|
|
The installation of a wireless network requires much the same basic planning as any wired network. The main difference is that the wireless signal requires some additional planning. This planning includes RF path planning, site preparation, and installation of outdoor components such as outdoor units, antennas, lightning protection devices, and cabling suitable for outdoor conditions. Usually, you also need to investigate the zoning laws as well as Federal Communications Commission (FCC) and Federal Aviation Administration (FAA) regulations.
Although the technology implemented in Cisco's broadband fixed wireless system can make use of multipath signals, reducing the effect of obstructions in the path, it is important that the characteristics of the path be carefully examined. With this knowledge, components and network requirements can be correctly planned for your specific application.
This chapter provides insight into the planning necessary to prepare your site for your broadband fixed wireless system.
A basic consideration is the physical location of the sites at each end of the link. Because microwave signals travel in a straight line, a clear line of sight between antennas is ideal. Frequently, however, the locations of the desired links are fixed. When a clear line of sight cannot be achieved, you must plan accordingly.
Other general site considerations include:
The planning of a wireless link involves collecting information and making decisions. The following sections will help you determine which information is critical to the site and will be an aid in the decision-making process.
It is important to research any unusual weather conditions that are common to the site location. These conditions can include excessive amounts of rain or fog, wind velocity, or extreme temperature ranges. If extreme conditions exist that may affect the integrity of the radio link, Cisco recommends that these conditions be taken into consideration early in the planning process.
Except in extreme conditions, attenuation (weakening of the signal) due to rain does not require serious consideration for frequencies up to the range of 6 or 8 GHz. When microwave frequencies are at 11 or 12 GHz or above, attenuation due to rain becomes much more of a concern, especially in areas where rainfall is of high density and long duration. If this is the case, shorter paths may be required.
The systems discussed in this guide operate at frequencies below 6 GHz, so rain is not a concern.
In most cases, the effects of fog are considered to be much the same as rain. However, fog can adversely affect the radio link when it is accompanied by atmospheric conditions such as temperature inversion, or very still air accompanied by stratification. Temperature inversion can negate clearances, and still air along with stratification can cause severe refractive or reflective conditions, with unpredictable results. Temperature inversions and stratification can also cause ducting, which may increase the potential for interference between systems that do not normally interfere with each other. Where these conditions exist, Cisco recommends shorter paths and adequate clearances.
A relatively small effect on the link is from oxygen and water vapor. It is usually significant only on longer paths and particular frequencies. Attenuation in the 2 to 14 GHz frequency range is approximately 0.01 dB/mile, which is not significant.
Any system components mounted outdoors will be subject to the effect of wind. It is important to know the direction and velocity of the wind common to the site. Antennas and their supporting structures must be able to prevent these forces from affecting the antenna or causing damage to the building or tower on which the components are mounted.
Antenna designs react differently to wind forces, depending on the area presented to the wind. This is known as wind loading. Most antenna manufacturers will specify wind loading for each type of antenna manufactured.
 |
Note For definitions of wind loading specifications for antennas and towers, refer to TIA/EIA-195 (for antennas) or TIA/EIA-222 (for towers) specifications. |
The potential for lightning damage to radio equipment should always be considered when planning a wireless link. A variety of lightning protection and grounding devices are available for use on buildings, towers, antennas, cables, and equipment, whether located inside or outside the site, that could be damaged by a lightning strike.
Lightning protection requirements are based on the exposure at the site, the cost of link down-time, and local building and electrical codes. If the link is critical, and the site is in an active lightning area, attention to thorough lightning protection and grounding is critical.
To provide effective lightning protection, install antennas in locations that are unlikely to receive direct lightning strikes, or install lightning rods to protect antennas from direct strikes. Make sure that cables and equipment are properly grounded to provide low-impedance paths for lightning currents. Install surge suppressors on telephone lines and power lines.
Cisco recommends lightning protection for both coaxial and control cables leading to the wireless transverter. The lightning protection should be placed at points close to where the cable passes through the bulkhead into the building, as well as near the transverter.
Because the coaxial line carries a DC current to supply power to the transverter, gas-discharge surge arrestors are required. Do not use quarter-wave stub or solid-state type surge arrestors.
When the entire coaxial cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. However, local electrical codes require that the conduit be grounded where it enters the building.
When steel conduit is not used to encase the cable, each cable requires one surge arrestor within 2 feet of the building entrance, and another surge arrestor within 10 feet of the transverter.
When the entire control cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. Otherwise, each control cable requires one surge arrestor within two feet of the building entrance, and another surge arrestor within 10 feet of the transverter.
|
Note For installations with several radios, it may be more convenient to use a Type-66 punch block with surge arrestors. A Type-66 punch block can accommodate up to 25 conductor pairs. |
An important part of planning your broadband fixed wireless system is the avoidance of interference. Interference can be caused by effects within the system or outside the system. Good planning for frequencies and antennas can overcome most interference challenges.
