Methods and apparatus for airborne synthetic antennas

Embodiments of an airborne synthetic phased antenna array include a plurality of airborne antennas and phase shifters for the antennas. The antennas are operated coherently, and constant phase shifts between the antennas are maintained. In addition, methods for providing wireless communication services include providing a plurality of airborne platforms, providing RF antennas for wireless communication services and phase shifters for the RF antennas on at least two airborne platforms, and operating the phase shifters to maintain synchronous operation of the RF antennas.

FIELD

Embodiments of the present principles generally relate to methods and apparatus for airborne wireless communications, and in particular for enabling synthetic antennas using airborne platforms. Non-limiting examples include providing communication links between airborne platforms and ground based equipment using synchronized airborne antennas.

BACKGROUND

Global broadband wireless communications have been growing exponentially in recent years. Network coverage, however, remains incomplete in many regions of the world and even in some currently served regions; thus demand may soon exceed the supply of existing communication infrastructure. Current network technologies are generally too expensive, ineffective, and slow to respond to growing demand. In addition, further proliferation of existing ground-based wireless technologies increases radio-frequency (RF) pollution and human exposure to large amounts of RF energy.

Terrestrial cellular wireless networks are well suited for local area deployments. They are relatively inexpensive, as compared to optical fiber networks, and are the technology of choice in new and emerging markets where the physical infrastructure is limited. Terrestrial cellular wireless networks are appropriate for fixed and mobile users and may be interfaced to wired networks. However, as discrete components, they are range limited and have finite bandwidth. To meet an increasing customer demand, new towers are added to increase the coverage density, while reducing their range to enable increased frequency reuse.

Alternatively, it is possible to establish an aerial network that employs airborne platforms as its main communication hubs. Such hubs would be stationed at altitudes well above commercial airspace, where the line of sight coverage extends over large terrestrial areas. Such a network could work either separately from or together with existing terrestrial mobile phone communication systems. However, one of its potential drawbacks is the limited ability of the airborne antennas to project and concentrate RF emission in small areas on the ground. In wireless cellular communications, where a service area is divided into small cell areas for the purposes of frequency reuse, it is advantageous to be able to project and confine RF emission to these specific small areas, i.e. cell areas. Smaller cell areas allow better frequency reuse and higher throughput per user. The size of the cell area for an airborne antenna is inversely proportional to the antenna's size, so that a larger antenna is more beneficial, since it is capable of producing more concentrated RF emission and smaller cell sizes. Due to the size limitations of a single airborne platform, the size of a single RF antenna may be limited to few meters, which in turn restricts the size of the resulting RF cell area on the ground to no less than several kilometers.

The inventors recognize that this limitation may severely restrict applications of airborne wireless networks. Thus, the inventors believe that there is a strong need for a way to improve the performance of airborne antennas and propose several solutions.

SUMMARY

Embodiments of apparatus for providing distributed airborne wireless communications are provided herein.

In some embodiments, a method of providing wireless communication services includes providing a plurality of airborne platforms, providing RF antennas for wireless communication services and phase shifters for the RF antennas on at least two airborne platforms, and operating the phase shifters to maintain synchronous operation of the RF antennas.

In some embodiments, an airborne synthetic phased antenna array includes a plurality of airborne antennas and phase shifters for the antennas, wherein the antennas are operated coherently, and constant phase shifts between the antennas is maintained.

In some embodiments, a method of providing an airborne synthetic phased antenna array includes providing a plurality of airborne antennas and phase shifters for the antennas, operating the antennas coherently, and maintaining constant phase shifts between the antennas.

Other and further embodiments of the present principles are described below.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments or other examples described herein. However, it will be understood that these embodiments and examples may be practiced without the specific details. In other instances, well-known methods, procedures, components, and/or circuits have not been described in detail, so as not to obscure the following description. Further, the embodiments disclosed are for exemplary purposes only and other embodiments may be employed in lieu of, or in combination with, the embodiments disclosed.

