Antenna System And Radar System Incorporating The Same

Abstract

An antenna system comprising an array of antenna elements, the array comprising a plurality of groups of antenna elements wherein each group comprises one or more antenna elements arranged in series, and wherein the system further comprises first phase-control means for performing the function of introducing respective phase-shifts to transmitted or received signals passed to or received from each of said groups to provide beamforming and second phase-control means for performing said function with respect to a sub-set of said groups. An antenna system of the invention allows two radar beam patterns having different spatial characteristics to be generated using a single antenna system. The invention also provides a radar system incorporating an antenna system of the invention.

Description

[0001] The invention relates to antenna systems and to radar systems.

[0002] Radar systems for certain applications are required to have the capability to produce more than one radar beam pattern in order to perform more than one type of detection and ranging. For example, an automotive radar system may be required to produce a long-range beam pattern having a relatively narrow angular extent in azimuth for use in cruise control on motorways, and also a short-range beam pattern having a wider angular azimuthal extent for use in a parking sensor system and to monitor nearby vehicles for the purpose of collision avoidance. Existing automotive radar systems of this type use two separate radar sub-systems to provide such beam patterns; see for example "A Wideband Millimetre-Wave Front-End for Automotive Radar" by J. C. E. Mayock et al, 1999 IEEE MTT-S Digest. This approach requires four antenna sub-systems if the two radar sub-systems are bistatic. Accommodation of four antenna sub-systems within a vehicle body places significant constraints on vehicle body shape and/or size. Also, each sub-system requires dedicated operating electronics because the transmitted frequencies of the short- and long-range beam patterns are different. (Typically 24 GHz is used for the short-range beam and 77 GHz for the long-range beam). The complexity and size of these existing automotive radar systems results in substantial cost both to manufacturers and consumers, impeding the take-up of automotive radar technology. Similar problems arise in other applications where antenna and radar systems having more than one beam pattern are required.

[0003] A first aspect of the invention provides an antenna system comprising an array of antenna elements, the array comprising a plurality of groups of antenna elements wherein each group comprises one or more antenna elements arranged in series, and wherein the system further comprises first phase-control means for performing the function of applying respective phase-shifts to transmitted or received signals passed to or received from each of said groups to provide beamforming and second phase-control means for performing said function with respect to a sub-set of said groups.

[0004] A single antenna system of the invention allows two radar beam patterns having different spatial characteristics to be produced, thus reducing the number of antenna systems needed to provide such functionality compared to prior art systems. An antenna system of the invention may be used for transmission, reception or both transmission and reception. By operating the array, and the sub-set of groups, at substantially the same frequency the invention allows a reduction in the cost and complexity of operating electronics used with the antenna system.

[0005] One or more additional sub-sets of groups of antenna elements may be defined using further respective phase-shifting means, providing for one or more additional beam patterns to be generated.

[0006] Although arrangements of switches and conventional phase-shifters may be used to carry out phase-control, preferably either the first or the second phase-control means is a Rotman lens. More preferably, both the first and second phase-control means are Rotman lenses. Rotman lenses are described in, inter alia, the proceedings of the 22.sup.nd International Communications Systems Satellite Conference & Exhibition 2004 (American Institute of Aeronautics & Astronautics), paper AIAA 2004-3196 by P. S. Simon, and are advantageous because they can be realised in planar, cheap and reliable form and are less affected by aberration errors than other phase-control devices.

[0007] Conveniently, the array of antenna elements is substantially rectangular, and each of the groups of antenna elements is a row or column of the array. This provides for simpler manufacture.

[0008] In one embodiment, the sub-set of groups of antenna elements is made up of contiguous rows or columns of the array.

[0009] Preferably the array of antenna elements is substantially planar, allowing the array to be placed behind a registration plate of a vehicle, for example.

[0010] For ease of manufacture, preferably the array of antenna elements and the Rotman lenses are mounted on a common former. For example, the former may be substantially cuboid in shape, with the first and second Rotman lenses mounted on opposite faces of the cuboid former and the array of antenna elements mounted on a face of the cuboid former adjacent of the faces mounting the Rotman lenses. Alternatively the former may be laminar with the array of antenna elements, and the Rotman lenses, mounted on the same side, or opposite sides, thereof. Manufacture of an antenna system of the invention is further simplified if each antenna element is a patch antenna element.

[0011] A second aspect of the invention provides a radar system comprising an antenna system of the invention. For reasons mentioned above, preferably one of the phase-control means is a Rotman lens, or, more preferably, both of the phase-control means are Rotman lenses. In order to allow rapid directional scanning of the radar pattern of the whole array or the sub-set of groups (in either transmission or reception), inputs to the Rotman lenses may be coupled to respective RF switches. A RF signal for transmission is preferably provided by a monolithic microwave integrated circuit (MMIC) operable to provide the signal to either of the RF switches, as a MMIC may be integrated with the RF switches.

