PDF of this tutorialThe primary propagation issue in lower-frequency bands is multipath fading. At the LMDS frequencies, multipath fading should not be an important effect. First, LMDS frequencies are much more line-of-sight (LOS)–dependent, which means that shadowing and diffraction do not occur as often at lower frequencies. Second, cellular and personal communications service (PCS) systems typically have customer-premises locations within six feet of the ground, whereas LMDS systems have customer antennas located high on rooftops. The height of the customer-premises antenna plays a large role in reducing multipath effects. Third, the LMDS antennas are highly directional (pointing to a single cell site), whereas the cellular and PCS antennas have either omnidirectional or loosely sectorized characteristics. Using directional antennas reduces multipath effects. Fourth, in cellular and PCS systems the customer antenna may be moving, whereas LMDS antennas are fixed on a rooftop. Once an antenna becomes fixed, installers can choose better case locations on the rooftop, leading to improved performance.
Considering these factors, the cell coverage distance will vary depending on the rainfall statistics in the particular area. Foliage height in relation to commercial and residential building heights also needs to be examined to determine the percentage of building rooftops that can be illuminated from any particular base-station antenna sector.
It is interesting to consider the approximate cell sizes that are possible within the LMDS systems. Over time these cell sizes will increase based on the microwave-power amplifier technology advances. Cell sizes are strongly affected by the propagation environment. Items such as foliage, rainfall rates, height of the transmit (cell site) antenna, and height of the customer-premises antenna are primary factors that must be considered. When detailed cell-site coverage area planning is done, it is necessary to account for the local obstructions and terrain and topology details that may affect the distance that the cell site can support. The following maxims hold true:
- As the required availability of the link increases, the distance decreases. For example, if a system operator is providing a service that requires 99.9% availability, the link distance may be up to 14 km. Customers at the distance 8.5 miles (14 kms) from the cell site will not receive service for eight hours a year, and customers closer to the cell site will have availability, which is better. However, if the system operator chooses to provide services that require 99.99% availability, the cell coverage distance may be reduced to 3 miles (5 km). If the system operator provides services that require 99.999% availability, the cell coverage distance is reduced to 1.5 miles (2.5 km). These numbers are approximate and depend on details specific to each vendor's system design.
- The modulation choice affects the distance as well. For example, QPSK and 4–QAM distances may be 6 miles (10 km), whereas 16–QAM coverage distance could be 3 miles (5 km) and 64–QAM distances could be 1.5 miles (2.5 km).
- The coverage distance also depends on the rain region. For example, LMDS systems in Miami and New Orleans might support a distance of 2 miles (3 km) at 99.99%. The same LMDS system design in Denver might support a distance of 3 miles (5 km) or more. This implies that the economics of providing telecommunications services using LMDS technology will vary depending on the area being served.