Co-channel interference results when another RF link is using the same channel frequency. Adjacent-channel interference results when another RF link is using an adjacent channel frequency. In selecting a site, a spectrum analyzer can be used to determine if any strong signals are present at the site and, if they are, to determine how close they are to the desired frequency. The further away from your proposed frequency, the less likely they are to cause a problem. Antenna placement and polarization, as well as the use of high-gain, low-sidelobe antennas, is the most effective method of reducing this type of interference.
Each broadband fixed wireless system is a full-duplex system. Two frequency bands are used to achieve this two-way operation, with the higher frequency band considered the "high" band in the link, and the lower frequency considered the "low" band. The transmitter at one end of the link will use the high band; the transmitter at the other end will use the low band.
Antennas focus the radio signal in a specific direction and in a narrow beam. The increase in the signal power (compared to an omnidirectional antenna) when it is focused in the desired direction is called gain.
Antennas are tuned to operate on a specific group of frequencies. Other specific attributes such as beamwidth and gain are also fixed by the manufacturer. Antennas should be selected and placed according to your site and your application.
In general, the larger the antenna, the higher the gain and the larger the mast required. It is best to use the smallest antenna that will provide sufficient protection from interference and enough signal at the far end of the link to provide good reception even with fading.
Other considerations include antenna beamwidth, front-to-side ratios, front-to-back ratios, and cross-polarization rejection. Where interference from other licensees on the same channel or adjacent channels is an issue, narrow beamwidths, high front-to-back and front-to-side ratios, and high cross-polarization rejection are likely to be required. Even when other licensees are not an issue, if you are using a network deployment using the "cell" approach, all these considerations are still important to reduce interference between your own adjacent installations.
Several antenna types are appropriate for the type of installation discussed in this guide. Semi-parabolic grid antennas are usually used where wind loading is an issue. Solid antennas should have the option to add a radome to reduce wind loading, as a means of ice protection, where necessary, and to prevent birds from roosting on the antenna feeds.
For short U-NII links (or links where the appearance of the antenna is a problem) panel, patch or planar antennas might be appropriate. With these antenna types, the front-to-side, front-to-back, and cross-polarization response are not as good, so it is important to carefully examine interference potential.
Consult your antenna vendor and installer for specific information on the antenna types, their use, and their performance.
The orientation of the antenna will change the orientation of the signal. The transmitting and receiving antennas should be both polarized either horizontally or vertically. Adjacent antennas on different frequencies can be cross-polarized to help reduce interference between the two, if your operating license permits this.
When transmitted signals follow several paths between the transmitter and the receiver, a condition called multipath occurs. Signals reflect off buildings, water, and other objects, creating multiple paths to the receiver. On long point-to-point radio links, stratification of the atmosphere can create multiple paths by refracting the signals. Because of their longer path lengths, these reflected or refracted signals take longer to arrive at the receiver, where they can interfere with the main signal. The Cisco broadband fixed wireless system combines VOFDM technology with spatial diversity to take advantage of multipath.
The diversity feature requires the installation of two antennas separated vertically or horizontally (vertical separation works well for longer free-space line-of-sight links, while horizontal separation works best for partially obstructed or non-line-of-sight links). The signals received by both antennas are combined to greatly enhance the quality of the signal where multipath exists.
As a rule of thumb, the separation between antennas using this feature should be a minimum of 100 to 200 times the wavelength of the frequency. The greater distances are preferable. Table 2-1 shows a sample antenna separation calculation.
| Frequency (MHz) | Wavelength (cm) | Wavelength x 100 (m) | Wavelength x 200 (m) |
|---|---|---|---|
5000 | 6 | 6 | 12 |
When planning antenna placement, it might be necessary to build a free-standing tower for the antenna. Regulations and limitations define the height and location of these towers with respect to airports, runways, and airplane approach paths. These regulations are controlled by the FAA. In some circumstances, the tower installations must be approved by the FAA, registered with the FCC, or both.
To ensure compliance, review the current FCC regulations regarding antenna structures. These regulations (along with examples) are on the FCC web site at www.fcc.gov/wtb/antenna/.
To get the most value from a wireless system, path planning is essential. In addition to the fact that radio signals dissipate as they travel, many other factors operate on a microwave signal as it moves through space. All of these must be taken into account, because any obstructions in the path will attenuate the signal.
The characteristics of a radio signal cause it to occupy a broad cross-section of space, called the Fresnel Zone, between the antennas. Figure 2-1 shows the area occupied by the strongest radio signal, called the First Fresnel Zone, which surrounds the direct line between the antennas.
Because of the shape of the First Fresnel Zone, what appears to be a clear line-of-sight path may not be. As long as 60 percent of the First Fresnel Zone is clear of obstructions, the link behaves essentially the same as a clear free-space path.
