Topic on Microwave XPIC-B

Document No.

Product Name

Intended Audience

Microwave engineers

Product Version

Compiled by

Optical Network Product Technical Service Dept.

Document Version

Topic on Microwave XPIC

Prepared by

Chen Hu

Date

Reviewed by

Date

Reviewed by

Date

Approved by

Date

2007-11-07

Huawei Technologies Co., Ltd. All Rights Reserved

V1.0


Confidentiality Level: Internal L

Revision History Date 2007-11-11

2012-03-23

Version V1.0

Description Draft completed.

Author Chen Hu

Huawei confidential. No spreading without permission.

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Table of Contents Chapter 1 Overview ...........................................................................................................................6 1.1 Overview of XPIC ..................................................................................................................6 1.2 Key Technical Terms .............................................................................................................6 1.2.1 Co-Frequency Dual-Polarization (Cross Polarization) ................................................6 1.2.2 Dual-Polarized Antenna ..............................................................................................6 1.2.3 Port Isolation ...............................................................................................................7 1.2.4 Static Cross-Polarization Discrimination (XPD) ..........................................................7 1.2.5 Using Two Uni-Polarizd Antennas to Implement the Functions of a Dual-Polarized Antenna ................................................................................................................................7 Chapter 2 Implementation Principles of XPIC ................................................................................9 2.1 Problems During Cross-Polarization Applications .................................................................9 2.2 Implementation Principle of XPIC ..........................................................................................9 2.3 Improvement by XPIC ......................................................................................................... 11 Chapter 3 Deployment Guide of Equipment Configured with XPIC Feature ............................ 13 3.1 Antenna Installation ............................................................................................................ 13 3.2 Equipment Installation ........................................................................................................ 14 3.3 Antenna Adjustment ........................................................................................................... 14 3.4 Service Configuration ......................................................................................................... 15 3.5 Precautions for the Configuration ....................................................................................... 15 Chapter 4 Common XPIC Faults and Troubleshooting Methods .............................................. 16 4.1 General Description ............................................................................................................ 16 4.2 Handling the Fault That XPD Cannot Meet the Specification ............................................. 16 4.3 Handling the XPIC LOS Alarm............................................................................................ 18 4.4 Handling the Fault That Bit Errors on One Polarization or on the Two Polarizations ......... 18 4.5 Precautions for Maintenance When XPIC Is Used ............................................................. 19 Chapter 5 Conclusion .................................................................................................................... 20

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List of Figures Figure 2-1 Principle block diagram of XPIC ............................................................................ 10 Figure 2-2 Comparison between the effects before and after the XPIC function is used ....... 12

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Key words: Microwave

XPIC

XPD

cross-polarization

interference

counteracted,

polarization discrimination multipath fading dual-polarized antenna Abstract: The cross-polarization interference counteracted (XPIC) is used to effectively improve the transmission capacity and the utilization of the frequency spectrums. It is widely used in the trunk microwave. As the split-mount microwave develops and the frequency resources become insufficient in recent years, some microwave manufacturers start to apply the XPIC technology in the split-mount microwave. This document describes the XPIC in terms of the principles, applications, precautions to be taken during the installation, and common troubleshooting methods. Acronyms and abbreviations: XPIC: Cross-Polarization Interference Counteracted References: None.

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Chapter 1 Overview 1.1 Overview of XPIC In normal cases, the following two methods can be used to improve the utilization of the frequency spectrums: 

Adopt a high modulation factor to improve the efficiency of the intermediate frequency (IF) modulation and to reduce the required radio frequency (RF) modulation bandwidth.



Adopt the XPIC technology.

Adoption of a high modulation factor requires complex components and good transmission conditions. Thus, it is difficult to implement a high modulation factor. Currently, a highest modulation factor can reach 512QAM. The XPIC technology can be used in flexible ways, and the cost of the components required for implementing the XPIC is low. Therefore, the XPIC becomes the major method of improving the frequency spectrum utilization in a large-capacity transmission system.

