Patent Publication Number: US-2023163483-A1

Title: Compact Modular Active-Passive Antenna Systems with Minimized Antenna Blockage

Description:
TECHNICAL FIELD 
     Various embodiments relate generally to antennas, and more particularly to active-passive antennas. 
     BACKGROUND ART 
     Active-Passive Antennas (APAs) (or equally APA systems) are multiband passive antennas which integrate 5G active features. APAs are used, e.g., in 5G base stations. Typically, APAs are antenna systems which integrate (5G) active massive MIMO antennas (i.e., massive antenna arrays or panels integrated with radio transceiver elements to form a single unit) with (4G or lower) passive antennas. The electronics, radio frequency components and chassis are shared between the active and passive antennas of the APA. Such an arrangement provides multiple benefits such as reduced bill of materials, lowered overall weight and reduced overall wind load. Some present APA solutions employ modular structures where the passive and/or active parts of the APA form separate but electrically (and physically) connected modules which may be independently removable and replaceable, even “in the field” (i.e., on site). However, the process of replacing said modules of the APA is often complicated and time consuming as this requires, first, removing all RF connections between the active and passive antenna modules. For example, in some solutions, the passive antenna module is not be detachable as a single part, but must, before detaching, be split into multiple smaller parts. There is a need for APA solutions enabling simple replacement of the active and/or passive antenna module of the APA even in the field while still maintaining the benefit of the compact form factor of known modular APA systems. This goal should be achieved such that the active and passive antenna modules do not cause significant blockage to each other even when beam scanning/forming is carried out. 
     BRIEF DESCRIPTION 
     According to an aspect, there is provided the subject matter of the independent claims. Embodiments are defined in the dependent claims. 
     One or more examples of implementations are set forth in more detail in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
     Some embodiments provide a passive antenna module for an active-passive antenna system and an active-passive antenna system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, some example embodiments will be described with reference to the accompanying drawings, in which 
         FIG.  1    illustrates an example of a communications system to which embodiments may be applied; 
         FIGS.  2 A,  2 B and  3    illustrate active-passive antenna systems according to embodiments; 
         FIG.  4    illustrates a passive antenna module for an active-passive antenna system according to embodiments; 
         FIGS.  5  and  6    illustrate, respectively, active-passive antenna systems employing electromagnetic coupling and waveguiding elements according to embodiments; and 
         FIGS.  7 A,  7 B and  7 C  illustrate, respectively, an example of a unit cell of a second antenna array of an active antenna module, an example of a unit cell of a ground plane layer and capacitive coupling element of a passive antenna module and two unit cells arranged on top of each other. 
     
    
    
     DETAILED DESCRIPTION OF SOME EMBODIMENTS 
     The following embodiments are exemplary. Although the specification may refer to “an”, “one”, or “some” embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. 
     In the following, different exemplifying embodiments will be described using, as an example of an access architecture to which the embodiments may be applied, a radio access architecture based on long term evolution advanced (LTE Advanced, LTE-A) or new radio (NR, 5G), without restricting the embodiments to such an architecture, however. It is obvious for a person skilled in the art that the embodiments may also be applied to other kinds of communications networks having suitable means by adjusting parameters and procedures appropriately. Some examples of other options for suitable systems are the universal mobile telecommunications system (UMTS) radio access network (UTRAN or E-UTRAN), long term evolution (LTE, the same as E-UTRA), wireless local area network (WLAN or WiFi), worldwide interoperability for microwave access (WiMAX), Bluetooth®, personal communications services (PCS), ZigBee®, wideband code division multiple access (WCDMA), systems using ultra-wideband (UWB) technology, sensor networks, mobile ad-hoc networks (MANETs) and Internet Protocol multimedia subsystems (IMS) or any combination thereof. 
       FIG.  1    depicts examples of simplified system architectures only showing some elements and functional entities, all being logical units, whose implementation may differ from what is shown. The connections shown in  FIG.  1    are logical connections; the actual physical connections may be different. It is apparent to a person skilled in the art that the system typically comprises also other functions and structures than those shown in  FIG.  1   . 
     The embodiments are not, however, restricted to the system given as an example but a person skilled in the art may apply the solution to other communication systems provided with necessary properties. 
     The example of  FIG.  1    shows a part of an exemplifying radio access network. 
       FIG.  1    shows user devices  100  and  102  configured to be in a wireless connection on one or more communication channels in a cell with an access node (such as (e/g)NodeB)  104  providing the cell. The physical link from a user device to a (e/g)NodeB is called uplink or reverse link and the physical link from the (e/g)NodeB to the user device is called downlink or forward link. It should be appreciated that (e/g)NodeBs or their functionalities may be implemented by using any node, host, server or access point etc. entity suitable for such a usage. 
     A communications system typically comprises more than one (e/g)NodeB in which case the (e/g)NodeBs may also be configured to communicate with one another over links, wired or wireless, designed for the purpose. These links may be used for signaling purposes. The (e/g)NodeB is a computing device configured to control the radio resources of communication system it is coupled to. The NodeB may also be referred to as a base station, an access point, an access node or any other type of interfacing device including a relay station capable of operating in a wireless environment. The (e/g)NodeB includes or is coupled to transceivers. From the transceivers of the (e/g)NodeB, a connection is provided to an antenna unit that establishes bi-directional radio links to user devices. The antenna unit may comprise a plurality of antennas or antenna elements (possible forming an antenna array). The (e/g)NodeB is further connected to core network  110  (CN or next generation core NGC). Depending on the system, the counterpart on the CN side can be a serving gateway (S-GW, routing and forwarding user data packets), packet data network gateway (P-GW), for providing connectivity of user devices (UEs) to external packet data networks, or mobile management entity (MME), etc. 
     The user device (also called UE, user equipment, user terminal, terminal device, etc.) illustrates one type of an apparatus to which resources on the air interface are allocated and assigned, and thus any feature described herein with a user device may be implemented with a corresponding apparatus, such as a relay node. An example of such a relay node is a layer  3  relay (self-backhauling relay) towards the base station. 
     The user device typically refers to a portable computing device that includes wireless mobile communication devices operating with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: a mobile station (mobile phone), smartphone, personal digital assistant (PDA), handset, device using a wireless modem (alarm or measurement device, etc.), laptop and/or touch screen computer, tablet, game console, notebook, and multimedia device. It should be appreciated that a user device may also be a nearly exclusive uplink only device, of which an example is a camera or video camera loading images or video clips to a network. A user device may also be a device having capability to operate in Internet of Things (loT) network which is a scenario in which objects are provided with the ability to transfer data over a network without requiring human-to-human or human-to-computer interaction. The user device may also utilize cloud. In some applications, a user device may comprise a small portable device with radio parts (such as a watch, earphones or eyeglasses) and the computation is carried out in the cloud. The user device (or in some embodiments a layer  3  relay node) is configured to perform one or more of user equipment functionalities. The user device may also be called a subscriber unit, mobile station, remote terminal, access terminal, user terminal or user equipment (UE) just to mention but a few names or apparatuses. 
