WO2014085345A1

WO2014085345A1 - Built-in passive intermodulation detector for base station equipment

Info

Publication number: WO2014085345A1
Application number: PCT/US2013/071747
Authority: WO
Inventors: Scott Carichner; Jeremy BAXTER
Original assignee: P-Wave Holdings, Llc
Priority date: 2012-11-30
Filing date: 2013-11-25
Publication date: 2014-06-05
Also published as: CN104756419B; CN104756419A; EP2926480A4; EP2926480A1

Abstract

A new invention and the preferred embodiments of that invention that allow in-situ testing passive inter-modulation (PIM) in cellular base stations equipment on a continuous basis a disclosed.

Description

BUILT-IN PASSIVE INTERMODULATION DETECTOR FOR BASE STATION

EQUIPMENT

BACKGROUND OF THE INVENTION

[0001] Frequency division duplexing (FDD) wireless cellular base stations (BTS) utilize relatively high power transmitters whilst simultaneously detecting very low-level receive signals near the noise floor of the system. Increasingly, the receive band is within the range of the intermodulation signals from the transmit band because of new frequency bands and wider transmit bandwidths. Under these conditions the low-level non-linearities that can usually be ignored in a passive electro-mechanical system can create receiver de-sensitization that can decrease system performance and which is difficult to detect, identify, and fix.

[0002] While specialized test equipment exists to detect and identify passive intermodulation (PIM), the equipment is easiest to use in the laboratory and factory environment. Portable versions of the equipment can be brought to the BTS in the field, but can only be effectively used during the initial check-out of the installation. Subsequent degradation in either the RF path equipment or changes in the environment (e.g. rusted metal within the antenna beam pattern, or added structures) may create detrimental PIM that will go undetected.

[0003] FIGURES la and lb are block diagrams illustrating the limitations of current PIM detection systems in the prior art. In order to run the test the coaxial cables used to feed the antenna system must be disconnected from the base station and connected to the test equipment. In order to make that change the system has already been disturbed possibly changing the PIM performance in a way that prevents the further detection of the problem. Additionally the base station is taken out of use for customers. This operation is very undesirable as it needs to be done at a time that will affect the fewest customers, such as late at night. Finally, the PIM tester uses narrowband CW signals that are typically swept across the band, interfering with all the other licensed users in the band at a very strong output level, high enough to interfere with users not only in this cell but at nearby cells as well. This is not only undesirable but potentially a violation of the operator's regulatory license. This leads to the desire to be able to make the test without the interruption to current operations and without using special test signals with the risk of the interference they would cause.

SUMMARY OF THE INVENTION

[0004] The present disclosure relates to a system and method for detecting unwanted passive intermodulation built-in to base station equipment that can operate in-situ to identify and alert the operator to the problem. In one aspect, the hardware to acquire the needed raw data is implemented within the RF portion of the base station electronics. This RF portion can be included in the base station electronics or it can be included in the RF portion of the base station that has been moved closer to the antenna to decrease the RF losses entailed between the power amplifier and low noise amplifier of the transmitter and receiver, respectively. In some cases, the entire base station is moved to a location close to the radiating antenna such as in so-called small cells or picocells. In addition to the RF portion for collecting the raw signals, a digital signal processing element is added or re-used if already available to implement the method in which the PIM is identified and announced to the network operator. Further embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The foregoing aspects and many of the attendant advantages of this disclosure will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGURES la and lb show how a prior art PIM detector can be used only at initial installation time since the BTS must be disconnected and an external test signal and detector connected;

FIGURE 2 illustrates a block diagram of the built-in PIM detector within a remote radio-head (RRH) of a base station;

FIGURES 3a, 3b, 3c and 3d illustrate that the embodiment of FIGURE 2 can be applied in multiple ways with the same hardware since the RRH can be used as an active antenna 3 a of the type referred to as an antenna with integrated RRH, as a small cell 3 b or what is sometimes called a picocell, as a tower mounted RRH 3c, or as a standard base station 3d at the base of the antenna pedestal;

FIGURE 4 illustrates an embodiment of the invention where the tower-mounted amplifier (TMA) is modified to include the built-in PIM detector and the power and data

communications to the detector is provided within the Antenna Interface Standards Group (AISG) Industry standard protocol;

FIGURE 5 illustrates an embodiment of the invention where the passive antenna is modified to include the built-in PIM detector and the power and data communications to the detector is provided within the AISG Industry standard protocol;

FIGURE 6 illustrates a system and method for using the hardware to detect the PIM; and FIGURES 7a-7e illustrate results from typical scenarios using the method of FIGURE 6.