1.2 Key Technical Terms You need to understand the following technical terms before learning the XPIC technology.

1.2.1 Co-Frequency Dual-Polarization (Cross Polarization) When sufficient frequency resources are available, different frequencies are selected for different services to minimize the intra-station and inter-station interferences. When the frequency resources become insufficient, the orthogonal feature of the polarization must be used to improve the utilization of the frequency spectrums. Co-frequency dual-polarization means that different polarization modes are used for transmission at the same frequency.

1.2.2 Dual-Polarized Antenna Based on the polarization, antennas are classified into two types: dual-polarized antenna and uni-polarized antenna. A dual-polarized antenna can transmit and receive RF signals on the two polarizations at the same time.

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Only one RF interface exists at the rear end of a uni-polarized antenna, of which the polarization orientation can be adjusted. Two RF interfaces, however, exist at the rear end of a dual-polarized antenna. By default, the polarization orientation of the left interface is vertical, and the polarization orientation of the right interface is horizontal. In the case of the two RF interfaces, the polarization rotation is implemented inner the dual-polarized feeder. The two RF interfaces face the same direction, which facilitates the installation of feed lines.

1.2.3 Port Isolation In addition to common antenna specifications, a key specification of a dual-polarized antenna is the port isolation. The port isolation means the polarization isolation of the antenna, which can be expressed as T/R. "T" indicates the RF power that is transmitted at a polarized port by the dual-polarized antenna, and "R" indicates the signal received by the other port of the dual-polarized antenna. The antenna port isolation reflects the resistance of a dual-polarized antenna to the interferences from the received and transmitted signals.

1.2.4 Static Cross-Polarization Discrimination (XPD) In addition to the port isolation, the static XPD is another key specification of antennas. When an antenna receives the signal from a polarization, it also receives the signal component of this polarization on the other polarization. The ratio of the received signal to the signal component is the static XPD. The XPD is closely related to many factors, and the flatness of the XPD in the operating frequency band is involved. In this case, the XPD needs to be emphasized as static. The methods for testing the static XPD should be accurate and detailed, and thus are not described in this document. The XPD specification holds the key to the improvement of the XPIC of the entire system.

1.2.5 Using Two Uni-Polarizd Antennas to Implement the Functions of a Dual-Polarized Antenna In the theory, dual-polarized signals should be completely orthogonal, and have no component and interference. In this case, you can use two uni-polarized antennas to replace a dual-polarized antenna.

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In actual applications, however, two uni-polarized antennas cannot be used to replace a dual-polarized antenna. The causes are as follows: 

Installation of two antennas leads to higher costs and heavier workload than installation of one antenna.



The isolation of two uni-polarized antennas is weaker than the isolation of a dual-polarized antenna in terms of the electric performance, since the feeder of the dual-polarized antenna is decoupled.

Therefore, this solution is not described in this document.

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Chapter 2 Implementation Principles of XPIC 2.1 Problems During Cross-Polarization Applications As described in the preceding chapter, the cross-polarization mode can improve the utilization of frequency spectrums. The implementation process, however, is not as simple as using a dual-polarized antenna to send out the RF signals at the same frequency. Problems may occur during the implementation process. Due to limitations of the components, a dual-polarized antenna cannot ensure that the signals of the two polarities are completely orthogonal. The signals are correlated in most cases. When the RF signals are transmitted in the space, due to the effect of climate and terrain, part of the signal on a polarization is converted into the signal on the other polarization, in most cases. Thus, the two signals that are originally orthogonal start to interfere with each other. This interference is the cross-polarization interference. The XPD of the system may deteriorate due to the following causes: 

Impacts of fallouts such as rain, fog, and ice crystal



Multipath propagation effects

In the case of pure air, the multipath fading is the main cause for the deterioration of the XPD. In most cases, the most severe deterioration of the XPD is associated with the multipath fading of the uni-polarized signal. Statistic results show that the XPD value changes slightly if the signal fading is less than 10 dB. When the signal fading is more than 10 dB, the XPD deteriorates violently as the strength of the signal is decreased and even the circuit may be interrupted. Therefore, if the cross polarization is not implemented based on the XPIC technology, the fading margin of the system is decreased and the system availability is reduced. To solve the problem that the system availability is reduced when the XPD deteriorates during the transmission, use a dual-polarized antenna and ensure that the equipment can implement the XPIC function. The XPIC function can be used to improve the resistance of the entire transmission system to the XPD deterioration.

