WiMax Operator's Manual

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CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 121 Mesh Networks and NLOS Chapter 3 has already discussed the principles of mesh network operation fairly extensively. This section focuses on the NLOS capabilities of meshes and how a mesh architecture relates to the previously mentioned techniques. Using a mesh topology does not exclude any of the methods for facilitating NLOS mentioned earlier. A mesh network can use OFDM and/or adaptive array antennas, and in fact one system manufactured by Radiant Networks (now out of business) did utilize a rather crude mechanically steered antenna that was the functional equivalent of a smart array in certain respects. Indeed, if one were to combine a smart antenna with a mesh, one would achieve a powerful synergy, but so far no manufacturer has attempted this. By itself a mesh offers a compelling solution to NLOS problems in that it allows the network to route around obstacles—provided enough subscriber terminals are present to permit circuitous routing paths. The drawback, of course, is that the multiple hops required to describe such paths rob the network of capacity and can introduce latencies that compromise time-sensitive applications. As indicated earlier, the future of the mesh in public networks remains in doubt. The companies advocating the architecture are, with one exception, all very small and may not be able to survive in the overall broadband wireless marketplace where there have been too few deployments to date to support a large and diversified population of equipment manufacturers. And there are also issues involving the installation of the terminals that could compromise the effectiveness of what is essentially a sound networking strategy. In a mesh, as you have seen, the subscriber terminal and the base station are one and the same. This means that the radio and antenna are likely to be self-installed and the antenna placed indoors at low elevation. The result will be that the individual radio will be provided with a number of possible transmission paths but none that are likely to be particularly good, especially, if, as is the case with most current mesh systems, the antennas are omnidirectional. Such antennas also give rise to a generally high level of background interference because the signal is being propagated everywhere. But without adaptive array antennas, or physically steered antenna such as Radiant makes, an omni is the only way to ensure that the signal can reach multiple nodes in the network. The choice of a mesh over a point-to-multipoint architecture is fundamental and will affect every aspect of network performance and evolution afterward. It is a choice not to be undertaken lightly. My view is that mesh products are less proven than point-to-multipoint but may be indicated in certain circumstances. If, for instance, the network is being planned for a dense urban core where tall buildings ring every conceivable base station location and prevent the establishment of links with choice customer locations, then one may want to explore what may be done with a mesh, in particular whether line-of-sight connections can be established among initial nodes that would enable future prime customers to be easily reached. Unfortunately, this exercise will have to be done manually, at least in part, because not many software tools are designed for planning meshes. And, in any case, the fact that mesh equipment itself is designed to be self-organizing rather than operator configurable limits the degree to which the network can be planned. As is the case with any other radio, the operator would want to examine the mesh radio in terms of its other capabilities—can it support QoS, voice telephony, legacy protocols such as Asynchronous Transfer Mode (ATM), and so on? NLOS is of little use if one cannot offer the services and reliability the customer expects.
122 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES Adaptive Modulation and Cell Planning Adaptive modulation is a term denoting the ability of a radio to change its modulation scheme on the fly to adapt to varying signal conditions. In existing commercial products the radio is not going to shift from CDMA to OFDM or anything that fundamental. Instead, the choice will probably be between quadrature phase shift keying (QPSK) and 64 quadrature amplitude modulation (QUAM) or perhaps 16 QUAM. Here I will provide only the briefest definition of the two basic techniques. Phase shift keying is somewhat akin to frequency modulation. The phase of the carrier wave is retarded or advanced by so many degrees, and the phase shifts are used to represent bits. By providing, for example, four different degrees of phase shift, the signal can carry 4 bits of information per hertz or wave cycle, though with normal losses this rate is unlikely to be consistently realized in practice. QUAM adds amplitude gradations to those of phase—in other words, the intensity of the carrier wave as well as its phase position is made to vary by so many discrete steps, and the more steps, the more bits can be encoded in a single waveform. In theory hundreds of bits per hertz could be encoded by this means, but with normal losses and overhead involved in radio transmissions, 5 bits per hertz is about the maximum throughput that can be achieved today. The greater the number of states that can be represented by a single waveform, the smaller the differences between states and the greater likelihood that the encoded information will be lost to interference or noise. Thus, QUAM modulation systems can only be used when signal strength is very high. A radio capable of adaptive modulation will constantly evaluate signal quality and will shift from high-order QUAM to lower-order PSK when a more robust signal is needed. The throughput rate will drop accordingly. In some cases adaptive modulation can be overridden in a radio, but the network operator can generally expect higher bit error rates when that is done. The effects of adaptive modulation on throughput rates, especially over distance and in the presence of multipath environments, must be taken into account when the network is being planned. Generally, adaptive modulation will degrade capacity while improving signal integrity, but the operator needs to know exactly to what degree for each within each individual cell and sector. Frequency-Agile Radios and Network Mapping Chapter 3 also alluded to frequency-agile radios. I am reasonably confident that they will play a major role in the networks of the future. They will certainly increase the capacity and spectral efficiency of any broadband wireless network in which they are used, but they will also add enormously to the complexity of cell mapping and network organization. I know of no network planning software capable of modeling networks based on this facility, so the operator today has no real means of planning for the arrival of a technology that is three to five years off in a commercial sense. My assumption at this point is that frequency agility would only be an asset when combined with adaptive array antennas and with the overall network intelligence that would enable every base station to coordinate its activities with every other base station and solve extremely complex problems concerning frequency allocation and reuse within the entire 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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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 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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