Field Test of Passive Intermodulation in POI System

Preface

In cellular communication systems, in order to fully share resources and reduce investment, operators have adopted a large number of multi-system access platforms (Point of Interface, referred to as POI systems). POI systems have been widely used in indoor distribution systems and subways, tunnel communications and other occasions. The structure of POI systems is very complex. According to different system requirements, they can be divided into structures such as co-channel bidirectional and unidirectional transmission and reception, and bidirectional and unidirectional transmission and reception split channels.

In the POI system, the second-generation mobile communication system CDMA800, GSM900, DCS1800 and the third-generation mobile communication system WCDMA, TD-SCDMA, CDMA2000 and the fourth-generation mobile communication system LTE coexist, and WLAN (wireless local area network) will also be added.

We know that when there are more than two carrier frequencies in any passive device, passive intermodulation will occur; when the intermodulation product reaches a certain amplitude, it will affect the normal communication of the system. In cellular communications, passive intermodulation is a problem that operators are very concerned about.

Before the POI system leaves the factory, the passive intermodulation index of each port will be measured. These measurements are performed on a certain frequency band, such as the 880-915MHz/925-960MHz frequency band of the GSM900 system. In this article, an online test method for passive intermodulation of the POI system is discussed through actual test cases.

Typical POI system

Figure 1 is a typical POI system diagram, which includes 8 carrier frequencies including CDMA800, GSM900, DCS1800, TD-SCDMA and WCDMA, which are finally synthesized into one output. These systems belong to different operators, and the figure indicates the standards of each operator and the working frequency band of each channel.

Figure 1. Typical multi-system access platform (POI system)

To simplify the analysis, you can consider the various paths in Figure 1 as filters of different frequency bands and synthesize them at the ANT port. This is enough to understand the ideas described in this article. For more information about the POI system, please refer to relevant materials or product manuals [1].

Passive intermodulation issues in POI systems

Due to the existence of multiple carrier frequencies, the passive intermodulation analysis of the POI system becomes more complicated compared to a single frequency band. When the POI device is connected to the indoor distribution system, the situation changes as follows:

1) The factory test condition of the POI system is 2×43dBm, but in actual use, the ANT port may have to withstand 8×43dBm (Figure 1) or even higher power. Usually, the POI is designed to withstand an average power of up to 1kW. At present, there is a preliminary understanding of the change law of the intermodulation amplitude under the condition of two carrier frequencies in engineering and academia [2]. However, the intermodulation law under the condition of multiple carrier frequencies needs further discussion;

2) The factory test of POI system intermodulation is conducted in the same frequency band, while in actual use, multiple carrier frequencies coexist;

3) Some second harmonic interference problems will occur.

There are two different viewpoints on the passive intermodulation measurement of POI systems. The first viewpoint is that as long as the reflected intermodulation of each input port of the system is measured separately, the intermodulation problem of the entire system can be explained. This test method is easy to implement. Just add low intermodulation loads to all idle ports and use a standard passive intermodulation measurement system to measure the reflected intermodulation of the input port separately.

The second view requires adding the corresponding power from the input of the system and measuring all possible intermodulation at the output port. System operators prefer this measurement method because it simulates the real working environment. To implement this measurement method in the laboratory, multiple high-power signal sources are required. In the field test of the POI system, the RRU (Remote Radio Unit) in the system can be used as the signal source. This method truly reflects the situation of the system under test and is also the topic to be discussed in this article.

Field test method of passive intermodulation in POI system

Figure 2 is similar to the test method of transmission intermodulation [3]. A low intermodulation directional coupler is connected to the output end of the POI under test. Part of the carrier frequency and intermodulation are output from the coupling end. The filter in the intermodulation frequency band is used to filter out the carrier frequency signal and extract the intermodulation product of concern to the tester. The low noise amplifier is used to compensate for the coupling loss and send the measured intermodulation signal to the spectrum analyzer.

Figure 2: Field test method for passive intermodulation of POI system - transmission intermodulation

In the test of Figure 2, the RRU and POI are considered as a whole, and the RRU in the system is directly used as the test signal source. According to the actual situation, the tester can turn on any RRU in the system and observe the intermodulation products falling into different frequency bands by changing the filter at the coupling end. This method can be used to evaluate the passive intermodulation performance of the entire system.

Figure 3 shows a measurement method for further locating the intermodulation source. This method can measure the reverse intermodulation falling into the RRU uplink frequency band. This intermodulation product will affect the normal operation of the entire system, so it is more concerned by operators.

Figure 3: Field test method for passive intermodulation in POI system - reverse intermodulation

Reverse intermodulation is caused by the fact that one RRU signal passes through the POI system and enters the other RRU in reverse, generating intermodulation at its output end [4]. If the passive device connected to the back end of the POI device generates reflected intermodulation, it will also be measured in the test system of Figure 3.

POI system passive intermodulation field test case

The test was conducted in an indoor distribution system, in which China Unicom reported that its WCDMA uplink frequency band (1940-1955MHz) was interfered, causing the system to fail to work properly.

Figure 4: Field test of passive intermodulation in POI system - synthesis end

We connected the test system as shown in Figure 4 and tested the intermodulation at the synthesized output of the POI system under normal working conditions of the system (Figure 5).

