high capacity, and low latency. Second, most of them will be mobile. Therefore, both their network links and topologies will vary with time, some faster than others. Third, the distances between any two adjacent nodes will vary significantly, from hundreds of meters to tens of kilometers. Fourth, the power supplied to any node would be limited. Consequently, as in the case for terrestrial networks, the energy efficiency of each node not only impacts the operation costs, but also the commercial viability 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.
1.3 From Digital to Hybrid Multiple Beamforming
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.
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.
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.
When analog beamforming is performed using analog phase shifters and other equivalent devices, significant cost reductions can be achieved immediately due to the decrease in the number of complete RF chains required to form the beams. However, the number of simultaneously supported data streams or beams in a hybrid array is lower in comparison to a full‐blown digital array. In practice, the actual antenna array design depends on the beamforming capabilities required along with the system’s total complexity and budget considerations, both issues being influenced directly by factors such as the number of steerable beams and costs. Although reducing the number of RF chains also limits the number of data streams, per‐user performance can be designed to come close to that attained with a fully digital beamformer. Owing to the nature of line of sight radio propagation and smaller numbers of users per cell, the hybrid beamforming strategy is definitely the more practical beamforming approach for mm‐wave systems in the near future [4, 11].
Figure 1.5a shows the basic architectures of both transmitting and receiving hybrid arrays. Their schematics illustrate the whole array being divided into many analog subarrays [12]. Each subarray includes N antennas and an RF/IF (intermediate frequency) unit. These components can be shared by different antenna elements in different ways, depending on their actual implementations. For convenience, we have simply denoted an array with M subarrays with N antenna elements in each subarray as an N × M hybrid array. Typically, given the dimension of the whole array, the decision on the size of the subarray, or the selection of N and M, is a trade‐off between the system cost and performance. If N is large, a high antenna gain can be achieved at a lower cost. If N is too large, however, the number of users the array can support would be limited. The distance between corresponding elements in adjacent subarrays is called the subarray spacing. It is determined by the desired multiple beam performance and the allowed physical area of the array. Each subarray is connected to a baseband processor via a digital‐to‐analog convertor (DAC) in the transmitter and an analog‐to‐digital convertor (ADC) in the receiver. The signals from all of the subarrays are interconnected and processed centrally in the baseband processor.