DETAILED DESCRIPTION OF THE INVENTION

[0006] Generally described, the present disclosure relates to a system and method for detecting unwanted passive intermodulation built-in to base station equipment that can operate in-situ to identify and alert the operator to the problem. Referring to FIGURE 2, in one aspect, the hardware to acquire the needed raw data is implemented within the RF portion 110 of the base station electronics. This RF portion can be included in the base station electronics FIGURE 3d, or it can be included in the RF portion of the base station that has been moved closer to the antenna to decrease the RF losses entailed between the power amplifier and low noise amplifier of the transmitter and receiver, respectively, FIGURES 3 a and 3 c. In some cases the entire base station is moved to a location close to the radiating antenna such as in so-called small cells or picocells FIGURE 3b. In addition to the RF portion for collecting the raw signals a digital signal processing element 112 is added or reused if already available to implement the method in which the PIM is identified and announced to the network operator.

[0007] One skilled in the relevant art will appreciate that the disclosed embodiments and examples are illustrative in nature. Accordingly, the disclosed embodiments and examples should not be construed as limiting. Additionally, although various aspects of the present disclosure have been identified and may be described together, the present disclosure is not limited to embodiments in which all the identified aspects must be considered or combination of any described aspects should be required.

[0008] FIGURE 2 illustrates an embodiment of the hardware that is used to collect the signals and information necessary to identify and quantify any destructive intermodulation that could be created at the site. The destructive intermodulation can occur anywhere starting inside the transmit filters 114, tracing a path 105 to the antenna and then within the antenna 100 (such as in the antenna distribution network 102 or the antenna elements 101), to even beyond the antenna through PIM causing conductive materials within the antenna beam that can create reflections that will be received back by the antenna and show up within the receive band.

[0009] It should be noted that a potentially new source of PIM can occur during the lifecycle of the antenna site if a new nearby structure is added or if an existing intermetallic connection within the antenna beam gets oxidized in a way that creates PIM.

[0010] The embodiment in FIGURE 2 is preferred because much of the system in place for normal base station operation can be reused for generating the high power source signals 115 and filtering them properly 114 to remove active intermodulations. This signal will be reflected back into the receiver where the filter will reject the linear portion of the signal enough so that there is no need to reduce the gain on the receiver or any risk of the LNA in the system regenerating active 3^rd order intermodulations at a problematic level.

[0011] A sample of the transmitted signal and a sample of the received signal is created in 116 and available to 112 on a continuous basis. This allows the processing method to use very long correlation times to extract a sensitive measurement out of an otherwise noisy environment.

[0012] FIGURES 3a, 3b, 3c and 3d show that the preferred embodiment of FIGURE 2 can be used in multiple ways and locations that are often given different names but are essentially the same from the point of view of this invention. In the traditional method 3d the RF electronics are positioned in the same location as the base station's baseband processing electronics, typically, but not always, in an enclosed rack mount system. In this case a coaxial cable for the RF will be driven from the bottom to the antenna. In more recent designs, loss-sensitive components (namely the PA and LNA) are moved closer to the antenna to improve the link budget of the system. In this case, a system such as 3c maybe more common whereby the coaxial cable is only used as a jumper to the antenna and a larger portion of the distance between base station and antenna is comprised of an optical fiber digital connection. Taken further, this leads to the system in 3 a whereby the RRI I is physically attached to the antenna. This configuration is often called an Active Antenna (AA) or sometimes an antenna integrated RRI I. Finally, the base station baseband processing electronics are sometimes integrated into this active antenna as shown in 3b in which case only an Ethernet connection is required to feed down from the antenna location. PIM is a risk in all of these implementations and therefore the built-in in-situ detection of PIM is very advantageous.

[0013] FIGURE 4 is an embodiment of the invention that requires a different hardware configuration to collect the raw signals necessary to check for PIM. This is a preferred embodiment if the system is built-in to the TMA. A TMA is generally a low noise amplifier (LNA) placed near the antenna to minimize the cable losses between the antenna and the RF front end LNA. In this topology, a duplexer provides access to the receive path which is then amplified and duplexed back into the single cable containing the transmit and receive signal. In a TMA some of the features necessary for collecting the PIM data already exist in the enclosure, such as the transmit filters 211, 212 that provide enough band isolation between transmit and receive that the signals can be efficiently separated and recombined, and the LNA 214 that boosts the received signal to prevent further loss on the feed cable, and the AISG power and data interface, and the entire environmental enclosure 210 that isolates the electro-mechanical system from the elements and acts as an RF shield.

