WO2016071681A1

WO2016071681A1 - Antennas

Info

Publication number: WO2016071681A1
Application number: PCT/GB2015/053301
Authority: WO
Inventors: Mark Barrett
Original assignee: Bluwireless Technology Limited
Priority date: 2014-11-06
Filing date: 2015-11-03
Publication date: 2016-05-12
Also published as: GB201419801D0; GB2532206A

Abstract

An antenna (2) for a radio frequency communications device comprises a substrate (20), and a plurality of discrete antenna elements (21) located on the substrate (20). The antenna elements (21) are arranged on the substrate (20) in a plurality of groups (a, b, c, d, e, f), the groups (a, b, c, d, e, f) being spaced apart from one another across the substrate (20) in a first direction (x), and each group (a, b, c, d, e, f) having one or more antenna elements (21) therein. The antenna elements (21) for each group (b, c, d, e) having a plurality of elements (21) therein are arranged in a second direction (y) substantially perpendicular to the first direction (x), and at least two of the groups (a, b, c, d, e, f) have different respective numbers of antenna elements (21) therein.

Description

ANTENNAS

The present invention relates to antennas for radio communications systems. BACKGROUND OF THE INVENTION

As is well known and understood, radio communications systems enable wireless communications using transmission of electromagnetic signals in the radio frequency (RF) spectrum of approximately 3kHz to 300GHz.

An important part of this RF spectrum is the millimetre-wave radio frequency band, from 30 to 300 GHz. A particularly useful practical portion of this range for communications purposes is a band around 60GHz. Communications in such this band are suited to short range (in the hundreds of meters range) applications, such as wireless backhaul applications for mobile communications (mobile telephony and data). In one such application, mobile communications base stations are connected to a core network using 60GHz band communications instead of optical fibre connections. Such systems are required to operate over typical ranges of 200m with a mesh of interconnecting nodes, also known as a mesh network. In such a mesh network, each node or device in the network is operable to communicate with at least one other device over a relatively short distance, and it is desirable to provide a device for use in a mesh network that is able to direct a transmission to a desired receiving device in order to avoid interference problems.

Radio communications systems use antennas for the transmission and reception of the electromagnetic signals. The use of phased array antennas is highly desirable to allow automatic installation, beam alignment and adaption in case of changes in the operating environment. A phased array antenna is an antenna device that includes a plurality of individual antenna elements arranged in a square or rectangular array. An exemplary phased array antenna 1 is illustrated in Figure 1 of the accompanying drawings. In such an example, a plurality (in this case twelve) antenna elements 11 are arranged on a substrate 10 in a rectangular array having four columns A, B, C and D. Each column has the same number of antenna elements (in this case, three), so that the resulting array of antenna elements is rectangular.

For transmission of an RF signal the antenna elements 1 1 of the phased array antenna 1 are coupled to drive circuits 12. The drive circuits 12 receive the RF signal to be transmitted and supply respective drive signals to the groups A, B, C and D of antenna elements 11.

Characteristics of the signals transmitted by each individual element are varied across the array in order to vary the pattern of the radiation transmitted from the array. For example, in current phased array antennas used in 60GHz applications, the relative phases of the transmitted signals are varied across the array. In one example, the phase variation is achieved by the routing of the drive signal from one element 1 1 to the next in a given group (using connections 14 and 15). Each element 1 1 introduces a delay for the drive signal, resulting in relative phase differences between the drive signals for each element in the group. All of the elements in the array are supplied with a drive signal during operation of such an antenna array. In order to provide a desired output signal profile, each antenna element is controlled as to the phase and amplitude of its output signal. However, such existing designs have undesirably high sidelobe levels. A sidelobe is a peak of transmitted radiation that is directed in a direction away from the main desired

transmission direction (the "main lobe") of the antenna. Low antenna sidelobe levels are important to reduce interference between wireless nodes and improve frequency re-use and, therefore, total traffic capacity in a network. Reduction in sidelobe levels in the previously- considered antenna design is not possible without the use of amplitude control in addition to phase control, which leads to increasingly complex drive circuitry.

It is, therefore, desirable to provide a phased array antenna that addresses the drawbacks of the previously-considered designs.

SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided an antenna for a radio frequency communications device, the antenna comprising a substrate, and a plurality of discrete antenna elements located on the substrate, wherein the antenna elements are arranged on the substrate in a plurality of groups, the groups being spaced apart from one another across the substrate in a first direction, and each group having one or more antenna elements therein, and wherein the antenna elements for each group having a plurality of elements therein are arranged in a second direction substantially perpendicular to the first direction, and wherein at least two of the groups have different respective numbers of antenna elements therein.

