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OTHER BAND ARCHITECTURE—OVERVIEW CHAPTER 8: MINIMIZING SITE INTERFERENCE • Dissimilar metals - Connection of dissimilar metals should only be made after reviewing section 4.5.2, “Galvanic Corrosion” on page 4-36. for each metal. The connections must be rigid and tight. • Cable ties – Bare metallic cable ties shall not be used unless they are stainless steel. • Power line insulators (glass type) – Cracked insulators are a very likely source of broadband noise. If broadband noise cannot be eliminated by implementing the above recommendations, contact the local utility company and ask them to perform a noise sweep of the general area. 8.3 OTHER BAND ARCHITECTURE—OVERVIEW Some additional RF site design techniques are required to mitigate radio frequency interference for sites utilizing frequencies below 700 MHz. This is typically referred to as Other Band Architecture (OBA) and may include trunking, conventional, data, and paging systems. With the limited number of frequencies available in the 800 MHz and 900 MHz bands, expansion into other bands is occurring. The 450 MHz and 150 MHz bands are being utilized for trunking. This utilization of older bands has introduced multi-carrier operation in bands that were originally used primarily for single carrier or limited multi-carrier. Understanding the interference obstacles within these bands requires different solutions than those used at 700 MHz and above. 8.3.1 SPECTRUM USAGE HISTORY The 700 MHz, 800 MHz and 900 MHz frequency spectrum was coordinated to facilitate combining and trunking system design. These frequency band plans facilitate the use of multiple transmitters connected to a single antenna, as well as receivers connected to a single receive antenna. This allows for the use of standard combining and multicoupling schemes. These bands are divided by a defined spacing between the transmit and receive channel pairs. (45 MHz for 800 MHz, 39 MHz for 900 MHz and 30 MHz for 700 MHz). This spacing helps isolate the intermodulation interference that occurs when two or more frequencies mix in non-linear devices, producing carriers on the existing receiver frequencies. The VHF and UHF bands were not organized with the same standards as the 700 MHz, 800 MHz and 900 MHz bands. Therefore standard combining and multicoupling schemes cannot be utilized and extensive frequency planning must occur within the system design. These items alone can add significant design time and cost to a project. The UHF and VHF frequency bands have far less immunity to interference because the spacing between transmit and receive channels is not a defined constant. The result can be catastrophic interference if systems are not properly designed. Thus interference will have an effect on sites with OBA systems if not properly addressed. 8.3.2 FORMS OF INTERFERENCE Interference can have many sources. All forms of interference must be monitored regularly for reliable communication. Interference is normally classified as internal or external. Internal interference is generated within the components of the system. External interference is generated externally and radiated into the affected system. External interference can be prevented with good radio system design practices. All external interference must be eliminated at the source. 8-4 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES OTHER BAND ARCHITECTURE—OVERVIEW Examples of interference include internal and external intermodulation (IM), transmitter sideband noise (TN), receiver desensitization (RD), Co-channel interference and passive intermodulation (PIM). These are briefly described below. • IM: normally caused locally within the system or on site. This form of interference is caused by the mixing of multiple transmitters producing an on frequency carrier. • Internal IM is generated within system components. This can occur within the transmitter power amplifier, within the transmitter combining isolator and within the receiver, when strong off-frequency signals are allowed to enter the front end of the receiver and mix. • External IM is generated external to the transmitter or receiver system components. This interference can be produced in non-linear components on the site. Rusty tower joints, rusty fences, guy wires, corroded connectors, and inadequate grounding can also be sources of intermodulation. In all cases the only solution for external intermodulation is to find the source and eliminate it. • Transmitter Sideband Noise: All transmitters emit some noise. Transmitters should have bandpass filtering added to reduce this noise and protect collocated receivers. • Receiver Desensitization: - A receiver’s ability to process weaker signals is reduced by the influence of a strong signal occurring on a closely spaced channel. Unlike the other types of interference, there are no audible indications of interference; There is only a reduction in receiver sensitivity. • Co-channel: Multiple transmitters within the same coverage area assigned the same frequency. • Adjacent Channel: Interference from users on a frequency assignment next to the operating frequency is adjacent channel interference. • PIM: Another type of intermodulation, evident in non-active (thus passive) system components. Common mixing points are connectors, cables, surge protectors, combiners and antennas. Use of high quality (PIM rated) components can mitigate these issues. 8.3.3 OBA INTERFERENCE MITIGATION When designing a new system utilizing OBA frequencies the following items should be considered and practiced: • Frequency Layout • Create Band Plan (Specific Tx/Rx Pairs) • Perform Intermodulation Analysis • Perform TN / RD Analysis • Antenna Network Components PIM analysis and PIM sourcing • Optimization / Testing • RF Maintenance Each of these items is discussed in more detail below. 68P81089E50-B 9/1/05 8-5
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TABLE OF CONTENTS Chapter 1. Introd
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STANDARDS AND GUIDELINES FOR COMMUN
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CHAPTER 1 INTRODUCTION 1 This manua
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CHAPTER 2 SITE DESIGN AND DEVELOPME
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CHAPTER 3 COMMUNICATION SITE BUILDI
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CHAPTER 4 EXTERNAL GROUNDING (EARTH
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CHAPTER 5 INTERNAL GROUNDING (EARTH
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CHAPTER 6 POWER SOURCES 6 6.1 LOCKO
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CHAPTER 7 SURGE PROTECTIVE DEVICES
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APPENDIX B SOIL RESISTIVITY MEASURE
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APPENDIX C PROTECTING AGAINST ELECT
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APPENDIX D GROUNDING (EARTHING) ELE
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APPENDIX E GENERAL CONVERSIONS AND
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APPENDIX F R56 COMPLIANCE CHECKLIST
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I NDEX Index Index AC power cabling
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