WO2019170827A1

WO2019170827A1 - High efficiency e-band antenna system

Info

Publication number: WO2019170827A1
Application number: PCT/EP2019/055760
Authority: WO
Inventors: Ali AL-RAWI; Aleksei DUBOK; Mathieu H.A.J. HERBEN; Adrianus B. Smolders
Original assignee: Technische Universiteit Eindhoven
Priority date: 2018-03-07
Filing date: 2019-03-07
Publication date: 2019-09-12

Abstract

Improved antenna configurations for high frequency RF point to point links are provided. Instead of using a reflector to provide a sharp focus where the feed array is placed, the reflector is used to provide a moderate level of reflector gain for all elements of a large and dense feed array. This is done by placing the feed array at a location with an appropriately defocused electromagnetic field configuration. With amplitude and phase control of the elements of the feed array, improved beam steering and tracking can be provided.

Description

High Efficiency E-Band Antenna System

FIELD OF THE INVENTION

This invention relates generally to millimeter-wave wireless point to point wireless communications, such as backhaul wireless communications for the cellular network.

BACKGROUND

Mobile broadband traffic is expected to show a two fold increase in the next five years. That is going to set a high demand for cellular backhaul capacity. Existing microwave backhauling solution represents a bottleneck for the required backhaul capacity, since they offer a limited bandwidth. On the other hand, fiber-optic systems provide virtually unlimited capacity, but such option comes with high costs of licensing and installation. Microwave remains an alternative cost efficient option.

Therefore, currently we see a surge in millimeter-wave links research to reach a data rate of tens of Gbps. The millimeter-wave E-Band (70/80 GHz) provides a large

bandwidth up to 10 GHz, split in two channels of 5 GHz each separated by a 5 GHz guard band. Such allocation enables full-duplex communication which is required in wireless backhaul links.

The main challenge for systems operating at E-Band is the relatively large signal losses due to precipitation and communication range. In addition, high-performance power amplifiers are very expensive. Such factors limit the potential of E-Band high-capacity backhaul networks and other point-to-point high-data rates applications. For example to establish a 5 km communication link with 99.99% availability of the time, an average antenna gain of 46 dBi is required.

Such high gain levels can be achieved with a parabolic reflector with a diameter of at least 0.4 m or with a phased-array antenna which may require more than 2000 active antenna elements. The extreme high power consumption and high cost, make phased-arrays unsuitable for E-band backhaul systems. Therefore, reflector antennas are the most suitable solution from an economic and manufacturing point of view. However, there are several disadvantages associated with high-gain reflector antennas. The wind induced-load on structures causes the antenna mast to twist and sway, which in return causes a signal loss due to the resulting misalignment. In addition, due to the narrow beamwidth at E-band, typically around 1.2 degrees, the alignment between antennas separated by 5 km is quite cumbersome during the installation of the wireless link.

In addition to the problem of small beamwidth, traditional reflector-based E-band backhaul systems require expensive high-power amplifiers to realize the required range. The availability of these kind of amplifiers is limited and they usually come with high costs. On the other hand we see that silicon based amplifiers are cost

effective, but they provide a limited output power in the range of 8-12 dBm with a moderate efficiency.

Accordingly, it would be an advance in the art to provide reflector-based E-beam antennas that are more suitable for E-beam point to point link applications. SUMMARY

We believe that focal plane array (FPA) systems represent a cost-effective solution for E-Band backhaul links. FPAs can provide traditional reflector antennas limited electronic scan capabilities in the range of +/- 4 degrees in azimuth and elevation. This scan range is sufficient to compensate with tracking methods the mis alignment due to antenna-mast twist and sway, and can automate the point to point installation procedure.

Also, the FPA can serve as a spatial power combiner, meaning that the combined effect of the output power from several relative low-power silicon-based amplifiers can provide a high EIRP (effective isotropic radiated power) that is required for a reliable long-range wireless

backhauling .