     Various techniques described herein may also be applied to a cyber-physical system (CPS) (a system of collaborating computational elements controlling physical entities). CPS may enable the implementation and exploitation of massive amounts of interconnected ICT (information and communications technology) devices (sensors, actuators, processors microcontrollers, etc.) embedded in physical objects at different locations. Mobile cyber physical systems, in which the physical system in question has inherent mobility, are a subcategory of cyber-physical systems. Examples of mobile physical systems include mobile robotics and electronics transported by humans or animals. 
     Additionally, although the apparatuses have been depicted as single entities, different units, processors and/or memory units (not all shown in  FIG.  1   ) may be implemented. 
     5G enables using multiple input-multiple output (MIMO) antennas, many more base stations or nodes than the LTE (a so-called small cell concept), including macro sites operating in co-operation with smaller stations and employing a variety of radio technologies depending on service needs, use cases and/or spectrum available. 5G mobile communications supports a wide range of use cases and related applications including video streaming, augmented reality, different ways of data sharing and various forms of machine type applications (such as (massive) machine-type communications (mMTC), including vehicular safety, different sensors and real-time control. 5G is expected to have multiple radio interfaces, namely below 6 GHz, cmWave and mmWave, and also being integradable with existing legacy radio access technologies, such as the LTE. Integration with the LTE may be implemented, at least in the early phase, as a system, where macro coverage is provided by the LTE and 5G radio interface access comes from small cells by aggregation to the LTE. In other words, 5G is planned to support both inter-RAT operability (such as LTE-5G) and inter-RI operability (inter-radio interface operability, such as below 6 GHz-cmWave, below 6 GHz-cmWave-mmWave). One of the concepts considered to be used in 5G networks is network slicing in which multiple independent and dedicated virtual sub-networks (network instances) may be created within the same infrastructure to run services that have different requirements on latency, reliability, throughput and mobility. 
     The current architecture in LTE networks is fully distributed in the radio and fully centralized in the core network. The low latency applications and services in 5G require to bring the content close to the radio which leads to local break out and multi-access edge computing (MEC). 5G enables analytics and knowledge generation to occur at the source of the data. This approach requires leveraging resources that may not be continuously connected to a network such as laptops, smartphones, tablet computers and sensors. MEC provides a distributed computing environment for application and service hosting. It also has the ability to store and process content in close proximity to cellular subscribers for faster response time. Edge computing covers a wide range of technologies such as wireless sensor networks, mobile data acquisition, mobile signature analysis, cooperative distributed peer-to-peer ad hoc networking and processing also classifiable as local cloud/fog computing and grid/mesh computing, dew computing, mobile edge computing, cloudlet, distributed data storage and retrieval, autonomic self-healing networks, remote cloud services, augmented and virtual reality, data caching, Internet of Things (massive connectivity and/or latency critical), critical communications (autonomous vehicles, traffic safety, real-time analytics, time-critical control, healthcare applications). 
     The communication system is also able to communicate with other networks, such as a public switched telephone network or the Internet  112 , or utilize services provided by them. The communication system may also be able to support the usage of cloud services, for example at least part of core network operations may be carried out as a cloud service (this is depicted in  FIG.  1    by “cloud”  114 ). The communication system may also comprise a central control entity, or a like, providing facilities for networks of different operators to cooperate for example in spectrum sharing. 
     Edge cloud may be brought into radio access network (RAN) by utilizing network function virtualization (NFV) and software defined networking (SDN). Using edge cloud may mean access node operations to be carried out, at least partly, in a server, host or node operationally coupled to a remote radio head or base station comprising radio parts. It is also possible that node operations will be distributed among a plurality of servers, nodes or hosts. Application of cloudRAN architecture enables RAN real time functions being carried out at the RAN side (in a distributed unit, DU  104 ) and non-real time functions being carried out in a centralized manner (in a centralized unit, CU  108 ). 
     It should also be understood that the distribution of labor between core network operations and base station operations may differ from that of the LTE or even be non-existent. Some other technology advancements probably to be used are Big Data and all-IP, which may change the way networks are being constructed and managed. 5G (or new radio, NR) networks are being designed to support multiple hierarchies, where MEC servers can be placed between the core and the base station or nodeB (gNB). It should be appreciated that MEC can be applied in 4G networks as well. 
     5G may also utilize satellite communication to enhance or complement the coverage of 5G service, for example by providing backhauling. Possible use cases are providing service continuity for machine-to-machine (M2M) or Internet of Things (IoT) devices or for passengers on board of vehicles, or ensuring service availability for critical communications, and future railway/maritime/aeronautical communications. Satellite communication may utilize geostationary earth orbit (GEO) satellite systems, but also low earth orbit (LEO) satellite systems, in particular mega-constellations (systems in which hundreds of (nano) satellites are deployed). Each satellite  106  in the mega-constellation may cover several satellite-enabled network entities that create on-ground cells. The on-ground cells may be created through an on-ground relay node  104  or by a gNB located on-ground or in a satellite. 
     It is obvious for a person skilled in the art that the depicted system is only an example of a part of a radio access system and in practice, the system may comprise a plurality of (e/g)NodeBs, the user device may have an access to a plurality of radio cells and the system may comprise also other apparatuses, such as physical layer relay nodes or other network elements, etc. At least one of the (e/g)NodeBs may be a Home(e/g)nodeB. Additionally, in a geographical area of a radio communication system a plurality of different kinds of radio cells as well as a plurality of radio cells may be provided. Radio cells may be macro cells (or umbrella cells) which are large cells, usually having a diameter of up to tens of kilometers, or smaller cells such as micro-, femto- or picocells. The (e/g)NodeBs of  FIG.  1    may provide any kind of these cells. A cellular radio system may be implemented as a multilayer network including several kinds of cells. Typically, in multilayer networks, one access node provides one kind of a cell or cells, and thus a plurality of (e/g)NodeBs are required to provide such a network structure. 
     For fulfilling the need for improving the deployment and performance of communication systems, the concept of “plug-and-play” (e/g)NodeBs has been introduced. Typically, a network which is able to use “plug-and-play” (e/g)Node Bs, includes, in addition to Home (e/g)NodeBs (H(e/g)nodeBs), a home node B gateway, or HNB-GW (not shown in  FIG.  1   ). A HNB Gateway (HNB-GW), which is typically installed within an operator&#39;s network may aggregate traffic from a large number of HNBs back to a core network. 
     In some embodiments, the system illustrated in  FIG.  1    may be a system comprising one or more active-passive antenna (APA) system. Specifically, the access node  104  may comprise an APA system. An APA system may be defined as an antenna system which integrates (5G) active MIMO antenna array with a (4G or lower) passive antenna array (or a singular antenna). An active antenna array is defined generally as an antenna array into which one or more active electronics components (i.e., active circuitry) have been integrated. Specifically, an active antenna array may be defined, here and in the following, as an antenna array to which a radio unit (being a radio transmitter, receiver or transceiver or a part thereof comprising active as well as passive elements) has been integrated. 
