WiMax Operator's Manual

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CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 115 such a regime. Thus, the presence of some degree of NLOS capability in a receiver does not mean that one can ignore line-of-sight considerations. Diffraction Diffraction, which occurs when radio waves bend around the edges of an object, results in a transmitted beam becoming off-axis in relationship to the receiver antenna. Since radio waves are vastly greater in wavelength than visible light waves, diffraction can occur when there is still optical line of sight between two radio terminals. The effect of diffraction is to reduce signal strength substantially at the receiver and also to introduce a possible source of unwanted early reflections. Here again, a diffracted signal is not necessarily useless, but it is certainly less useful than a direct signal. NLOS: A Truer Conception It should be obvious by now that, absent line of sight, no radio functions optimally, and the mere fact that a radio has some measure of NLOS capability does not mean that the network operator can place base stations and subscriber haphazardly. NLOS technologies are partial remedies (misleading might be a better word) at best. They do not rewrite the laws of physics. Before I leave this discussion, I should mention the term near-NLOS, industry jargon that surpasses simple NLOS in its sheer ambiguity. As the term is most commonly employed, it refers to radio equipment capable of dealing with consequences of obstructions that occur within the Fresnel zone but do not block optical line of sight. What is really being claimed here is not the ability to reach completely obstructed sites but the ability to cope with multipath with a high degree of effectiveness. NLOS Technologies Radios are able to function in NLOS environments by using a number of technologies. These involve the basic functioning of the radio, the modulation system employed, the design of the antenna, and the use of certain extraneous signal processing techniques normally involving multiple antennas. Basic Radio Performance and NLOS Increasing radio sensitivity and channel selectivity while providing the radio front end with high overload capabilities and a very low noise circuitry will in and of itself endow the receiver with an enhanced ability to use the weakened signals upon which one is forced to rely on a NLOS installation. In other words, basic high quality RF engineering and circuit implementation, most of which, incidentally, is still analog, will go a long way toward the aiding in the recovery of a marginal signal. Modulation Technique and NLOS Modulation technique can also play a major role in enabling links where line of sight is not present. All of the spread spectrum techniques in common use in broadband wireless, including frequency hopping, direct sequence, CDMA, and OFDM, can immunize the signal to some extent from multipath, with frequency hopping perhaps performing best in this regard. However, interestingly frequency hopping figures in the IEEE standard only for WLANs and 802.11,
116 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES not in the 802.16 standard. OFDM also has very good immunity from multipath, and OFDM radios can generally operate in the presence of weaker signals than is the case with most other modulation techniques, thus permitting the radio to retrieve a usable signal in marginal settings. Nevertheless, OFDM, in and of itself, will not allow a link to be maintained in the face of complete line-of-sight obstructions, as is the case when a target terminal is located behind a hill, a large masonry building, or a dense grove of trees. It is actually better suited to near-NLOS conditions. OFDM is the only advanced modulation technique specified in the 802.16 standard, and only in the 802.16a amendment. Single-carrier modulation is permitted within the 802.16a standard as well, but no company currently active in wireless broadband within the lower microwave region is making a single-carrier system any longer except for wireless bridges. OFDM Since OFDM figures so prominently in the standard, and because its advocates make so many claims in its behalf, the network operator should acquire at least a rudimentary understanding of how the technology works, of its real strengths and weaknesses (and it has weaknesses), and the degree to which it truly advances the art. OFDM has a lengthy patent history and a long incubation period, but in its present form it is quite a recent development. Indeed, prior to 1990 the basic technology that would permit the concept to be realized scarcely existed. In simplest terms, OFDM is a technique by which a message is assigned to a number of narrowband subcarriers, usually numbering in the hundreds or thousands, simultaneously. In 802.16a the specified number of subcarriers is 256. The same information is not replicated on all of the subcarriers; rather, it is parceled out, with some bits going to one subcarrier and some to another and to another. Some redundancy will normally be present; that is, some bits will be shared among some sets of subcarriers, but, in the main, the data will be widely scattered over a considerable expanse of spectrum. Incidentally, OFDM may not be considered a modulation technique in the strictest sense because there is nothing in the technology itself that determines how a signal is impressed on a carrier wave, and OFDM can be—and in fact always is—combined with amplitude modulation, phase modulation, or combinations thereof. It can also be combined with direct sequence or CDMA, both of which involve an initial modulation of phase to impress the signal on a carrier and then remodulation of the resulting signal to impose an additional coding sequence upon it. OFMD is in fact combined with frequency hopping in Flarion Technologies’ Flash OFDM system. ■Note For those who lack a technical background in radio engineering, a carrier is a wave of fixed frequency that is made to vary in some way; the variations constitute the encoding of information on the carrier. In essence OFDM represents a fairly simple idea, and the basic concept goes back to the 1950s, but when attempted with 1950s RF technology, it did not provide a lot of benefits except to military radio personnel attempting to render transmissions difficult to intercept. What made OFDM both feasible and intriguing was the development of cost-effective digital signal
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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 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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