This work provides designs for increasing the EIRP in FPA systems. The method uses a reformation of the focal field distribution in reflector antennas. We will show that the focal-field distribution can be controlled in such a way to optimize the realized EIRP level. We will apply this method in parabolic reflectors. However, our concept can be extended to include more complex single or double reflector configurations. Since the resulting antenna configurations have the feed array disposed away from the focal plane of the reflector system, we use the more general term 'feed array' in the following description, instead of 'focal plane array'.

The new antenna system can achieve higher transmitted power compared to conventional systems. This is sufficient to establish a wireless link with a capacity of 10s of Gbps in a 5 km wireless communication scenario. In addition, the new system is cheaper than traditional wireless backhaul systems since it is designed to utilize low-cost silicon ICs and has a much smaller antenna size.

The mechanism of the new antenna system is based on the focal fields of quasi-optical systems such as reflector and lens antennas. These systems focus the fields in what is known as focal plane. Conventionally the field is concentrated in a very small spot that is very challenging to fit more than one practical antenna. This results in a very limited transmitted power. We found a way in which this field can be extended or broadened and then sampled with a larger number of practical antennas. This results in a much higher transmitted power. This technique produces a shaped quasi-optical device and a defocused focal plane antenna system. An important trade-off has to be made between the aperture efficiency and the input power.

In one embodiment, the focal field broadening using the axial defocusing technique uses an array of antennas in which the antenna elements are connected to low-cost silicon ICs. Displacing the array towards a reflector or a lens antenna, the reflected (or refracted in the case of a lens) rays can be intercepted at an earlier stage, before converging in a focal point. By sampling the broadened field of the reflector or lens the number of active

antennas can be increased. An advantage of this method is that it results in an increase in the radiated power in the transmit mode, and increases the system sensitivity in the receive mode. This is realized without increasing the size or cost of the reflector or lens antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a point to point RF link. FIG. 2A shows a reflector antenna having the feed array disposed at a defocused location.

FIG. 2B shows the example of FIG. 2A having the feed array disposed at a focused location.

FIGs. 3A-D show examples of various ways to provide a defocused field configuration at the feed array.

FIG. 4A is a side view of an exemplary antenna system.

FIG. 4B is a perspective end view of the example of FIG. 4A.

FIG. 5 is an exemplary block diagram showing some features of a preferred embodiment of the invention.

FIG. 6 shows simulated effective isotropic radiated power results relating to an exemplary embodiment of the invention .

FIG. 7 shows simulated efficiency results relating to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

Millimeter-wave wireless communication in E-Band can use a substantially large bandwidth. As a result, high data rate communication up to 100 Gbps (Gigabit per second) becomes possible. The backhaul wireless communications in the mobile cellular network or the point to point wireless connection in the cellular network needs such data rate. There are two problems associated with E-Band backhaul communication; the first is that the signals weaken

significantly during rainy weather. The second issue arises during the installation and operation of a high-gain antenna system. The pointing errors including the antenna- mast deflection, requires on-site calibration by a skilled technician. In addition, the installation and aligning of the point-to-point link is cumbersome and requires skilled personnel too.

FIG. 1 schematically shows a point to point link.

Here towers 102 and 104 have antennas 106 and 108 disposed on them, respectively. Weather induced swaying 110 of tower 104 can lead to a corresponding motion 112 of

received beam 114 at antenna 106. Since these antennas tend to be highly directional, this beam motion can lead to significant loss of performance if it isn't compensated for. Of course the same issue can affect signals received at antenna 108 from antenna 106, but for simplicity this is not shown on the figure. As indicated above, the

attenuation experienced by beam 114 can also depend

significantly on weather conditions.

FIG. 2A schematically shows the main concept of this work. A reflector 202 brings incident radiation to a focus as shown by the dashed lines. Instead of being disposed at the focus, feed array 204 is disposed away from the focus, as shown. The more conventional placement of feed array 204 at the focus is shown on FIG. 2B. In the arrangement of FIG. 2B, high reflector gain is only provided for one (or a few) elements of the feed array (i.e., the elements coupled to the focused radiation) , with all other elements of the feed array having very low reflector gain, since they are laterally displaced away from the focus as shown.