2.2 Implementation Principle of XPIC The overall implementation principle of XPIC is simple. At the receiving end, the two polarizations send an interference counteracted signal that has the

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same size as the interference signal and a phase opposite to the interference signal to each other, depending on the receive level and a series of decision mechanisms. In this way, a pure signal with the local polarization is synthesized at

the

receiving

end.

The XPD of the entire transmission system changes all the time, and thus the XPIC must be adaptive. Figure 2-1 shows the implementation principles of XPIC. Received signal Interfering signal

Vertical MO MOD D

A/D L LO O

TRSV FILTER FILTER

+

DE DEC C

CRC TRSV FILTER FILTER

Canceling signal

Cross-polarization inteference

A/D CONTRL LOGIC LOGIC

L LO O

CONTRL CONTRL LOGIC LOGIC

A/D Cross-polarization inteference

TRSV FILTER TRSV FILTER

Canceling signal

CRC Horizontal MO MOD D

A/D

TRSV FILTER FILTER

+

DE DEC C

Received signal

L LO O

Interfering signal

Figure 2-1 Principle block diagram of XPIC

A/D: Analog/Digital Converter CRC: Carrier Recovery Circuit DEC: Decision Circuit LO: Local Oscillator MOD: Modulator TRANSV FILTER: Transversal Filter The signal flow of the vertically polarized signal and the processing methods are described as follows. The signal is radiated to the antenna, and then the antenna at the opposite station extracts the RF signal. During the transmission process, the horizontally polarized signal interferes with the vertically polarized signal that is then received on the vertical polarization at the opposite station.

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At this time, the receiver on the vertical polarization sends a signal sample with the same received power, phase, and size as the vertically polarized signal to the receiver on the horizontal polarization. It also receives the corresponding signal sample from the receiver on the horizontal polarization. The signal sample is sent to the logic control circuit after the analog/digital conversion. The logic control circuit compares the received digital signal with the output signal, and then outputs the reference information to the TRANSV FILTER (transversal filter). The transversal filter outputs a signal that has the same size as the interfering signal and has the phase opposite to the interfering signal, according to the reference information from the logic control circuit and the signal sample from the other polarization. The interference counteracting signal and the signal with the normal received signal level are overlaid in the decision circuit. Then a pure vertically polarized signal is output. Note: The implementation of XPIC varies depending on the manufacturer. This section provides only a typical example.

2.3 Improvement by XPIC The XPIC function can be used to improve the performance (including the reliability and availability) of the system that adopts cross polarization. Figure 2-2 shows the test results of the XPIC from a manufacturer. The effects after the XPIC function is used are illustrated.

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50


128QAM C/N vs. XPD BER = 1 x 10-3

- 3

(in the case of flat fading)

40 C/N (dB)

W/O XPIC

E/W XPIC

30

20 0

10

20

30

40

XPD (dB)

Figure 2-2 Comparison between the effects before and after the XPIC function is used

As shown in Figure 2-2, when the signal-to-noise ratio is 30 dB and the bit error –3

rate is 10 , the equipment that is not configured with the XPIC function requires a 26 dB XPD of the system, while the equipment that is configured with the XPIC function requires only a 6 dB XPD of the system. That is, the XPD of the system is improved by 20 dB in the case of the equipment that is configured with the XPIC function.

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Chapter 3 Deployment Guide of Equipment Configured with XPIC Feature 3.1 Antenna Installation In most cases, XPIC is used for large-diameter antennas. Hence, this chapter describes how to assemble and mount a large-diameter dual-polarized antenna. Most small-diameter dual-polarized antennas integrate feeders, and thus feeders need not be assembled on site. The installation of a small-diameter dual-polarized antenna is the same as the installation of a uni-polarized antenna. A dual-polarized antenna uses a dual-polarized feeder. Hence, you need to check whether the dual-polarized feeder is damaged and whether there is moisture on the feeder interface when unpacking the cabinet. By default, the antenna is installed with "V" on the left and "H" on the right. The polarization identifier "V" or "H" should be located under the feeder interface. Strictly follow the instructions to install the feeder. Adjust the torque on the thre e or four feed lines to ensure that the feed horn is at the focus of the reflective surface. This is a specification required for dual-polarized antennas, but is not required for uni-polarized antennas. The two RF ports of the feeder must be aligned in a horizontal line. In certain cases, you need to use a gradienter to ensure that the two RF ports are in a horizontal line. The antenna pole must be installed vertically on the earth. The XPD performance may be affected when the antenna pole is installed leftward, rightward, forward, or backward from the vertical. It is recommended that you use a plummet to check whether the antenna pole stands vertically on the earth before mounting the antenna. Prevent the feeder interface from colliding with the pole when suspending the antenna. Determine the orientation of the feeder horn after the antenna is suspended to the expected height. The split-mount microwave equipment needs to be mounted separately if no dual-polarized combiner is used. In this case, the following conditions must be met before you install the antennas:

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

There is sufficient space for installing the two polarized antennas.



There is space for using installation tools.



Waterproof measures and grounding can be conducted normally.


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3.2 Equipment Installation To maximize the XPIC effects, the receivers and transmitters of the two polarizations each need to have a reference oscillator so that he receivers and transmitters of the two polarizations can work at the same frequency. Thus, a local oscillator (LO) is added between the two polarizations in the principle block diagram. In the case of split-mount microwave, the ODU is the only tool to ensure the XPIC effects. The equipment installation indoors is similar to the equipment installation in common cases. Note that the XPIC signal cable must be cross-connected between the XPIC workgroups to ensure that the reference signal can be sent to the other polarization. Determine the position for cross-connecting the signal cables and check the quality of the signal cables. Configure the XPIC-related functions on the NMS before adjusting the antenna. After you confirm that the configurations are correct, align the two antennas. Make the polarization marks when connecting the IF cables to avoid misconnections due to polarization confusion. In the case of split-mount microwave, determine the polarizations of the ODUs and the mapping relation between the ODUs and the IF boards before the installation. Otherwise, the polarizations of the ODUs may be misconnected on site.

3.3 Antenna Adjustment After installing the antennas, IF cables, ODUs, and powering on the equipment, then adjust the antennas. When you adjust a dual-polarized antenna, close the output of one polarization (it is recommended that you power off the ODU) to decrease the interference and to facilitate the commissioning. Enable only one polarization at both sides of the HOP, and adjust as the uni-polarized antenna. W hen you adjust antenna, take care not to damage the feeder or the RF cables. When the received signal level reaches the design value, it indicates that the antenna orientation is properly aligned. In this case, fix the antenna. Then, complete the commissioning process that is required only in the case of dual-polarized antennas — polarization decoupling. At this time, close the output of this polarization at the opposite station, and enable the output of the other polarization at the opposite station. Adjust the feeder according to the received signal level by tuning the feeder slightly leftward or rightward until the minimum received signal level is obtained. The angle of the feeder at the

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opposite station is slightly adjusted in the same way as at the local station until the minimum received signal level is obtained. Then, the polarization decoupling work is complete. To check whether the adjustment is correct, replace a polarization at the local station and at the opposite station, check the received signal level, and check the decoupling effects. Note that the feeder cannot be rotated to a larger angle, and can be adjusted only between –5° and +5°. The received signal levels of the two polarizations may be different if the insertion losses of received and transmitted signals of the two feeders are different or if the impacts of the space on the two polarizations are different. In most cases, the difference is within 1 –2 dB, which is normal and thus can be neglected.

3.4 Service Configuration After all the preceding configuration tasks are complete, the service is available. In normal cases, the services that are transmitted by the XPIC workgroup can be regarded as two independent services and can be groomed at random. In normal cases, however, as the space transmission quality of one channel falls, the transmission of the other channel is affected, due to the mutual influences within the XPIC workgroup, it is not suggest to configuring the two services of the XPIC workgroup mutual protection.

