Chapter 18 Terrestrial Microwave Communication Systems Objectives 18.1 Introduction 18.2 Terminal and Repeater Siting 18.3 Path Calculations 18.4 Fixed Microwave Links 18.5 Local Microwave Distribution systems
Chapter 18 Terrestrial Microwave Communication Systems Objectives 18.1 Introduction 18.2 Terminal and Repeater Siting 18.3 Path Calculations 18.4 Fixed Microwave Links 18.5 Local Microwave Distribution systems
Chapter 18 Terrestrial Microwave Communication Systems Objectives 18.1 Introduction 18.2 Terminal and Repeater Siting 18.3 Path Calculations 18.4 Fixed Microwave Links 18.5 Local Microwave Distribution systems
Objectives After completing this chapter, you should be able to: Describe the basic structure and uses of microwave radio links, Explain the methods used in choosing site for repeaters, Calculate the signal strength at the receiver for a variety of transmitter, antenna, and terrain configurations Calculate the required clearance of a microwave path from an obstacle, Calculate the noise temperature and carrier-to-noise level of a microwave system, •
• •
•
•
Objectives After completing this chapter, you should be able to: Describe the basic structure and uses of microwave radio links, Explain the methods used in choosing site for repeaters, Calculate the signal strength at the receiver for a variety of transmitter, antenna, and terrain configurations Calculate the required clearance of a microwave path from an obstacle, Calculate the noise temperature and carrier-to-noise level of a microwave system, •
• •
•
•
Objectives •
•
•
•
•
Calculate the energy per bit per noise density ratio for a microwave system, Explain fading and describe the diversity schemes that are used to overcome it, Describe the transmitting and receiving equipment used for FM, SSB, and digital systems, Describe the types of repeaters used for analog and digital systems and perform frequency calculations for these systems, Describe the place of microwave systems for terrestrial broadcasting.
Objectives •
•
•
•
•
Calculate the energy per bit per noise density ratio for a microwave system, Explain fading and describe the diversity schemes that are used to overcome it, Describe the transmitting and receiving equipment used for FM, SSB, and digital systems, Describe the types of repeaters used for analog and digital systems and perform frequency calculations for these systems, Describe the place of microwave systems for terrestrial broadcasting.
Chapter 18 Terrestrial Microwave Communication Systems Objectives 18.1 Introduction 18.2 Terminal and Repeater Siting 18.3 Path Calculations 18.4 Fixed Microwave Links 18.5 Local Microwave Distribution systems
Chapter 18 Terrestrial Microwave Communication Systems Objectives 18.1 Introduction 18.2 Terminal and Repeater Siting 18.3 Path Calculations 18.4 Fixed Microwave Links 18.5 Local Microwave Distribution systems
Electronics Rewind
•
Microwaves in Long-Distance Telephony
The first fixed microwave link for telephone communications in North America was established in 1947 between New York and Boston.
•
Microwaves in Long-Distance Telephony
The first fixed microwave link for telephone communications in North America was established in 1947 between New York and Boston.
•
Microwaves in Long-Distance Telephony
By 1951 a transcontinental system was in place using 107 repeaters to cover the distance from New York to San Francisco.
•
Microwaves in Long-Distance Telephony
By 1951 a transcontinental system was in place using 107 repeaters to cover the distance from New York to San Francisco.
18. 1 Introduction •
Point-to-point microwave radio links have many uses. They can be used as studio-to-transmitter (STL) links for radio and television broadcasting stations, and they can also link the head-ends (antenna sites) of many cable television installations to their distribution systems. Another very common application of microwave links is as part of a communications network involving telephone, data, or television signals.
18. 1 Introduction •
Point-to-point microwave radio links have many uses. They can be used as studio-to-transmitter (STL) links for radio and television broadcasting stations, and they can also link the head-ends (antenna sites) of many cable television installations to their distribution systems. Another very common application of microwave links is as part of a communications network involving telephone, data, or television signals.
18. 1 Introduction •
Fiber-optic systems are being installed in preference to microwave radio for some fixed point-to-point services. The bandwidth available with fiber-optic systems is greater than with radio, and they require less maintenance. On the other hand, microwave relays are needed only at intervals of approximately 40 km, so microwave systems are easier to install in difficult terrain, such as in mountainous or wilderness areas. There is no doubt that terrestrial microwave systems will continue to be part of the evolving communications grid.
18. 1 Introduction •
Fiber-optic systems are being installed in preference to microwave radio for some fixed point-to-point services. The bandwidth available with fiber-optic systems is greater than with radio, and they require less maintenance. On the other hand, microwave relays are needed only at intervals of approximately 40 km, so microwave systems are easier to install in difficult terrain, such as in mountainous or wilderness areas. There is no doubt that terrestrial microwave systems will continue to be part of the evolving communications grid.
18. 1 Introduction •
Some microwave systems use only one link or hop, while others are multihop systems that use repeaters to extend the system beyond the line-of-sight range of a single link. Figure 18.1 shows the two types of systems.
18. 1 Introduction •
Some microwave systems use only one link or hop, while others are multihop systems that use repeaters to extend the system beyond the line-of-sight range of a single link. Figure 18.1 shows the two types of systems.