In accordance with embodiments of the present principles, airborne systems are provided for enabling wireless communications services on the ground and in the air. The systems comprise a fleet of specially equipped unmanned airborne vehicles (UAVs). UAVs carry a distributed payload that comprises wireless communication equipment. A UAV with an associated payload represents an airborne communication platform (ACP).FIG. 1depicts a schematic view of an airborne communication platform (ACP)100in accordance with an embodiment of the present principles. In some embodiments of the present principles, an apparatus, such as the ACP100schematically shown inFIG. 1, may comprise a wing110, a fuselage120and a tail130. The ACP100may also include one or more RF antennas140, as a part of its payload for the purposes of providing communication services to end users on the ground and elsewhere. Other elements of the ACP100that are not shown inFIG. 1may for example include a propulsion system, flight control elements, and additional parts of the payload. The RF antennas140may be attached to or integrated with the skin of the airframe and located on the surface of either the wing110, fuselage120or tail130, as shown inFIG. 1. The RF antennas140may also be located inside the airframe body, provided the airframe skin is RF transparent. In addition to fixed-wing aircraft, other airborne vehicles may be used as a basis for an ACP, including helicopters, vertical-take-off-and-landing (VTOL) aircraft, lighter-than-air (LTA) aircraft, tethered aircraft and others.

Different types of RF antennas may be used for the ACP100, including isotropic (or nearly isotropic) and directional antennas. The former are characterized by a low gain, whereas the latter are characterized by a high gain. The low gain antennas may be used for close range (<1 km) communications between different ACPs, such as air-to-air (ATA) communications. The high gain antennas may be used for long range (>1 km) communications between an ACP and ground-based equipment, such as air-to-user (ATU) communications. The antennas may be tuned to specific carrier frequencies (e.g. 2.0 GHz), or alternatively may be broadband and characterized by a large bandwidth (e.g. from 2.0 to 3.5 GHz).

As an illustrative example,FIG. 2depicts a directional phased array antenna200that may be used on board of the ACP100ofFIG. 1in accordance with an embodiment of the present principles. The directional phased array antenna200may include a substrate210and patch antennas220. The directional phased array antenna200may also include an additional circuitry, which is not shown inFIG. 2, for feeding electrical RF signals to the patch antennas220. In addition, the phases of the RF signals to different patch antennas220may be independently controlled allowing RF beam steering and spatial multiplexing. The RF beam shape and direction may thus be digitally controlled, enabling beam stabilization during flight and synchronization with other antennas. In general, a larger phased array antenna could produce a narrower RF beam, i.e. a larger antenna gain. Therefore, it is more beneficial to have a larger number of patch antennas in an array for long range communications. The patch antennas may be also more widely spaced to further increase the antenna gain. For airborne applications with low weight requirements, a sparse phased array antenna may comprise a smaller number of patch antennas that are spaced at larger than normal distances.

FIG. 3illustratively depicts the operation of an exemplary ACP300using the directional phased array antenna200ofFIG. 2to project an RF beam310onto the ground below and, as a result, forming a single cell area320. In terrestrial cellular networks the cell boundaries are often determined by the propagation distance of the RF signal, the strength of which quickly decreases with the distance from the source (a cell tower). However, in the case of the airborne network, the cell boundaries and a size of the cell may be determined by the shape and size of the RF emission patterns from the airborne antennas, since most of the coverage area may be roughly at the same distance from the airborne antennas. Due to a limited size of the directional phased array antenna200, the RF beam spread may be large and so the size of the cell320may be also relatively large. For example, for the ACP300operating at an altitude of 20 km and the RF beam divergence of 30 degrees, the resulting cell size is about 10 km. In order to reduce the cell size, the RF beam divergence must be reduced, which may be achieved by either using a larger antenna or by synthesizing a larger antenna aperture from a number of individual smaller antennas.

FIG. 4schematically depicts an ACP fleet400comprised of multiple ACPs410equipped with directional phased array antennas200producing RF beams directed towards the ground in accordance with an embodiment of the present principles. The RF beams may be coherent with each other, operating at essentially the same frequency, polarization and modulation format. The RF beams may be synchronized and stabilized, so as to keep a constant phase shift between antennas on different ACPs. As a result, a much larger effective antenna aperture may be synthetized as determined by the distance between different ACPs rather than the size of any individual antenna. Thus a much smaller cell area420may be produced on the ground as shown inFIG. 4compared to the scenario illustrated inFIG. 3. The size of the cell area may correspond to the size of either macrocells, metrocells, microcells, or picocells that are typically realized in terrestrial cellular networks. In general, the size of the cell produced by the ACP fleet400may be adjustable and varied in response to the demand from the users on the ground. This adjustability of the airborne cellular mapping is enabled by the beam-forming ability of the ACP's antennas. Furthermore, other characteristics of the RF signals, such as carrier frequencies, bandwidth, modulation formats, control logic, and others, may be matched to those in existing terrestrial networks in order to facilitate closer integration and interoperability with existing user equipment, such commercial cellular phones. In addition, the ATU links from the ACP fleet400may be tailored for other applications utilizing for example IoT (Internet of Things) automated equipment requiring wireless links with low-bandwidth and wide coverage.

