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Fixed broadband networks can provide far higher data rates and capacity than the currently envisioned 3G and 4G mobile cellular systems. Achieving higher data rates is due to the unique technical properties of fixed systems, in particular, the use of high gain and adaptive antennas, wide frequency bands, dynamic data rate and channel resource allocation, and advanced multiple access techniques.
Fixed Broadband Wireless System Design is a comprehensive presentation of the engineering principles, advanced engineering techniques, and practical design methods for planning and deploying fixed wireless systems, including:
At present, there are no titles available that provide such a concise presentation of the wide variety of systems, frequency bands, multiple access techniques, and other factors that distinguish fixed wireless systems from mobile wireless systems. Fixed Broadband Wireless System Design is essential reading for design, system and RF engineers involved in the design and deployment of fixed broadband wireless systems, fixed wireless equipment vendors, and academics and postgraduate students in the field.
The theoretical origin of communications between two points using electromagnetic (EM) waves propagating through space can be traced to James Maxwell's treatise on electromagnetism, published in 1873, and later to the experimental laboratory work of Heinrich Hertz, who in 1888 produced the first radio wave communication. Following Hertz's developments at the end of the nineteenth century, several researchers in various countries were experimenting with controlled excitation and propagation of such waves. The first transmitters were of the 'spark-gap' type. A spark-gap transmitter essentially worked by producing a large energy impulse into a resonant antenna by way of a voltage spark across a gap. The resulting wave at the resonant frequency of the antenna would propagate in all directions with the intention that a corresponding signal current would be induced in the antenna apparatus of the desired receiving stations for detection there. Early researchers include Marconi, who while working in England in 1896 demonstrated communication across 16 km using a spark-gap transmitter, and Reginald Fassenden, who while working in the United States achieved the first modulated continuous wavetransmission. The invention of the 'audion' by Lee DeForest in 1906 led to the development of the more robust and reliable vacuum tube. Vacuum tubes made possible the creation of powerful and efficient carrier wave oscillators that could be modulated to transmit with voice and music over wide areas. In the 1910s, transmitters and receivers using vacuum tubes ultimately replaced spark and arc transmitters that were difficult to modulate. Modulated carrier wave transmissions opened the door to the vast frequency-partitioned EM spectrum that is used today for wireless communications.
Radio communications differed from the predominate means of electrical communication, which at the time was the telegraph and fledgling telephone services. Because the new radio communications did not require a wire connection from the transmitter to the receiver as the telegraph and telephone services did, they were initially called wireless communications, a term that would continue in use in various parts of the world for several decades. The universal use of the term wireless rather than radio has now seen a marked resurgence to describe a wide variety of services in which communication technology using EM energy propagating through space is replacing traditional wired technologies.
1.2 EVOLUTION OF WIRELESS SYSTEMS
As the demand for new and different communication services increased, more radio spectrum space at higher frequencies was required. New services in the Very High Frequency (VHF) (30-300 MHz), Ultra High Frequency (UHF) (300-3,000 MHz), and Super High Frequency (SHF) (3-30 GHz) bands emerged. Table 1.1 shows the common international naming conventions for frequency bands. Propagation at these higher frequencies is dominated by different mechanisms as compared to propagation at lower frequencies. At low frequency (LF) and Mediumwave Frequency (MF), reliable communication is achieved via EM waves propagating along the earth-atmosphere boundary - the so-called groundwaves. At VHF and higher frequencies, groundwaves emanating from the transmitter still exist, of course, but their attenuation is so rapid that communication at useful distances is not possible. The dominant propagation mechanism at these frequencies is by space waves, or waves propagating through the atmosphere. One of the challenges to designing successful and reliable communication systems is accurately modeling this space-wave propagation and its effects on the performance of the system.
The systems that were developed through the twentieth century were designed to serve a variety of commercial and military uses. Wireless communication to ships at sea was one of the first applications as there was no other 'wired' way to accomplish this important task. World War I also saw the increasing use of the wireless for military communication. The 1920s saw wireless communications used for the general public with the establishment of the first licensed mediumwave broadcast station KDKA in East Pittsburgh, Pennsylvania, in the United States using amplitude modulation (AM) transmissions. The 1920s also saw the first use of land-based mobile communications by the police and fire departments where the urgent dispatch of personnel was required.
