WiMax Operator's Manual

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CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 103 It also typified the first attempts to deploy commercial broadband data networks in the United States. The limitations of simple peer-to-peer, citizens band radio being a prime example, as opposed to the similar but much more sophisticated mesh, are fairly obvious. Each individual radio can reach adjacent radios only if it is not to broadcast interference far and wide, so such an architecture cannot form the basis of a high-speed access network. The limitations of the second approach are not so obvious, but they are nonetheless real. Simply stated, a single base station has only a certain amount of spectrum to utilize to reach all the subscribers in a given geographical area. Thus, in early mobile telephone systems where at most a few dozen channels were available, a few hundred subscribers were all the network could support using a widely accepted ratio of ten subscribers to every one of the available channels. (Such a ratio is based on the concept of statistical multiplexing, where statistically there may be only a 10 percent chance of any one subscriber transmitting at any given instant.) Obviously, a mass market service such as cellular telephony must support not hundreds, but thousands or even tens of thousands of subscribers within a metropolitan area, so it demands something more, either a vast amount of spectrum or some way of reusing a more limited number of channels so that more than one user can occupy a given channel simultaneously. Given the total demand for spectrum on the part of a huge array of powerful interests, the vast amount option is not really available; at least it was not until the emergence of ultra- wideband radio recently. The only choice the operator had was to find some means of reusing spectrum, and the cellular approach, described next, spoke to that need. So here, in brief, is how cells figure in a radio network and permit extensive frequency reuse: Radio cells themselves are relatively small areas surrounding base stations. Both the base station and the subscriber terminals within the cells transmit at very low power so that the signal quickly fades out to the point where it will not interfere with someone else on the same channel a fairly short distance away. Thus, each cell becomes in effect a subnetwork. In principle this is simple enough, but, for the scheme to succeed, one has to work with quite high frequencies that will fade quickly over distance, and during the first two decades following Bell Labs’ enunciation of the principal of cellular reuse, the only devices that could transmit at such frequencies were exotic vacuum tubes such as klystrons and traveling wave tubes, neither of which was suitable for inclusion in a subscriber terminal. Moreover, if one wanted to use cells in a mobile network, further problems presented themselves because a terminal in motion would always be passing out of range of particular base station and had to have some means of instantly acquiring an unoccupied channel in an adjacent cell if a transmission was to be maintained. Such “handoffs,” as they have come to be known in the cellular industry, require the use of powerful computers and specialized software at the base station and some computing ability in the terminal itself. The microprocessors that could support such functionality in a handset were not available much before the opening of the first network in 1978. Since cellular telephones established themselves in the early 1980s, the principle of cellular reuse has also been applied to WLANs and internal wireless phone systems called wireless PBXs and of course to fixed broadband wireless metro networks. In the case of the latter, handoffs are not ordinarily required, so the design of the network is somewhat simplified over that of a mobile network, but the basic design principles are much the same, as are the benefits, chief among them reduction of transmit power requirements for terminals and vastly increased spectral efficiency over the entire network because of aggressive frequency reuse.
104 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES I point out, however, that the cells deployed within broadband wireless networks are usually larger than those in cellular telephone networks. A Note on Channel Assignments from One Cell to Another Concerning this matter of frequency reuse, it should be understood that channels, according to standard engineering practice, cannot be reused from cell to cell because a signal fades only gradually over distance and will not be sufficiently attenuated so as not to cause grave interference if another user tries to occupy the same channel in an adjacent cell. Therefore, a channel would normally be reused only one cell diameter away at best. Figure 5-1 shows frequency reuse patterns in a cellular network; for simplicity’s sake, it displays only four channels. The exception would occur when the cell was divided into sectors, which are discussed next. Figure 5-1. Frequency reuse within a cellular architecture Today this standard engineering practice has been subject to some modification because of the appearance of advanced modulation techniques such as direct sequence, Code-Division Multiple Access (CDMA), and orthogonal frequency division modulation (OFDM); the introduction of polarization diversity; and the emergence of smart antenna technology. All these techniques, described in detail later in this chapter, increase the immunity of a transmission from interference and allow reuse of a channel at reduced distances—in other words, less than one cell diameter away. Reuse, at least within certain sectors in adjacent cells, does then in fact become possible, though certain minimum distances still have to be maintained. Coincidentally, simple rules of thumb for channel spacing become increasingly difficult to formulate. As it happens, numerous schemes and formulae have been developed for cellular telephone networks for optimizing frequency reuse patterns, but all of these, understandably, are optimized for mobility and must take into account the fact that a subscriber terminal must be able to “see” two base stations simultaneously at the boundary of a cell in order to initiate a handoff. As a result, such techniques cannot be imported into the broadband fixed wireless arena without extensive modification. Considerably less attention has been given to maximizing frequency reuse in fixed broadband networks through improved calculations for base station placement or through new protocols for dynamic channel assignment. Part of the problem has to do with the ad hoc nature of most fixed wireless broadband networks to date. Typically, wireless broadband networks have been the product of startup companies with limited resources that have tended to add capacity as needed with no overall plan of what a fully loaded network would look like and with no staff with either the training, inclination, or mandate to refine the formulae used by cellular network engineers to perform cell mapping and base station distribution. The tendency instead has been to rely on relatively crude cell-splitting techniques to accommodate increased traffic and, ironically, on ultrasophisticated adaptive modulation and beam steering technologies for the same purpose, neither of which, incidentally, require much engineering skill on the part of the network
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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 152 and 153: CHAPTER 6 ■ ■ ■ Beyond Access
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Page 162 and 163: Figure 6-4. Wireless public network
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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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