FIG. 5depicts an ACP fleet500comprised of multiple ACPs510, each of which is equipped with RF antennas520and530in accordance with another embodiment of the present principles. The RF antennas520may be used for providing ATU communications, while the RF antennas530may be used for providing ATA communications. Alternatively, free-space optical communications may be used for ATA communications. The ATA communications allows ACPs510to synchronize the operation of the RF antennas520, so that they function together as a single interconnected phased array of separate antennas. ATA communications provide channels for passing user data and control data between different ACPs. User data may include voice calls, text messaging, digital data, and so on. Control data may include information about user location, locations about other ACPs510in the ACP fleet500, reference locations, timing information, as well as specific tasking commands enabling shared distributed operations of the ACP fleet500as a single network node.

FIG. 6depicts a high level block diagram of an exemplary ACP600, comprising an aircraft605, a corresponding flight control system610, and a communication payload620in accordance with an embodiment of the present principles. The flight control system610ofFIG. 6may at least include different sensors, including position sensors606and an autopilot system607. The position sensors606may provide precise information about current aircraft position and attitude and the rates of their changes in time (velocity and acceleration), e.g. using an inertial measurement unit (IMU) and Global Positioning System (GPS) (not shown). The autopilot system607enables the aircraft to fly autonomously and follow different flight regimes and missions. In addition, the flight control system610may also include an accurate clock608and a mechanism for synchronizing the clock608to an external clock (not shown), for example, by using GPS satellites. The ACP's communication payload620may represent only a section of the total payload associated with a fleet of ACPs. It may include at least an ATA communications portion625and an ATU communications portion630. The ATA portion625may include an ATA antenna621, a control channel subsystem622, and a signal channel subsystem623. The signal channel subsystem623may be used for relaying user data, such as voice, for example during calls. The control channel subsystem622can be used for exchanging control data between different ATPs within a same fleet. The ATU communications portion630may include an ATU antenna626, a transceiver627for sending and receiving RF signals, a phase-shifter628, an RF signal splitter629and RF signal combiner631. The transceiver627for sending and receiving RF signals may in turn including software-defined radios (SDR) and at least some elements of eNodeB for establishing GSM, LTE, and similar cellular links with existing terrestrial wireless equipment (all not shown). The phase-shifter628is used to apply a common phase shift or different phase shifts to the RF signals emitted by the ATU antenna626or the constituent parts, respectively. The RF signal splitter629and RF signal combiner631are used to split and combine RF signals, respectively, that may be sent to and originate from different ACPs. In general, the number of incoming and outgoing RF signals may be as large as the number of ACPs in a fleet.

FIG. 7depicts a block diagram of a process for sending and receiving signals from a fleet of ACPs to user equipment on the ground in accordance with an embodiment of the present principles. As depicted inFIG. 7, at702an RF signal is split into N parts by a splitter, such as the RF signal splitter629ofFIG. 6. The N split parts of the RF signal are communicated, in some embodiments, to N different ACPs via, for example, an ATA communications link. The splitting function may be realized either in the analog or digital domain. The analog splitting may be performed by, for example, an isotropic or low-gain ATA RF antenna in simultaneous transmission of the same signal to multiple ACPs. At each ACP a respective phase-shifter, such as the phase-shifter628ofFIG. 6, adjusts the phase of the RF signal, which is then transmitted at704by an ATU antenna, such as the ATU antenna626ofFIG. 6, to a user on the ground. In addition, the signal amplitude, timing and modulation may be also adjusted or modified. In such embodiments and arrangements, N different phased antennas may be implemented to accomplish the signal transmission.

Similarly,FIG. 8depicts a block diagram of a process of receiving a signal from a user on the ground in accordance with an embodiment of the present principles. In the embodiment depicted inFIG. 8, the user signal is received at N different ACPs via N respective ATU antennas. The phases of the N received signals are adjusted at802using phase shifters, such as the phase shifter628ofFIG. 6. InFIG. 8, the adjusted signals are combined at804using a signal combiner, such as the signal combiner631ofFIG. 6. In various embodiments in accordance with the present principles, the value of N may range from two to the number of aircraft in a ACP fleet. In some embodiments, various components may possess multiple functions and may be used redundantly for both transmitting and receiving functions. For example, in some embodiments an ATU antenna may be used in a duplex mode, so that the ATU antenna may send and receive RF signals. Alternatively, separate ATU antennas may be used for sending and receiving functions, in order to, for example, reduce RF interference and noise.

