WiMax Operator's Manual

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CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 117 processors capable of performing digital fast Fourier transforms (FFTs), covered shortly, on very high frequency waveforms, and such digital signal processors (DSPs) did not exist until as recently as a few years ago. To explain the significance of digital FFTs to OFDM, you need first to look at the design goals in engineering an OFDM radio, as follows. First, one wants to pack available spectrum very efficiently; second, one wants to achieve a very high immunity to interference and multipath; and third, one wants to reuse spectrum aggressively. OFDM does all of these things thanks to a DSP programmed to perform the FFT. A Fourier transform, for those lacking a grounding in the mathematics of wave theory, is a computational method for deriving the single-frequency constituents of a complex waveform, and it enables a radio to tune to various frequencies entirely in the digital domain since a microprocessor is actually running the computations. This is in marked contrast to the operation of a traditional analog radio where electrical filters separate individual frequencies and no computing functions are performed. A Fourier transform requires a powerful processor, which adds considerable expense to the system, but it can disentangle transmissions that are so close together in frequency as to overlap, and it eliminates the expedient of empty guard bands between channels that are required in all analog radio systems (most so-called digital radios used in cell phones and WLANs still use conventional analog front-end tuning circuitry). The end result is that the FFT-based OFDM system can pack the available spectrum with a great multitude of overlapping channels and tease them out of a blizzard of interference by finding the repetitive patterns that signify a carrier wave while simply ignoring extraneous information. Because the resulting system is at once so robust and so spectrally efficient, it lets the network operator reuse subcarriers very aggressively and tolerates degrees of multipath that would cripple most other systems. It also allows the receiver to recover a weaker signal than is possible with most other modulation techniques and thus to operate in the presence of obstructions that would render other radios useless. The fact that the bit rate for individual subcarriers is relatively low also contributes to the system’s high immunity from multipath. Because individual bits endure for relatively long durations, antiphase reflections are less likely to result in complete cancellations of an individual bit. The drawbacks? The main problem with OFDM is electrical inefficiency. The DSP gobbles current, and the subcarriers are always transmitting regardless of whether any data is assigned to them. For this reason, OFDM has never been used in a commercial mobile system to date and is not favored by most of the proponents of 802.20. Nevertheless, OFDM is favored by the architects of the fourth-generation cellular telephone system and may see deployment there by the end of this decade. Another limitation with OFDM is that the technology is not really applicable above a few gigahertz. Existing logic circuits simply cannot switch fast enough to deal with signals in the millimeter microwave region, and even if they could, it is not clear what OFDM would buy the operator there since multipath is not a problem, and signal attenuation through the lightest obstructions is so severe that no modulation technique is going to be much help in preventing it. Nevertheless, OFDM will probably gain ever-wider acceptance in the future and may even find its way into mobile networks. The distant future is difficult to predict, though. OFDM faces challenges, at least within the 802.20 standard, from the older technology wideband CDMA, and may face challenges in all broadband wireless applications from the rapidly
118 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES evolving ultrawideband RF technology. What I can say with utter certainty is that OFDM does provide a means of building useful NLOS links at least in some circumstances, and when combined with other technologies, such as adaptive array antennas (covered next), can enable high degrees of synergy. Multiple Antennas: Diversity Antennas, Phased Arrays, and Smart Antennas For the worst-case scenarios, something more is required than advanced modulation, though even that something may not be enough in all cases. The following sections refer to multiple antennas, conjoined in most cases with a sophisticated signal processor that can sample the different antenna feeds and construct useful signals where none may be present with a single antenna element. Diversity Antennas The simplest kind of multiantenna array is what is known as a diversity antenna system. Here two or more antennas, generally simple omnidirectional rods spaced some distance apart, are deployed. These work essentially by choosing between or among different samples of the same signal. At the wavelengths used for broadband wireless services, the signal quality may differ considerably from one antenna to another. The radio behind the diversity antenna will have circuitry for detecting the signal least afflicted with multipath distortion and will select that antenna receiving such a signal. In cases of changes over time in the incidence of multipath from one antenna element to another, the circuitry will simply choose again. Phased Arrays A much more sophisticated way of using multiple antenna elements is to construct what is called a phased array. Here the outputs or inputs of the various elements are constructively or destructively merged to form beams of almost any desired shape, and this can be done at both the transmitter and receiver. Beams can even be steered over many degrees of arc to direct energy off to one side. Phased arrays have existed for decades, but until rather recently they were manually configured, and the phase relationships were relatively fixed. Much more recently, adaptive array antennas, or smart antennas, have been developed that utilize a computing engine to shape beams dynamically on a channel-by-channel basis in order to concentrate energy in whatever direction and at whatever intensity is desired. Some such antennas can even steer the beam dynamically and track a moving object, thus providing the receptor terminal with a strong direct signal at all times. Adaptive Array “Smart Antennas” Essentially two types of adaptive array antennas have been developed: the switched-beam antenna and the beam-steering type. The switched-beam antennas can combine only the beams from the different element in a finite number of juxtapositions. The beam can assume a few fixed widths and a few fixed angles and is not infinitely variable. The beam-steering type is, on the other hand, infinitely variable and is far more flexible. It also requires far more processing power to operate effectively. Smart antennas excel in NLOS applications for a number of reasons, not all of which are pertinent to all designs. Most significantly—and this does apply to all design variants—the antenna array has the ability to focus a beam very tightly toward each subscriber unit on a packet-by-packet basis. The RF energy in that beam is not dispersed through the atmosphere
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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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