WiMax Operator's Manual

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CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 109 switching or routing capability; it simply extends the reach of an individual base station in a given direction, and is often used to reach a few isolated customers whose numbers do not justify the creation of a complete new base station. The installation of repeaters is a tactical move on the part of the network operator, one that is indicated only in certain circumstances. As a rule, a repeater is less expensive than a fullfledged base station because of the lack of switching and/or routing equipment, though one still has to pay for the site and the radio. Because of its lack of intelligence, the repeater cannot really augment the capacity of a network appreciably, but what it can do is enable the operator to extend the boundary of the cell in one direction in order to encompass a few subscribers who would otherwise be unreachable. Sectorization I have already touched upon sectorizing slightly. In this section I will attempt a more complete explanation. Sectorizing is using an array of highly directional antennas to direct intense radio frequency (RF) energy into a designated area of the cell and little energy elsewhere. The sector defined by the antenna array appears like a pie slice when depicted on a diagram. Sectorization itself is a species of spatial diversity, of which adaptive beam steering is another. In both cases the operator is able to define subchannels in three-dimensional space rather than by frequency division or the use of sequential time slots. Figure 5-3 shows how a directional antenna defines a sector by sweeping a narrow arc. The figure shows a directivity polar plot for such an antenna. Figure 5-3. Directional antenna directivity polar plot Sectorizing is somewhat akin to cell splitting and indeed may be viewed as a sort of poor man’s cell splitting inasmuch as it allows the operator to reuse spectrum aggressively without installing a new base station and defining a new cell. It is a standard tactic in the majority of broadband wireless networks. Sectoral antenna arrays vary according to the number of sectors they form, with three to eight sectors being the usual range and four and six being the most common numbers. Obviously, the more sectors, the narrower the beam width of each antenna in the array. Some arrays are configurable to cover the whole range from three to eight, that is, the number of sectors that can be made to vary by adjustments in the antenna itself. Since each antenna in a sectoral array will be provided with a separate radio, sectorization definitely entails higher costs for the network operator. Sectors are akin to cells in that they ordinarily permit a channel to be reused one sector away but not in intervening sectors. This means that when four sectors are present, the reuse
110 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES factor is two, and for six the factor is three, and so on. As indicated earlier, advanced modulation techniques loosen such reuse constraints to some extent. In very large cells, as you have seen, the narrow beams formed by sectoral antennas spread out over distance, so actual frequency reuse capabilities will be reduced at the outer periphery. Naturally the bigger the cell, the greater the spreading and the more the sectors will overlap. Sectoral antennas can be used in both the lower microwave and millimeter microwave regions, though the physical form of the antenna will differ with frequency, with horns and waveguides being employed at the highest frequencies and arrays of spaced omnidirectional pole antennas at the lower frequencies. Sectoral antennas of whatever type are considerably more expensive than the omnis used in WLAN applications, but, given the vastly increased spectral efficiency that they confer upon the network, the cost is trifling. Indeed, using such devices has almost no downside, though they do concentrate the radiated energy, somewhat increasing the potential for interference outside the cell. The cure for that is to polarize the antennas in the horizontal plane so that only horizontally disposed magnetic fields are propagated. The whole array is then tilted downward so that beyond a certain distance the radiation will simply be absorbed into the ground. The following section explains polarization itself. Polarization Diversity The term polarization refers to a property common to all radio waves, namely that the magnetic waves emanating from the antenna tend to propagate outward in a shallow ellipse. If the antenna element is upright, the wave propagation will be in the horizontal plane, and if the antenna itself is horizontal, a vertical propagation pattern will occur. In either case the wave front is said to be polarized in one or another dimension—or linearly polarized, to use the technical term. The third dimension, through which the airlinks extend, is occupied by wave fronts in either state of polarization as they make their way toward the receiver site. Both transmit and receive antennas are generally polarized in the same dimension, and if they encounter signals of the opposite polarization, that is, offset by 90 percent, they will interact with relatively few magnetic lines of force and will not develop signals of much strength. The result is that signals of opposing polarization will not interfere with one another even if they occupy the same channel. Signals of opposite polarization, incidentally, are said to be orthogonal to one another. In self-installs, particularly those involving indoor antennas, polarization is apt to be haphazard, which argues against the use of an airlink based on simple linear polarization. One solution is circular polarization, described next. Now it is also possible to tilt antenna elements at intermediate angles and thus create a multitude of polarization states, but in such cases, the various states will not be orthogonal to one another and will interfere. Nevertheless, radios have been created that could resolve several nonorthogonal polarization states and reuse spectrum very aggressively in this manner. At this time, however, no such radio is available for broadband operators. What is available are radios that offset two signals 45 degrees from the vertical so that both antenna elements are tilted. The total separation is still 90 degrees and thus fully orthogonal, but the propagation patterns tend to be more useful, though dual vertical and horizontal polarizations are employed as well in broadband networks, such an arrangement being the aforementioned circular polarization. Circular polarization is usually provided to a single radio, and its purpose, as indicated previously, is to afford the best reception in a random polarization environment. Figure 5-4 shows the various polarization states.
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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 79 and 80: 58 CHAPTER 3 ■ STRATEGIC PLANNING
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 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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