[0014] With continued reference to FIGURE 4, it is shown that additional circuits and components need to be added to provide the PIM testing function. The transmit reference signal is sampled with a directional coupler 213 to provide a reference to use for the modeling. The receive signal is sampled with a directional coupler 215 to provide the source for evaluating the PIM that falls into the receive band. In some embodiments of the system it may be preferred to provide a non-directional coupler, or the opposite direction coupler for either the transmit 213 or receive path 215 coupler so that the sensitivity for PIM generated at various points along the transmission path on the base station or antenna side of the TMA can be emphasized or de-emphasized. RF switches 216 are used to switch different input paths in multiple antenna systems (e.g. receive diversity, MIMO, space-time diversity) into a single capture system. If it is desired to improve the sensing speed it would be possible to add additional capture systems and remove some or all of the switches 216 so that the data could be captured and processed in parallel. To overcome the coupling loss from the sampling process the weaker receive signal is fed into another LNA 214 prior to a lossy filter stage 217. The sampled transmit signal is at a very high level and needs no further amplification. In fact it needs to be attenuated significantly so that it can be combined with the received signal at similar levels for digitization.

[0015] In order to simplify the processing and lower the cost, the transmit and receive signal can be combined with any of the common signal summers 218 appropriate for RF signals at different frequencies. The signal needs to be downconverted to a lower IF by a local oscillator (LO) 219 and mixer to allow efficient digitization by currently available Analog-to- digital converters (ADC). In some embodiments of the design, no downconversion may be necessary and the signal can be directly digitized with an ADC. The appropriately filtered and amplified signal is digitized by an ADC 220 that may have a speed and resolution such as 500 MSPS and 12 bits. However, many other speeds and resolutions can be used as appropriate to provide the necessary inputs for processing.

[0016] The sampled signals, either from a single ADC or multiple ADCs are provided to a digital signal processing (DSP) device 221 such as an FPGA, DSP, ASIC or microprocessor for identification of the PIM. This device implements the computationally intensive part of the method which is described later in reference to FIGURE 6. The results after processing on the DSP 221 are examined by the microprocessor. If the results indicate that there is PIM present, the magnitude is reported to the operator through the AISG protocol. This protocol exists within a standard TMA device so that it can obtain power and also report the status of the TMA over the data status and alarm message in the AISG protocol. Other proprietary and non-proprietary methods may be used to obtain power and report the data results and still remain within the scope of this disclosure.

[0017] FIGURE 5 illustrates an embodiment of the invention for use in a passive antenna. In this embodiment, there is no existing hardware available for re-use except the AISG data and power ports that the antenna may use to communicate and power an electrical tilt function in the antenna if one exists. In this embodiment a carefully environmentally and RF shielded housing 501 is included in the antenna housing. This housing must be carefully designed so that the sensitive antenna signals are not interfered with. In so doing, the couplers needed to sample the transmit and receive signals 502, 503 are carefully fed into the housing through filter and attenuation structures sufficient to prevent coupling of any unwanted signals back onto the antenna lines. This is done by having special cavity combline filters 504 on the receive path. These filters provide many functions. They support surge protection necessary for antennas mounted on towers and structures to protect the electronics. They are built in a way such that they filter all the signals that enter or leave the enclosure so that no out-of- band signals can leak in our out of the structure. They provide low-loss in the passband which will represent some or all of the receive band that the PIM is to be detected so that the PIM sensitivity can be maximized. They provide sufficient rejection for the very strong transmit signals that are primarily creating the PIM but which need to be attenuated until they are low enough so that the receive band electronics do not either saturate losing their sensitivity or create non-linearities due to the IIP3 that would mimic and mask the PIM that is trying to be detected.

[0018] On the transmit side, the coupler 503 should provide as much attenuation as possible without creating a leakage path that is strong enough to degrade the sample. This may be around 50dB of attenuation. A special compartment is made to ensure that these signals will be entering the RF enclosure at a low enough level so that they will not leak into the receive path and de-sensitize or create intermods. This special compartment may include shielding such that low cost surface acoustic wave (SAW) filters can be used to pass only the transmit band signal which are of interest in sampling for checking of PIM. In this case loss is unimportant as the signals are very strong and must be attenuated prior to entry into the enclosure. A switch on both paths allows a single set of electronics to process all the multiple antenna RF paths. While two paths are common and shown in FIGURES 2, 4, and 5, it should be obvious that the number of paths can vary according to the design of the cellular system. It would be common to use 1, 2, 4, 8 paths, and any other value is readily possible.

