What is WiMAX?
With WiMAX, WiFi-like data rates are easily supported, but the issue of interference is lessened. WiMAX operates on both licensed and non-licensed frequencies, providing a regulated environment and viable economic model for wireless carriers.
WiMAX can be used for wireless networking in much the same way as the more common WiFi protocol. WiMAX is a second-generation protocol that allows for more efficient bandwidth use, interference avoidance, and is intended to allow higher data rates over longer distances.
While many technologies currently available for fixed broadband wireless can only provide line of sight (LOS) coverage, the technology behind WiMAX has been optimized to provide excellent non line of sight (NLOS) coverage. WiMAX’s advanced technology provides the best of both worlds – large coverage distances of up to 50 kilometers under LOS conditions and typical cell radii of up to 5 miles/8 km under NLOS conditions.
NLOS versus LOS Propagation
The radio channel of a wireless communication system is often described as being either LOS or NLOS. In a LOS link, a signal travels over a direct and unobstructed path from the transmitter to the receiver. A LOS link requires that most of the first Fresnel zone is free of any obstruction, see Figure 1 if this criteria is not met then there is a significant reduction in signal strength, see [Ref 1]. The Fresnel clearance required depends on the operating frequency and the distance between the transmitter and receiver locations. WiMAX Base StationLocationWiMAXCPELocationAll
In a NLOS link, a signal reaches the receiver through reflections, scattering, and diffractions. The signals arriving at the receiver consists of components from the direct path, multiple reflected paths, scattered energy, and diffracted propagation paths. These signals have different delay spreads, attenuation, polarizations, and stability relative to the direct path.
The multi path phenomena can also cause the polarization of the signal to be changed. Thus using polarization as a means of frequency re-use, as is normally done in LOS deployments can be problematic in NLOS applications.
How a radio system uses these multi path signals to an advantage is the key to providing service in NLOS conditions. A product that merely increases power to penetrate obstructions (sometimes called “near line of sight”) is not NLOS technology because this approach still relies on a strong direct path without using energy present in the indirect signals. Both LOS and NLOS coverage conditions are governed by the propagation characteristics of their environment, path loss, and radio link budget.
There are several advantages that make NLOS deployments desirable. For instance, strict planning requirements and antenna height restrictions often do not allow the antenna to be positioned for LOS. For large-scale contiguous cellular deployments, where frequency re-use is critical, lowering the antenna is advantageous to reduce the co channel interference between adjacent cell sites. This often forces the base stations to operate in NLOS conditions. LOS systems cannot reduce antenna heights because doing so would impact the required direct view path from the CPE to the Base Station.
NLOS technology also reduces installation expenses by making under-the-eaves CPE installation a reality and easing the difficulty of locating adequate CPE mounting locations. The technology also reduces the need for pre installation site surveys and improves the accuracy of NLOS planning tools.
The NLOS technology and the enhanced features in WiMAX make it possible to use indoor customer premise equipment (CPE). This has two main challenges; firstly overcoming the building penetration losses and secondly, covering reasonable distances with the lower transmit powers and antenna gains that are usually associated with indoor CPEs. WiMAX makes this possible, and the NLOS coverage can be further improved by leveraging some of WiMAX’s optional capabilities.
WiMAX technology, solves or mitigates the problems resulting from NLOS conditions by using:
• OFDM technology.
• Sub-Channelization.
• Directional antennas.
• Transmit and receive diversity.
• Adaptive modulation.
• Error correction techniques.
• Power control.
SUMMARY:
WiMAX technology can provide coverage in both LOS and NLOS conditions. NLOS has many implementation advantages that enable operators to deliver broadband data to a wide range of customers. WiMAX technology has many advantages that allow it to provide NLOS solutions, with essential features such as OFDM technology, adaptive modulation and error correction. Furthermore, WiMAX has many optional features, such as ARQ, sub-channeling, diversity, and space-time coding that will prove invaluable to operators wishing to provide quality and performance that rivals wireline technology. For the first time, broadband wireless operators will be able to deploy standardized equipment with the right balance of cost and performance; choosing the appropriate set of features for their particular business model.
WiMAX technology can provide coverage in both LOS and NLOS conditions. NLOS has many implementation advantages that enable operators to deliver broadband data to a wide range of customers. WiMAX technology has many advantages that allow it to provide NLOS solutions, with essential features such as OFDM technology, adaptive modulation and error correction. Furthermore, WiMAX has many optional features, such as ARQ, sub-channeling, diversity, and space-time coding that will prove invaluable to operators wishing to provide quality and performance that rivals wireline technology. For the first time, broadband wireless operators will be able to deploy standardized equipment with the right balance of cost and performance; choosing the appropriate set of features for their particular business model.
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