White Paper Calculating Passive Intermodulation for Wideband Antenna Arrays | July 2018 Page 1 of 16 White Paper ABSTRACT In mobile communication, the base station antenna is an essential element for pro- viding wide area coverage and high data throughput for the user equipment (UE). Passive intermodulation (PIM), in the present context a key performance characteristic, can substantially degrade the overall perfor- mance of mobile communication systems. Such a degradation would be visible to the end customer typically via a higher call drop rate, lower data throughput and reduced coverage, but can also lead to effects such as shorter battery lifetime. These unfavourable end customer expe- riences due to passive intermodulation CONTENTS ▪ Abstract ▪ PIM Measurement ▪ In-band PIM ▪ Cross-band PIM ▪ PIM Simulation ▪ Simulation model ▪ PIM analyser simulation model ▪ Test of simulation model ▪ Simulation of Base Station Antennas ▪ PIM caused by external combining ver- sus internal combining ▪ Kathrein antenna 80011878 ▪ In-band comparison of simulated and measured PIM ▪ Cross-band PIM simulation ▪ Conclusion ▪ References Calculating Passive Intermodulation for Wideband Antenna Arrays
Transcript
White Paper Calculating Passive Intermodulation for Wideband Antenna Arrays | July 2018 Page 1 of 16
White Paper
ABSTRACT
In mobile communication, the base station antenna is an essential element for pro-viding wide area coverage and high data throughput for the user equipment (UE). Passive intermodulation (PIM), in the present context a key performance characteristic, can substantially degrade the overall perfor-mance of mobile communication systems. Such a degradation would be visible to the end customer typically via a higher call drop rate, lower data throughput and reduced coverage, but can also lead to effects such as shorter battery lifetime.
These unfavourable end customer expe-riences due to passive intermodulation
CONTENTS Abstract
PIM Measurement
In-band PIM
Cross-band PIM
PIM Simulation
Simulation model
PIM analyser simulation model
Test of simulation model Simulation of Base Station Antennas PIM caused by external combining ver-
sus internal combining Kathrein antenna 80011878 In-band comparison of simulated and
Calculating Passive Intermodulation for Wideband Antenna Arrays
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shall be prevented by careful and systematic design and manufacturing. In addition, with the advent of LTE and the increasing number of bands which a base station antenna must support, PIM is becoming a major criterion for selecting the right antenna.
Passive intermodulation is a result of non-linearity in the antenna system. In general, non-linearity can appear inside the antenna system and in the area around the antenna. An example of an “outside” PIM source would be a rusty chimney in close proximity to the base station antenna. Inside the antenna, non-linearity stems from many different factors. Among them, improper material selection (such as ferromagnetic materials), thermal effects, mechanical stress or faulty crafts-manship during manufacturing. Base station antenna signals often achieve high power levels. High transmission power levels increase the risk of de-sensitising the receiver chan-nels of the base station due to passive intermodulation in the antenna.
This paper compares simulated and measured passive inter- modulation effects, covering in-band and cross-band scenarios. The investigation also outlines the effects of com-bining spectrum outside and inside the antenna system, followed by a discussion about which combining position brings the best PIM performance, subsequently leading to an architectural discussion about macro base station antenna concepts for better PIM performance.
Ampli
tude
Frequency
–150 dBc
–107 dBm
2 * f1 – f2 f1 f2 2 * f2 – f1
43 dBm
Figure 1: Intermodulation products (blue) of two CW signals (green), specified PIM value in dBc
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In mobile communication systems, the focus is mainly on 3rd order intermodulation products because they are a commonly used and comparable measure for system non-linearity. In commercial LTE scenarios, 3rd order intermodulation products often fall into network operator receive frequency allocations.