     The APA system may specifically be an antenna system which integrates (5G) active massive MIMO antenna array with (4G or lower) passive antenna array (or a singular antenna). The term “massive MIMO antenna array” refers to a MIMO antenna array with a large number of individual antenna elements. In a massive MIMO (mMIMO) system, the number of antenna elements in a MIMO antenna array of an access node may be assumed to be larger than the number of terminal device served by that access node. For example, a massive MIMO antenna array may be defined, here and in the following, as a MIMO antenna array with at least 8, 16 or 32 antenna elements. 
     Some present APA solutions employ modular structures where the passive and/or active parts of the APA form separate but electrically (and physically) connected modules which may be independently removable and replaceable, even “in the field” (i.e., on site). However, as multiple electrical connections typically exist between the active and passive parts or modules of the APA, the process of replacing said modules of the APA is often complicated and time consuming as this requires, first, removing all of electrical connections between the active and passive antenna modules. For example, in some solutions, the passive antenna module is not be detachable as a single part, but must, before detaching, be split into multiple smaller parts. 
     The embodiments seek to provide modular APA systems and modules thereof where the active antenna module (equally called the active module) and the passive antenna module (equally called the passive module) are separate entities not connected electrically (i.e., via RF connectors) for facilitating their removal and replacement. The active antenna module is, in contrast to conventional APA solutions, not specifically designed for integration with the passive antenna module and may, as a consequence, be used also separately from the passive antenna module. The same applies, mutatis mutandis, for the passive antenna module which is formed as a singular module, as opposed to multiple individual modules as in some conventional APA solutions. By eliminating the need for RF connectors between the active and passive antenna modules, the overall APA system is rendered less complex and may be capable of improved performance (e.g., in terms of gain and/or passive intermodulation). Such an APA system according to embodiments should still, however, be compact (e.g., to minimize weight and wind load) and provide good performance. Moreover, said APA system according to embodiments should be preferably configured such that the APA system is able to operate in a satisfactory manner without its performance being significantly deteriorated due to any antenna blockage caused by the passive antenna module to the active antenna module and vice versa. 
       FIGS.  2 A and  2 B  provides a schematic illustration of an APA system  200  according to embodiments with fully separatable and independently operatable passive and active antenna modules  201 ,  211 . Specifically,  FIG.  2 A  illustrates a simplified APA system  200  in a cross-sectional side view while  FIG.  2 B  shows a view from above.  FIG.  2 A  corresponds to a cross section ‘A’ illustrated in  FIG.  2 B . The APA system  200  may form a part of a terminal device such as one of terminal devices  100 ,  102  of  FIG.  1    or of an access node such as an access node  104  of  FIG.  1   . 
     It should be noted that  FIGS.  2 A and  2 B  show a very simplified view where many of the elements of the APA system  200  (e.g., any power distribution elements, a radio unit and radomes) have been omitted. A more detailed view of an exemplary APA system is provided in  FIG.  3    which are to be discussed later. 
     Referring to  FIGS.  2 A and  2 B , the APA system  200  comprises a passive antenna module  201  and an active antenna module  211 . The passive antenna module  201  comprises a first antenna array  204  comprising a plurality of first antenna elements (of which two and four first antenna elements  205  are shown in  FIGS.  2 A and  2 B , respectively) and the active antenna module  211  comprises a second antenna array  213  comprising a plurality of second antenna elements  214 . Said two first antenna elements  205  are specifically arranged on opposing sides of the active antenna module  211 . The second antenna array  213  may be a (5G) active massive MIMO antenna array and the first antenna array  204  may be a (4G or lower) passive antenna array (or even just a singular antenna). 
     The passive antenna module  201  further comprises a first chassis  202  (or equally a first frame), and the active antenna module  211  further comprises a second chassis  212  (or equally a second frame). The first chassis  202  may, in practice, surround the active antenna module  211 , fully or partly, such that an (elongated) opening or cavity is provided in the first chassis  202  for receiving the active antenna module  211 . Said two first antenna elements  205  may be specifically arranged on opposing (elongated) sides of said opening or cavity. The first chassis  202  may be, at least in part, made of metal so as to implement a (planar) metallic ground plane  207 . The first chassis  202  may be detachably attachable or mountable onto the second chassis  212  of the active antenna module  211 . However, no (wired) electrical connection may be provided (or needs to be provided) between the passive and active antenna module  201 ,  211 . In other words, the passive and active antenna module  201 ,  211  may be fully independent radio modules connected to each other (only) mechanically. The first chassis  202  and said mounting action is discussed, in more detail, in connection with  FIG.  3   . 
     In general, the first antenna array  204  may be adapted to operate at a first (operational) frequency band while the second antenna array  213  may be adapted to operate a second (operational) frequency band higher than the first (operational) frequency band. The first frequency band may be a radio frequency band, e.g., within the super high frequency (SHF) band and/or the ultra high frequency (UHF) band and the second frequency band may be a radio frequency band, e.g., within the extremely high frequency (EHF) band and/or any higher frequency band. In some embodiments, the center frequency of the second frequency band may be equal to or larger than the center frequency of the first frequency band times two, three or four. For example, the first antenna array  204  may be adapted to operate below 1 GHz (e.g., at 694-960 MHz frequency band), while the second antenna array  213  may be adapted to operate at 3.3-3.8 GHz frequency band or at 3.3-4.2 GHz frequency band or other frequency band lowest frequency of which is at least three times the highest frequency of the first operational frequency band of the first antenna array  204 . 
     The first antenna array  204  may be specifically a one- or two-dimensional planar array with uniform antenna spacing. The first antenna array  204  is arranged, at least partially, adjacent to the second antenna array  213  of the active antenna module  211 . In general, the first antenna elements  205  of the first antenna array  204  may be arranged adjacent to one (longitudinal) side of the active antenna module  211  or adjacent to two opposing (longitudinal) sides of the active antenna module  211  (longitudinal direction being the vertical or up/down direction in  FIG.  2 B ). As shown in  FIGS.  2 A and  2 B , the first antenna elements  205  extend partially over the active antenna module  211  (or equally over an opening or cavity provided in the passive antenna module  201  or over a metallic grid  221  within said opening or cavity and a two-dimensional array of metallic patches  222  arranged within said metallic grid). The plurality of first antenna elements  205  of the first antenna array  204  may be arranged, at least for the most part, over a (planar) metallic ground plane  207  of the passive antenna module  201  (being different from the ground plane  215  of the active antenna module  211 ). 
     The plurality of first antenna elements  205  of the first antenna array  204  may be any conventional resonant antenna elements used in antenna arrays such as patch or crossed-dipole antennas of any known design. The plurality of first antenna elements  205  may be dual-polarized antenna elements. Preferably, the first antenna element(s)  205  should be designed such that the antenna blockage caused by them to the second antenna array  213  is minimized. This may be achieved, in general, by minimizing the metallic or metallized (or in general electrically conductive) surface area of the first antenna element(s)  205 . For example, a crossed-dipole-type or patch-type antenna design may be used for the first antenna elements  205 . In an embodiment, the one or more first antenna elements  205  are crossed-dipole type antenna elements with one or more slots in each dipole arm for minimizing blockage caused to the second antenna array  213 . The first antenna element(s)  205  may be, for example, microstrip antennas (without a ground plane), i.e., printed circuit board (PCB)-based printed antennas, or antennas formed of separate (thin) metal sheets. Said first antenna element(s)  205  may be specifically omnidirectional and/or dual-polarized antenna elements. The first antenna element(s)  205  may be made, at least partially, of a metal or an alloy. 