In contrast, the arrangement of FIG. 2A provides a moderately high reflector gain to all elements of the feed array, since no element of the array is effectively outside the relevant radiation path. Here reflector gain is defined relative to a fictitious reference situation where the same power is radiated isotropically. More

specifically, the reflector gain of a feed array element is the far field on-axis beam intensity from that feed array element divided by the above-defined isotropic reference intensity. E.g., a 112% reflector gain means that the far field on-axis beam intensity from the array element is 1.12 times greater than the reference intensity.

For detailed designs, a trade-off is made between the size of the feed array and the minimum reflector gain seen by a feed array element. The smaller the lateral size of the array (in wavelengths) , the larger the minimum

reflector gain can be made. Similarly, as the lateral size of the array is increased, the minimum reflector gain for the array will need to decrease (assuming, in both cases, a fixed reflector size) . For detailed designs of E-band point to point links as described herein, we have found that requiring reflector gain of 112% or more for each element in the feed array has led to good results in simulations. Although this may seem like a very low reflector gain requirement, it can be better understood by referring to FIG. 2B where the central array element would have a reflector gain much higher than 100% while the remaining array elements would have reflector gain much less than 100%, effectively not contributing at all to reception or transmission. Thus a configuration as on FIG. 2B could not meet this reflector gain requirement. It is also noted that the EIRP of the antenna as a whole will be much higher than this 112% per-element EIRP, because the outputs of multiple feed array elements will combine constructively due to per-element amplitude and phase control as described below. Another point worth noting is that focal plane array antenna designs are commonly used in radio astronomy, where a feed array of any reasonable size can fit within the lateral size of the focus provided by the antenna. In that application, the undesirable configuration of FIG. 2B, where the antenna array is larger than the beam focus, cannot arise and is therefore not a problem.

Accordingly, an exemplary embodiment of the invention is an electromagnetic antenna including one or more

reflective elements (e.g., 202 on FIG. 2A) , a feed array having multiple radiative elements disposed in an array (e.g., 204 on FIG. 2A) , and a processor configured to provide individual amplitude and phase adjustment of each of the radiative elements of the feed array (e.g., 402 on FIGs. 4A and 5, as described below) . Here the

electromagnetic antenna is configured to operate at one or more operating frequencies in a range from 20 GHz to 140 GHz. The operating frequencies have corresponding operating wavelengths, where frequency f and operating wavelength l are related to each other by the usual free space

relationship: lί = c, where c is the free space speed of light .

An important aspect of this work is that the feed array is densely packed. More specifically, let l_± be the shortest of the operating wavelengths. Then the lateral spacing of the radiative elements of the feed array is l_±/2 or less. An exemplary number of elements in the feed array is 81 elements.

The feed array is disposed at a location that has a defocused configuration of electromagnetic fields formed by the one or more reflective elements as described above.

More specifically the defocused configuration of

electromagnetic fields is such that each radiative element of the feed array has 112% or more reflector gain, where reflector gain is as defined above. This defocused configuration of electromagnetic field can be provided by: moving the feed array away from a focal plane of the reflector ( s ) , changing the shape of the reflector (s) to provide a less sharp quasi-focal plane where the feed array is disposed, or any combination of these two approaches.

For example, the one or more reflective elements can provide a focal plane, and the defocused configuration of electromagnetic fields can be formed by axially displacing the feed array from the focal plane. As another example, the one or more reflective elements can provide a quasi- focal plane because shapes of the reflective elements differ from conic sections. Here the defocused

configuration of electromagnetic fields can be formed by disposing the feed array at or near the quasi-focal plane. FIGs. 3A-D show some examples of these ideas.

In FIG. 3A the feed array 306 is disposed at a

defocused configuration 308 of the electromagnetic field provided by reflector 302, where the defocusing is from an axial displacement 310. In FIG. 3B the focal field

broadening effects can also be a result of the shaping 312 of the reflector 302, in addition to or as an alternative to axial displacement 310.