PIM analysers typically power the device under test with 2 * 43 dBm carriers and measure the amplitude of the 3rd order intermodulation product. Such measurements are stand-ardised in IEC 62037 [1] giving a limit of –150 dBc as industry standard. Since PIM values are typically given in dBc, which is the distance from the carrier, the limit has a dBm value of –107 dBm (–150 dBc + 43 dBm = –107 dBm), see Figure 1. Every Kathrein antenna is tested to a limit of −153 dBc, confirming the high standard of Kathrein’s antenna products. As a rule of thumb, for each 1 dB increase of transmit power, 3rd order intermodulation products increase by 3 dB.
This contribution is structured as follows: To start with, the difference between in-band and cross-band PIM is described as well as the required measurement systems. After that, our PIM simulation method is de-scribed with simple circuits including PIM sources. In the following section, we concen-trate on simulating a base sta-tion antenna and give a com-parison of the measured and simulated PIM performance before we conclude.
PIN
P PIM
+1 dB
+3 dB
Figure 2: Ratio between input power and 3rd order IM product
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PIM MEASUREMENTIn-band PIM
Every port of a macro base station antenna is dedicated to a certain frequency spectrum, usually including transmit (TX) and receive (RX) channels of FDD systems. We talk about in-band PIM if at least two transmit signals within the same frequency band are fed by the base station transceivers into one or more ports of the antenna and one or more intermodulation products are visible on the receive port. This case is shown in Figure 3.
Figure 4 shows the setup for a typical PIM measurement and how it is used for qualifying a base station antenna according to IEC 62037. There are two signal generators, which create two different frequencies. These get amplified, combined and are sent through the duplexer to the device under test (DUT). The reflected PIM products travel back through the duplexer and the RX filter and finally get amplified ahead of the receiver for analysis.
PortA
Combiner
Signal generator
Signal generator
Signal analyser
Duplex filter
PA
PA
LNA RX filter
A [d
B]
f [MHz]
f1 and f2 in-band
2 * f1 – f2
2 * f1 – f2f1 f2
f1 f2
RX
Port A
TX
Figure 3: Simplified spectrum of TX and RX bands with carriers f1 and f2. The in-band intermodulation product 2 * f1 – f2 appears in the RX band
Figure 4: PIM measurement setup for in-band measurement scenario
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Cross-band PIM
We talk about cross-band PIM if at least two transmit signals from at least two frequency bands are fed by the base station transceivers into one or more ports of the antenna and one or more IM products are visible on the RX port. This scenario is sketched in Figure 5. Given the fact that network opera-tors have an increasingly high number of frequency bands allocated, it becomes obvious that for designing multi- port and multi-band antennas, cross-band scenarios have to be considered.
Cross-band PIM measurements usually have a slightly different setup than in-band PIM measurements. Typically, cross-band intermodulation is measured on different antenna ports, so the applied carriers do not have to be combined. A possible setup is shown in Figure 6.
A [d
B]
f [MHz]
f1 and f2 cross-band
2 * f1 – f2
2 * f1 – f2f1 f2
f1 f2
RX A RX B
PortA
PortB
TX A TX B
Figure 5: Simplified spectrum of TX and RX bands with carriers f1 at port A and f2 at port B. The cross-band intermodulation product 2 * f1 – f2 appears in the RX band of port A
PortB
Port A
Signal generator
Signal generator
Signal analyser
Duplex filter
RXfilter
PA
PA
LNA Figure 6: A possible PIM measurement confi- guration for cross-band measurement on two different antenna ports
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Intermodulation measurement setups are usually cost inten-sive due to stringent requirements on amplifier, RX filter, du-plex filter and receiver. Taking into account that multi-band antennas have many ports, qualifying such products are time and cost intensive.
PIM SIMULATIONSimulation model
In our simulations, we use a non-linear black box acting as a PIM source, as shown in Figure 7. The non-linear block is de-signed to satisfy the following dynamic characteristic. A 1 dB increase of input power leads to a 3 dB increase of the 3rd or-der intermodulation product (IM3), as shown in Figure 2. The non-linearity of the black box can be adjusted to any arbitrary value. For the following calculations, a setting was used which generates −150 dBc at 43 dBm input power and IM products follow bidirectional propagation.