3.5 Precautions for the Configuration The XPIC workgroup is an integrity in which the members influence each other. Thus, inconsistent settings may affect the circuit decision or even result in a misoperation. When you configure the XPIC workgroup, make sure that the configuration contents are consistent with the configuration items, such as the transmitted power, definitions of the SD and SF, and enabled state of the ATPC function (it is recommended that you set the ATPC function to disabled). When the received signal level of a polarization decreases to a certain level, the XPIC signal sent to the other polarization becomes the interfering signal, thereby affecting the normal transmission on the other polarization. Therefore, some manufacturers add the automatic shutdown function. When the received signal level of one polarization is overly low, the XPIC signal to be sent to the other polarization is automatically shut down to prevent impacts on the propagation.

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Chapter 4 Common XPIC Faults and Troubleshooting Methods 4.1 General Description XPIC can effectively improve the utilization of the spectrums. When XPIC is used, however, the construction and maintenance become more complex. In actual applications of XPIC, various problems may occur. This chapter describes only several typical faults of XPIC, and focuses on the methods and thoughts of analyzing and troubleshooting the faults. Thus, this chapter can be used as reference for troubleshooting XPIC-related faults.

4.2 Handling the Fault That XPD Cannot Meet the Specification The XPD value of the system can be affected by many factors, including the antenna static XPD, port discrimination, flatness of the antenna static XPD within the operating frequency band (consistency of the static XPD values within the entire operating range), and the consistency of the antennas. In addition, the weather and transmission terrain affect the XPD value of the system. This section describes the troubleshooting thoughts when the XPD value does not meet the specification on site. Check whether the antennas are properly assembled and mounted, whether the engineering is implemented in strict compliance with the engineering drawing, and whether an overall check is performed after the installation is complete. Make sure that each installation procedure is correct. As mentioned in the preceding chapters, the techniques required for mounting a dual-polarized antenna are complex than the techniques required for mounting a uni-polarized antenna. Make sure that each connector is firmly connected. If any component is not installed in compliance with the installation regulations, re-install the component to ensure the long-term running quality of the circuit. If the antennas are properly assembled and mounted, check whether the received signal level meets the design requirements. First, check whether the relevant settings are correct, and whether the parameters are set according to the design file (including whether the ATPC function is disabled). Then, check whether the antennas are properly aligned, and whether the received signal level is adjusted to the design value. The XPD of the system is associated with the received signal level. When the received signal level does not reach the

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design value, the XPD of the system is affected. When the received signal level decreases to a certain level, the XPD starts to violently deteriorate. Hence, you need to check whether the received signal level is adjusted to the design value. Check whether the polarization of the feeder is properly decoupled. When the received signal level is adjusted to the design value, the polarization decoupling adjustment needs to be performed for the feeder. During the engineering, the XPD of the system may be affected by the following factors: 

Changes in the circuit route and the transmission terrain



Adjustment of the angle of the antenna



Adjustment of the position for installing the pole

After the antenna adjustment is complete, perform the polarization decoupling adjustment, which is equal to adjusting all the preceding factors to maximize the XPD under the current transmission conditions. Check whether the fault exists in the software or hardware of the equipment. During the standard commissioning, the adjustment of the received signal level and the polarization decoupling of the feeder should not involve the equipment. At the opposite station, the signal source (beacon) generates a single carrier. At the local station, a spectrum analyzer or another receiver meter is used to adjust the orientation of the antennas and to decouple the polarization of the feeder. Then, you can connect the equipment. In this way, the standard installation of the antennas is ensured. In actual engineering implementation, the adjustment effects are checked only according to the equipment instructions (for example, AGC voltage, and the received signal level value that is read on the NMS), since the technologies and costs required for the standard installation cannot be met. Thus, if the equipment instructions are incorrect or the hardware of the equipment is faulty, the judgment of the adjustment effects may be affected. If none of the faults described in steps 1 –3 exists, replace the relevant components to troubleshoot the fault. Check whether the design is correct, and whether the antenna specifications meet the relevant requirements. If the design does not meet the requirements after all the preceding efforts are made, check whether the circuit design is correct. The transmission terrain may change, for example, the lake surface freezes or new high buildings exist on the circuit route. In this case, request the network planning personnel to reproduce and calculate the circuit design. If required, conduct onsite inspection along the transmission route to check whether any case that affects the system specifications occurs. The antenna specifications may not meet the requirements. Since large workload is required to troubleshoot this fault and this fault occurs in rare cases, check whether this fault occurs only after all the preceding faults do not exist. In addition, the latitude and longitude on the radome are related to the polarization decoupling.