Such an antenna arrangement is able to provide an output signal that has improved sidelobe characteristics compared with previously-considered designs.

In one example, in which each group of elements has a predetermined number of elements therein, the numbers of elements in the groups are determined by quantisation of a desired output profile from the antenna. The desired output profile may be represented by symmetrical tapering function, such as (1- cos(0)) where Θ ranges from 0 to 2ττ, and the quantisation may be base 2 quantisation.

In one example, the elements of each group having a plurality of elements are arranged on the substrate in a series. In such an example, respective first elements of the groups may be substantially aligned with one another in the second direction.

An antenna embodying the present invention may further comprise drive circuitry operable to provide a drive signal to each group of elements. In such an example, for each group having a plurality of elements therein, each element in a group of elements may be operable to pass a received drive signal onto a next element in the group. The drive circuitry may be operable to supply substantially the same drive signal to each group of elements.

In one example, the antenna elements are distributed across the substrate so as to form a symmetrical pattern in the first direction, about a centre line which extends in the second direction. In such an example, the antenna elements may also be distributed across the substrate so as to form a symmetrical pattern in the second direction, about a centre line which extends in the first direction.

An antenna embodying aspects of the present invention is able to provide enhanced performance by reducing the magnitude of transmitted sidelobes. In one particular example, sidelobe magnitude can be decreased by 6dB.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of a previously-considered phased array antenna;

Figure 2 is a schematic view of a first example phased array antenna embodying principles of the present invention; Figure 3 is a schematic view of a second example phased array antenna embodying principles of the present invention;

Figure 4 is a schematic view of a third example phased array antenna embodying principles of the present invention;

Figure 5 illustrates an example radio frequency communications device, and Figures 6 and 7 illustrate a mesh network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 2 of the accompanying drawings illustrates a first example antenna embodying principles of the present invention. The antenna 2 comprises a substrate 20 on which a plurality of discrete antenna elements 21 are arranged. The antenna elements 21 are arranged in groups a, b, c, d, e, and f, and the groups are distributed across the substrate 10 in a first direction x. The groups a, b, c, d, e, and f have respective numbers of antenna elements therein, and at least two of the groups have different respective numbers of antenna elements therein. In the example of Figure 2, groups a and f have a single antenna element 21 each, groups b and d have two antenna elements 21 each and groups c and d have four antenna elements 21 each. It will be appreciated that the groups can have any number of antenna elements 21 therein.

The antenna elements 21 of each group are arranged linearly in a second direction y substantially perpendicular to the first direction x. In the example of Figure 2, the antenna elements 21 are arranged in each group in a linear series, with the first element in the series being substantially aligned in the second direction with the first element of each of the other series. In the example of Figure 2, the groups of antenna elements 21 are arranged so that the overall pattern of elements on the substrate is symmetrical about a centre line which extends in the second direction. The antenna elements 21 of the Figure 2 arrangement are driven by drive circuits 22. In one example, each group of elements has an associated drive circuit 22. The drive circuit 22 supplies a drive signal to the associated group of elements, with the drive signal being passed along the series of elements in the group (for groups having a plurality of elements), such that the elements in the group receive phase shifted versions of the signal relative to the other elements in the group. The groups may be supplied with the same drive signal, or may be supplied with respective different drive signals.

It is to be noted that the distribution of the antenna elements on the substrate results in a desired output signal from the antenna without the need for amplitude control of the signals supplied to the individual antenna elements. As such, the drive circuitry is greatly simplified over previously-considered designs of antenna.

Figure 3 illustrates a second example antenna embodying the principles of the present invention. The antenna elements are arranged in groups (columns) 1 to 12, distributed across the substrate in a first direction. The groups have respective numbers of antenna elements therein, and at least two of the groups have different respective numbers of antenna elements therein.

In the example of Figure 3, each group comprises a number of antenna elements equal to a power of 2 (2ⁿ). In the example shown in Figure 3, groups 1 and 12 have 1 (2°) element each, groups 2 and 1 1 have 2 (2¹) elements each, groups 3, 4, 9 and 10 have 4 (2²) elements each, and groups 5 to 8 have 8 (2³) elements each.

The antenna elements of each group are again arranged linearly in a second direction substantially perpendicular to the first direction. In the example of Figure 3, the antenna elements are arranged in each group in a linear series, with the first element in the series being substantially aligned in the second direction with the first element of each of the other series.

