Planning Link Budgets for Wireless ISP Cell Sites

Although the technology implemented in most broadband fixed wireless systems 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.

General Requirements

A basic requirement 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 requirements include:

• Ability to install one or more antennas—Is the roof adequate to support the antenna(s) or will it require structural reinforcement? Will a tower have to be constructed? Are permits required?
• Possibility of future obstructions—Will trees grow high enough to interfere with the signal? Are there plans to erect buildings between the sites that may obstruct the path?
• Availability of grounding—Good grounding is important in all areas of the world, but in areas prone to lightning, it is especially critical.
• Availability of power—Are redundant power systems available if the area is prone to outages?

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.

Weather

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, BWE recommends that these conditions be taken into consideration early in the planning process.

Rain and Fog

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, BWE recommends shorter paths and adequate clearances.

Atmospheric Absorption

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.

Wind

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.

Interference

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 and Adjacent Channel Interference

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.

Frequency Band Division

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

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.

Types

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 wireless 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.

Antenna Polarization

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.

Diversity and OFDM

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 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.

Towers

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, make sure you visit the FCC's Antenna Structure Registration website to review regulations regarding antenna structures.

Path Planning

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.

Fresnel Zone

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.

Here Are Some Easy to Use Link Budget Calculators