Patent Application
20210203422
The present invention relates generally to methods and apparatus for location of a source of passive intermodulation (PIM) within an antenna array assembly.
Passive intermodulation (PIM) may be generated in a wireless network when one or more signals are transmitted along a signal path including a passive component having a non-linear transmission characteristic. PIM products typically differ in frequency from the signal or signals from which they were generated, and may potentially cause interference to other signals. The generation of non-linear products is becoming a problem of increasing importance in modern wireless communication systems, and in particular cellular wireless systems, since the radio frequency spectrum available has been steadily expanded as additional bands have become available, and the pattern of allocation of uplink and downlink bands within the available spectrum for use by various cellular systems, such systems using GERAN (GSM EDGE Radio Access Network), UTRAN (UMTS Terrestrial Radio Access Network) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) radio access networks, and by various operators, is complex and territorially dependent. Non-linear products generated from transmitted carriers in one or more downlink bands may fall as interference within an uplink band in which signals are received at the base station. This interference may limit the capacity of the radio system, and so it is important to minimise the level of PIM generated in a wireless system. Antennas and their feed networks may exhibit a non-linear transmission characteristic to some degree which may generate PIM, for example due to an oxide layer at a metal to metal contact, or a poor solder joint during manufacture. An antenna array may be provided, for example base station antennas typically comprise a vertical array of antenna elements fed by a feed network to produce a narrow beam in elevation and a broader beam in azimuth. The elevation angle of the beam is typically adjusted on installation of the antenna and may be further adjusted in use. Typically, the beam is given a certain angle of down-tilt from the horizontal, to limit interference to the areas of coverage of other base stations. In order to facilitate the adjustment of the tilt angle, a Remote Electrical Tilt (RET) facility may be provide for an antenna array, by which the relative transmission and/or reception phase of antenna elements or groups of antenna elements (sub-arrays) may be adjusted, by providing a incremental phase shift across the array, which has the effect of tilting the beam angle. Typically, an antenna array assembly may be provided with a controllable multi-element phase shifter, which may be an electro-mechanical device comprising signal splitters/combiners and sliding capacitive contacts, which may adjust the phase of a plurality of transmission paths by a change of path length. The controllable multi-element phase shifter may be set by use of an electric motor.
There are many locations within an antenna array assembly comprising a controllable multi-element phase shifter at which PIM may be generated. It may be desired to locate a PIM source within the antenna array assembly for diagnosis of a fault condition, or as a factory test, for example. Existing methods of locating PIM in a signal path involve using a swept frequency excitation and deriving a distance between the PIM source and the receiver from a delay value derived from the phase gradient of the received PIM. This technique may be useful to detect sources of PIM in the wireless propagation path, such as a PIM source on a rusty part of an antenna tower, but such techniques may not be able to distinguish between PIM sources in a branched structure such as an antenna array assembly in which a single connection port may be connected to several branches of the antenna array, each branch for example being a feed to a sub-array, and each branch being provided with a respective phase shift by a controllable multi-element phase shifter.
It is an object of the invention to address at least some of the limitations of the prior art systems.
In accordance with a first aspect of the present invention there is provided a method of identifying a location of at least one PIM (passive intermodulation) source within an antenna array assembly comprising a plurality of sub-arrays, a connection port, and a controllable multi-element phase shifter configured to apply a respective phase shift to a respective path between the connection port and each sub-array, the method comprising:
This allows identification of the signal path within the antenna array assembly in which the PIM source is likely to be located.
In an embodiment of the invention, the first and the further measurements are of the amplitude and phase of the PIM product, and the method comprises determining from the first and further measurements a dependence of the amplitude and phase of the PIM product on the state of the multi-element phase shifter, and said comparing comprises:
This may allow more accurate identification of the path in which the PIM source is likely to be located by taking into account amplitude as well as phase variation, which may result for imperfect impedance matching in the antenna array causing reflections.
In an embodiment of the invention, said comparing comprises a cross-correlation
This provides an efficient method of identifying which path a PIM source is likely to be located on, in particular in the case of a single PIM source.
In an embodiment of the invention, said comparing comprises a Linear Least Squares process, which may comprise identifying the location of one or more PIM sources by solution of Ax=b,
This provides an efficient method of identifying which path or paths a PIM source or sources are likely to be located on, particularly in the case of more than one PIM source.
In embodiments of the invention, the controllable multi-element phase shifter is a device for applying a Remote Electrical Tilt (RET), which may comprise a plurality of power dividers and a plurality of controllable phase shifting elements, and each sub-array may comprise one or more antenna elements for radiation and/or reception.
In an embodiment of the invention, the excitation waveform comprises a first and a second signal, wherein at least the first signal is a continuous wave (CW) signal.
This provides a convenient way of implementing an excitation waveform.
In an embodiment of the invention, the second signal is a continuous wave (CW) signal.
This provides a convenient way of implementing an excitation waveform for generating PIM of an expected frequency.
In an embodiment of the invention, the second signal is a modulated signal. The second signal may be modulated with a noise-like waveform having a bandwidth in the range 10 MHz to 40 MHz.
