Читать книгу Advanced Antenna Array Engineering for 6G and Beyond Wireless Communications - Richard W. Ziolkowski - Страница 19

Given an antenna array, digital beamforming is the ultimate way to achieve optimal performance. It is the most flexible approach to generating individually steerable and high‐quality multiple beams. With a single antenna array of large enough size and the same set of RF circuits, one can effectively create as many beams as desired by applying different complex weights (amplitude and phase) to each element of the array in the digital domain. More advanced digital beamforming schemes employ algorithms such as eigen‐beamforming to obtain the maximum SINR values [11]. Fully digital beamforming with massive antenna arrays serves as a powerful technology to meet some of the most challenging desired features of future wireless communication networks including capacity, latency, data rates, and security.

A high‐level digital beamformer for reception is shown in Figure 1.4. It consists of an array of antennas, each antenna element being connected with an RF receiver. The RF receiver includes a filter, a low noise amplifier, a down converter, and an analog‐to‐digital converter (ADC). Thus, a signal chain is formed for each antenna.

Figure 1.4 High‐level architecture of a digital beamformer (DBP) for reception.

The signals from all of the signal chains are fed into a digital beamformer (DBF). The DBF can form, in principle, as many beams as required. Theoretically it can realize real‐time beamforming via real‐time signal processing. However, in practice, this approach will generally incur prohibitive costs, including computing resources and hardware expenditures in both the RF circuits and digital devices such as ADCs and field‐programmable gate arrays (FPGAs). In fact, the cost of the RF components is almost independent of the desired bandwidth whereas the cost of digital signal processing is approximately proportional to it in terms of both hardware and computing requirements. While those system costs are extensive, the necessary amount of energy to run the system may be even a higher outlay. The energy consumption of a large scale digital beamformer can easily amount to hundreds and even thousands of watts.

These significant practical issues mean that to achieve all of the desired functionalities of future ultra‐high data rate communication systems, fully digital beamforming using massive antenna arrays is simply unaffordable for most application scenarios. Moreover, it is actually not even acceptable for many base station antennas for 5G with the current state of the art of device technologies [9]. These factors lead to the conclusion that some kind of hybrid system based on both digital and analog beamforming might serve as a good solution to large scale antenna arrays with multiple steerable beams in the foreseeable future.