]

Beamforming is a mature technology used in cellular communications and other applications. Beamforming was originally developed based on various analog signal chain techniques and processes. Generally speaking, beamforming combines antenna array elements together to steer the signal at a controlled angle so that a specific receiver can receive the maximum signal.

Analog beamforming can improve the spatial selectivity and efficiency of transmitters and receivers. However, it is still limited by the next generation of beamforming technology based on digital technology to enhance analog processing. Digital beamforming or hybrid digital/analog beamforming can overcome the limitations of analog beamforming, including:

The antenna element is fed by a single data stream, limiting the data rate;

Not very flexible;

The number of available beams is fixed in the hardware;

Digital beamforming can overcome these limitations and provide greater flexibility because different signals are optimized for each antenna in the digital domain. With digital beamforming, different powers and phases can be allocated to different antennas and different frequency bands. In addition, digital beamforming supports spatial multiplexing, so that different frequency bands (subcarriers) have different directivities. Frequency domain beamforming includes the ability to support different beams for different subcarriers, but the implementation of frequency domain beamforming requires digital beamforming.

In the case of time-domain beamforming, the same beam is applied to the entire frequency carrier. Time-domain beamforming is usually implemented using analog beamforming techniques, although it can also be implemented using digital beamforming. However, there are some technical limitations that have prevented the full adoption of digital beamforming.

Today, two significant concerns with full mmWave digital beamforming are the cost and power requirements of the baseband processor. To mitigate these concerns, full mmWave digital beamforming must be implemented with lower-resolution converters to keep the power consumption of the front end at a manageable level. As a result, the spatial multiplexing gains expected from a fully digital approach can only be achieved at the expense of unacceptable signal degradation.

To make matters worse, today’s communications protocols are designed with the assumption of using analog or hybrid beamforming. This limits the ability to fully exploit the potential benefits of digital beamforming. In the future, the specifications for control and data channel protocols may be modified to enable fully digital beamforming to deliver the required low-latency communications and high quality of service (QoS).

A promising solution to the problems and limitations of all-digital beamforming or all-analog beamforming is hybrid beamforming. Hybrid beamforming uses a combination of baseband digital processing and RF domain analog processing. Several hybrid beamforming solutions are being developed, using different system partitioning approaches between the digital and analog domains and within the domains. The goal is to find the best combination of digital baseband processing and analog RF signal chain processing.

A tactile hybrid beamforming architecture combining digital baseband processing with analog phase shifters in the RF domain

Of the many possible hybrid beamforming architectures, two examples are fully connected and sub-connected. In a fully connected architecture, each RF chain is connected to all antennas. The transmit signal from each digital transceiver passes through a dedicated RF path (with mixers, power amplifiers, phase shifters, etc.). It is summed before connecting to the antenna. This approach offers higher performance, but also comes with higher complexity and power consumption. Using this approach, each transceiver implements full beamforming and is expected to be used in base stations and similar fixed installations.

Another approach, called a sub-link beamforming architecture, is better suited for mobile phones and other applications, such as automotive. In sub-link beamforming, each RF signal chain is connected to only a subset of the available antennas. This makes the sub-link architecture simpler and more energy-efficient, but it is less spectrally efficient and not suitable for large installations where spectral efficiency is important.

Currently, to complement existing analog and hybrid methods, researchers are also developing new technologies for beamforming and millimeter wave transmission. For example, holographic beamforming can also shape the radio pattern of the antenna under software control. You can think of it as a software-defined antenna. Recently developed electronically scanned antenna technology, called metamaterial surface antenna technology, is based on the concept of diffractive metamaterials and uses high birefringence liquid crystals to achieve electronic scanning.

In addition to metamaterial antennas and holographic beamforming, there is also interest in developing metamaterials to make smart, digitally controlled reflective surfaces for 5G and future 6G networks. But using smart reflective surfaces to control the wireless propagation environment is a big challenge. To this end, researchers need to develop a two-dimensional metamaterial array of smart reflective surfaces whose interaction with electromagnetic waves can be controlled, for example by adjusting the change in surface impedance. In 6G, these surfaces can direct wireless signals from transmitters to receivers. The ultimate idea is to describe the behavior of smart reflective surfaces based on wireless signal data and develop a control algorithm to configure the reflective surface to assist wireless communication.

Digital beamforming and related technologies for 5G communications are developing rapidly. These technologies, along with related technologies such as intelligently controlled reflective surfaces, are expected to become important elements in the design of future 5G installations.