In the example of Figure 3, the groups of antenna elements are arranged so that the overall pattern of elements on the substrate is symmetrical about a centre line which extends in the second direction. The distribution of the antenna elements results in a pattern mirrored about this centre line of the antenna.

The number of antenna elements in each group is determined by quantising (to the nearest power of two) a desired output signal profile for the antenna. In one example, the number of elements is determined by rounding up the result of the function (1 - cos (Θ)) to the nearest power of 2 (base 2 quantisation), where Θ is in the range 0 to 2ττ. An example of the calculation for the example antenna of Figure 3 is shown below in Table 1.

Column No Angle Cos(Angle) l-Cos(Angle) Elements Roundup Sub-Array Elements

0 0.261799 0.966 0.017 0.136297 1 1

1 0.785398 0.707 0.146 1.171573 2 2

2 1.308997 0.259 0.371 2.964724 3 4

3 1.832596 -0.259 0.629 5.035276 6 4

4 2.356194 -0.707 0.854 6.828427 7 8

5 2.879793 -0.966 0.983 7.863703 8 8

6 3.403392 -0.966 0.983 7.863703 8 8

7 3.926991 -0.707 0.854 6.828427 7 8

8 4.45059 -0.259 0.629 5.035276 6 4

9 4.974188 0.259 0.371 2.964724 3 4

10 5.497787 0.707 0.146 1.171573 2 2

11 6.021386 0.966 0.017 0.136297 1 1 The antenna elements of the Figure 3 arrangement are once again driven by drive circuits (not shown for the sake of clarity). In one example, each group of elements has an associated drive circuit. Each drive circuit supplies a drive signal to the associated group of elements, with the drive signal being passed along the series of elements in the group, such 5 that the elements in the group receive phase shifted versions of the signal relative to the other elements in the group. The groups may be supplied with the same drive signal, or may be supplied with respective different drive signals.

It is to be noted that the distribution of the antenna elements on the substrate results in a desired output signal from the antenna without the need for amplitude control of the signals 10 supplied to the individual antenna elements. As such, the drive circuitry is greatly simplified over previously-considered designs of antenna.

Figure 4 illustrates a third example antenna 3 embodying the principles of the present invention. As in the example of Figures 2 and 3, the antenna 3 comprises a substrate 30 on which a plurality of discrete antenna elements 31 are arranged. The antenna elements 31 15 are arranged in groups a', b', c', d', e', and Γ, distributed across the substrate 30 in a first direction x. The groups have respective numbers of antenna elements therein, and at least two of the groups have different respective numbers of antenna elements therein.

In the example of Figure 4, groups a' and f have two antenna elements 21 each, and groups b', c', d' and e' have four antenna elements 21 each. The antenna elements 31 of each

20 group are again arranged linearly in a second direction y substantially perpendicular to the first direction x. In the example of Figure 4, the antenna elements 31 are arranged in each group in a linear series. The overall pattern of elements 31 on the substrate 30 is symmetrical about a centre line which extends in the second direction y, as before. In addition, the pattern of elements 31 is also symmetrical about a second centre line which

25 extends in the first direction x.

The groups of elements 31 can be understood as being comprised of respective pairs of subgroups of elements 31. Each sub group receives a drive signal from drive circuitry (not shown for the sake of clarity, via a drive connection 32. This drive signal is passed from one element 31 to another in the sub group (for those subgroups having a plurality of elements 30 31 ). The drive signal received by each individual element is a phase shifted, with respect to the other elements in the group, version of the drive signal. There is no amplitude control required for each individual element in the array, however, as the arrangement of elements on the substrate enables an RF signal with a desired low sidelobe profile to be transmitted from the antenna. By way of example, a previously considered antenna arrangement, such as that shown in Figure 1 , can have a scan angle of +/- 60deg, with sidelobes of -8.9dB. Using an antenna embodying an aspect of the present invention, a scan angle of +/- 45deg and sidelobes of - 15.2dB can be achieved. Figure 5 is a schematic illustration of a radio frequency communications device that includes an antenna embodying an aspect of the present invention. Such a device 4 includes a data processing unit 40 that is operable to receive data for transmission and to encode that data to produce and encoded data stream. The data may be encoded using any desired coding scheme, such as a low density parity check (LDPC) coding scheme. The device 4 includes a radio frequency unit 42 which receives the encoded data stream from the data processing unit 40, and which is operable to modulate that data stream for transmission over a radio frequency channel. The radio frequency channel is preferably a channel in the 60GHz transmission band, and the modulation scheme can be any appropriate scheme. Possible modulation schemes include binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), orthogonal frequency division multiplexing (OFDM), and quadrature amplitude modulation (QAM), and appropriate combination thereof.

