GB2346013A

GB2346013A - Calibration method for a phased array

Info

Publication number: GB2346013A
Application number: GB9927003A
Authority: GB
Inventors: Magnus Sundberg
Original assignee: Radio Design Innovation TJ AB
Current assignee: Radio Design Innovation TJ AB
Priority date: 1998-11-27
Filing date: 1999-11-15
Publication date: 2000-07-26
Also published as: GB9927003D0; SE513340C2; SE9804091D0; SE9804091L

Abstract

A method of calibrating a phased array, having a number of signal paths (1 to 8) corresponding to the antennae of the array, comprises injecting a constant reference signal into one of the signal paths and then injecting calibration signals into the other paths and measuring the resultant signals in these paths. To calibrate the array the transfer function of the reference path is set to a predetermined value (Adj) and the transfer paths of the other paths are adjusted so that the same phase and amplitude relations exists between the reference path and each of the other paths. Preferably the paths of the phased array are connected at their feed ends to a lode shaping network (LSN) and the reference and calibration signals are injected by an adjustment unit (Cal.) having variable phase and amplitude adjustment. The signals may be injected into the paths either near the lode shaping network B, adjacent the antennae A or may be injected into the paths via remote radio coupling to the antenna C and the measurement of the resultant signals may also occur at any of these three locations. Preferably the calibration is improved by iterating the steps of the above method using different signal paths as the reference path and averaging over the resulting transfer functions.

Description

CALIBRATION METHOD FOR PHASED ARRAY ANTENNA FIELD OF THE INVENTION The present invention relates to a calibration method for phased array antennas. Phased array antennas are for instance used in telecommunication systems utilizing narrow lobes for communication between mobile stations and base stations.

The phased array antenna is controlled by a lobe shaping network having a fixed matrix for creating and shaping the narrow lobes. The lobe shaping network contains phase and amplitude functions applied to the signals going to and coming from the individual antenna elements of the phased array antenna. The signal paths of the antenna elements should be tuned together so that the lobes can be shaped with accuracy and in the desired manner.

The present invention relates to a method for calibrating the signal paths of the antenna elements of the phased array antenna, so that they are tuned together. A calibration signal is injected in the individual signal paths between the antenna elements and lobe shaping network. Each signal path has an adjustment unit, which is capable of adjusting the transfer function of the individual signal path. Sionals caused by the injected calibration signal are measured and adjusted with reference to one of the signal paths, so that a uniform relationship exists between all the signal paths and the reference signal path.

STATE OF THE ART Various methods for calibration of phased array antennas are known in the prior art of which three examples are given below.

US 4,488,155 relates to a method and apparatus for self-calibration and phasing of an array antenna. The method uses an external pilot signal ; is transmitted to an antenna element and the response is used as a reference.

US 5,477,229 relates to an active antenna near field calibration method. The method measures and calculates transfer functions of antenna elements in a phased array antenna by a measurement in the near field in front of each antenna element using a special measurement probe. Then, the transfer functions are used to calibrate the system.

W095/34103 also relates to an antenna calibration method. A signal is transmitted through each antenna section with the associated antenna element. The transmitted signai is compared with the input signal and correction factors are calculated.

The present invention is a further development and simplification of the prior art methods, in that the transfer function of one signal path is used as a reference for the calibration of the other signal paths. Thus, the antenna elements are calibrated and tuned tooether in a simple and efficient manner.

SUMMARY OF THE INVENTION Thus, the present invention provides a calibration method for transmission and reception paths of a phased array antenna having a plurality of antenna elements.

Each antenna element has an individual signal path connected to a lobe shaping network and includes an adjustment unit having variable phase and amplitude adjustment factors.

According to the invention, the transfer function of a first signal path of a first antenna element is set at a fixed value and a calibration signal is injected into one end of the first signal path. Then, a calibration signal is sequentially injected into one end of other signal paths. A first signal caused by the injected calibration signal through the first signal path is measured. Thereafter, signals through the other paths are measured sequentially and the transfer functions of other signal paths are adjusted, so that the same phase and amplitude relationships exists between the first signal and other signals caused by the injected calibration signal.