PIM analyser simulation model
Figure 8 shows our harmonic balance simulation setup which works similar to a standard PIM analyser, performing a fre-quency step analysis. Two signal generators are connected by a lossless combiner to the device under test. The DUT could either be a port of a multi-band antenna or of any other RF structure. Port A is the port with fstep. This port performs a step-through in the selected TX band. Port B has a fixed frequency, which is in this example at the lower boundary of the TX band. The harmonic balance simulation observes port A, where the reflected signals from the DUT are received. Moving fstep, fIM3
PIM sourcec1x + c3x3 ...
Figure 7: Non-linear black box simulation model
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moves in the same direction. This simulation setup equals a PIM analyser and is more flexible, since up and down stepping and wideband analysis can be done without hardware limitations.
Test of simulation model
To verify the simulation setup and the PIM source, a simple model is built. Figure 9 shows the DUT, in this case the PIM source, with a coaxial cable, a −150 dBc PIM source and a 50 Ω load.
The coaxial cable has a length of 1 metre and is used in the test with two attenuation options. The first option has zero atten-uation and the second option is set to 1 dB/m attenuation at 2.7 GHz. The stepped frequency range for this simulation goes from 1.7 GHz to 2.7 GHz, with 1.7 GHz being the fixed frequency.
Figure 10 shows the simulation result with both cable attenuation options. The result with zero cable attenuation is −150 dBc, which is the value of the PIM source. The result is constant over the whole frequency range because the model has no frequency dependence. With attenuation from the coaxial cable, the IM3 value is –154 dBc at 2.7 GHz. The input power at the PIM source is 2 * 42 dBm because of the 1 dB loss caused by the cable. The reflected power goes back through the cable with 1 dB loss, which results in an IM3 value of −154 dBc at the analyser port. The frequency dependence of the cable can be observed as having a slight slope throughout the IM3 spectrum.
Analyser Coax1 m 50 ohm load
PIM source–150 dBc @2 * 43 dBm Figure 9: DUT model with simple network
Combiner
Port Afstep
Analyser Device under Test
Port Bffix
DUT
A [d
B]
f [MHz]
2 * f1 – f2
f1 Port 1 f2 Port 2
Figure 8: PIM analyser simulation model
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Another model, with a 3 dB splitter, is built to verify whether splitters are calculated correctly. The coaxial cable from the analyser to the splitter is set to a length of 2 metres and all other cables after the splitter are set to 1 metre. Again, the simulation is done with and without losses of the coaxial cables.
Firstly, we calculate without losses and one PIM source with −150 dBc placed into the circuit. We expect that the simulation result is −162 dBc because the signal (2 * 43 dBm) is divided using a 3 dB splitter. Therefore, the input power at the PIM source is 2 * 40 dBm, which decreases the PIM source value to −159 dBc. The reflected IM3 product runs back through the 3 dB splitter, which results in a decreased value of −162 dBc.
Secondly, we incorporate cable attenuation and expect that the reflected PIM value is −174 dBc. Due to an input power of 2 * 43 dBm, cable losses of 2 dB at the first cable, 3 dB at the splitter and another 1 dB loss at the second cable, there is 37 dBm input power at the PIM source. With the 1:3 power ratio, −6 dB : −18 dB, the PIM value decreases to −168 dBc. With the same losses of 6 dB on the return path, the IM3 product has a value of −174 dBc at 2.7 GHz at the analyser port.
Analyser
Coax1 m 50 ohm load
Coax1 m 50 ohm load
PIM source–150 dBc @2 * 43 dBm
Coax2 m
Figure 11: DUT model with diverse network and one PIM source
dBc
Frequency [MHz] without attenuationwith attenuation
-120
-125
-130
-135
-140
-145
-150
-155
-160
-165
-170800 1000 1200 1400 1600
IM 3
1800 2000 2200 2400 2600
Figure 10: Simulation results for DUT model with simple network
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The next step is to verify the same configuration with two PIM sources. In this case, we expect −156 dBc. The reflected PIM with values of −159 dBc from both paths arrives at the com-biner. In this case, the phases are the same, so the power is added and a PIM value of −156 dBc is created, as shown in Figure 14.