Transmitter
Receiver Antenna
Antenna (a) Single link
Repeater
Transmitter Antenna
Antenna
Receiver Antenna
Antenna (b) System with repeater
Transmitter
Receiver Antenna
Antenna (a) Single link
Repeater
Transmitter Antenna
Antenna
Receiver Antenna
Antenna (b) System with repeater
18. 1 Introduction •
Microwave systems can also be classified by the modulation technique used. Older systems are analog; most employ frequency modulation, though some use single-sideband AM. Many of the newer systems use digital modulation schemes, generally quadrature amplitude modulation (QAM). All of these techniques have been discussed in previous chapters. The analog and digital systems are similar in many aspects, since the radio propagation part of the system is the same for both. The modulation and demodulation techniques and the design of repeaters are the main difference between the two types.
18. 1 Introduction •
Microwave systems can also be classified by the modulation technique used. Older systems are analog; most employ frequency modulation, though some use single-sideband AM. Many of the newer systems use digital modulation schemes, generally quadrature amplitude modulation (QAM). All of these techniques have been discussed in previous chapters. The analog and digital systems are similar in many aspects, since the radio propagation part of the system is the same for both. The modulation and demodulation techniques and the design of repeaters are the main difference between the two types.
18. 1 Introduction •
This chapter will first review microwave propagation and then discuss the components used in microwave links, both analog and digital. Because these systems are used in commercial applications were reliability is of utmost importance, we will also consider the means of ensuring the greatest possible reliability. Practical systems often have reliability in the range of 99.99%, that is the system may be “down” for about one hour per year, or even less.
18. 1 Introduction •
This chapter will first review microwave propagation and then discuss the components used in microwave links, both analog and digital. Because these systems are used in commercial applications were reliability is of utmost importance, we will also consider the means of ensuring the greatest possible reliability. Practical systems often have reliability in the range of 99.99%, that is the system may be “down” for about one hour per year, or even less.
18. 1 Introduction •
Recently there has been considerable interest in the use of terrestrial microwave links for broadcasting. Most of these systems are used as substitutes for television distribution using coaxial cable, but two-way systems that allow interactive television and Internet access have also been introduced. This chapter concludes with a look at some of these systems.
18. 1 Introduction •
Recently there has been considerable interest in the use of terrestrial microwave links for broadcasting. Most of these systems are used as substitutes for television distribution using coaxial cable, but two-way systems that allow interactive television and Internet access have also been introduced. This chapter concludes with a look at some of these systems.
Chapter 18 Terrestrial Microwave Communication Systems Objectives 18.1 Introduction 18.2 Terminal and Repeater Siting 18.3 Path Calculations 18.4 Fixed Microwave Links 18.5 Local Microwave Distribution systems
Chapter 18 Terrestrial Microwave Communication Systems Objectives 18.1 Introduction 18.2 Terminal and Repeater Siting 18.3 Path Calculations 18.4 Fixed Microwave Links 18.5 Local Microwave Distribution systems
18.2 Terminal and Repeater Siting •
In general, a microwave system should use as few repeaters as possible. Repeaters cost money, of course, and one increases the chances of an equipment breakdown that can disable the link. More importantly, additional links contribute to noise levels in analog systems and increase the jitter in digital ones.
18.2 Terminal and Repeater Siting •
In general, a microwave system should use as few repeaters as possible. Repeaters cost money, of course, and one increases the chances of an equipment breakdown that can disable the link. More importantly, additional links contribute to noise levels in analog systems and increase the jitter in digital ones.
18.2 Terminal and Repeater Siting •
On the other hand, repeater stations must not be located beyond line-of-sight propagation range from each other, and all sorts of practical considerations can prevent certain sites from being used for repeaters. Land must be acquired, access and electrical power must be arranged, and the topography must be inspected for the best repeater sites, preferably on high points of the terrain.
18.2 Terminal and Repeater Siting •
On the other hand, repeater stations must not be located beyond line-of-sight propagation range from each other, and all sorts of practical considerations can prevent certain sites from being used for repeaters. Land must be acquired, access and electrical power must be arranged, and the topography must be inspected for the best repeater sites, preferably on high points of the terrain.
18.2 Terminal and Repeater Siting •
Terrestrial microwave systems use relatively low power transmitters with high-gain parabolic or hog-horn antennas. By concentrating the transmitted power into a narrow beam, these antennas increase the effective power and reduce interference to and from other systems. There are limits to antenna gain, however. Gain figures greater than 45 dB should be avoided because antennas with this much gain have such narrow beamwidth (less than one degree) that mounting requirements are severe—slight motion of an antenna tower due to wind can be sufficient to cause signal loss with such narrow antenna beams.
18.2 Terminal and Repeater Siting •
Terrestrial microwave systems use relatively low power transmitters with high-gain parabolic or hog-horn antennas. By concentrating the transmitted power into a narrow beam, these antennas increase the effective power and reduce interference to and from other systems. There are limits to antenna gain, however. Gain figures greater than 45 dB should be avoided because antennas with this much gain have such narrow beamwidth (less than one degree) that mounting requirements are severe—slight motion of an antenna tower due to wind can be sufficient to cause signal loss with such narrow antenna beams.
18.2 Terminal and Repeater Siting •
Feedlines between transmitters and/or receivers and antennas are almost always constructed from waveguide at frequencies at 2 GHz, to reduce losses. At lower frequencies, coaxial cable may be used.
18.2 Terminal and Repeater Siting •
Feedlines between transmitters and/or receivers and antennas are almost always constructed from waveguide at frequencies at 2 GHz, to reduce losses. At lower frequencies, coaxial cable may be used.
18.2 Terminal and Repeater Siting Path Calculations
18.2 Terminal and Repeater Siting Path Calculations
Path Calculations •
To be continued by the next reporter…
