Avoiding Static for Spread Spectrum

The use of spread spectrum technology in telecommunication systems was originally developed during World War 11. Most notably, the technology has inherent features that provide for a very secure means of communicating even in unfriendly RF environments.

Despite widespread military use, the technology was not made available for commercial use until l995 when the U.S. Federal Communications Commission (FCC) issued ruling 15.247. This ruling permitted the use of spread spectrum technology for commercial applications in the 900, 2400 and 5800 MHz frequency bands.

 

Spread Spectrum Hastes

The basic concept of spread spectrum refers to an RF modulation technique that spreads the transmit signal over a wide spectrum (or bandwidth). 

Contrary to conventional narrow band modulation techniques that are evaluated by their ability to concentrate a signal in a narrow bandwidth, spread spectrum modulation techniques use a much wider bandwidth. 

A typical spread spectrum transmitter will integrate the actual signal with a sequence of bits that codes (referred to as pseudo-random code) and spreads the signal over a bandwidth usually from 20 to 30 Mllz. The spreading is actually accomplished using one of two different methods - direct sequence or frequency hopping. 

Direct sequence spread spectrum uses the pseudo-random code, integrated with the signal, to generate a binary signal that can be duplicated and synchronized at both the transmitter and receiver. 

The resulting signal evenly distributes the power over a wider frequency spectrum. Direct sequence spread spectrum is usually used to transmit higher speed digital data for E1, T1, or high-speed wireless data networks. 

The second popular type of spread spectrum modulation is frequency hopping. 

This technique is similar to a conventional narrow band carrier with a narrow transmit bandwidth. The difference to the former is a random hopping sequence within the total channel bandwidth. 

Spread spectrum modulation techniques have a principal advantage over other radio techniques: the transmitted signal is diluted over a wide bandwidth, which minimizes the amount of power present at any given frequency. 

The net result is a signal that is below the noise floor of conventional narrow band receivers, but is still within the minimum receiver threshold for a spread spectrum receiver. 

While the receiver is able to detect very low signal powers, the receivers are also designed to reject unwanted carriers, including signals which are considerably higher in power than the desired spread spectrum signal. Each transmitter and receiver is programmed with unique spreading sequences which is used to de-spread the desired signal and spread the undesired signal, effectively canceling the noise. 

Clear Line-of-Site

System planning is critical to the successful installation, operation and proper performance of any communication system. 

Unless your proposed microwave link will be operating over a very long path, you should be able to confirm whether a visible line-of-sight path exists between the two proposed antenna sites. This is only a first-step process, and is often accomplished by using a combination of strobe lights, mirrors (which reflect the sun), binoculars and spotting scopes. Being able to see one site from the other will not guarantee that the visible path is appropriate for a microwave signal, but at least you who know that the possibility of such a path exists. 

In many instances there may be obstacles to overcome such as buildings, trees, small hills and elevated roads, and it may not be possible to confirm line-of-sight exists without additional aid. 

Keep in mind that even a "perfectly clear= visual path may not actually be so. As an example, small branches of deciduous trees, barren in the winter, may not be visible until spring or summer when growth appears. Even the skeleton of a new building may not be visible until the sides go up! 

When establishing line-of-sight, it is extremely important to plan for the future. In urban areas, new building construction may result in total path obstruction. In areas where construction is not anticipated, the rapid growth of trees or foliage may severely effect the path over time. 

While a number of software products are available for assisting with path work, combining a topographical mapping of the path with a subsequent path walk or drive is often an excellent way to start the line-of-sight confirmation process. For long paths where earth curvature and other elements also come into play, topographical mapping is likely to be a requirement. 

Assuming an appropriate line-of-sight path from radio site to radio site can be established, both the feasibility and viability of a point-to-point microwave radio link will be dependent upon the gains, losses and receiver sensitivity corresponding with the system. 

Gains are associated with the transmitter power output of the radio, and the gains of both the transmitting and receiving antennas. Losses are associated with the cabling between the radios and their respective antennas, and with the path between the antennas. Other losses can also occur if the path is partially obstructed, or if path reflections cancel a portion of the normal receive signal. 

One of the first items to consider for any microwave path is the actual distance from antenna to antenna. The further a microwave signal must travel, the greater the signal loss. This form of attenuation is termed free space loss (FSL). Assuming an unobstructed path, only two variables need to be considered in FSL calculations: 

• The frequency of the microwave signal
• The actual path distance. 

Compared with visible light, a 2.4 GHz RF signal is not appreciably affected by fog, clouds, rain, snow, hail, smoke, and smog. Since the wavelength of the RF signal is much larger than the physical size of the components which make up these items (for example, water droplets and smoke particles), all of these items are virtually transparent to the radio signal. 

When RF energy is transmitted from a parabolic antenna, the energy spreads outward, much like the beam from a flashlight. This microwave beam can be influenced by the terrain between the antennas, as well as by objects in or along the path. 

When the centerline of a beam from one antenna to another antenna just grazes an obstacle along the path, some level of signal loss will occur due to diffraction. The amount of signal loss can vary dramatically, influenced by the physical characteristics and the distance of the object from the antenna. 

A microwave beam can also be reflected by water or relatively smooth terrain, very much in the same way a light beam can be reflected from a mirror. Again, since the wavelength of a microwave beam is much longer than that a visible light beam, the criteria for defining "smooth terrain" is quite different between the two. 

While a light beam may not reflect well off of an asphalt road, a dirt field, a billboard, or the side of a building, to a microwave beam these can all be highly reflective surfaces. Even gently rolling country can prove to be a good reflector. 

A microwave beam arriving at an antenna could effectively be canceled by its own "mid-path" reflection, causing tremendous signal loss. Long microwave paths can also be affected by atmospheric refraction, the result of variations in the dielectric constant of the atmosphere. 

The dielectric constant is controlled by atmospheric pressure, temperature and relative humidity, and actually affects the velocity of propagation. Under normal circumstances, a microwave beam is bent downward a slight amount by atmospheric refraction, so the "microwave horizon" will be slightly further away than the "visible horizon." 

Changes in the dielectric constant make the microwave horizon appear to shift closer or farther away If the surface temperature goes up and/or the humidity increases with altitude, the microwave beam will curve slightly away from the earth. Conversely, a rise in temperature with increasing altitude and/or decreasing humidity will cause the microwave beam to follow the curvature of the earth more closely. 

An abrupt change in dielectric constant could also result in a reflective surface, such as the upper surface of low ground fog in a valley. 

As one can see, the nature of microwave propagation is fairly complicated. Although microwave paths are usually referred to as line-of-sight, and are drawn as straight lines between antennas, in reality they are neither lines nor straight. To minimize the possibility of signal loss due to refraction, diffraction and reflection, as well as by other effects of obstructions and terrain, microwave paths must be properly engineered to address all of these issues. 

For relatively short 2.4GHz microwave paths, only reflection points and obstructions are usually of real concern. The effects of atmosphere and earth curvature will not usually come into play, so the engineering of these paths is quite straightforward. For long or unusual paths, however, all aspects of path engineering must be considered

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