This proves that this calculation is correct. With the attenua-tion, the value is −168 dBc because it is the same difference of −12 dB as in the first test with one PIM source.
Analyser
Coax1 m 50 ohm load
Coax1 m 50 ohm load
PIM source–150 dBc @2 * 43 dBm
PIM source–150 dBc @2 * 43 dBm
Coax2 m
Figure 13: DUT model with diverse network and two PIM sources
dBc
Frequency [MHz] 1 source without attenuation1 source with attenuation
-140
-145
-150
-155
-160
-165
-170
-175
-180800 1000 1200 1400 1600
IM 3
1800 2000 2200 2400 2600
Figure 12: Simulation results for DUT model with diverse network and one PIM source
dBc
Frequency [MHz] 2 sources without attenuation2 sources with attenuation
-140
-145
-150
-155
-160
-165
-170
-175
-180800 1000 1200 1400 1600
IM 3
1800 2000 2200 2400 2600
Figure 14: Simulation results for DUT model with diverse network and two PIM sources
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In this ideal simulation setup, we see that the reflected PIM is added up due to phase coherence of the PIM sources. In real application, such a setup could yield any arbitrary result due to the lack of phase coherence. The simulation therefore shows the worst case scenario values. On the other hand, two parallel PIM sources could remain undetected using PIM analyser hardware if the reflected PIM was cancelled due to destructive interference.
SIMULATION OF BASE STATION ANTENNASPIM caused by external combining versus internal combining
In this section, we apply the previously described simulation method to a typical multi-port and multi-band base station antenna in practice.
At first, a comparison is made showing the PIM performance of the feeding network of the antenna system, either applying external or internal combining of frequency bands. Operating multiple bands over one antenna system, while not upgrad-ing the individual antennas, is a common practice for network operators. Band combining in this case is commonly achieved by using external band combiners.
Using an external combiner and an improper band combina-tion increases the probability of reducing the base station’s RX performance due to passive intermodulation for two reasons. Firstly, the number of potentially PIM-critical RF interfaces is increased. Secondly, the higher aggregate power level of the combined signals in the feeding network leads to higher PIM levels, as can be seen in Figure 15. This may be caused by faulty craftsmanship during installation, by violating minimum cable bending radii, interface torque requirements, etc.
Figure 15 shows that a PIM source of −150 dBc at the antenna port leads to a result of −150 dBc, assuming that there are negligible losses in the cable path. The level of a possible PIM source in the feeding cable will dominate the system performance.
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Internal combining means that the different frequency bands are combined within the antenna architecture, as shown in Figure 16. This leads to an intermodulation level below practical relevance. For cross-band PIM, such a setup shows significant performance advantages compared to external combining. For in-band PIM, significant advantages are possible by smart architecture choices, where power tapering is applied in front of band combining and as close to the radiating elements as possible. If PIM sources are present within the antenna, the overall effect of IM products impacts RX performance less, due to lower power levels within the antenna system.
Kathrein antenna 80011878
The Kathrein multi-band, multi-port antenna 80011878, as shown in Figure 17, is part of a new generation of antennas which has been designed to minimise the risk of passive intermodulation with the help of internal frequency combining. This antenna architecture reduces the risk of faulty tower assembly or similar workmanship deficiencies for the network operator. Additionally, it supports a “tidy mast” strategy due to the reduction of additional boxes, such as band combining products on the installation site.
Combiner
PIM critical feeding path
Antenna
f1 f2
Port
Port
PortPIM source–150 dBc
PortPIM source–150 dBc Figure 16: No PIM critical path with antenna
internal combining
Combiner
External antenna
PIM critical feeding path
Antenna
f1 + f2
PortPort
Port
PIM source–150 dBc
Figure 15: Significant PIM critical path with base-station-side combining
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This antenna is used as a state-of-the-art base station antenna example for our PIM simulation.