The modulated encoded data signal is supplied by the radio frequency unit 42 to an antenna 44 for transmission across an air interface 46. The antenna 44 is an antenna as described with reference to Figures 2 to 4, and provides the transmission with desirably low sidelobe levels.

It will be readily appreciated that Figure 5 shows only the transmission elements of a communication device, and that the device will be operable to receive modulated encoded data, and to demodulate and decode that data.

Figure 6 illustrates a mesh network in which devices having an antenna embodying the principles of the present invention may be utilised. The mesh network includes a plurality of network nodes arranged to communicate with adjacent nodes of the network. In the example shown in Figure 6, the nodes are arranged in a rectilinear pattern, although it will be readily appreciated that the nodes may be arranged in any pattern. In the example of Figure 5, each node is illustrated as being able to communicate on four channels f1 , f2, f3, and f4, and these channels are directed to specific adjacent nodes in the network.

Figure 7 illustrates the mesh network of Figure 6 in use. In the example case shown, node N7 communicates directly with node N1 1 on the first channel f1. This communication is labelled S_{7J 1} in Figure 7, and represents the desired communication between nodes N7 and N11. Other first channel f1 communications are shown in Figure 7, and three of these, in the example shown, may interfere with the communication from node N7 to node N 11 , due to sidelobe transmissions. These sidelobe transmissions are indicated as interference transmissions. A transmission from node N2 to node N3 on channel f1 can produce a sidelobe interference signal l₂_n at node N11. Similarly, transmissions from nodes N4 and N5 can produce sidelobe interference signals l_{4 11} and l_{5 11} at node N11.

An antenna embodying an aspect of the present invention enables reduced sidelobe transmission magnitude, and so reduces the magnitude of the interference signals received by node N11 from nodes N2, N4 and N5. Such reductions in sidelobe signal magnitude increases the signal to interference ratio (SIR) at node N11.

It will be appreciated that the mesh network of Figures 6 and 7 is merely an example to illustrate the principles of interference signals, and how reducing sidelobe signal magnitude can enhance the SIR at the receiving node. Using nodes that include antennas embodying an aspect of the present invention allows for greater flexibility in positioning of nodes of the mesh network.

Claims

CLAIMS:

1. An antenna for a radio frequency communications device, the antenna comprising: a substrate, and a plurality of discrete antenna elements located on the substrate, wherein the antenna elements are arranged on the substrate in a plurality of groups, the groups being spaced apart from one another across the substrate in a first direction, and each group having one or more antenna elements therein, and wherein the antenna elements for each group having a plurality of elements therein are arranged in a second direction substantially perpendicular to the first direction, and wherein at least two of the groups have different respective numbers of antenna elements therein.

2. An antenna as claimed in claim 1 , in which each group of elements has a

predetermined number of elements therein, wherein the numbers of elements in the groups are determined by quantisation of a desired output profile from the antenna.

3. An antenna as claimed in claim 2, wherein the desired output profile is represented by symmetrical tapering function.

4. An antenna as claimed in claim 3, wherein the symmetrical tapering function is (1- cos(0)) where Θ ranges from 0 to 2ττ, and the quantisation is base 2 quantisation.

5. An antenna as claimed in any one of the preceding claims, wherein the elements of each group having a plurality of elements are arranged on the substrate in a series.

6. An antenna as claimed in claim 5, wherein respective first elements of the groups are substantially aligned with one another in the second direction.

7. An antenna as claimed in any one of the preceding claims, further comprising drive circuitry operable to provide a drive signal to each group of elements.

8. An antenna as claimed in claim 7, wherein, for each group having a plurality of elements therein, each element in a group of elements is operable to pass a received drive signal onto a next element in the group.

9. An antenna as claimed in claim 7, wherein the drive circuitry is operable to supply substantially the same drive signal to each group of elements.

10. An antenna as claimed in any one of the preceding claims, wherein the antenna elements are distributed across the substrate so as to form a symmetrical pattern in the first direction, about a centre line which extends in the second direction.

11. An antenna as claimed in claim 10, wherein the antenna elements are distributed across the substrate so as to form a symmetrical pattern in the second direction, about a centre line which extends in the first direction.

12. A radio frequency communications device including an antenna as claimed in any one of the preceding claims.

13. An antenna substantially as hereinbefore described with reference to, and as shown in, Figures 2 to 4 of the accompanying drawings.

14. A radio frequency communications device substantially as hereinbefore described with reference to, and as shown in, Figure 5 of the accompanying drawings.