[0019] Once the signals are attenuated and filtered they may be combined with a summer 218 into a single signal, separated naturally by transmit and receive frequencies and mixed to a lower frequency by a LO 219 such that they can be more appropriately sampled by the ADC 220. As in the other embodiments a DSP device 221 processes the signal into using a method that identifies PIM and the microprocessor measures the existence and magnitude of the PIM for reporting as an alarm and message to the operator. This can be sent over an existing AISG channel if it exists or by some other proprietary or non-proprietary alternative protocol.

[0020] FIGURE 6 illustrates the method used to identify in-situ PIM. Because there is no opportunity to disconnect the cable and insert a test signal as shown in the prior art of FIGURE 1, the signals used are actual transmitted data. These may be any of the standard wireless protocols used (e.g. GSM/EDGE, WCDMA/HSPA, 1XRTT EVDO, LTE, etc.) or a combination of them. The particulars are unimportant as a sample of the sum of the signals is collected by a coupler 603 and will provide the reference for comparing to the received samples. In all the systems mentioned this composite transmit signal is processed and amplified to provide the desired cellular performance. The signal is filtered 601 to meet emission mask requirements, and to ensure that any active intermods or noise in the receive band are attenuated to a point sufficiently below the power spectral density (PSD) of thermal noise which is approximately -174 dBm/Hz in this case. A diagram of the transmission path 650 shows a simple model that has been used to develop and verify the method used to detect the PIM. It includes linear delay elements 651, between which various PIM sources represented as mild non-linearities 652 are modeled. Additionally, the real existing receive signals 653 are added in to the receive path since they will act as an interferer to detecting the PIM. The modeled time-delayed non-linearities and linear received signals are summed with additive white Gaussian noise (AWGN) 654 to represent the thermal noise floor captured by the receiving antenna and components.

[0021] The received signal is filtered 602 as a real system would do to eliminate out-of-band signals and specifically to substantially lower the level of the transmitted signal power which if not attenuated will lead to active intermods and/or receiver de-sensitization. The receive signal 604 is amplified and digitally down converted. The transmit sample 603 is interpolated to a high enough sample rate to include the 3^rd order intermod frequencies. Then the transmit sample is processed through a 3^rd order non-linearity model 605 to generate the same non-linearities that would be expected in PIM. While the PIM contains many orders of non-linearities with multiple, unknown time delays, the PIM is detected by correlating just the 3^rd order portion of the transmit signal against the potential PIM over different time delays. The two signals (sampled transmit 3^rd order non-linearity and the received PIM sample) are aligned in frequency through any further digital frequency conversion necessary. A basic time alignment is done to set a zero delay point to calibrate where a correlation would occur if the PIM generating non-linearity was immediately at the transmitter output. Finally a cross-correlation 606 is run between the two complex signals. This complex cross- correlation is described as:

Where * is the complex conjugate operator,

2N+1 is the length of the sequences used for the cross-correlation,

x is the modified transmit sample signal, and

y is the modified receive sample signal.

[0022] The output is evaluated 607 and the peaks are evaluated. If any peaks are detected above a pre-defined noise threshold that is dependent on the duration of the measurement and the averaging that is used, they are identified by the magnitude of z which represents the value of PIM and m which represents 2 times the distance to the PIM generating source in terms of the inverse of the sample rate used for the correlation. The determination of the value and distance to various PIM sources is provided to the alarm message processing software which will send the message to the operator.

[0023] FIGURES 7a, 7b, 7c, 7d, and 7e represent plots of various signals at certain points in the system of FIGURE 6. FIGURE 7a illustrates a sample transmitted signal if there were two carriers of a signal similar to 1XRTT where the signals are spaced with several empty carriers between them. FIGURE 7a would represent such an example signal at 603.

FIGURE 7b illustrates the signal of FIGURE 7a after the PIM has been created, such as immediately after 652. The PIM is represented by the intermodulation that is substantially lower than the transmitted signals. FIGURE 7d shows transmit sample of 603 after it has been run through a simulated non-linearity and filtered with a simulated receive filter and is represented as the transmit sample to enter the cross-correlator 606. FIGURE 7e represents the receive sample after the receive filter and with the AWGN added. This represents the receive sample to enter the cross-correlator 606. Finally, FIGURE 7c represents the output of a running mean of multiple cross-correlations on 1024 point samples over 10 ms. In FIGURE 7c it is evident that there are 2 strong PIM sources of similar amplitude but delayed by about 1.5 us. This is consistent with the model that was used to create the non-linearities 651, 652 in this example. The evaluator block 607 would evaluate this input periodically and extract meaningful peaks and classify them as PIM. Meaningful peaks are those above a certain threshold set individually per case based on the averaging of measurements, noise in the system, the sample rates, and bandwidths of the system.