In-band comparison of simulated and measured PIM
For use in our simulation framework, the DUT’s internal architecture is based on elaborated S-parameter models. In the following, we purposely degrade the simulation model and the off-the-shelf 80011878 base station antenna. This is done by placing respective hardware or software PIM sources in the internal signal distribution at worst case positions. Figure 19 to Figure 21 present a comparison of simulated and measured IM3 products.
The results of simulated and measured in-band IM3 products match well in amplitude and shape. Obviously, the measured results are spectrally narrower than the simulated results due to the limited bandwidth of the IM3 measurement equipment. Nevertheless, the spectrally wider simulated results give a bet-ter understanding of the location of the maximum IM3 level over the antenna’s supported bandwidth.
measurementTX Band 3Figure 19: Comparison of simulated and measured IM3 product for Band 3
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Cross-band PIM simulation
The antenna under test (AUT) is simulated for a relevant cross-band combination shown in Table 1.
As an example, the simulated PIM level results of band combi-nation 1 & 7 (see Figure 22) are in the range between −159 dBc and −174 dBc. These low PIM levels are plausible because of the power distribution function of the splitter. Thus, the posi-tive effect of combining frequency bands inside the antenna is confirmed. Measurements of the simulated cross-band PIM scenario are omitted since they do not show any additional insight.
dBc
Frequency [MHz]RX Band 2 simulation
-140
-145
-150
-155
-160
-165
-170
-175
-1801840 1860 1880 1900 1920 1940 1960 1980 2000
measurementTX Band 2Figure 20: Comparison of simulated and measured IM3 product for Band 2
dBc
Frequency [MHz]simulation
-140
-145
-150
-155
-160
-165
-170
-175
-1802450 2500 2550 2600 2650 2700 2750
measurementRX Band 7 TX Band 7Figure 21: Comparison of simulated and measured IM3 product for Band 7
Table 1
Band Port Fixed Tone [MHz]
Sweep Range [MHz] IM3-Rx Band
1 & 7 B2 + Y2 2170 2555–2630 3
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CONCLUSION
The objective of this work is to create a simulation framework where the PIM behaviour of a mobile network base station antenna can be predicted. The external and internal combining of LTE signals and its effects on the PIM behaviour are described. We demonstrate that using a qualified multiband antenna with internal combining reduces the effects of PIM.
The Kathrein multiband antenna 80011878 is tested in the simulation environment and compared with measurements of in-band IM3. The simulation results are very close to the measurement results. Using the simulation framework, we have no limitations in terms of the bandwidth to be analysed or the number of antenna ports. Simulating cross-band IM3 provides a better view of what happens between antenna columns if more than one antenna port is fed. In theory, with the PIM source (assuming constant PIM level over frequency and same power levels) which we used for simulation, the cross-band PIM values cannot exceed the specified in-band PIM values.
We conclude that it is possible to create an antenna archi- tecture capable of handling in-band and cross-band IM, without the need for a separate antenna column.
[3] Dmitry S. Kozlov, Alexey P. Shitov, Alexander G. Schuchinsky and Michael B. Steer “Passive Intermodulation of Analog and Digital Signals on Transmission Lines With Distributed Nonlinearities: Model-ling and Characterization” IEE Trans. Microw. Theory Techn., Vol. 64, No. 12, May 2016
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ABOUT KATHREIN
Kathrein is a specialist for reliable, high-quality communication technologies.
The company is driving innovation and technology in today’s connected world. Its ability to provide solutions and services enables people all over the world to communicate, access information and use media, whether at home, at the office or on the road.
The business covers a broad spectrum: from mobile commu-nication, RFID and special solutions, to satellite reception and broadcast technology, to transmission and reception systems in vehicles.
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