     The plurality of first antenna elements  205  may be separated from this first ground plane  207  by free space (i.e., air) or by a substrate (on which the plurality of first antenna elements  205  may be printed and other side of which may be metallized to form the first ground plane  207 . The first antenna array  204  may be arranged, at least in some embodiments, substantially at a distance of λ/4 from the first chassis  202  acting as its ground plane (or specifically from a first ground plane  207  formed by the first chassis  202 ), where λ is a first wavelength being a wavelength corresponding to a frequency (e.g., a center frequency) within a first operational frequency band of the first antenna array  204 . The first ground plane  207  may serve as a primary ground plane for the first antenna array  204  (as it lies, at least for the most part, directly below the first antenna elements  205 ). The passive antenna module  201  comprises a ground plane layer  220 . 
     Said ground plane layer  220  may be arranged within or over said opening or cavity in the passive antenna module  201 . Said ground plane layer  220  may be fixed to the first chassis  202  of the passive antenna module  201 . Said ground plane layer  220  may be configured such that it is capable of acting as a ground plane for the first antenna array  204  while allowing a (significant) part of the (higher frequency) electromagnetic waves radiated by the second antenna array  213  to pass through it. In other words, the ground plane layer  220  may serve as a secondary ground plane for the first antenna array  204  and be transparent or at least semitransparent at frequencies above a certain pre-defined frequency (being a frequency above the operational frequencies of the first antenna array  204 ) or at least at the second operational frequency band of the second antenna array  213  of the active antenna module  211 . The ground plane layer  220  may be, at least in some embodiments, substantially aligned (vertically) with the first ground plane  207 . In other embodiments, the ground plane layer  220  may be arranged to be at a lower level compared to the first ground plane  207 . The first antenna array  204  may be arranged, at least in some embodiments, substantially at a distance of λ/4 from the ground plane layer  220  acting as its ground plane, where λ is the first wavelength being a wavelength corresponding to a frequency (e.g., a center frequency) within the first operational frequency band of the first antenna array  204 . Thanks to the ground plane layer  220 , the passive antenna module  201  is fully operational even without the active antenna module  211  (i.e., the first antenna array  204  does not have to depend on using the ground plane of the active antenna module  211  as its ground plane). 
     Said ground plane layer  220  comprises, in the illustrated example, a metallic or metallized grid  221  and a two-dimensional array of metallic patches  222  arranged within the metallic grid  221 . Each metallic patch of the two-dimensional array  222  may be located within a particular unit cell of the metallic grid  221 . In general, a unit cell of a grid may be defined as the smallest repeating unit of the grid. In some alternative embodiments, a metallized grid (i.e., a grid made of a non-metallic material but having a metallized (outer) surface) may be employed instead of a metallic grid  221 . The metallic or metallized grid  221  may be electrically connected to the first ground plane  207 . 
     The metallic or metallized grid  221  (equally called a mesh) may be of any type of grid. The metallic or metallized grid  221  may be a regular grid or an irregular grid. For example, the metallic or metallized grid  221  may be a square grid, a rectangular grid, a rhombus grid, a triangular grid or a regular or irregular polygonal grid. The metallic or metallized grid  221  may have a first period along a first direction and a second period along a second direction orthogonal to the first direction. 
     The metallic or metallized grid  221  may have a period or multiple periods along different directions which are electrically small in view of the first antenna array  204  and its operating frequencies. The largest dimension of a unit cell (or the period or the largest period if multiple different periods are definable) of the metallic or metallized grid  221  may be defined such that the metallic or metallized grid  221  is capable of acting as a ground plane for the first antenna array  204 . For example, the largest dimension of a unit cell (or the period or the largest period if multiple different periods are definable) of the metallic or metallized grid  221  may be, e.g., smaller than a second wavelength divided by five, by six, by seven, by eight, by nine, by ten, by eleven or by twelve, where said second wavelength is a (free-space) wavelength corresponding to a frequency (e.g., a center frequency) within a first operational frequency band of the first antenna array  204 . Additionally or alternatively, the largest dimension of a unit cell (or the period or the largest period if multiple different periods are definable) of the metallic or metallized grid  221  may be, e.g., larger than a third wavelength divided by five, four, by three or by two, where said third wavelength is a (free-space) wavelength corresponding to a frequency (e.g., a center frequency) within a second operational frequency band of the second antenna array  213 . The selected period(s) of the metallic or metallized grid  221  may correspond to a compromise between effective ground plane behavior at the first operational frequency band and effective (semi)transparent behavior at the second operational frequency band taking possibly also into account the dimensions of the second antenna elements  214  of the second antenna array  213  (assuming that the second antenna array  213  has the same period as the metallic or metallized grid  221 ). 
     The metallic patches of the two-dimensional array  222  may located within the (unit) cells of the metallic or metallized grid  221  (each or most of the unit cells of the metallic or metallize having within them a metallic patch). The metallic patches may be shaped so that they substantially fill the unit cell of the metallic or metallized grid  221  or at least fill most of the unit cell of the metallic or metallized grid  221 . The metallic patches may be, for example, square patches, rectangular patches, circular patches, elliptical patches or any patches having a shape of a regular or irregular polygon. 
     The ground plane layer  220  (or at least the two-dimensional array of metallic patches  222  thereof) may be implemented, for example, as at least one printed circuit board (not shown in  FIGS.  2 A and  2 B ). 
     As long as the ground plane layer  220  is arranged to be sufficiently near the second antenna array  213  of the active antenna module  211  when the passive and active antenna modules  201 ,  211  are connected (i.e., when the first chassis  202  of the passive antenna module  201  is mounted onto the active antenna module  211 ), the propagation characteristics of the active antenna array  213  (e.g., S11, gain and radiation pattern) remain relatively unchanged and thus the active antenna module  211  is still functional. The ground plane layer  220  may be arranged, e.g., so that it is located at least within an (electrically) small distance from the second antenna array  213  of the active antenna module  211  when the passive and active antenna modules  201 ,  211  are connected. Such an electrically small distance may be, for example, at least equal to or smaller than λ/4, λ/5, λ/6, λ/7, λ/8, λ/9, λ/10, λ/12 or λ/15 (depending on the particular desired performance), where λ is a (free-space) wavelength corresponding to a frequency (e.g., a center frequency) within the second operational frequency band of the second antenna array  213 . 
     In some embodiments, the two-dimensional array of metallic patches  222  may be omitted. In such embodiments, the ground plane layer  220  may comprise only the metallic or metallized grid  221  (implemented, e.g., as at least one PCB or as a separate grid made of metal or made of a non-metallic material having a metallized surface). It is well known that even a simple metallic grid is capable of approximating a ground plane. 