This provides a convenient way of implementing an excitation waveform for generating PIM, which may provide improved resilience to phase distortion from reflections and element mutual coupling, and may facilitate delay measurements to determine range to a PIM source or sources as well as path.
In an embodiment of the invention, the method comprises:
This allows more precise determination of which part or parts of the antenna array assembly is the location of a PIM source, by allowing a location to be determined in terms of distance along a path in addition to identifying on which signal path within the antenna array assembly the PIM source or sources are located.
In an embodiment of the invention, the plurality of predetermined dependences of at least the phase of the PIM product on the state of the multi-element phase shifter include effects of mutual coupling between sub-arrays.
This may allow more accurate identification of which path is causing PIM in the presence of mutual coupling.
In an embodiment of the invention, the plurality of predetermined dependences of at least the phase of the PIM product on the state of the multi-element phase shifter include effects of reflections between the phase shifter and the sub-arrays.
This may allow more accurate identification of which path is causing PIM in the presence of reflections within the antenna array assembly.
In an embodiment of the invention, the plurality of predetermined dependences of at least the phase of the PIM product on the state of the multi-element phase shifter includes dependencies for reflective paths.
This may allow the identification of reflective paths in addition to direct paths which may further assist in identifying a location or locations of sources of PIM.
In an embodiment of the invention, each state of the phase shifter represents a tilt angle for the antenna array.
In accordance with a second aspect of the invention, there is provided test apparatus for identifying a location of at least one PIM (passive intermodulation) source within an antenna array assembly comprising a plurality of sub-arrays, a connection port, and a controllable multi-element phase shifter configured to apply a respective phase shift to a respective path between the connection port and each sub-array, the test apparatus comprising:
Further features and advantages of the invention will be apparent from the following description of preferred embodiments of the invention, which are given by way of example only.
By way of example, embodiments of the invention will now be described in the context of identifying a location of at least one PIM (passive intermodulation) source in an antenna array assembly for use in in cellular wireless networks such as GSM, 3G (UMTS) and LTE (Long Term Evolution) networks comprising GERAN, UTRAN and/or E-UTRAN radio access networks, but it will be understood that embodiments of the invention may relate to other types of branched radio frequency device and to other types of radio access network, and that embodiments of the invention are not restricted to cellular wireless systems or to base station antennas.
In a cellular wireless network, PIM may be generated in a component due to a passive non-linear characteristic, albeit a relatively weak non-linear characteristic. The non-linear characteristic may be caused by an oxide layer between metallic parts, for example in an antenna array assembly at a base station. The antenna array assembly may be impinged upon by the downlink transmitted signals, and then the generated PIM may be transmitted back into an uplink receiver at the base station. The generation of PIM is by radio frequency mixing between, in this example, the two signals at frequencies f1 and f2, or between different frequency components of a modulated signal, such as an OFDM signal, which may be relatively wideband, occupying for example 10% or more of the passband of a frequency selective device. As a result of the radio frequency mixing, PIM product may be generated at various frequencies, but it is PIM products which fall at a frequency in a receive band of the cellular wireless system which may be problematic, since the PIM may be received as interference. PIM products generated by intermodulation within a wideband modulated signal may fall within or adjacent to the signal bandwidth and so may be seen as interference. For example, PIM products may be third order products appearing at frequencies 2 f1-f2 and 2 f2-f1. So, antenna array assemblies are typically tested on manufacture, and potentially also in the field, to determine whether they meet stringent specifications for the generation of PIM. In the event that an antenna array assembly is found to be generating PIM, it may be required to locate the source of PIM within the assembly, so that corrective action may be taken.
In the embodiment of the invention illustrated by
The processor/controller may then set the multi-element phase shifter to a succession of further states, the respective phase shift applied to each of the respective paths being dependent on the state, and make a further measurement of at least the phase of the PIM product emitted from the connection port for each of the further states. The processor/controller may then determine from the first and further measurements a dependence of at least the phase of the PIM product on the state of the multi-element phase shifter. The function representing the dependence of at least the phase of the PIM product on the state of the multi-element phase shifter may be referred to as a “cisoid”, which is a complex representation in inphase and quadrature components at baseband of the phase and/or amplitude of the received PIM product as a function of the phase shifter state, which may be expressed in terms of tilt angle.
The received PIM product may be a PIM product selected to be of interest for the test, typically a product of two or more signals or signal components in a downlink band for transmission by the antenna which fall within an uplink band of the antenna, and so would potentially appear as interference to received signals in use. The PIM receiver is accordingly tuned to receive the expected PIM product, for example a low side third order two tone product of the form f1-2 f2, where f1 and f2 are the respective carrier frequencies of the downlink band signals causing the PIM.
