WiMax Operator's Manual

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CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 111 Figure 5-4. Polarization states The property of polarization can be exploited to reuse spectrum within the same space by setting up airlinks of opposing polarity, a tactic known as polarization diversity. Reuse, however, will reach only a factor of two by this means, so polarization must rank among the weaker methods for enhancing spectrum reuse. Dual polarization can be used in tandem with spatial diversity via sectorization or adaptive arrays, but I know of no commercial product with such capability, though an experimental system developed by Lucent was so enabled. In most systems one or another strategy is adopted, that is, polarization diversity or spatial diversity. In sum, polarization diversity is part of the network operator’s bag of tricks for extracting the best performance from a particular radio in a particular RF environment. The aforementioned EDX software has subprograms for plotting the effects of polarization diversity on reception. Cell Splitting Cell splitting consists of decreasing the radii of existing cells and adding new ones. Cell splitting has been one of the principal means by which cellular telephone operators increased the capacity of their networks, and it will also be a standard tactic for broadband wireless operators, although it will be supplemented by NLOS technologies that were not available to cellular operators during the period of greatest growth in cellular networks. Cell splitting should properly be considered a species of cell mapping or planning and refers to a process by which the network operator redetermines the minimum number of cells required to provide the desired coverage and capacity as the network attracts more subscribers. One does not just split a cell into two neat halves; one has to construct entirely new coverage patterns for each of the resulting base stations. It is a fairly involved process because of the very indefiniteness of cells themselves, a fact that may not be immediately apparent to the individual without extensive knowledge of RF propagation. When a diagram is made of a cellular network—and I use the term broadly here, not just with respect to mobile telephone networks—the cells are often represented as a sort of honeycomb pattern, a hexagonal arrangement of spaces where everything is clearly defined and fits
112 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES neatly together, though occasionally a checkerboard pattern is substituted. Both patterns are abstractions, and misleading ones at that. A cell radius is always an arbitrary value. A radio signal does not abruptly cease to propagate at so many yards from the transmitter; indeed it continues to the very edge of the cosmos, though growing steadily weaker over distance. What this means is that not only do adjacent cells overlap, but that every cell in the network overlaps with every other cell. Picture the propagation of signals as ripples or wavelets spreading over the surface of a pond. Each pattern of ripples travels everywhere, and each reflection begets new ripples. This being the case, cells should be considered as concentrations of RF energy rather than as well-defined geographical areas. Nevertheless, subscriber units within a point-to-multipoint network architecture must treat the cells as if they were well-defined entities; that is, subscriber units located at the arbitrary boundary separating cells must communicate with only one base station even though they are receiving signals from several, albeit at reduced levels. In other words, they must lock onto the base station with which they are registered and reject interference from all others, and for that to happen transmit power levels must be strictly controlled throughout the network. This, as it happens, has important implications for cell splitting. Because the average radii of all the cells in the network decrease with cell splitting, so perforce does the transmit distance from the subscriber terminal to the base station, and vice versa. And because of the shorter distances involved, transmitting power must be reduced at both the base station and the subscriber terminals to avoid interference throughout the network. Because in a fixed broadband network (excepting the mesh variety, which really does not have cells as such) subscriber units are normally assigned to a specific cell (an assignment that is ultimately based on the strength of the signal in either direction), redetermining the optimal transmit power levels becomes extremely important during cell splitting. Cell splitting may also necessitate the reassignment of channels within each cell in the network since the new cells will be establishing new channel relationships with surrounding cells. Altogether, it is not a process to be undertaken lightly and without a thorough reexamination of the entire network. As indicated earlier, it is advisable to plot out the location and capacity of every base station that the network will ever need at the time the network is being launched, though that may not always be possible, and the time may come when the network operator is forced to consider unanticipated microcells to meet demand. At that point, the network operator is faced with the task of essentially reengineering and rearchitecting the entire network. Obviously, there are limits to what can be done here. For practical reasons one is not going to relocate existing base stations. But power levels and channel assignments will all have to be redone, and the same software tools used in the initial planning process will have to be used all over again. Line of Sight and Non–Line of Sight I have previously alluded to NLOS many times in this book. In this section I present a fuller explanation of how it is achieved and what it means in terms of network mapping. First a bit of background: Radios operating in the region above 2GHz, the region that has generally been assigned to broadband wireless services, are, as you have seen, easily obstructed. This is a simple matter of the physics of wave propagation at such frequencies. Since the degree to which such high-frequency transmissions are obstructed has a major bearing on the broadband wireless operator’s ability to sign up customers, the equipment segment
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WiMax Operator’s Manual Building
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This book is dedicated to my wife.
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Contents About the Author . . . . .
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■CONTENTS ix Performing Site Surv
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■CONTENTS xi Cyberwarfare . . . .
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About the Technical Reviewer ■ROB
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Introduction Broadband wireless has
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CHAPTER 1 ■ ■ ■ Wireless Broa
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CHAPTER 1 ■ WIRELESS BROADBAND AN
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14 CHAPTER 2 ■ ARCHITECTING THE N
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34 CHAPTER 3 ■ STRATEGIC PLANNING
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Page 81 and 82: 60 CHAPTER 3 ■ STRATEGIC PLANNING
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Page 85 and 86: 64 CHAPTER 4 ■ SETTING UP PHYSICA
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Page 123 and 124: 102 CHAPTER 5 ■ STRATEGIES FOR SU
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Page 152 and 153: CHAPTER 6 ■ ■ ■ Beyond Access
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Page 156 and 157: CHAPTER 6 ■ BEYOND ACCESS 135 (Qo
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Page 162 and 163: Figure 6-4. Wireless public network
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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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