In FIG. 3C the feed array 306 is placed in a double reflector configuration formed by main reflector 302 and sub-reflector 304, and defocused by axial displacement 310 to achieve similar broadening effects. In FIG. 3D the sub- and main reflector 304 and 302 are shaped (314 and 312 respectively) to broaden the focal field distribution across the feed array, in addition to or as an alternative to axial displacement 310. In addition, the magnification properties of the double reflector configuration can also be utilized to broaden the field distribution across the feed array, meaning that the sub-reflector or the main reflector can be displaced axially or in off-set configuration. However the extent of the displacement or the off-set angle are subject to the trade-off between sub-reflector blockage, array antenna power loss due to spill-over efficiency, and the power loss due to cross-polarization discrimination.

In all cases, the defocused configuration of

electromagnetic fields is such that each radiative element of the feed array has 112% or more reflector gain, no matter how many reflectors (or other focusing elements, such as lenses) are present in the system, or how the defocusing is actually implemented.

An example of an operational antenna system with adaptive capabilities as described below where the feed array is placed in the defocused field is shown in

FIGs. 4A-B. Here FIG. 4A is a side view and FIG. 4B is a corresponding perspective end view. The feed array 204 can be in the center of the main reflector 302 or be laterally displaced where needed for an optimal design. The reflected wave from the sub-reflector 304 toward the feed array arrives with a broadened field distribution related to defocusing as described above. Here the defocusing is provided by shaping the main reflector 302 and the sub reflector 304 as schematically shown by 312 and 314 respectively. An additional advantage of this

configuration is that the main reflector back-side can function as the heat-sink for the electronics module 402.

An important aspect of this work is full control of the phase and amplitude of all feed array elements. Such control can provide various functions, and can be better understood with reference to the example of FIG. 5. The main capabilities provided by this exemplary system are 1) compensating for weather effects to maximize the efficiency of the communication channel during all-weather condition and 2) tracking capability to offset pointing errors.

The system includes a reflector 302, and feed array 204 placed in the defocused configuration of the

electromagnetic field provided by reflector 302. Here it is noted that FIG. 5 is a block diagram, and no attempt is made here to show the proper positioning of feed array 204 relative to reflector 302. The radiative elements of feed array 204 are connected to the transmitting and the

receiving modules (RF modules) . The RF modules can be in the form of analog or digital beam-formers that contain phase shifters and amplifiers that have amplitude

adjustment .

An exemplary operating process starts by transmitting a beacon signal to the far-end in the point to point wireless system. The antenna elements receive the beacon signal via the reflector at the far-point of the point to point wireless system, the beacon signal is then fed into the processor 402 which runs a correlation algorithm that calculates the weighting coefficients for the phase

shifters and amplitude controls in the transmitting and receiving chains.

Initially, the algorithm correlates the received amplitude and phase state from the beacon signal with the stored phase and amplitude distributions of the calibrated state. The algorithm finds the new weighting complex coefficients that maximize the correlation between the two sets. The new coefficients are then fed to the calibration register that used in the next iteration of the algorithm.

Block 402 on FIG. 5 schematically depicts this process. The next received beacon signal then overwrites the stored measurement .

We can change the output power in an adaptive manner by measuring the strength of the received signal in the receiver units. The correlation algorithm finds the value and compares it with the threshold value. Therefore by means of correlation the system can make logic decisions to maintain or increase the output transmitting power.

The beacon signal can arrive with an angle during the alignment process or during the operation due to the twist and sway of the antenna mast. Reflector 302 converts the beacon signal angle of arrival into an image on the antenna array. The lateral displacement of the maximum of the field distribution (i.e., the image) is equivalent to the

incoming wave direction. When the correlation algorithm maximizes the correlation between the calibrated state and the beacon signal, the generated weighting coefficients are causing the local antenna system to track the motion of the far-end transmitter.

To increase the margin for which the antenna system can maximize the transmitted power we need to increase the number of simultaneously active antenna array elements, which is the primary motivation for placing the feed array at a defocused location as described above.