The invention is defined in the independent claims 1 and 6 while preferred embodiments are set forth in the independent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described below with reference to the accompanying drawings in which: Fig. 1 is a schematic view of the signal paths of the phased array antenna, and Fia 2 is a schematic view of eight antenna elements and illustrating the interelement coupling.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As is known, the performance and capacity of mobile telecommunication systems may be enhanced by using spatial division multiplexing, that is instead of omnidirectional or tri-sector antennas, narrow lobes are set up between base stations and mobile stations. This increases the spectrum efficiency and decreases interference in the system. Narrow lobes require phased antenna arrays comprising a number of antenna elements. A signal to be transmitted (or received) in a narrow lobe is passed through a lobe shaping network applying phase and amplitude modulations to the signal, which is split up and transmitted to a number of antenna elements. For certain lobe widths and directions, the phase and amplitude modulation coefficients are fixed. However, the transmission and reception paths from the lobe shaping network to antenna element may vary with time and there has to be a method of calibrating or tuning the antenna elements together so that the lobe shaping network produces accurate lobes. This is the purpose of the present invention.

A schematic view of a path of a phased array signalling system is shown in Figure 1. Signals to be transmitted in a lobe are input to a lobe shaping network LSN. The lobe shaping network controls the splitting and phase and amplitude modulations in order to create the different lobes. The lobe shaping network may include a fixed matrix for the phase and amplitude modulation factors of the different lobes. From the lobe shaping network (LSN) there are individual signal paths to each antenna element. Each signal path includes a phase and amplitude modulation or adjustment unit Adj. As an alternative, the lobe shaping network may consist of digital individually controllable phase and amplitude modulation (PAM) units. In this case, the adjustment unit is in practice an integral part of the PAM unit or the PAM is adjusted directly. The signal paths may also include filters (not shown). It is understood that even though only one signal path is shown for each antenna element, there are in fact two signal paths for reception and transmission, respectively.

According to the invention, a calibration unit Cal. controls the phase and amplitude factors of the adjustment units. The calibration unit injects a calibration signal at a point in the system and measures signals at other points in the system, as is discussed more in detail below. The calibration signal is not critical and may be a pilot tone or a signal having a an arbitrary modulation such as CW noise, pseudorandom noise etc. Suitably, the calibration unit is located indoors in the same frame as the lobe shapina network.

First, the transmission case is discussed, that is calibration of the transmission paths from the lobe shaping network to the individual antenna elements. A first transmission signal path is selected as reference, e. g. No. 4. The adjustment unit of this path is set at a suitable value which is held fixed through the calibration. The fixed value is not critical as long as it is not so extreme that the calibration of the other signal paths will be impossible. Thus, a suitable flat or centred value should be preferable. The calibration signal is injected at a point B in the transmission path close to the output of the lobe shaping network. The injected calibration signal gives rise to a signal that can be measured. The measurement can take place at different points as is discussed below.

Then, a calibration signal is transmitted through the other transmission signal paths to be calibrated, i. e. from corresponding points B to the antenna elements in the respective transmission paths. For the purpose of illustrating the invention we here look at antenna element number 5.

In accordance with a first embodiment of the invention, the signals caused by the injected calibration signal is measured through another antenna element. e. g. No.

3. The signal mav be tapped out at a point A close to the antenna element, e. g. by means of a directional coupler providing a calibration output of the antenna box. or at a point B close to the lobe shaping network. Then, a reception path from the antenna element to point B is used for measuring the signal. It is most preferred that point A is used.

In another embodiment of the invention, the signals caused by the injected calibration signals are measured at a point C in the antenna field of the phased array antenna.

The calibration measurements and calculations are similar to the reception case set forth below.

We will now discuss the reception case, that is the calibration of the reception paths from the individual antenna elements to the lobe shaping network LSN of the phased array antenna. The reception case is similar to the transmission case, except that the injection points of the calibration signal corresponds to the measurement points in the transmission case and vice versa.

