Advanced Antenna Array Engineering for 6G and Beyond Wireless Communications. Richard W. Ziolkowski

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


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of the entire network. Fifth, it is highly desirable for antennas on most airborne platforms to be conformal in order to meet their aerodynamic requirements and to maintain their mechanical integrity.

      All of the noted, desirable ISTN features pose a number of significant and interesting challenges for future 6G antennas and antenna arrays. The antennas, for example, must be compact, conformal, and high‐gain. They must be reliable, lightweight, and low‐cost. The corresponding arrays must provide individually steerable multiple beams; dynamic reconfiguration of their patterns, polarizations, and frequencies to cope with the movement of the platforms; and overall high energy efficiency. The biggest challenge among all of them is arguably the reduction of the overall energy consumption. One promising solution is to employ analog steerable multi‐beam antennas. Hybrid beamforming is another. Since beamforming and beam scanning can be done by antenna reconfiguration through electronic switching or tuning, the energy required is negligible in comparison to employing a full digital beamforming approach.

      There are several ways to form multiple beams from an array. Major schemes can be categorized into digital, analog, and crossover strategies. We begin by describing digital beamforming and a major crossover of much recent excitement, hybrid beamforming.

      1.3.1 Digital Beamforming

      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.

Schematic illustration of 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.

      1.3.2 Hybrid Beamforming

      Hybrid beamforming is a strategy that combines the advantages of both analog and digital beamforming techniques. The motivation for employing hybrid beamformers is now clear. One wants to reduce hardware costs and processing complexities while retaining nearly the optimal performance that is achievable with optimized digital designs.

      The hybrid beamforming approach does not treat every antenna element as a completely independent one. The key concept is to partition a large antenna array into smaller subarrays. This type of array is also known in the 5G literature as an array of subarrays (AOSA) [4]. Each subarray consists of a conventional analog antenna array that forms its beam in the analog domain [12, 13]. The number of sub‐arrays into which the whole array is partitioned determines its degrees of freedom.

Schematic illustration of hybrid antenna arrays. (a) The basic architectures of transmitter and receiver systems. (b) Two types of array configurations for uniform square hybrid arrays: interleaved (upper) and localized (bottom). Each square represents an antenna element and squares with the same color represent antenna elements in the same analog subarray.
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