From that point the growth in commercial wireless communication was relentless. Mediumwave AM broadcasting was supplemented (and now largely supplanted) by frequency modulation (FM) broadcasting in the VHF band (88-108 MHz). Television appeared on the scene in demonstration form at the 1936 World Fair in New York and began widespread commercial deployment after World War II. Satellite communication began with the launch of the first Russian and American satellites in the late 1950s, ultimately followed by the extensive deployment of geostationary Earth orbit satellites that provide worldwide relay of wireless communications including voice, video, and data.
Perhaps the most apparent and ubiquitous form of wireless communication today are cellular telephones, which in the year 2002 are used by an estimated one billion people worldwide. The cellular phone concept was invented at Bell Labs in the United States in the late 1960s, with the first deployments of cell systems occurring in the late 1970s and early 1980s. The so-called third generation (3G) systems that can support both voice and data communications are now on the verge of being deployed.
Fixed wireless systems were originally designed to provide communication from one fixed-point terminal to another, often for the purpose of high reliability or secure communication. Such systems are commonly referred to as 'point-to-point (PTP)' systems. As technology improved over the decades, higher frequency bands could be successfully employed for fixed communications. Simple PTP telemetry systems to monitor electrical power and water distribution systems, for example, still use frequencies in the 150- and 450-MHz bands. Even early radio broadcast systems were fixed systems, with one terminal being the transmitting station using one or more large towers and the other terminal the receiver in the listener's home. Such a system could be regarded as a 'Point-to-Multipoint (PMP)' system. Similarly, modern-day television is a PMP system with a fixed transmitting station (by regulatory requirement) and fixed receive locations (in general). Television can also be regarded as 'broadband' using a 6-MHz channel bandwidth in the United States (and as much as 8MHz in other parts of the world), which can support transmitted data rates of 20 Mbps or more.
The invention of the magnetron in the 1920s, the 'acorn' tube in the 1930s, the klystron in 1937, and the traveling wave tube (TWT) in 1943 made possible efficient ground and airborne radar, which saw widespread deployment during World War II. These devices made practical and accessible a vast new range of higher frequencies and greater bandwidths in the UHF and SHF bands. These frequencies were generically grouped together and called microwaves because of the short EM wavelength. The common band designations are shown in Table 1.2. Telephone engineers took advantage of the fact that PTP microwave links used in consecutive fashion could provide much lower signal loss and consequently higher quality communication than coaxial cables when spanning long distances. Although buried coaxial cables had been widely deployed for long-range transmission, the fixed microwave link proved to be less expensive and much easier to deploy. In 1951, AT&T completed the first transcontinental microwave system from New York to San Francisco using 107 hops of an average length of about 48 km. The TD-2 equipment used in this system were multichannel radios manufactured by Western Electric operating on carrier frequencies of around 4 GHz. Multihop microwave systems for long-distance telephone systems soon connected the entire country and for many years represented the primary mechanism for long-distance telecommunication for both telephone voice and video. The higher frequencies meant that greater signal bandwidths were possible - microwave radio links carrying up to 1800 three-kilohertz voice channels and six-megahertz video channels were commonplace.
On the regulatory front, the Federal Communications Commission (FCC) recognized the value of microwave frequencies and accordingly established frequency bands and licensing procedures for fixed broadband wireless systems at 2, 4, and 11 GHz for common carrier operations. Allocations for other services such as private industrial radio, broadcast studio-transmitter links (STLs), utilities, transportation companies, and so on were also made in other microwave bands.
Today, these long-distance multihop microwave routes have largely been replaced by optical fiber, which provides much lower loss and much higher communication traffic capacity. Satellite communication also plays a role, although for two-way voice and video communication, optical fiber is a preferred routing since it does not suffer from the roughly 1/4 s round-trip time delay when relayed through a satellite in a geostationary orbit 35,700 km above the Earth's equator.
Today, frequencies up to 42 GHz are accessible using commonly available technology, with active and increasingly successful research being carried out at higher frequencies. The fixed broadband wireless systems discussed in this book operate at frequencies in this range. However, it is apparent from the foregoing discussion of wireless system evolution that new semiconductor and other microwave technology continues to expand the range at which commercially viable wireless communication hardware can be built and deployed. Frequencies up to 350 GHz are the subject of focused research and, to some extent, are being used for limited military and commercial deployments.