A first reception signal path is selected as reference. e. . No. 4. The adjustment unit of this path is set at a suitable value which is held fixed through the calibration. The fixed value is not critical as long as it is not so extreme that the calibration of the other signal paths will be impossible. Thus. a suitable flat or centered value should be preferable. The calibration signal is injected into the reception path and propagates from the antenna element to a point B in the reception path close to the input of the lobe shaping network. The injection can take place at different points as is discussed below. The injected calibration signal gives rise to a signal that can be measured Then, a calibration signal is transmitted through the other reception signal paths to be calibrated, i. e. from the antenna elements to corresponding points B in the respective reception paths. For the purpose of illustrating the invention we here look at antenna element number 5.

In accordance with a first embodiment of the invention, the signals caused by the injected calibration signal is injected through another antenna element, e. g. No.

3. The signal mav be injected at a point A close to the antenna element, e. g. by means of a directional coupler providing a calibration input of the antenna box, or at a point B close to the lobe shaping network. Then, a transmission path from point B to the antenna element is used for injecting the signal. It is most preferred that point A is used.

In another embodiment of the invention, the signals caused by the injected calibration signals are injected at a point C in the antenna field of the phased array antenna.

If the injected calibration signal is injected through another antenna element, the match calibration uses the known interelement coupling between the elements in the antenna array as reference. The coupling between the elements is measured when the antenna is installed. The coupling is known as sij.

A calibration signal is injected e. g. in the directional coupler of the low noise amplifier in the antenna box located in the mast (not shown) connected to antenna element 3. The calibration signal is then received through each signal path corresponding to element 1,2,4 and 5. The signal received through element 4 is used as reference. The other signal paths are then matched to the reference element.

The calibration measurements and calculations using element 4 as reference are as follows.

The output power on element 3 is P3 The input signals on elements 4 and 5 are P4 = P3 . s43 P5 = P3 . S53 The measured signals are then M4 =p4. 4-P3. 543. Hd M5 = P5 . H5 = P3 . S53 . H5 where Hn is the transfer function of the individual reception signal path or channel and contains both the phase and amplitude behaviour.

Therefore:

It will be understood that H and M are affected by the adjustment units each having a controllable adjustment factor. Using the reception signal path 4 as the reference, the adjustment factor of this signal path is held fixed while adjusting the adjustment factor of the signal path 5, such that the relationship Hs/H4 obtains a certain value. Then, this is repeated for the other signal paths, such that the relationships Hi/H4 obtain the same value.

If the injected calibration signal is injected via the antenna field at point C, the match calibration uses the known coupling between the elements in the antenna array and point C as reference. This coupling may also be measured when the antenna is installed. Generally, the amplitude characteristics are set equal between the elements and the phase characteristics are given by the geometrical relationships. The coupling is known as ti. Otherwise, the calibration measurements and calculations are the same as above, substituting ti for sij.

Since it is common that the phased array antenna consists of panels having eight antenna elements, it is preferred that the calibration method is iterated using different antenna elements as the reference. For instance, antenna elements 6 and 5 may be used as references. The calibration signal is then transmitted with element 6 as reference and elements 4,5.7 and 8 are measured. The calibration signal is finally transmitted with element 5 as reference and elements 3, 4, 6 and 7 are measured.

This is in order to reach and measure the signal paths of the various antenna elements.

Also, a phased array antenna may comprise multiple panels arranged in a closed ring. In the multiple panel excitation case the calibration signal is injected in element 1 and measured in element 8 in the adjacent panel and vice versa.

Thus, the antenna elements close to the adjacent panel are used as references for reaching the adjacent panels.

It will be appreciated that some signal paths will be measured more than once with different references, each reference forming a group of calibration measurements. In that case, all the measurement results mav be used bv averaging the adjustment factors belon, to more than one calibration group.

The injection of the calibration signals at the various points in the phased array antenna system ma be accomplished by means of suitable switches and directional couplers. A person skilled in the art will appreciate that the calibration unit and calibration signal injection can be implemented in various way by means of suitable hardware and software combinations which all are intended to fall within the scope of the claims below.