Figure 5. POI system passive intermodulation field test - test results of the synthesis end

The test results show that there is no intermodulation signal falling into the 1940-1955MHz range at the synthesis end of the POI. The 1960-1966MHz signal shown in the figure is not the frequency band of interest to the tester, so it will not be described here.

To further find intermodulation, we connected the test system to the RRU output of WCDMA according to the test principle in Figure 3 (Figure 6).

The signals reflected by China Unicom falling into the 1940-1945MHz frequency band were found (Figure 7a). These signals flow directly to China Unicom's WCDMA RRU, and their amplitude exceeds -90dBm, which is enough to interfere with the WCDMA uplink.

Figure 6: Field test of passive intermodulation in POI system - reverse intermodulation

To prove that the signal was a product of system intermodulation rather than a call from an end user, we turned off the mobile RRU (2300-2483MHz). At this time, the signal in the 1940-1945MHz band disappeared (Figure 7b), which proved that the signal was generated by the system.

Figure 7a), China Mobile 2300-2483MHz/China Unicom 2130-2145MHz are turned on at the same time

Figure 7b), China Mobile 2300-2483MHz off/China Unicom 2130-2145MHz on

Figure 7. POI system passive intermodulation field test - reverse intermodulation test results

The test results show that the interference signal comes from the POI system and the subsequent indoor distribution system. To further determine the source of the interference, we reproduced the test system shown in Figure 6 in the laboratory.

Laboratory Reproduction of Field Test of Passive Intermodulation in POI System

Figure 8 shows the test method reproduced in the laboratory. We connected a standard low intermodulation load, a long cable plus a standard low intermodulation load, and a long cable plus an indoor distribution system antenna behind the POI system. The reverse intermodulation values ​​measured at a frequency of 1945 MHz in these three cases were -116.91 dBm, -108.87 dBm, and -80.06 dBm, respectively (Figure 9).

The different test results of changing the load connected to the terminal show that the intermodulation of the system is related to the passive intermodulation value of the load connected to the terminal. Figure 9a is a standard low intermodulation load with good PIM performance; Figure 9b connects a long cable between the POI and the standard load, and the test results show that the PIM introduced by the long cable is slightly deteriorated; and Figure 9c uses an antenna commonly used in indoor distribution systems, and the results show that the PIM is sharply deteriorated. In this case, the system can no longer work properly.

Figure 8. Laboratory reproduction of POI system passive intermodulation field test

Figure 9a), terminated with standard low intermodulation load

Figure 9b), terminating long cables and adding standard low intermodulation loads

Figure 9c), terminating long cables and indoor distributed antennas

Figure 9. PIM performance of POI system when connected to different loads

Introduction to POI System Passive Intermodulation Field Test System

The above test uses a field passive intermodulation and spurious test system developed and produced by BXT Technologies (Figure 10), model named GTR-0727LIM, which is divided into two versions: built-in spectrum analyzer and external spectrum analyzer. It can measure the intermodulation interference signals generated by the downlink frequency band of FDD cellular base stations or POI systems, various TDD carrier frequencies, and fall into other cellular communication uplink frequency bands and TDD frequency bands; it can also measure the reverse crosstalk between base stations under co-site conditions.

Figure 10. GTR-0727LIM on-site intermodulation and spurious test system

GTR-0727LIM is designed in accordance with the IEC63027 standard and various field test requirements. Its maximum test power is 1kW (CW), which can measure the intermodulation of a 10-channel POI system with a power of 100W per channel. The test frequency range is 0.7-2.7GHz, and the insertion loss of its main channel is less than 0.2dB. GTR-0727LIM has four commonly used intermodulation filters built in and a test channel with an external filter. It deeply suppresses the carrier frequency of the base station under test, and adopts a 0dB loss design for the frequency band of interest, which can measure intermodulation or stray interference as low as -127dBm.

GTR-0727LIM adopts low passive intermodulation technology and supports multi-carrier input. Its residual passive intermodulation product is less than -165dBc@2*20W, which ensures the credibility of the test results. It supports online measurement to ensure the normal operation of the tested base station and POI system during the test process. GTR-0727LIM can be connected to any point of the tested system to determine whether the interference source comes from the left or right side of the test point. This feature is very suitable for the following scenarios:

1) During the installation and commissioning of the POI and indoor distribution system, the installed system can be tested section by section. When the system is installed, the test is also completed.

2) When interference occurs in an operating system, GTR-0727LIM can be used to accurately measure and distinguish whether the interference comes from the POI system or the subsequent indoor distribution system.

GTR-0727LIM provides customized design and can be used by mobile communication engineers and operators, radio monitoring stations and other units.

Compared with the common passive intermodulation measurement system with its own signal source, the biggest feature of GTR-0727LIM is that it uses the RRU in the system under test as the signal source. In addition to greatly reducing the cost, the more important point is that it truly reflects the passive intermodulation situation of the entire system under test.

Conclusion

This article discusses a very practical test method and test equipment through actual cases. This method can accurately determine whether the intermodulation of an indoor distribution system is generated by the POI device or by the passive devices in the subsequent main channel.