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Initial deployments of IEEE 802.16 standard-based networks will probably target fixed access connectivity to unserved and underserved markets where wireline broadband services are insufficient to fulfil the market need for high-bandwidth Internet connectivity.
Precertification WiMAX and prestandards WiMAX-type proprietary implementations exist today, which are addressing this fixed access service environment. On completion, standardization and certification will help accelerate the ramp for these fixed access solutions by providing interoperability amongst equipment and economies of scale resulting from high-volume standard-based components.
While many technologies currently available for fixed broadband wireless can only provide LOS coverage, the technology behind WiMAX has been optimized to provide excellent NLOS coverage. WiMAX’s advanced technology provides the best of both worlds -large coverage distances of up to 50 km under LOS conditions and typical cell radii of up to 5 miles/8 km under NLOS conditions (Figure 4.1).
Figure 4.1 LOS and NLOS installation
BENEFITS OF NLOS
The radio channel of a wireless communication system is often described as being either LOS or NLOS. In an LOS link, a signal travels over a direct and unobstructed path from the transmitter to the receiver.
In an NLOS link, a signal reaches the receiver through reflections, scattering and diffractions. The signals arriving at the receiver consist 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 multipath phenomena can also cause the polarization of the signal to be changed. Thus, using polarization as a means of frequency reuse, as is normally done in LOS deployments, can be problematic in NLOS applications. How a radio system uses these multipath signals to advantage is the key to providing service under NLOS conditions.
A product that merely increases power to penetrate obstructions (sometimes called ‘near LOS’) 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.
WiMAX technology can provide coverage under both LOS and NLOS conditions. NLOS has many implementation advantages that enable operators to deliver broadband data to a wide range of customers.
4.2 BENEFITS OF NLOS
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 reuse is critical, lowering the antenna is advantageous to reduce the cochannel interference between adjacent cell sites. This often forces the base stations to operate under 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 preinstallation 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 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 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, subchannelling, diversity and space-time coding that will prove invaluable to operators wishing to provide quality and performance that rivals wireline technology.
4.3 SELF-INSTALL CPE
Self-install CPEs are NLOS units which do not require specialized installation. The user can install such CPE with ease as no special skills are required. The biggest benefit of self-install CPE is that it is very cost-effective as no expenditure is incurred for installation and maintenance. Other benefits include user comfort, higher reliability and simplicity.
The challenge is that the industry will need to generate CPE -especially indoor, consumer-installable CPE - at viable prices. A real stumbling block in the residential space is the self-install issue. Without such user-friendly equipment, it would be quite difficult, not only for WiMAX, but for any technology to become a mass-market success. Yet a device that can work anywhere within a home presents a difficult engineering task. The challenge of meeting system-level requirements becomes larger as you try to penetrate walls and compensate for different materials in those walls. Terrain also plays a big role, as does the service provider’s coverage in that area. All said, an outdoor antenna will always provide better reception.