[0012] A radar system of the invention may be a frequency-modulated, continuous-wave (FMCW) radar system. In order to provide low-phase noise on transmit and highly coherent operation (and hence high range accuracy) at low cost, preferably the radar system comprises an RF oscillator, a direct digital synthesiser (DDS) arranged to provide a frequency-modulated signal and a mixer arranged to mix respective outputs of the RF oscillator and the DDS to provide a FMCW signal for transmission, wherein the clock signal of the DDS is derived from the RF oscillator. By locking the DDS to the RF oscillator, a free-running source (e.g. a free-running dielectric resonator oscillator (DRO)) may be used because any jitter in the output of the RF oscillator then corresponds to the same jitter in the DDS clock signal. Additionally, in embodiments of the invention in which one or more analogue-to digital converters (ADCs) are provided to digitise received signals, the clock signal for each ADC is preferably derived from the RF oscillator. The various clock signals may be obtained by frequency-dividing the output of the RF oscillator. Use of a free-running source as the RF oscillator, rather than a phase-locked source, provides optimum phase noise at offset frequencies that cause most effect in FMCW radars (typically 100 kHz and 1 MHz from the carrier frequency). The use of a DDS as described above provides very high frequency-sweep linearity and hence very high range resolution.

[0013] In automotive radar applications it is important to avoid the problem of interference between radars systems of vehicles which are adjacent or nearly adjacent and travelling in the same direction, as occurs for example on motorways when vehicles are moving in adjacent lanes. If automotive radar systems fitted to adjacent vehicles use the same frequency then each system will return spurious results. To avoid this problem, the frequency of radiation transmitted from a radar system of the invention is preferably a function of transmission direction.

[0014] A radar system of the invention may be monostatic, in which case a single antenna system of the invention is needed for both transmission and reception, or bistatic, in which case two antenna systems of the invention are required. The same operating electronics can be used to operate the whole array of antenna elements and to operate the sub-set of groups of antenna elements.

[0015] Embodiments of the invention are described below by way of example only and with reference to the accompanying drawings in which:

[0016] FIGS. 1 & 2 show perspective views of an antenna system of the invention;

[0017] FIG. 3 shows an array of patch antenna elements comprised in the antenna system of FIGS. 1 and 2;

[0018] FIG. 4 shows drive electronics for use with the antenna system of FIGS. 1 & 2 to provide azimuthal scanning in transmission or reception; and

[0019] FIGS. 5 & 6 show advantageous arrangements for generating a transmitted FMCW signal and for digitising radar returns within a radar system of the invention.

[0020] Referring to FIGS. 1, 2 and 3, an antenna system of the invention is indicated generally by 100. The antenna system 100 comprises a substantially cuboid former 114 mounting first 108 and second 116 Rotman lenses on opposite faces thereof and a rectangular array 102 of antenna elements on a face of the former 114 adjacent to the faces mounting the Rotman lenses 108, 116. The array 102 comprises 69 groups of patch antenna elements, each group comprising a linear array of 11 elements connected in series forming a column of the array 102. Rotman lens 108 has 69 outputs each of which is coupled to a respective group of patch antenna elements via a connecting line. The 69 connecting lines are indicated collectively by 106. Rotman lens 116 has 19 outputs, each of which is coupled by a connecting line to a respective column of antenna elements in a sub-set 104 of columnar groups of the array 102, the sub-set being the central 19 columns (shown shaded in FIG. 3) of the array 102. The 19 connecting lines coupling outputs of the lens 116 to respective columns in the sub-set 104 of groups of antenna elements of the array 102 are indicated collectively by 112. Connecting lines to the various inputs of the Rotman lenses 108, 116 are indicated by 110, 118 respectively.

[0021] The antenna system 100 may operate either as a transmitter or a receiver, or both when used in a monostatic radar system. A long-range beam pattern with a narrow angular extent in azimuth may be produced by operating the system 100 so that only the sub-set 104 of groups of antenna elements are activated. The transmit/receive direction of the system in this mode of operation may be varied in azimuth by applying a transmission signal to an appropriate input of lens 116 in the case of transmission, or by processing a signal from an appropriate input of the lens 116 in the case of reception. The azimuthal direction of a short-range beam pattern may be varied similarly, i.e. by applying a transmission signal to an appropriate input of lens 108 in the case of transmission, or by processing signals from an appropriate input of the lens 108 in the case of reception.

[0022] FIG. 4 schematically illustrates an arrangement for producing a scanning, transmitted, long-range radar beam pattern using the antenna system 100. Input lines 118 to the Rotman lens 116 (the outputs 112 of which are connected to the sub-set 104 of groups of the array 102) are connected via an alumina drop-in unit 120 to an RF switch 122. A monolithic microwave integrated circuit (MMIC) 124 provides an RF transmission signal and a direction-scanning signal to the RF switch 122. At a particular instant, the RF transmission signal appears on a particular output of the RF switch 122 according to the status of the direction-scanning signal. The RF transmission signal is applied to the Rotman lens 116 via a particular input 118. Drive signals appear at each of the outputs 112 of the Rotman lens 116 with appropriate phasing to generate a plane wave having an azimuthal transmission direction corresponding to that particular input of the Rotman lens 116. A second RF switch and drop-in unit (not shown) are provided for the Rotman lens 108 to allow transmission of a short-range beam pattern from the whole of the array 102 of antenna elements. The MMIC 124 is arranged for switching between the RF switches so that a long- or short-range beam may be transmitted as required.