     The plurality of first antenna elements  205  may be fed by feeding means or elements  206  which may form a part of first power distribution means of the passive antenna module  201  for enabling beamforming for the first antenna array  204 . Other elements of the first power distribution means may comprise one or more phase shifters forming a first phase shifter network located, e.g., inside the element  202 . Said feeding elements  206  may act also as supporting elements for the plurality of first antenna elements  205  (e.g., in microstrip line feeding, the PCB(s) may provide support) or alternatively they may be integrated into separate supporting elements. 
     The feeding may be arranged, e.g., with coaxial cables (using baluns) or with microstrip lines. In some embodiments, the passive antenna module  201  may comprise a balun integrated into the power distribution means or forming a part thereof. A balun is an electrical device which converts balanced signals and unbalanced signals and vice versa. Specifically, a balun may be used here for converting an unbalanced signal of a coaxial cable to a balanced signal to be fed to the first antenna element  205  (e.g., a crossed-dipole-type antenna) in transmission and providing opposite operation in reception. The balun may be, for example, a sleeve balun configured to operate at the first frequency band (or at least configured to operate optimally at a frequency within the first frequency band). 
     The second antenna array  213  may be specifically a one- or two-dimensional planar array with uniform antenna spacing. The plurality of second antenna elements  214  of the second antenna array  213  may be arranged over the (planar) ground plane  215 . The plurality of second antenna elements  214  may be separated from the ground plane  215  by free space (i.e., air) or by a substrate (on which the plurality of second antenna elements  214  may be printed and other side of which may be metallized to form the ground plane  215 ). The plurality of second antenna elements  214  may be fed by feeding elements  216  which may form a part of second power distribution means of the active antenna module  211  (other elements being, e.g., inside element  214 ) for enabling beamforming for the second antenna array  213 . Each feeding element  216  may correspond, for example, to a one or more coaxial cables or other transmission lines for feeding a corresponding second antenna element  214  at one or more feed points (with the outer conductor of the coaxial cable being connected to the ground  215 ) or one or more pairs of feed points. The ground plane  215  may be mounted on the second chassis  212  of the active antenna module  211 . 
     All of the plurality of second antenna elements  214  of the second antenna array  213  have the same geometry and dimensions. Said plurality of second antenna elements  214  may be any conventional resonant antenna elements used in (5G) antenna arrays such as patch or crossed-dipole antennas of any known design. The plurality of second antenna elements  204  may be dual-polarized antenna elements. Said plurality of second antenna elements  214  may be microstrip antennas, i.e., printed circuit board (PCB)-based printed antennas, or antennas formed of separate (thin) metal sheets. Said plurality of second antenna elements  214  may be specifically omnidirectional and/or dual-polarized antenna elements. The plurality of second antenna elements  214  may be made, at least partially, of a metal or an alloy. 
     The plurality of second antenna elements  214  may be assumed to be considerably smaller (or specifically electrically smaller) than any operational wavelength of the first antenna array  204  so that the plurality of second antenna elements  214  are capable of interacting only weakly with any electromagnetic waves transmitted by the first antenna array  204  or receivable via the first antenna array  204 . However, some antenna blockage may still be caused by the first antenna array  204  to the second antenna array  213 , especially when large beam scanning angles are employed. 
     While not shown in  FIG.  2 A , the active antenna module  211  may comprise a radio unit operatively coupled to the second antenna array  213  for radio reception and/or transmission via the second antenna array  213  and/or other at least partially active circuitry. Said radio unit may be a radio receiver, transmitter or transceiver. As mentioned briefly above, the active antenna module  211  also comprises second power distribution means for distributing power to and from the plurality of second antenna elements  214  of the second antenna array  213 . The second power distribution means may provide one or more input/output ports. 
     Finally, it should be noted that, vertical metallic walls  203 ,  217  extending orthogonally from the first and second ground planes  207 ,  217  are provided in the illustrated embodiment in order to better isolate the first and second antenna arrays  204 ,  213  from each other. The metallic walls  203 ,  217  may be provided along one direction or along two orthogonal directions (thus forming a grid of walls). The metallic walls  217  of the active antenna module  211  may substantially align with the metallic or metallized grid  221  of the passive antenna module  201  when the passive and active antenna modules  201 ,  211  are connected (as shown in  FIG.  2 A ). In other embodiments, elements  203 ,  217  may be omitted. 
       FIG.  3    illustrates, in a more detailed view compared to  FIGS.  2 A and  2 B , an APA system  300  comprising passive and active antenna modules  301 ,  311  according to embodiments. Specifically,  FIG.  3    illustrates the APA system  300  according to an exemplary embodiment in a perspective view when the passive and active antenna modules  301 ,  311  are not yet attached to each other and in another perspective view when the passive and active antenna modules  301 ,  311  are attached to each other. In general, the APA system  300  may correspond to the APA system  200  of  FIGS.  2 A and  2 B . 
     Referring to  FIGS.  3 A and  3 B , the passive antenna module  301  comprises a first chassis (or frame)  302  which is suitable for detachably mounting (or detachably attaching) onto an active antenna module  311  of the APA system  300 . The first chassis  302  may, at least for the most part, be made of a metal or an alloy. For enabling this, the first chassis  302  comprises a cavity  303  adapted to extend over the active antenna module  311  when the first chassis  302  is mounted onto the active antenna module  311  for minimizing antenna blockage caused by the passive antenna module  301  (predominantly by the first chassis  302  thereof). The cavity  303  may specifically penetrate through the first chassis  302  in a direction orthogonal to a plane of the first chassis  302  (or equally orthogonal to the plane of the first antenna array  304 ). The cavity may be formed onto a lateral side of the first chassis  302 . The arrow in  FIG.  3 A  indicates the mounting direction. The cavity  303  may extend specifically at least partially over a second antenna array of the active antenna module  311  when the first chassis  302  is mounted onto the active antenna module  311 . Once mounted, the first chassis  302  of the passive antenna module  301  is adapted to substantially surround the active antenna module  311  (i.e., surround it from three sides with one lateral side being left open). In other words, the active antenna module  311  is embedded into the first chassis  302  of the passive antenna module  301 . 
     In other embodiments, an opening (or a hole) may be provided in the first chassis  302 , instead of a cavity. The difference between the opening and a cavity is that the opening is surrounded from all sides by the first chassis while the cavity may be open at one side (as shown in  FIG.  3   ). Said opening may extend over the active antenna module  311  when the first chassis  302  is mounted onto the active antenna module  311  for minimizing antenna blockage caused by the passive antenna module  301  (predominantly by the first chassis  302  thereof). The opening  303  may extend specifically at least partially over a second antenna array of the active antenna module  311  when the first chassis  302  is mounted onto the active antenna module  311 . The opening  303  may be, for example, a rectangular opening. Once mounted, the first chassis  302  of the passive antenna module  301  is adapted to surround the active antenna module  311 . In other words, the active antenna module  311  is embedded into the first chassis  302  of the passive antenna module  301 . 
     As shown in  FIG.  3   , both the first chassis  302  and the opening or cavity  303  may have a shape which is elongated along the same direction. Further, the one or more first antenna elements may be arranged specifically adjacent to one or more longitudinal sides of the opening or cavity  302  (i.e., not necessarily adjacent to a lateral side of the opening or cavity  302 ). 
     The ground plane layer may be fitted into the opening or cavity  303 . In this particular example, the ground plane layer comprises only a metallic or metallized grid  321 . 