The determined dependence of at least the phase of the PIM product on the state of the multi-element phase shifter is then compared, under control of the controller/processor of the test apparatus, with a plurality of predetermined dependences of at least the phase of the PIM product on the state of the multi-element phase shifter, each predetermined dependence being for a PIM source located in a respective path between the multi-element phase shifter and a respective sub-array, including the respective sub-array. That is to say, the measured cisoid is compared with pre-determined cisoids for each path. The processor/controller then determines the location within the antenna array assembly of the at least one PIM source 10, shown in
The signal processing circuit comprising a processor 5 as shown in
The dependencies shown in
The measures and pre-determined dependencies may be either of phase only or phase and amplitude representations of the PIM product. So, the first and the further measurements may be of the amplitude and phase of the PIM product, and the method comprises determining from the first and further measurements a dependence of the amplitude and phase of the PIM product on the state of the multi-element phase shifter. In this case the comparison process comprises comparing the determined dependence of the amplitude and phase of the PIM product on the state of the multi-element phase shifter with a plurality of predetermined dependences of the amplitude and phase of the PIM product on the state of the multi-element phase shifter, each predetermined dependence being for a PIM source located in a respective path between the multi-element phase shifter and a respective sub-array, including the respective sub-array. This may allow more accurate identification of the path in which the PIM source is likely to be located by taking into account amplitude as well as phase variation, which may result from imperfect impedance matching in the antenna array causing reflections.
Using a cross-correlation technique for the comparison of the measured and predetermined dependencies may be an efficient method of identifying which path a PIM source is likely to be located on, in particular in the case of a single PIM source.
Using a Linear Least Squares process for the comparison of the measured and predetermined dependencies may comprise identifying the location of one or more PIM sources by solution of Ax=b,
The equation Ax=b may be solved for x by well-known linear algebra techniques. For example, the equation may be solved by matrix inversion or by Gaussian elimination and back substitution. The solution to the equation may be calculated by using signal processing chips, or by software running on a general purpose computer or by other digital signal processing hardware, software, and/or firmware.
In an alternative embodiment, the second signal may be a continuous wave (CW) signal. This provides a convenient way of implementing an excitation waveform for generating PIM of an expected frequency, and may conform to existing PIM test requirements.
The location of at least one PIM source may be determined in dependence on the determined delay, based on a known relationship between delay and propagation distance for the transmission medium through which the signals propagate. The location can be described in combination with a path determined by comparing measured and pre-determined dependencies on the state of the phase shifter as already described.
So the plurality of predetermined dependences of at least the phase of the PIM product on the state of the multi-element phase shifter includes dependencies for reflective paths. This may allow the identification of reflective paths in addition to direct paths which may further assist in identifying a location or locations of sources of PIM.
Alternatively or in combination, a combined pre-determined dependency may be determined by combination of the direct and phantom dependencies for each setting of the phase shifter, and the combined dependency may be used for comparison with the measured dependency to determine on which path the PIM is located.
So, the plurality of predetermined dependences of at least the phase of the PIM product on the state of the multi-element phase shifter may include effects of mutual coupling between sub-arrays and may include effects of reflections between the phase shifter and the sub-arrays. This may allow more accurate identification of which path is causing PIM in the presence of reflections within the antenna array assembly.
As shown in
Measurements are made of the complex reverse PIM response vs tilt angle and a wideband reverse PIM response obtained. The detection process is then run which compares the measurements with the candidate fault location signatures. The output of this process are the likely fault locations, levels and confidence metrics. The latter is an indication of how closely the measurements correspond with the detected fault locations. Remedial action is then taken to repair the faulty locations and the antenna re-tested to confirm if the repair has been successful, then this loop may need to be repeated.
A range of data may be collected during the overall process and this data used to improve the overall detection reliability by tuning it on the basis of the success and failure of the detections as the available result history is built up over multiple antenna testings. For example it may be found over time that the fault signatures are not closely matching the measured results. For the example of single detected faults, corresponding to actual faults identified at locations known by their successful repair, the fault signatures may be updated so as to more accurately match the measured data.
Secondly the reliability of certain detections can be monitored by examining the repair success and the detection confidence process tuned to more accurately reflect the measured detection probability and false alarm rates. It is possible that certain fault locations may also generate a spurious or phantom detection at a second location. Over multiple antenna measurements and corresponding good or bad detections it will become clearer where these phantom detections are likely to occur and the corresponding detection confidence levels modified accordingly. A further benefit of this process is that the frequency of individual fault conditions may be monitored and a high occurrence of specific faults may then be investigated to ascertain if a certain manufacturing process is at fault and corrective action taken to remedy this
It may already be known from the antenna design that certain fault locations are likely to occur more often. For example certain sections of the feed network are likely to have more solder joints and or be subject to greater RF power and hence be more likely to exhibit a greater propensity to fault conditions.
The detection confidence process may be pre-primed to take advantage of this using, for example, Bayesian statistics. In addition to the historical learning process outlined on the previous chart a fast learning mode could be of benefit in accelerating the process. One way this may be accomplished is by taking a known good antenna and then introducing faults node by node and updating the corresponding fault signatures to better match the measured data.
The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1810769.8 | Jun 2018 | GB | national |
This application is a continuation of International Application No. PCT/GB2019/051868, filed Jul. 1, 2019, which claims priority to GB Application No. 1810769.8, filed Jun. 29, 2018, under 35 U.S.C. § 119(a). Each of the above-referenced patent applications is incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/GB2019/051868 | Jul 2019 | US |
| Child | 17127478 | US |