Claims

1. A calibration method for transmission paths of a phased array antenna having a plurality of antenna elements, each with an individual signal path connected to a lobe shaping network (LSN) and including an adjustment unit having variable phase and amplitude adjustment factors, characterised by the steps of : setting the transfer function of a first transmission signal path to a first antenna element at a fixed value; injecting a calibration signal into one end of the first transmission signal path ; sequentially injecting a calibration signal into one end of other transmission signal paths; measuring a first signal caused by the injected calibration signal through the first transmission signal path and sequentially measuring signals through the other transmission signal paths ; adjusting the transfer functions of the other signal paths by varying the respective adjustment factors, so that the same phase and amplitude relationships exist between the first signal and all the other signals caused by the injected calibration signal.

2. A calibration method according to claim 1, characterised by: measuring a signal at a point (A) electrically close to a second antenna element caused by the calibration signal injected into one end of said first signal path and said other signal paths by radio coupling between the thus excited antenna elements and the second antenna element; and adjusting the transfer functions taking account of the known interelement coupling (sij) between the excited antenna elements and the second antenna element.

3. A calibration method according to claim 1, characterised by: measuring a signal using the reception path of a second antenna element at a point (B) close to the lobe shaping network (LSN) caused by the calibration signal injected into one end of said first signal path and said other signal paths by radio coupling between the thus excited antenna elements and the second antenna element; and adjusting the transfer functions taking account of the known interelement coupling (six) between the excited antenna elements and the second antenna element.

4. calibration method according to claim 1, characterised by : measuring by radio a signal at a point (C) in the antenna field of the phased array antenna, caused by the calibration signal injected into one end of the first and the other sisnal paths; and adjusting the transfer functions taking account of the known transfer factor (ti) of the radio path between the point (C) in the antenna field and the respective antenna element.

5. A calibration method according to any one of claims 1 to 4, characterised by: iterating the steps of the method sequentially using different signal paths as the first signal path, each iteration forming a group ; estimating the adjustment factors of the other signal paths in the respective groups ; and averaQ-ing the adjustment factors of signal paths belonging to more than one group.

6. A calibration method for reception paths of a phased array antenna having a plurality of antenna elements, each with an individual signal path connected to a lobe shaping network (LSN) and including an adjustment unit having variable phase and amplitude adjustment factors, characterised by the steps of : settinD the transfer function of a first reception signal path from a first antenna element at a fixed value; injecting a calibration signal into one end of the first reception signal path; sequentially injecting a calibration signal into one end of other reception signal paths; measuring a first signal caused by the injected calibration signal through the first reception signal path and sequentially measuring signals through the other reception signal paths; adjusting the transfer functions of the other reception signal paths by varying the respective adjustment factors, so that the same phase and amplitude relationships exist between the first signal and all the other signals caused by the injected calibration signal.

7. A calibration method according to claim 6, characterised by: injecting a calibration signal at a point (A) electrically close to a second antenna element in order to excite the antenna element, such that the calibration signal is injected into one end of said first and other signal paths by radio coupling between the second antenna element and the first and other antenna elements; and adjusting the transfer functions taking account of the known interelement coupling (sij) between the second antenna element and the other antenna elements.

8. A calibration method according to claim 6, characterised by: injecting a calibration signal using the transmission path of a second antenna element at a point (B) close to the lobe shaping network (LSN) in order to excite the second antenna element, such that the calibration signal is injected into one end of said first and other signal paths by radio coupling between the second antenna element and the other antenna elements; and adjusting the transfer functions taking account of the known interelement coupling (sij) between the second antenna element and the other antenna elements.

9. A calibration method according to claim 6, characterised by: injecting a calibration signal at a point (C) in the antenna field of the phased array antenna, such that the calibration signal is injected into one end of the first and the other the signal paths by radio; and adjusting the transfer functions taking account of the known transfer factor (ti) of the radio path between the point in the antenna field and the respective antenna element.

10 A calibration method according to any one of claims 6 to 9, characterised by: iterating the steps of the method sequentially using different signal paths as the first signal path, each iteration forming a group; estimating the adjustment factors of the other signal paths in the respective groups; averaging the adjustment factors of signal paths belonging to more than one group.

11. A method substantially as herein described with reference to the accompanying drawings.