Wimax standards and Security The Wimax

Fixed Broadband Access

Since our focus is on fixed broadband access, we should emphasize that the propagation modeling of a fixed radio link has some fundamental differences with that of a mobile link.
The problem of collecting fixed data for an empirical model is not trivial; and many experimenters present methods to locally average data (over one- half of a wavelength) to remove small-scale fading due to multipath. Small- scale fading is difficult to quantify accurately, and even a large number of fixed data points would provide insufficient sampling to be able to evaluate its impact.
Another important issue is that of antenna beamwidth (or directivity). Mobile data collections are conducted using an omnidirectional antenna (isotropic with respect to azimuth). It has long been known that the antenna beamwidth and more specifically the distribution of angles of arrival with respect to the direction of motion of a mobile are important parameters to quantify the fading of a mobile link [16].
Consequently, fixed data models may differ in some cases from the usual empirical models. One contribution to IEEE 802.16 [14] analyzes these details and proposes models based on a large PCS data campaign and associated model [13].
Good fixed models would be welcome by the industry, but the current use of cellular and PCS models is likely to continue for a number of rea- sons: first, they provide a good estimate for initial design (site-specific models and simulations are used for more precise predictions); second,


^∗ Typically, some frequency extensions may be obtained by adding a frequency dependence in f ^2.6

×
(ora 26 log f term in dB) as suggested by Ref. 19, and used for instance in the Okumura–Hata model [8] and the 802.16 contribution [14].

some time is necessary to roll out large fixed wireless systems that can be used and analyzed to provide a wide modeling range; lastly, by the time these fixed models exist, the focus of WiMAX is likely to turn again toward mobility.

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