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If required, you can adjust the latitude and longitude on the radome to be in the same direction as the polarization of the feeder. This can improve the XPD effects.

4.3 Handling the XPIC LOS Alarm The XPIC LOS alarm occurs only when XPIC is used. It occurs when a board cannot receive the XPIC signal from the paired board. The XPIC LOS alarm occurs due to various factors. Different manufacturers produce different types of equipment. This section introduces the procedures for handling the XPIC LOS alarm. Check the relevant connections. In normal cases, there should be XPIC signal cables between two XPIC boards. The receiving cable and the transmitting cable are cross-connected. Check whether the connections of the IN port and the OUT port are correct. Then check the quality of the signal cables. Equipment from certain manufacturers supports the automatic shutdown function. In this case, check whether the opposite polarization received signal level is lower than the threshold value for automatic shutdown. If yes, determine whether further analysis and handling measures are required depending on the actual situation. Check whether the ODU on the opposite polarization at the local station or opposite station is powered on, whether any RF signal is radiated from the opposite station, whether the boards relevant to the opposite polarization at the local station are powered on, and whether the equipment hardware is normal. Check whether the software and hardware of the board on this polarization runs normally, and whether the board on this polarization can detect the signal from the opposite polarization.

4.4 Handling the Fault That Bit Errors on One Polarization or on the Two Polarizations Troubleshooting of bit errors, especially sporadic bit errors, in the microwave transmission system is complex. This section describes the simple methods of handling the bit errors that occur when XPIC is used. Check whether the test connection cable or baseband signal cable are properly connected, and whether virtual connection exists.

Check whether the XPD value meets the specifications, and whether any change occurs in the XPD value. If the XPD does not reach the design

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value, refer to " Handling the Fault That XPD Cannot Meet the Specification ", and troubleshoot the fault. View the bit errors on the NMS. Compare the bit errors before the error correction with the bit errors after the error correction to check whether a large number of bit errors occur before the error correction. If yes, it indicates that the fault exists in the transmitter at the opposite station or in the space. If you can view the bit errors and the signal levels, especially the received signal level, on the NMS, you can determine whether the bit errors result from fading or other factors. If the bit errors last for a long time, check whether polarization interference exists, that is, whether the two polarizations interfere with each other. If the conditions permit, close a polarization, and then test bit errors. When deep fading occurs on one polarization, check whether the polarization selective fading is exit if the equipment hardware is not faulty. If deep fading occurs on the two polarizations at the same time, it indicates that flat fading may occur. In this case, request the network planning personnel to review the network design and re-plan the network. The fault may also occur due to external factors. The external factors may affect one polarization or both the polarizations. The troubleshooting methods are the same in the two cases. You can locate and then troubleshoot the fault as follows: 

Close the polarizations at the opposite station, and then observe bit errors.



Observe the results on the spectrum analyzer.



Change the frequency and switch the Tx high/low station.

4.5 Precautions for Maintenance When XPIC Is Used In the case of cross polarization, the two polarizations are related to e ach other. Therefore, if one polarization becomes unavailable or other faults occur when XPIC is used, it is recommended that you close both ends of the local hop and all the components related to this polarization. Otherwise, the operation of the other polarization may be affected. Replace spare parts when the received signal level is normal. When the received signal level is norm al, the XPD of the antenna has certain resistance to interferences, thus protecting the normal operation of the service. Collect the data of both the polarizations. Otherwise, the comparison analysis does not work. After spare parts are replaced, power on the ODU on this polarization at both ends at the same time, thus minimizing the mutual interferences between the two polarizations. Note the spare parts are all related to the space transmission, such as ODU, IF boards, and IF cables, with service boards excluded.

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Chapter 5 Conclusion This document describes the XPIC in terms of principles, implementation modes, installation commissioning, and common troubleshooting methods. In addition to the performance of the equipment, the XPIC effects depend on whether the equipment is correctly installed and whether the circuit design is proper. If yes, critical faults do not occur when XPIC is used. This document is also intended to familiarize readers with the XPIC basics and to provide instructions to the engineering implementation and maintenance.

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Topic on Microwave XPIC-B

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