FIG. 9depicts a block diagram of an airborne wireless system900for sending wireless signals in accordance with an embodiment of the present principles. The airborne wireless system900ofFIG. 9illustratively includes several ACPs910-913, in which the ACP910includes a signal splitter920, and the ACPs911-913include phase shifters931-933and ATU antennas941-943, respectively. In addition, airborne wireless system900ofFIG. 9includes ATA communication links951-953for transmitting signals from the ACP910to ACPs911-913, respectively. The signals may be transmitted as data packets for easier routing. Additional data may be transmitted along with the signals, including the target location (or a direction) of the user for which the signals are intended. To facilitate the synchronization and cross-referencing among ACPs911-913, the signal splitter920may also provide a reference position or reference plane (e.g. a geometric reference surface with a normal originating from a location of a user or a cell center) for the ATU antennas941-943and an accurate time reference, which may be obtained from a flight control module. Synchronized and cross-referenced ATU beams961-963may then form a common RF radiation pattern producing a cell area on the ground encompassing a target user equipment970. In alternate embodiments, the airborne wireless system900may not require a separate ACP910for hosting the signal splitter920. Instead, the signal splitter920may be collocated with other elements of the airborne wireless system900on one of the ACPs911-913.

FIG. 10depicts a block diagram of an airborne wireless system1000for receiving wireless signals in accordance with an embodiment of the present principles. The airborne wireless system1000ofFIG. 10consists of several ACPs1010-1013, in which the ACP1010includes a signal combiner1020, and the ACPs1011-1013include phase shifters1031-1033and ATU antennas1041-1043, respectively. In addition, the airborne wireless system1000includes ATA communication links1051-1053for transmitting signals from the ACPs1011-1013to the ACP1010, respectively. Additional data may be transmitted along with the signals, including the location of the user, a reference position and a time reference. ATU beams1061-1063from the user1070to the ACPs1011-1013are received separately by the antennas1041-1043, respectively. In alternate embodiments, the airborne wireless system1000may not require a separate ACP1010for hosting the signal combiner1020. Instead, the signal splitter1020may be collocated with other elements of the airborne wireless system1000on one of the ACPs1011-1013. Furthermore, the elements of both systems900and1000can be combined into one system capable of both sending and receiving wireless signals from ground-based users.

FIG. 11depicts a block diagram of the operation of a phase shifter1128in accordance with an embodiment of the present principles. The phase shifter1128provides an offset for one or more RF signals to be sent or received by an ATU antenna. The phase offset (or offsets) is calculated using information about the location1105of the target area on the ground (e.g. where a projected cell area or a target user is located). To impart a correct phase offset, additional information is required, such as location of a reference point (or a plane)1115and an exact location1110of an aircraft (i.e. the ATU antenna of the aircraft). The latter is subject to constant changes due to aircraft movement and rotation, therefore continuous and real-time updates of the aircraft position are necessary to produce an accurate phase offset. Therefore, the phase offset may be a fast varying parameter (or parameters) applied to RF signals and updated at rates for example ranging from 10 Hz to 10 kHz, as determined by the rate of change in the aircraft position vs the reference position. The phase offset ensures a constant phase difference is maintained between different ATU antennas, resulting in a well-defined RF beam direction towards a predefined cell area on the ground and the cell boundaries. The ATU antennas located on different ACPs may be similar to each other to simplify beam forming. The ATU antennas themselves may be phased array antennas, allowing simultaneous application of multiple phase shifts and thus correcting instantaneous changes in an aircraft's attitude.

FIG. 12depicts a diagram of an ACP fleet in accordance with an embodiment of the present principles. The ACP fleet1200ofFIG. 12illustratively comprises multiple ACPs1210equipped with ATU antennas producing RF beams1215directed towards the ground. The RF beams1215may be coherent, synchronized and stabilized, so that a much larger effective antenna aperture may be synthetized as determined by the size of the ACP fleet1200rather than the size of any individual antenna. As a result, a cell area1220may be produced on the ground forming an ATU link. In addition, the ACP fleet1200may include an ACP1230, which may carry either a splitter, a combiner or both sections of the ACP fleet communication payload. The ACP1230may be also have an ATU antenna and directly participate in the formation of the cell area1220. The ACPs1210and1230may also have means for establishing wireless ATA links, e.g. such as links1235for linking the ACP1230with every other ACP1210. The links1235may then be used for relaying user data to and from the ACP1230and controlling RF beams on the ACP1210. The ACP1230may fly at the same altitude as the rest of the ACP fleet1200or at a different altitude, e.g. above or below the average altitude of the fleet1200. Flight at different altitudes may improve ATA communications, for example, by enabling either the use of high gain antennas in the case RF communications are used for establishing the ATA links, or the use of point-to-point free-space optics in the case of using optical communications for establishing the ATA links.

