WiMax Operator's Manual

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CHAPTER 3 ■ STRATEGIC PLANNING OF SPECTRUM AND SERVICES 35 exceeding 100GHz are currently used only for imaging (radio photography). Electromagnetic waves occurring above 300GHz are conventionally considered infrared light, but there is no arbitrary frequency where the characteristics of wave propagation change drastically and the radio wave suddenly assumes the properties of radiant light. Commercial broadcasts commence at hypersonic frequencies in the hundreds of kilohertz (thousands of cycles per second), frequencies that are assigned to AM broadcast stations in the United States. These frequency bands are wholly unsuitable for high-speed data for a number of reasons. Wavelengths span literally hundreds of yards and require immense amounts of power to propagate at detectable signal levels. And because a radio signal can convey only data rates that are a few multiples of the carrier frequency, such low-frequency signals simply cannot transmit data very quickly. As you proceed up into the megahertz (millions of cycles per second), the bands become increasingly well suited to the transmission of data, but most of these bands have long ago been assigned to what are now well-entrenched commercial and governmental users and therefore are effectively unavailable. Only as you approach the low microwave region from 1GHz to about 10GHz do bands become available that can, on the one hand, support highspeed data traffic and, on the other hand, have not been assigned to users who are so influential that they cannot be made to surrender the spectrum for new uses. Much of the vast amount of radio spectrum located between 2GHz and approximately 100GHz lends itself to data transmission simply because high frequencies enable high data throughputs. What is not useful are those regions of the spectrum where atmospheric conditions conspire to limit range. In the following sections, you will examine the microwave region much more closely, and I will discuss the characteristics of those bands that have already been allocated for data use in the United States and elsewhere. Beachfront Property: The Lower Microwave Frequencies Spectrum available for high-speed data starts in the ultrahigh frequency (UHF) bands beginning at 300MHz and extending to 3GHz. In the United States the lowest frequencies currently available for broadband wireless transmissions reside between the 700MHz and 800MHz spectrum formerly assigned to television. Further spectrum is available in the United States between 902MHz and 928MHz, at 2.3GHz, at 2.4GHz, from 2.5GHz to 2.7GHz, and in several bands from 5GHz to 6GHz. Bands located at 2.4GHz and at 5.8GHz are widely available across the globe. Throughout most of the world, though not in the United States, a band centered at 3.5GHz is also available for public access data networks and is fairly widely used. Early in 2005 the Federal Communications Commission (FCC) approved new unlicensed spectrum for broadband data services located between 3650MHz and 3700MHz. The spectrum between 3GHz and 30GHz is termed super high frequency (SHF) but is not all of a piece in regard to the characteristics of RF transmissions within this frequency range. Transmissions occurring from 3GHz to approximately 10GHz and occupying the lower third of the SHF region really have more in common with UHF in that they are relatively limited in throughput, do not readily conduce to high degrees of frequency reuse, and, perhaps most important, share a vulnerability to what is known as multipath distortion. Multipath distortion is a condition in which the signal interferes with itself because reflections off physical boundaries converge with the direct signal, causing the signal level at the receiver to swell or fade depending on the phase alignments of the converging waveforms at the moment they interact with one another. In addition, multipath results in errors in the
36 CHAPTER 3 ■ STRATEGIC PLANNING OF SPECTRUM AND SERVICES bitstream at the receiver inasmuch as the reflected signals are delayed relative to the direct signal, causing bits to appear out of sequence. Multipath varies enormously according to the position of the transmitting antenna in relationship to the ground and to large obstructions, and according to the position of the receiving antenna in relationship to the direct and reflected signals impinging on it. If the receiving antenna is not in an area where significant cancellations or reinforcements are taking place, reception will be fine. But move it a few inches, and the signal may become almost unrecoverable. In the past, multipath has been an endemic problem for networks operating in the lower microwave region. This is a problem that could be mitigated but never entirely solved by careful installation and by using diversity antenna systems that consisted of two or more spaced antennas and associated smart circuitry that would choose the optimal signal—that is, the one least afflicted with multipath. Today various new and sophisticated modulation techniques, as well as adaptive antenna technologies, are emerging that confer a considerable degree of immunity from multipath on broadband receivers and render placement fairly noncritical. The greatest reductions in the effects of multipath are to be had with certain types of adaptive antennas; however, such technology remains expensive to implement and is by no means widely present in the marketplace. One modulation technique, known as frequency hopping, provides almost absolute immunity to multipath; however, it is not specified in the 802.16 standard and is not particularly spectrally efficient either. Multipath afflicts transmissions from 300MHz all the way up to about 10MHz, but as we enter the SHF bands (3GHz–30GHz) a new problem manifests itself: increasing susceptibility to blockage from walls. Beyond about 2.5GHz, transmitting through walls becomes increasingly difficult at reasonable power levels and at reasonable distances. Beyond 3GHz, the problem becomes fairly acute, and in-building antenna mounting becomes essentially impractical for receiving outdoor transmissions in the popular 5.8GHz band. Another problem appearing as low as 2GHz and worsening progressively with frequency thereafter is vulnerability to signal interruption in the presence of foliage. Trees may be regarded as vessels filled with water, and microwaves tend to give up their energy to water when they encounter it directly—this in fact is the principle behind microwave ovens. A customer whose terminal is blocked by trees is therefore unlikely to be able receive a signal of adequate strength. In such cases the network operator has two choices: elevate the transmitting antenna sufficiently that the signal clears the treetops or see whether the owner of the trees can be prevailed upon to trim or remove them. In spite of the vulnerability of lower microwave transmissions to physical obstructions, spectrum in this region, especially below 4GHz, is extremely valuable, affording the user a combination of high throughput, fairly long distances, and some ability to pass through walls. This spectrum, whether licensed or unlicensed, is the overwhelming choice of broadband network operators the world over and is apt to remain so for the foreseeable future. Millimeter Microwave: Bandwidth at a Price As we move past 10GHz, we enter what is known as the millimeter microwave region. In this region, multipath ceases to be a problem because the radiated energy can be tightly focused with a small, passive, dish-shaped antenna. A number of other constraints begin to manifest themselves, however, as well as a couple of singular advantages.
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WiMax Operator’s Manual Building
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This book is dedicated to my wife.
Page 8 and 9: Contents About the Author . . . . .
Page 10 and 11: ■CONTENTS ix Performing Site Surv
Page 12: ■CONTENTS xi Cyberwarfare . . . .
Page 16: About the Technical Reviewer ■ROB
Page 20 and 21: Introduction Broadband wireless has
Page 22 and 23: CHAPTER 1 ■ ■ ■ Wireless Broa
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CHAPTER 6 ■ ■ ■ Beyond Access
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Figure 6-4. Wireless public network
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Specialized Content Distribution Pl
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Beyond the Central Office CHAPTER 6
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CHAPTER 6 ■ BEYOND ACCESS 149 out
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CHAPTER 7 ■ ■ ■ Service Deplo
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CHAPTER 7 ■ SERVICE DEPLOYMENTS O
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178 CHAPTER 8 ■ NETWORK MANAGEMEN
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CHAPTER 9 ■ ■ ■ Network Secur
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CHAPTER 9 ■ NETWORK SECURITY 189
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Index ■Numbers 2.4GHz to 928MHz b
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asic access and best-effort deliver
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■D dark fiber, features of, 75-76
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harmonics, impact on AC power, 188
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challenges to, 85-86 and NLOS, 121
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PONs (passive optical networks), fe
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signal diversity, providing with ad
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■V VDSL and entertainment service
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