[0023] The arrangement shown in FIG. 4 may also be used for reception. For example, a direction-scanning signal applied to RF switch 122 provides for each of the lines 118 to be coupled through the RF switch in turn, corresponding to scanning of the receive direction in azimuth. Switching means may be provided to allow switching between lenses 108, 116 so that reception may be carried out using either the whole array 102 or using the sub-set 104 of antenna elements, as desired.

[0024] Two separate arrangements of the type shown in FIG. 4 may be used (one for transmission and one for reception) in a bistatic radar system of the invention.

[0025] FIG. 5 shows an arrangement for providing high range range-accuracy in a FMCW radar incorporating an antenna system of the invention. A free-running DRO 132 operating at 9200 MHz is coupled to a mixer 134. Output from the DRO 132 is also down-converted by a first frequency-divider 136 to provide a clock signal to a DDS 138 which is arranged to output a signal having a frequency which periodically increases from 200 MHz to 250 MHz in a sawtooth form, as shown in the Figure. Mixing of the DDS and DRO outputs at the mixer 134 provides an FM RF transmission signal which also has a sawtooth frequency as a function of time, sweeping from 9400 MHz to 9450 MHz. Output from the first frequency-divider 136 is passed to a second frequency-divider 140 which provides a further down-converted signal to a complex programmable logic device (CPLD) 142 which in turn provides a clock signal to an ADC 144 which digitises signals received from the antenna system of the radar system. By locking the various clock signals to a single reference (DRO 132) the reference can be free-running because any jitter in the reference corresponds to the same jitter on the clock signals, thus providing highly coherent operation.

[0026] FIG. 6 show an alternative to the FIG. 5 arrangement, in which the output of a mixer 154 is up converted to provide an RF transmission signal having a frequency which is an order of magnitude greater than that of the arrangement shown in FIG. 5.

Claims

1. An antenna system comprising an array of antenna elements, the array comprising a plurality of groups of antenna elements wherein each group comprises one or more antenna elements arranged in series, and wherein the system further comprises a first phase-controller arranged to perform the function of applying respective phase-shifts to transmitted or received signals passed to or received from each of said groups to provide beamforming and a second phase-controller for performing said function with respect to a sub-set of said groups.

2. An antenna system according to claim 1 wherein at least one of the first and second phase-controllers comprises a Rotman lens.

3. An antenna system according to claim 2 wherein both the first and second phase-controllers each comprise a Rotman lens.

4. An antenna system according to claim 1 wherein the array of antenna elements is substantially rectangular and each of said groups of antenna elements is a row or column of the array.

5. An antenna system according to claim 4 wherein said sub-set of groups is made up of contiguous rows or columns of the array.

6. An antenna system according to claim 5 wherein the array of antenna elements is substantially planar.

7. An antenna system according to claim 4 wherein array of antenna elements and the Rotman lenses are mounted on a common former.

8. An antenna system according to claim 7 wherein the former is substantially cuboid and the first and second Rotman lenses are mounted on respective opposite faces of the cuboid former and the array of antenna elements is mounted on a face of the cuboid former adjacent to the faces mounting the Rotman lenses.

9. An antenna system according to claim 7 wherein the former is laminar.

10. An antenna system according to claim 1 wherein each antenna element is a patch antenna element.

11. A radar system comprising an antenna system according to claim 1.

12. A radar system according to claim 11 wherein at least one of the first and second phase-controllers comprises a Rotman lens.

13. A radar system according to claim 12 wherein both the first and second phase-controllers each comprise a Rotman lens.

14. A radar system according to claim 13 further comprising first and second RF switches coupled to the first and second Rotman lenses respectively.

15. A radar system according to claim 14 further comprising a monolithic microwave integrated circuit (MMIC) operable to provide a transmission signal to each RF switch.

16. A radar system according to claim 11 wherein the radar system is a FMCW radar system.

17. A radar system according claim 16 further comprising a RF oscillator, a DDS arranged to provide a frequency-modulated signal and a mixer arranged to mix respective outputs of the RF oscillator and the DDS to provide a FMCW signal for transmission, and wherein the clock signal of the DDS is derived from the Rb oscillator output.

18. A radar system according to claim 17 comprising one or more ADCs arranged to digitise received signals and wherein the clock signal of each ADC is derived from the RF oscillator output.

19. A radar system according to claim 17 and further comprising a frequency-divider arranged to frequency-divide the output of the RF oscillator to generate the, or each, clock signal.

20. A radar system according to claim 17 wherein the RF oscillator is a free-running DRO.

21. A radar system according to claim 11 and arranged to produce from the antenna array a transmitted signal having a frequency which is a function of transmission direction.

22. A radar system according to claim 11 wherein the radar system is monostatic.

23. A radar system according to claim 11 wherein the radar system is bistatic.

24-26. (canceled)