FIG. 13depicts a diagram of an ACP fleet in accordance with an alternate embodiment of the present principles. The ACP fleet1300ofFIG. 13illustratively comprises multiple ACPs1310equipped with ATU antennas producing RF beams1315directed towards the ground. The RF beams may be coherent, synchronized and stabilized, so that a cell area1320may be produced on the ground as shown inFIG. 13forming an ATU link. The ACP fleet1300may be also a part of a larger communication system, which may also include a communication satellite1330, which may carry either a splitter, a combiner or both sections used in support the ACP fleet communication payload that at least includes phase shifters and ATU antennas, as described above, distributed among different ACPs. The satellite1330may also have means for establishing wireless links1335with every ACP1310, using either RF antennas or free-space optics. The links1335may then be used for relaying user data to and from the satellite1330and controlling RF beams on the ACP1310. The satellite1330may also have communication links with other communications satellites and terrestrial communication systems for relaying user data and control signals.

FIG. 14depicts a diagram of an ACP fleet in accordance with an alternate embodiment of the present principles. The ACP fleet1400ofFIG. 14illustratively comprises multiple ACPs1410equipped with ATU antennas producing RF beams1415directed towards the ground. The RF beams may be coherent, synchronized and stabilized, so that a cell area1420may be produced on the ground forming an ATU link. The ACP fleet1400may be also a part of a larger communication system, which may also include a terrestrial station1430, which may carry either a splitter, a combiner or both used in support of the ACP fleet communication payload that at least includes phase shifters and ATU antennas, as described above, distributed among different ACPs. The station1430may also have means for establishing wireless air-to-ground (ATG) links1435with at least some ACP1410, for example, using high-gain RF antennas. Specifically the ATG links1435may be realized using phased-array antennas, which in turn may produce multiple RF beams focused on different ACPs1410. Alternatively, the station1430may produce a single RF beam covering the extent of the ACP fleet1400. The data and signals within such an RF beam may be provided to all linked ACPs simultaneously, wherein the information received from the station1430is essentially the same. Alternatively, separate communication channels can be established within the same RF beam, for example using frequency, time or code division multiplexing. The links1435may then be used for relaying user data to and from the station1430and controlling RF beams on the ACP1410. The station1430may also have communication links (wired or wireless) with other terrestrial communication systems for relaying user data and control signals.

In accordance with the present principles, an ACP fleet may serve multiple cell areas at the same time. Each cell area may be projected by a specific combination of RF beams produced by different ACPs with specific set of phase offsets between them. As a result, each ACP's ATU antenna may be emitting multiple RF beams with different phase offsets corresponding to different cell areas without interfering with each other. This signal overlay between multiple cells areas may be done in either the digital or analog domain. Generally, digital signal synthesis is more efficient and versatile.

In accordance with embodiments of the present principles, an ACP fleet may fly directly above its service area to minimize the signal propagation distance. The service area may move as well together with the ACP fleet. Alternatively, the service area may be stationary due to intrinsic beam steering capabilities of the synthetized ACP antenna array as described above, in which any relative and absolute changes in any or all of the ACP's position may compensated by the phase shifters on board the ACPs. The service area may comprise multiple cell areas, each characterized by size, shape and reference location. To minimize interference, neighboring cells areas may be serviced using different frequency or modulation format (e.g. different spreading codes). The relative positions among different ACPs in the fleet may be kept nearly constant to simplify operation and maintain the same level of performance. For example, a four-ACP fleet may maintain a continuous diamond formation as shown inFIG. 5. Many other formations are possible with the number of aircraft ranging from 2 to 10 or more. An ACP fleet as a whole may fly different paths. The preferred path may be a circular path at a constant altitude centered above the service area. Different formation patterns may be used and these patterns may be changed in flight depending on application requirements. Some formation patterns, such as close V-formations where ACPs may fly in each other's wake, may provide aerodynamic benefits by reducing drag and propulsion power for fixed-wing platforms. These ACP fleet formations may be particularly beneficial for airborne wireless systems, as they optimize the performance of both the airborne platforms and their communication payloads.