The following formula is used to calculate it:
where
H = Height of the First Fresnel Zone (in feet)
D = Distance between the antennas (in miles)
F = Frequency in GHz
When planning for paths longer than seven miles, the curvature of the earth might become a factor in path planning and require that the antenna be located higher off the ground. The additional antenna height needed can be calculated using the following formula:
where
H = Height of earth bulge (in feet)
D = Distance between antennas (in miles)
The minimum antenna height at each end of the link for paths longer than seven miles (for smooth terrain without obstructions) is the height of the First Fresnel Zone plus the additional height required to clear the earth bulge. The formula would be:
where
H = Height of the antenna (in feet)
D = Distance between antennas (in miles)
F = Frequency in GHz
A link budget is a rough calculation of all known elements of the link to determine if the signal will have the proper strength when it reaches the other end of the link. To make this calculation, the following information should be available:
A signal degrades as it moves through space. The longer the path, the more loss it experiences. This free-space path loss is a factor in calculating the link viability. Free-space path loss is easily calculated for miles or kilometers using one of the following formulas:
Lp = (96.6 + 20 log10 F) + (20 log10 D)
where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in miles
or
Lp = (92.4 + 20 log10 F) + (20 log10 D)
where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in kilometers
Antenna gain is an indicator of how well an antenna focuses RF energy in a preferred direction. Antenna gain is expressed in dBi (the ratio of the power radiated by the antenna in a specific direction to the power radiated in that direction by an isotropic antenna fed by the same transmitter). Antenna manufacturers normally specify the antenna gain for each antenna they manufacture.
There will always be some loss of signal strength through the cables and connectors used to connect to the antenna. This loss is directly proportional to the length of the cable and generally inversely proportional to the diameter of the cable. Additional loss occurs for each connector used and must be considered in planning. Your cable vendor can provide a chart indicating the loss for various types and lengths of cable. Table A-1 on page A-4 is an example of this kind of chart.
The example below is based on the following assumptions:
Frequency | 5.0 GHz (U-NII) |
Length of Path | 7 miles |
Free Space Path Loss | 131.9 dB |
Transmitter Power | 23.8 dBm (limited by FCC EIRP guidelines) |
Cable Length | |
Number of Connectors Used | 4 (~ 0.5 dB loss per connector) |
Antenna Gain | 29 dBi transmit, 29 dBi receive |
Receiver Threshold | -76 dBm |
Required Fade Margin | 20 dB (minimum) |
The following formulas can be used to determine if the fade margin meets the requirement:
fade margin = received signal - receiver threshold |
The received signal can be calculated with the formula:
received signal = | transmitter power - transmitter cable and connector loss + transmitter antenna gain - free space path loss + receiver antenna gain - receiver cable and connector loss |
Based on the assumptions in the example, the formula becomes:
received signal = | 23.8 dBm - 2 dB (10 ft) + 29.0 dB - 131.9 dB + 29.0 dB - 2 dB (50 ft) = -54.1 dBm |
The fade margin is then calculated as follows:
fade margin = -54.1 dBm - (-76.2 dBm) = 22.1 dBm |
A fade margin of 22.1 dBm is above the required fade margin minimum (20 dB) specified for this example.
|
Note The previous link budget calculation is only an example. The actual figures and requirements will vary with the installation. |
Availability represents the quality of a link. It is the ratio of the time that the link is available to the total time. This serves as a guide to the service that you can expect, on average, over a period of one year. Table 2-2 shows how percentage availability relates to outage time per year.
| Availability | Outage Time | Outage Per Year |
|---|---|---|
99.9% | 0.1% | 9 hours |
99.99% | 0.01% | 1 hour |
99.999% | 0.001% | 5 minutes |
99.9999% | 0.0001% | 30 seconds |
Your application determines what availability is required. A critical application where downtime adversely affects business and revenue requires a high percentage of availability. Somewhat lower availability might be acceptable by an application used to gather data, where occasional outages can be tolerated.
Availability is largely a function of fade margins and the amount of signal fading. Paths obstructed by trees have larger fades than paths with no trees. Longer paths tend to have more fading than shorter paths. Larger fade margins yield better link availability.
The International Telecommunications Union (ITU) publishes a reference for link planning, which is available at www.itu.ch. ITU Recommendation G.826 contains definitions for "availability" and related terms used to describe link quality. It also contains recommendations for link quality objectives. ITU Recommendation P.530 contains information on how to plan for high reliability in clear, line-of-sight links.
Availability is much more difficult to predict for non-line-of-sight links. It is best determined by field measurements.
|
Note You can lower the bit error rate (BER), resulting in greater reliability, by reducing the data throughput and increasing the latency. |
The FCC has identified the frequencies from 5.725 to 5.825 GHz as Unlicensed National Information Infrastructure (U-NII). This band can be used by anyone without having to obtain a license. However, you must use radio equipment that is "type approved" by the FCC for use within the specific band. If you are installing a U-NII band link between two buildings, across a parking lot, or across town, you will find that this type of system is much simpler to implement than licensed systems. By using very directional antennas in the installation, you are not likely to experience interference.
Posted: Thu Jul 18 17:35:10 PDT 2002
All contents are Copyright © 1992--2002 Cisco Systems, Inc. All rights reserved.
Important Notices and Privacy Statement.