The term wireless has generally applied only to those systems using radio EM wavelengths below the infrared and visible light wavelengths that are several orders of magnitude shorter (frequencies several orders of magnitude higher). However, free space optic (FSO) systems using laser beams operating at wavelengths of 900 and 1100 nanometers have taken on a growing importance in the mix of technologies used for fixed broadband wireless communications. Accordingly, FSO systems will be covered in some detail in this book.
1.3 MODELS FOR WIRELESS SYSTEM DESIGN
The process of designing a fixed broadband wireless communications system inherently makes use of many, sometimes complex, calculations to predict how the system will perform before it is actually built. These models may be based on highly accurate measurements, as in the case of the directional radiation patterns for the antennas used in the system, or on the sometimes imprecise prediction of the levels and other characteristics of the wireless signals as they arrive at a receiver. All numerical or mathematical models are intended to predict or simulate the system operation before the system is actually built. If the modeling process shows that the system performance is inadequate, then the design can be adjusted until the predicted performance meets the service objects (if possible). This design and modeling sequence make take several iterations and may continue after some or all of the system is built and deployed in an effort to further refine the system performance and respond to new and more widespread service requirements.
The ability to communicate from one point to another using EM waves propagating in a physical environment is fundamentally dependent on the transmission properties of that environment. How far a wireless signal travels before it becomes too weak to be useful is directly a function of the environment and the nature of the signal. Attempts to model these environmental properties are essential to being able to design reliable communication systems and adequate transmitting and receiving apparatus that will meet the service objectives of the system operator. Early radio communication used the LF portion of the radio spectrum, or the so-called long waves, in which the wavelength was several hundred meters and the propagation mechanism was primarily via groundwaves as mentioned earlier. Through theoretical investigation starting as early as 1907, an understanding and a model of the propagation effects at these low frequencies was developed. The early propagation models simply predicted the electric field strength as a function of frequency, distance from the transmitter, and the physical characteristics (conductivity and permittivity) of the Earth along the path between the transmitter and receiver. The models themselves were embodied in equations or on graphs and charts showing attenuation of electric field strength versus distance. Such graphs are still used today to predict propagation at mediumwave frequencies (up to 3000 kHz), although computerized versions of the graphs and the associated calculation methods were developed some years ago.
All wireless communication systems can be modeled using a few basic blocks as shown in Figure 1.1. Communication starts with an information source that can be audio, video, e-mail, image files, or data in many forms. The transmitter converts the information into a signaling format (coding and modulation) and amplifies it to a power level that is needed to achieve successful reception at the receiver. The transmitting antenna converts the transmitter's power to EM waves that propagate in the directions determined by the design and orientation of the antenna. The propagation channel shown in Figure 1.1 is not a physical device but rather represents the attenuation, variations, and any other distortions that affect the EM waves as they propagate from the transmitting antenna to the receiving antenna.
By using EM waves in space as the transmission medium, the system is necessarily exposed to sources of interference and noise, which are often beyond the control of the system operator. Interference generally refers to identifiable man-made transmissions. Some systems such as cellular phone systems reuse frequencies in such a way that interference transmitters are within the same system and therefore can be controlled. Cellular system design is largely a process of balancing the ratio of signal and interference levels to achieve the best overall system performance.
External noise sources may be artificial or natural, but are usually differentiated from interference in that they may not be identifiable to a given source and do not carry any useful information.
Excerpted from Fixed Broadband Wireless System Design by Harry R. Anderson Copyright © 2003 by John Wiley & Sons, Ltd. Excerpted by permission.
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Fixed Broadband Wireless Systems.
Electromagnetic Wave Propagation.
Propagation and Channel Modes.
Propagation Environment Models.
Fixed Wireless Antenna Systems.
Modulation, Equalizers, and Coding.
Traffic and Application Mix Models.
Single and Multilink System Design.
Point-to-Multipoint (PMP) Network Design.
Channel Assignment Strategies.
Appendix A: Atmospheric and Rain Data.
Appendix B: PDF of a Signal with Interference and Noise.