     The passive antenna module  301  further comprises a first antenna array  304  comprising a plurality of first antenna elements (here, specifically eight) arranged on two opposing sides of the cavity  303 . The first antenna array  304  (and associated feeding structure or element) may be mounted directly onto the first chassis  302  in this embodiment, as discussed above. The plurality of first antenna elements may be arranged, at least partially, adjacent to the cavity  303 . The plurality of first antenna elements may partially overlap or extend over the cavity  303  (though they may predominantly lie over the first chassis  302  as shown in  FIG.  3   ). The first antenna array  304  may be arranged substantially at a distance of λ/4 from the first chassis  302  acting as its ground plane and/or from the ground plane layer (comprising here the metallic or metallized grid  321 ), where λ is a first wavelength being a wavelength corresponding to a frequency (e.g., a center frequency) within the first frequency band of the first antenna array  304 . 
     In some embodiments, the passive antenna module  301  may comprise, in addition to the first antenna array  304 , also one or more other passive antenna arrays  307  arranged over the first chassis  302  and adjacent to the cavity  303  (i.e., not above it) and to the first antenna array  304 . Specifically, said one or more other passive antenna arrays  307  may be arranged adjacent to the cavity  303  in a longitudinal direction of the first chassis  302 , as opposed to being adjacent to the cavity  303  in a lateral direction of the first chassis  302  like the first antenna array  304 . 
     The passive antenna module  301  comprises also a first radome  331  for protecting the passive antenna module  301  as well as the active antenna module  311  when it is attached to the passive antenna module  301 . The first radome  331  may be made, e.g., of polycarbonate. 
     It should be noted that while  FIG.  3    shows the metallic or metallized grid  321  being arranged outside the first radome  331 , in other implementations, said metallic or metallized grid  321  (or the ground plane layer in general) may be arranged to be within the first radome  331  of the passive antenna module  301 . 
       FIG.  4    provides a schematic illustration of a passive antenna module  401  of an APA system (detached from the active antenna module). Specifically,  FIG.  4    illustrates passive antenna module  401  in a side view. It should be noted that  FIG.  4    shows a very simplified view where many of the elements of the passive antenna module  401  (e.g., any power distribution elements) have been omitted. The passive antenna module  401  may form a part of a terminal device such as one of terminal devices  100 ,  102  of  FIG.  1    or of an access node such as an access node  104  of  FIG.  1   . 
     The passive antenna module  401  may correspond, to a large extent, to the passive antenna module  201  of  FIGS.  2 A and  2 B  as indicated by the shared reference signs. Any of the features discussed in connection with  FIGS.  2 A and  2 B  and/or  FIG.  3    may apply, mutatis mutandis, also here. In the following, only the differences between the passive antenna modules of  FIGS.  2  and  4    are discussed. 
     As in previous embodiments, the passive antenna module  401  comprises a ground plane layer  420  which here comprises a metallic or metallized grid  421  and a two-dimensional array of metallic patches  422 . Here, the ground plane layer  420  is implemented as a printed circuit board  423 . The printed circuit board  423  is arranged within (in some alternative embodiments, over) the opening or cavity of the first chassis  202  and fixed to the first chassis  202 . The metallic or metallized grid  421  and the two-dimensional array of metallic patches  422  correspond, in this case, to a metallized pattern on a surface of said at least one printed circuit board  423  with an opposing surface to said surface being bare (i.e., not metallized). The metallization of the printed circuit board  423  may be electrically connected (e.g., via soldering) to the ground plane  207  of the first chassis  202 . While here the metallic or metallized grid  421  and the two-dimensional array of metallic patches  422  are printed on an upper surface of the printed circuit board, in other embodiments, they may be printed on the lower surface of the printed circuit board  423  or on two opposing surfaces of said at least one printed circuit board  423 , respectively. Any conventional PCB material may be used here. The printed circuit board  423  may be thin (e.g., less than 1 mm) and/or of low permittivity (e.g., relative permittivity less than 5 or less than 3) so that any detrimental effect of the dielectric of the printed circuit board on the propagation properties is minimized. 
     Moreover,  FIG.  4    illustrates, with a dashed line, a radome  430  surrounding the passive antenna module  401 . The radome  430  may be made, e.g., of polycarbonate. It should be noted that no specific connection exists between the radome  430  and the PCB implementation of the ground plane layer  420 , that is, a particular embodiment may implement one or both of said features. 
     While the APA systems as discussed in connection with  FIGS.  2 ,  3 A,  3 B and  4    may work adequately in many scenarios, they come with multiple disadvantages. Due to the passive antenna array extending partially over the active antenna array (as shown, e.g., in  FIGS.  2 A and  2 B ), the scanning capabilities of the active antenna array may be limited due to antenna blockage. In other words, if a large scanning angle (relative to broadside direction) is used, the beam is at least partially obstructed by the passive antenna module causing a deterioration of the performance of the active antenna array. Due to practical considerations relating to, e.g., mechanical design, it may not always be possible to adapt the active antenna module such that its ground plane aligns with the ground plane provided the chassis of the passive antenna module (as shown in  FIGS.  2 A and  2 B ), that is, the active antenna array may need to be arranged at a lower level relative to the passive antenna array compared to the APA system of  FIGS.  2 A and  2 B . This further exacerbates the aforementioned beam scanning problem leading to severely limited beam scanning capability. These problems could be overcome if the electromagnetic fields radiated by the active antenna module could be transferred or guided farther from the active antenna array and subsequently re-radiated. 
       FIG.  5    provides a schematic illustration of an APA system  500  according to embodiments. Specifically,  FIG.  5    illustrates an APA system  500  (comprising a passive antenna module  501  and an active antenna module  511 ) in a side view. It should be noted that  FIG.  5    shows a very simplified view where many of the elements of the passive antenna module  501  (e.g., any power distribution elements, a radio unit and radomes) have been omitted. The APA system  500  may form a part of a terminal device such as one of terminal devices  100 ,  102  of  FIG.  1    or of an access node such as an access node  104  of  FIG.  1   . 
     The APA system  500  may correspond, to a large extent, to the APA system  200  of  FIGS.  2 A and  2 B  as indicated by the shared reference signs. The active antenna module  211  of  FIG.  5    may correspond fully to the active antenna module  211  of  FIGS.  2 A and  2 B . Any of the features discussed in connection with  FIGS.  2 A and  2 B  and/or  FIG.  3    and/or  FIG.  4    may apply, mutatis mutandis, also here. In the following, only the differences between the passive antenna modules  201 ,  501  of  FIGS.  2  and  5    are discussed. 