[0024] It will be appreciated by those skilled in the art and others that all of the functions described in this disclosure may be embodied in software executed by one or more processors of the disclosed components and mobile communication devices. The software may be persistently stored in any type of non-volatile storage.

[0025] Conditional language, such as, among others, "can," "could," "might," or "may," unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment.

[0026] Any process descriptions, elements, or blocks in the flow diagrams described herein and/or depicted in the attached figures should be understood as potentially representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or steps in the process. Alternate

implementations are included within the scope of the embodiments described herein in which elements or functions may be deleted, executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those skilled in the art. It will further be appreciated that the data and/or components described above may be stored on a computer-readable medium and loaded into memory of the computing device using a drive mechanism associated with a computer readable storing the computer executable components such as a CD-ROM, DVD-ROM, or network interface further, the component and/or data can be included in a single device or distributed in any manner. Accordingly, general purpose computing devices may be configured to implement the processes, algorithms, and methodology of the present disclosure with the processing and/or execution of the various data and/or components described above.

[0027] It should be emphasized that many variations and modifications may be made to the above-described embodiments, the elements of which are to be understood as being among other acceptable examples. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Claims

What Is Claimed Is:

1. A system for passive intermodulation (PIM) detection associated with a wireless cellular base station, the system comprising:

a. a passive antenna;

b. a base station; and

c. a remote radio-head,

wherein the remote radio-head couples the base station to the antenna, the remote radio-head including a configuration to identify and quantify destructive intermodulation, wherein the remote radio-head is configured to receive signals from the base station, use those signals to generate high power source signals for PIM detection and reflect those signals to a filter arranged to reject a linear portion of the signal sufficient to reduce gain on a receiver thereof.

2. The system of Claim 1 wherein the remote radio-head is configured to create samples of transmitted and received signals on a continuous basis to enable signal processing using long correlation times.

3. The system of Claim 1 wherein the base station and the remote radio-head are co- located at a base of a tower of the antenna.

4. The system of Claim 1 wherein the base station is located at a base of a tower of the antenna and the remote-radio-head is located on the tower spaced apart from the antenna and the base station.

5. The system of Claim 1 wherein the base station is located at a base of a tower of the antenna and the remote-radio-head is co-located on the tower with the antenna.

6. The system of Claim 1 wherein the base station and the remote radio-head are co- located with the antenna.

7. A system for passive intermodulation (PIM) detection associated with a wireless cellular base station, the system comprising: a. a passive antenna;

b. a base station; and

c. a tower-mounted amplifier including PIM detection module,

wherein the tower-mounted amplifier couples the base station to the antenna, the PIM detection module including a first coupler for sampling a transmit reference signal to establish a reference for PIM modeling and a second coupler for sampling a receive reference signal to provide a source for evaluating the PIM in a receive band.

8. The system of Claim 7 wherein the first coupler and the second coupler are directional couplers.

9. The system of Claim 7 wherein either or both of the first coupler and the second coupler is a non-directional coupler or an opposite-direction coupler.

10. The system of Claim 7 further comprising one or more RF switches to switch different input paths into a single capture system.

1 1. A passive antenna comprising:

a. a housing;

b. a plurality of antenna elements arranged to send transmit signals and receive receive signals;

c. one or more transmit couplers coupled to the plurality of antenna elements; d. one or more receive couplers coupled to the plurality of antenna elements; and e. one or more combline filters connected to the one or more receive couplers arranged to prevent coupling of unwanted signals back to the plurality of antenna elements, wherein the one or more combline filters are selected to provide low-loss in the passband representing some or all of the receive band where the PIM exists so as to maximize PIM sensitivity.

12. The antenna of Claim 1 1 wherein the one or more transmit couplers are selected to provide attenuation without creating a leakage path sufficient to degrade sampling.

13. The antenna of Claim 12 wherein the one or more transmit couplers provides about 50dB of attenuation.

14. The antenna of Claim 1 1 wherein the housing includes a compartment for retaining the combline filters therein.

15. The antenna of Claim 14 wherein the compartment includes one or more surface acoustic wave filters arranged to pass only transmit band signals of interest in sampling for PIM.