The individual amplitude and phase adjustments provided by the processor can be configured to maximize an effective isotropic radiated power of the electromagnetic antenna. This amounts to providing a beam forming function (for both transmit and receive) with the active feed array elements in the defocused configuration. This provides scanning and tracking of a single beam of electromagnetic radiation. Alternatively, the electromagnetic antenna can be configured to provide scanning and tracking of two or more beams of electromagnetic radiation with the amplitude and phase adjustments of the feed array.

As indicated above, the individual amplitude and phase adjustments provided by the processor can be updated adaptively. For example, the individual amplitude and phase adjustments provided by the processor can be updated adaptively to maximize reception of a beacon signal from a remote antenna.

Adaptive control of both ends of the link is possible. The processor can further adaptively adjust amplitudes and phases of the remote feed array of the remote antenna in order to maximize reception of the beacon signal from the remote antenna at the local antenna.

Any kind of polarization can be used in these antennas and systems, including linear, circular and elliptical polarization. In preferred embodiments, two orthogonal polarizations are simultaneously used to increase capacity and/or to provide duplexing.

FIG. 6 shows the resulting increased the effective isotropic radiated power (EIRP) . The feed array is deployed in the defocused configuration of the electromagnetic field in such way the EIRP can be maximized in an adaptive manner. The solid line is when the feed array is in the defocused field configuration. The dashed line is when the feed array is in the focal plane. The results in the solid line can be realized by the feed array in the defocused configuration. Therefore, the feed array in the defocused configuration can increase the EIRP in an adaptive way.

On FIG. 7 the solid line relates to a half-wave length spaced feed array deployed in the defocused configuration of the electromagnetic field (FIG. 2A) . The dashed line relates to the same feed array deployed in the focal plane of the reflector antenna (FIG. 2B) . Due to the defocused configuration more elements are contributing and hence a higher reflector gain is seen for the defocused

configuration.

Claims

1. An electromagnetic antenna comprising:

one or more reflective elements;

a feed array having multiple radiative elements disposed in an array; and

a processor configured to provide individual amplitude and phase adjustment of each of the radiative elements of the feed array;

wherein the electromagnetic antenna is configured to operate at one or more operating frequencies in a range from 20 GHz to 140 GHz, wherein the operating frequencies have corresponding operating wavelengths; wherein l_± is a shortest of the operating wavelengths, and wherein a lateral spacing of the radiative elements of the feed array is l_±/2 or less; wherein the feed array is disposed at a location that has a defocused configuration of electromagnetic fields formed by the one or more reflective elements, wherein the defocused configuration of electromagnetic fields is such that each radiative element of the feed array has 112% or more reflector gain.

2. The electromagnetic antenna of claim 1, wherein the one or more reflective elements provide a focal plane, and wherein the defocused configuration of electromagnetic fields is formed by axially displacing the feed array from the focal plane.

3. The electromagnetic antenna of claim 1, wherein the one or more reflective elements provide a quasi-focal plane because shapes of the reflective elements differ from conic sections, and wherein the defocused configuration of electromagnetic fields is formed by disposing the feed array at or near the quasi-focal plane.

4. The electromagnetic antenna of claim 1, wherein the individual amplitude and phase adjustments provided by the processor are configured to maximize an effective isotropic radiated power of the electromagnetic antenna.

5. The electromagnetic antenna of claim 1, wherein the individual amplitude and phase adjustments provided by the processor are updated adaptively.

6. The electromagnetic antenna of claim 5, wherein the individual amplitude and phase adjustments provided by the processor are updated adaptively to maximize reception of a beacon signal from a remote antenna.

7. The electromagnetic antenna of claim 6, wherein the processor further adaptively adjusts amplitudes and phases of a remote feed array of the remote antenna in order to maximize reception of the beacon signal from the remote antenna .

8. The electromagnetic antenna of claim 1, wherein the electromagnetic antenna is configured to provide scanning and tracking of a single beam of electromagnetic radiation.

9. The electromagnetic antenna of claim 1, wherein the electromagnetic antenna is configured to provide scanning and tracking of two or more beams of electromagnetic radiation.

10. The electromagnetic antenna of claim 1, wherein the electromagnetic antenna is configured to simultaneously operate in two orthogonal polarizations.