     Referring to  FIG.  5   , the passive antenna module  501  corresponds to the passive antenna module  201  of  FIGS.  2 A and  2 B  with the addition of an array (or a set)  502  of electromagnetic coupling elements  503  arranged over the ground plane layer  220  (but separated from it by a certain distance) for coupling electromagnetic radiation received from the active antenna module to free space when the first chassis  202  is mounted onto the active antenna module. In other words, a coupling structure (e.g., a dipole or a loop) of the electromagnetic coupling elements  503  captures the electromagnetic fields radiated by the second antenna array  213 , and these captured electromagnetic fields are, then, re-radiated by a radiating structure (e.g., a dipole, a patch or a loop) of the electromagnetic coupling elements  503 . The electromagnetic coupling elements  503  may be configured to be excited at least at the second operational frequency band of the second antenna array  213  of the active antenna module  211  (or at least at some frequency or frequencies therein). Each of the electromagnetic coupling elements  503  may comprise one or more resonant elements having a resonance frequency within said second operational frequency band of the second antenna array  213 . 
     The array  502  of electromagnetic coupling elements  503  may be substantially aligned with openings in the metallic or metallized grid  221 . Additionally or alternatively, the array  502  of electromagnetic coupling elements  503  may be substantially aligned with the second antenna array  213  of the plurality of second antenna elements  214  (when the passive and active antenna modules  201 ,  211  are connected). The number of the electromagnetic coupling elements  503  may be equal to or lower than the number of the plurality of second antenna elements  214  in the second antenna array  213  (the former case being illustrated in  FIG.  5   ). 
     In the illustrated example of  FIG.  5   , the electromagnetic coupling elements  503  are based on capacitive coupling. The illustrated electromagnetic coupling elements  503  may correspond more specifically to a structure where two horizontal dipoles (being parallel with the plurality of second antenna elements  214  and arranged at different distances from them) are connected with at least one vertical (metallic or metallized) section. Alternatively, the illustrated electromagnetic coupling elements  503  may correspond to two horizontal dual-polarized dipoles (being parallel with the plurality of second antenna elements  214  and arranged at different heights) which are connected with at least one vertical (metallic or metallized) sections, as will be discussed below in connection with  FIG.  7   . 
     The plurality of electromagnetic coupling elements  503  may, in general, comprise one or more capacitive coupling elements and/or one or more inductive coupling elements. A capacitive coupling element may, for example, comprise one or more (connected) dipoles arranged substantially parallel to the plurality of second antenna elements  214 . An inductive coupling element may comprise, for example, one or more (connected) loops arranged along a plane substantially parallel to a plane of the plurality of second antenna elements  214 . 
     The distance between the array  502  of electromagnetic coupling elements  503  and the ground plane layer  220  may be (electrically) small. Such an electrically small distance may be at least equal to or smaller than λ/4, λ/5, λ/6, λ/7, λ/8, λ/9, λ/10, λ/12 or λ/15, where λ is a (free-space) wavelength corresponding to a frequency (e.g., a center frequency) within the second operational frequency band of the second antenna array  213  of the active antenna module  211 . The array  502  of electromagnetic coupling elements  503  may lie in the near field or the non-radiating near field of the second antenna array  213  at the second operational frequency band of the second antenna array  213 . The array  502  of the plurality of electromagnetic coupling elements  503  may be separated from the ground plane layer  220  by a spacer material layer such a dielectric layer (not shown in  FIG.  5   ). 
     The array  502  of the plurality of electromagnetic coupling elements  503  may be separated from the ground plane layer  220  by a spacer material layer such a dielectric layer (not shown in  FIG.  5   ). 
       FIG.  6    provides a schematic illustration of an APA system  600  according to embodiments. Specifically,  FIG.  6    illustrates an APA system  600  (comprising a passive antenna module  601  and an active antenna module  611 ) in a side view. It should be noted that  FIG.  6    shows a very simplified view where many of the elements of the passive antenna module  601  (e.g., any power distribution elements, a radio unit and radomes) have been omitted. The APA system  600  may form a part of a terminal device such as one of terminal devices  100 ,  102  of  FIG.  1    or of an access node such as an access node  104  of  FIG.  1   . 
     The APA system  600  may correspond, to a large extent, to the APA system  200  of  FIGS.  2 A and  2 B  as indicated by the shared reference signs. Any of the features discussed in connection with  FIGS.  2 A and  2 B  and/or  FIG.  3    and/or  FIG.  4    may apply, mutatis mutandis, also here. In the following, only the differences between the APA systems  200 ,  600  of  FIGS.  2  and  6    are discussed. 
     Referring to  FIG.  6   , the passive antenna module  601  corresponds to the passive antenna module  201  of  FIGS.  2 A and  2 B  with the addition of an array (or a set)  602  of electromagnetic waveguiding elements  603  (or equally an array  602  of electromagnetic waveguiding elements  603 ) arranged over the ground plane layer  220  (but separated from it by a certain distance) for guiding electromagnetic radiation received from the active antenna module to free space when the first chassis  202  is mounted onto the active antenna module. In other words, one end of the electromagnetic waveguiding elements  603  (e.g., one end of a waveguide) captures the electromagnetic fields radiated by the second antenna array  213 , these captured electromagnetic fields propagate along the electromagnetic waveguiding elements  603  and are, then, re-radiated at the other end of the electromagnetic waveguiding elements  603 . The electromagnetic waveguiding elements  603  may be configured to support electromagnetic waves at least at the second operational frequency band of the second antenna array  213  of the active antenna module  611  (or at least at some frequencies therein). 
     As mentioned above, the electromagnetic waveguiding elements  603  may be waveguides. For example, the electromagnetic waveguiding elements  603  may be hollow metallic (or metallized) waveguides or dielectric waveguides. The dielectric waveguides may be dielectric waveguides with or without metallic or metallized walls. The electromagnetic waveguiding elements  603  may be oriented, for example, substantially orthogonally to a plane of the second antenna array  213  for guiding the electromagnetic directly away from the second antenna array  213  (as shown in  FIG.  6   ). The number of the electromagnetic waveguiding elements  603  may be equal to or lower than the number of the plurality of second antenna elements  214  in the second antenna array  213  (the former case being illustrated in  FIG.  6   ). 
     The distance between the array  602  of electromagnetic waveguiding elements  603  and the ground plane layer  220  may be (electrically) small. Said distance may be at least equal to or smaller than λ/4, λ/5, λ/6, λ/7, λ/8, λ/9, λ/10, λ/12 or λ/15, where λ is a (free-space) wavelength corresponding to a frequency (e.g., a center frequency) within the second operational frequency band of the second antenna array  213  of the active antenna module  511 . In some embodiments, the array  602  of electromagnetic waveguiding elements  603  may even be in contact with the ground plane layer  220 . 
     The distance between the ground plane layer  220  and the second antenna array  213  may also be (electrically) small, similar to as discussed in connection with previous embodiments. The array  602  of electromagnetic waveguiding elements  603  may lie at least in the near field of the second antenna array  213  at the second operational frequency band of the second antenna array  213 . 
     The array  602  of the plurality of electromagnetic waveguiding elements  603  may be separated from the ground plane layer  220  by a spacer material layer such a dielectric layer (not shown in  FIG.  6   ). 
