Microwave Path Design

For many wireless carriers, microwave is becoming a popular choice over wireline transport. It is an attractive option for many reasons, especially as radio equipment costs decrease. Low monthly operating costs can undercut those of typical T1 expenses, proving it more economical over the long term.

Carriers also are attracted to its modular and expandable characteristics. Network operators like the fact that they can own and control microwave radio networks instead of relying on other service providers for network components.

Like many carriers, you may be planning to jump on the microwave bandwagon. But before you move forward, make sure you understand all of the design considerations that will affect your deployment.

Frequency Options

First, it is important to understand the relationship between capacity, frequency band, path distance, tower heights, radio equipment and antennas. In the United States, there are numerous licenses to operate microwave radio, including 2GHz, 6GHz, 7GHz, 8GHz, 10GHz, 11GHz, 13GHz, 15GHz,18GHz, 23GHz and 38GHz frequency bands.

Wavelengths in the lower frequencies are longer, which is important because the wavelength determines how the atmosphere affects transmission. The atmosphere may refract longer waves. Refraction can reduce the length of the path, or microwave hop. In developed countries, such as the United States, much of the available frequency spectrum already is in use. Competition for these frequencies has pushed use into higher bands, such as 38GHz.

Radios in the 2GHz to 6GHz frequencies can transmit over longer distances, which makes them more suitable for rural areas. High-frequency radios are a better fit for suburban and urban environments. For example, a low-frequency radio could carry a signal for more than 12.5 miles, while a high-frequency radio, such as a 23GHz radio, could cover a path distance of more than three miles.

Terrain & Weather

Because line of sight is a microwave requirement, terrain such as mountains, hills, trees and buildings can block a microwave signal and limit the distance of a microwave path. However, a bird or other object moving through the microwave path will not affect your design because transmission can go around a small, intermittent object.

Capacity is another important consideration. You can configure radios to carry a certain amount of traffic in a specific frequency. Capacities range from DS1 to OC-3, which is equivalent to three DS3s. So you could select a 16DS1-capacity radio operating at 38GHz to carry a significant amount of traffic over a path distance of less than five miles.

A microwave system includes an antenna, radio, multiplexers, waveguide (hollow metal conductor connecting the RF equipment to the antenna) and feed cables. Based on capacity and radio equipment, antenna size, tower heights and terrain elevation will play a major role in how you plan and construct the system. These four factors also will dictate system reliability, multi-path fading, fade margin calculations, fresnel zone clearance, interference analysis, system diversity and long-distance specifications.

You will use a large antenna (low frequency) when the path is longer. Large antennas require large towers and have higher wind and ice load factors. As a result, you also must consider existing tower loads to ensure that you can implement the design on existing or planned towers and structures.

You also must take into account attenuation, the reduction in energy as a signal travels through equipment, transmission lines or air. The term often refers to the impact of rain, snow or fog as well as normal signal loss in the waveguide and microwave system itself. Fog, snow, sand and dust have minimal influence in the frequency bands above 8GHz. Rainfall rates and storm duration can affect the availability of the path at 23GHz and 38GHz.

In many cases, design engineers can calculate rain effect to ensure customer requirements are met. However, the rate on the path will vary. Raindrops also vary in shape. For example, large drops change shape as they fall. As a result, a radio wave with vertical polarization is less attenuated than a wave that is horizontally polarized.

Ice and snow have little effect on high-frequency radio links, and antenna radomes are designed to prevent snow accumulation. In the United States, the National Weather Service Library provides detailed data on rain rate and drop-size distribution. Also, do not forget to consider temperature.

In some cases, you will need to locate an antenna indoors. Transmitting through glass causes attenuation. The attenuation will depend on metallic content, any exterior coating on the glass and the angle of incidence of the radiated beam. You will get the best results by placing the antenna at least 12 inches from the window at a 10-degree angle.