multiple cables between the radios and antennas, thus increasing their reliability by reducing part counts and handling, and simplifying their testing and installation. As a result, there has been an increasing need for effective antenna‐in‐package (AiP) solutions. In addition to managing the radiation performance of the antenna elements and arrays, one must consider several issues for AiP designs. These include, for instance, the materials; process selection and control; power and heat management; and new testing techniques. As an example, Figure 1.1 shows a 64‐element AiP system at 28 GHz. It has four flip‐chip‐mounted transceiver ICs that support its dual‐polarized operation [8]. For clarity, the heat sink below the ball‐grid‐array (BGA) interface is not shown.
Figure 1.1 An illustration of a 64‐element antenna‐in‐package (AiP) assembly breakout.
Source: From [8] / with permission of IEEE.
One particular new challenge associated with highly integrated 5G antenna arrays is obtaining accurate antenna beam patterns. Depending on their actual implementation, methods for testing active antennas vary. Current examples include the following [4]:
a) Sample Testing
This approach involves the fabrication of a number of fixed analog beamforming circuits that provide the requisite amplitude and phase excitations to the antenna array to produce the desired beams including narrow beams for user traffic and broad beams for user management. Each circuit produces one specific beam. This allows one to sample each of the desired beam types and steering directions. For practical reasons, it is difficult to perform a comprehensive test of all of the possible beams generated by a large array. Therefore, only those beams of greatest interest are likely to be tested.
b) Element‐by‐Element Testing
The far‐field vectorial pattern of each element, i.e., the amplitude and phase distribution in the far‐field of the array, can be measured with respect to a common reference. Any beamforming pattern can then be synthesized numerically by adding all the element patterns with the corresponding appropriate complex weights. This approach is the most flexible method since all possible patterns can be tested. Nevertheless, one can argue realistically that the synthesized beam patterns may differ from the real ones to a certain extent because all of the actual interactions are not explicitly included.
c) Employ Beam Testers
Beam testers are effectively flexible beamforming networks. By connecting a beam tester to an antenna array, one can test a variety of the beams defined by the beam tester using a traditional method for antenna pattern testing. The 3rd Generation Partnership Project (3GPP), which unites seven telecommunications standard development organizations (ARIB, ATIS, CCSA, ETSI, TSDSI, TTA, and TTC), has defined three Over‐the‐Air (OTA) test methods for MIMO antennas: the direct far‐field (DFF) method using a far‐field chamber, the indirect far‐field (IFF) method using a compact range, and the near‐field to far‐field transform (NFTF) method using a near‐field chamber. All three OTA approaches are conventional methods familiar to antenna engineers.
It must be recognized that when active electronics are added to a radiating aperture to form a MIMO antenna, the antenna ports are now embedded in the system. As a result, it becomes much more difficult to measure the true gain and antenna efficiency. Because a massive MIMO antenna has a large number of antenna elements and its radiating aperture can be excited in many ways to create different beams, both narrow and broad, it is truly difficult to fully test and validate beam performance in terms of conventional figures of merit, e.g., pattern characteristics, beam shapes, beam steering, side lobe levels, and null locations. Testing is further complicated because measurements for both the transmit case and the receive case must be performed to understand the operating characteristics of both RF chains.
To facilitate the manufacturing and adoption of large antenna arrays in 5G and beyond systems, the wireless industry is pushing to increase the level of integration of the system frontend modules (FEM). Figure 1.2 shows the AiP roadmap of the TMY Technology (TMYTEK) company for their 5G mm‐wave products [9]. Each enclosure block represents one particular level of component integration. The industry trend is to integrate the antenna arrays with all of the radio frequency (RF) and intermediate frequency (IF) modules into one package. Characterization of all of the beams produced by such modules is undoubtedly a new challenge for antenna designers.
Figure 1.2 Three levels of AiP implementation by TMYTECH.
Source: From [9] / with permission of TMY Technology Inc.
1.2 6G and Its Antenna Requirements
5G mobile and wireless systems are ground‐based. Consequently, they have coverage requirements similar to earlier generations of terrestrial networks. In contrast, space‐communication networks provide vast coverage for people and vehicles at sea and in the air, as well as in remote and rural areas. They are complementary to terrestrial networks. Clearly, future information networks must seamlessly integrate space networks with terrestrial networks to achieve significant advances beyond 5G. This integrated wireless ecosystem may become one of the most ambitious targets of 6G systems [10]. It is currently envisaged that 6G wireless systems will support truly global wireless communications, anywhere and anytime. An integrated space and terrestrial network (ISTN) is expected to be at the core of beyond 5G communication systems. As a consequence, the development of the technologies to achieve a high‐capacity, yet low‐cost, ISTN is of significant importance to all of the emerging 6G wireless communication systems.
Currently, there are a number of commercial and government spaceborne and airborne platforms that support various applications in communications and sensing. These include geostationary Earth orbit (GEO), medium Earth orbit (MEO), and low Earth orbit (LEO) satellites. As their names indicate, they operate at different altitudes relative to the Earth’s center. Various airborne platforms also operate at different altitudes such as high‐altitude platforms (HAPs), airplanes, and unmanned aerial vehicles (UAVs, otherwise known as drones). It is anticipated that any eventual 6G and beyond mobile wireless communication networks will thus consist of three network layers, namely the space network layer, the airborne network layer, and the terrestrial network layer. An illustration of a potential ISTN architecture is shown in Figure 1.3. Figure 1.3 clearly suggests that there will be a huge number of dynamic nodes constituting the mobile airborne networks, in addition to the dynamic nodes of the ground and space (satellite) networks [10].
Figure 1.3 An illustration of a potential ISTN architecture for 6G and beyond.
Source: From [10] / with permission of IEEE.
Airborne networks have a number of unique characteristics. First, most of their nodes would have