     The active antenna module  611  shown in  FIG.  6    differs from the active antenna module  211  of  FIGS.  2 A and  2 B  in that, instead metallic walls which are orthogonal to the ground plane  215  of the second antenna array  213 , the active antenna module  611  comprises tilted (or slanted) metallic walls  612  projecting from the ground plane  215  at an angle (i.e., a non-90° angle). Said tilted metallic walls  612  are specifically tilted away from the second antenna elements  214  so as to form (with the ground plane  215 ) cup-like shapes (or equally horn antenna-like shapes) around the plurality of second antenna elements  214  and their feeding elements  216 . In other words, a set of tilted metallic walls  612  formed around a particular second antenna element  214  may form the shape of an upside-down right frustum (e.g., with circular, square or regular polygonal bases) or have a shape formed by connecting a bottom base having a first shape (e.g., a circle) and a (larger) top base (substantially aligned with and parallel to the bottom base) having a second shape (e.g., square). These types of shape serve to more efficiently guide the electromagnetic fields radiated by the second antenna array  213  towards the desired direction (i.e., “up” in  FIG.  6   ). In other embodiments, the metallic walls may be configured to project orthogonally from the ground plane  215 , as in previous embodiments. 
       FIGS.  7 A,  7 B and  7 C  illustrate a practical non-limiting example of a capacitive coupling element of a passive antenna module arranged over a second antenna element  705  of a second antenna array of an active antenna module according to embodiments. Specifically,  FIG.  7 A  illustrates a unit cell structure  701  of an active antenna module,  FIG.  7 B  illustrates a capacitive coupling element  721  of a passive antenna module and  FIG.  7 C  illustrates a combination of said a unit cell structure  701  and said capacitive coupling element  721 . In  FIG.  7 C , some of the elements related to the capacitive coupling element  721  have been rendered transparent. 
     It should be noted that while  FIGS.  7 A,  7 B and  7 C  show rather large empty spaces surrounding the unit cell structure  701 , in a practical scenario, said elements  701 ,  721  may be arranged right next to each other, as shown, e.g., in  FIGS.  2 A and  2 B . 
     Referring to  FIG.  7 A , the unit cell structure  701  of the active antenna module comprises a second antenna element  705 , a feeding element  704  for the second antenna element  705  and a set of metallic walls  703  projecting from a ground plane  702  of the second antenna element  705  and surrounding the feeding element  704  and the second antenna element  705 . The second antenna element  705  is a dual-polarized crossed-dipole dipole antenna comprising a primary crossed-dipole antenna element  706  fed by the feeding element  704  and a parasitic crossed dipole antenna element  707  arranged over the primary crossed-dipole antenna element  706  (separated from it by a certain distance). The primary and parasitic crossed dipoles  706 ,  707  may be printed on different sides of a printed circuit board. The feeding element  704  is implemented here using microstrip lines. The set of tilted metallic walls  703  formed around the feeding element  704  and the second antenna element  705  have a shape formed by connecting a bottom base having a circular shape and a top base aligned with and parallel to the bottom base having a square shape (with a narrow straight or non-tilted section at the distal end). 
     Referring to  FIG.  7 B  (and  FIG.  7 C ), the capacitive coupling element  721  of the passive antenna module comprises a crossed dipole-type capacitive element  721  and metallic walls  722  surrounding said crossed dipole-type capacitive element  721 . The crossed dipole-type capacitive element  721  comprises a bottom crossed dipole element  723 , a top crossed dipole element  725  and a section  724  connecting said bottom and top crossed dipole elements. Each of the bottom crossed dipole element  723  and the top crossed dipole element  725  comprises two dipoles crossing each other (with each dipole having two distinct opposing &amp; parallel dipole arms) and thus forming an ‘x’ shape (as is characteristic for a crossed dipole). The crossed dipole-type capacitive element  721  may be made of a metal or an alloy. The metallic walls  722  projecting orthogonally from the ground plane layer  733  are used here for shaping of the electromagnetic field re-radiated by the crossed dipole-type capacitive element  721  (i.e., shaping of the antenna pattern produced by the second antenna element  705 ). In other words, the metallic walls  722  may serve a similar function to the walls  703 . 
     The layer  732  may correspond to a radome of the passive antenna module (made of, e.g., polycarbonate). 
     The upper surface of the layer  732  may be metallized or a separate metallic sheet may be provided to form the ground plane layer  733 . The ground plane layer  733  comprises a (continuous) metallic or metallized layer or surface  734  having an opening (or a hole)  735 . The opening  735  coincides with the metallic walls  722  surrounding the capacitive coupling element  721 , that is, the lower opening of the metallic walls  722  for facing the active antenna module corresponds to the opening  735  in the ground plane layer  733 . As mentioned above, in a practical scenario, the capacitive coupling elements  721  may be arranged right next to each other (as shown, e.g., in  FIG.  5   ) and thus the ground plane layer  733  having a plurality of openings  735  (e.g., one for each capacitive coupling element  721 ) forms a grid-like shape, as discussed above. 
       FIG.  7 C  shows the unit cell structures  701 ,  721  of  FIGS.  7 A and  7 B  arranged on top of each other. It should be noted that the layer  731  forming a part of the active antenna module was not previously shown in  FIG.  7 A  for clarity of presentation. Said layer  731  may correspond to a radome of the active antenna module while the layer  732  corresponds to the radome of the passive antenna module (similar to  FIG.  7 B ). 
     Moreover,  FIG.  7 C  further shows an electromagnetic directing element  726  (made of a metal or an alloy) in the form of a rectangular metallic patch arranged over the capacitive coupling element  721 . This metallic directing element  726  is employed here to tune the S11 parameter (i.e., the reflection coefficient) of the second antenna element  705  of the active antenna module. Other shapes for this element  726  may be used in other embodiments. 
     The exemplary structures shown in  FIGS.  7 A,  7 B and  7 C  may have, for example, the following dimensions for enabling operation at a frequency band of 3.2 to 3.8 GHz.
         The distance between the ground plane  702  and the crossed dipole antenna element  705  is 28 mm.   The dimensions of the mouth of the cup-like set of walls  703  are 40 mm×40 mm.   The lateral dimensions of the whole unit cell structure  701  (i.e., the periods of the second antenna array of the active antenna module in two orthogonal directions) are 42 mm×58 mm.   The thickness of the radome layer  731  of the active antenna module is 2 mm.   The thickness of the radome layer  732  of the passive antenna module is 2 mm.   The walls  722  surrounding the capacitive coupling element  721  have a height of 10 mm and lateral dimensions of 40 mm×40 mm.   The electromagnetic directing element  726  has the dimensions 27 mm×27 mm and is placed at a distance of 25 mm from the radome layer  732  of the passive antenna module.   Each dipole arm of each bottom crossed dipole element  723  of the capacitive coupling element  721  has the length of 15 mm and a maximum width of 5 mm (width being the dimension orthogonal to the longitudinal direction of the dipole arm).   Each dipole arm of each top crossed dipole element  725  of the capacitive coupling element  721  has the length of 15 mm and a maximum width of 5 mm (width being the dimension orthogonal to the longitudinal direction of the dipole arm).       

     As used in this application, the term “circuitry” may refer to one or more or all of the following: 
     (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and 
     (b) combinations of hardware circuits and software, such as (as applicable): 
     (i) a combination of analog and/or digital hardware circuit(s) with software/firmware and 
     (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and 
     (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation. 
     This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. 
     Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be combined with other embodiments in various ways.