Sunday, June 5, 2011


Recently, several channel models have been developed for various environments and system topologies such as the COST 231, Stanford University Interim (SUI), and WINNER models. Unfortunately, statistical models of this type make a number of general assumptions that are not always met in practical WiMAX scenarios. For example, the SUI models are suitable for FWA (rather than MWA) and they are based only on cellular-type measurements in the 1.9 GHz band. The COST 231 models do not formally apply beyond 2 GHz. The WINNER models do reach to frequencies at 5 GHz, but does not include frequency correction factors to enable their use in the 3.5 GHz band envisaged for WiMAX. Furthermore, there is little by way of empirical data collected in specifically multihop scenarios in the 3.5 GHz making an evaluation of these models in their intended deployments difficult. This part presents the results of a measurement campaign conducted in outdoor city environment to subject these models to empirical scrutiny.


Measurements were carried out along seven routes in Bristol city centre, United Kingdom, comprised of five RS locations as shown in Figure 1. These routes were identified from a ray-tracing analysis as being likely to exhibit deep shadowing from the BS (which was mounted on the roof of a building approximately 30 m above the top of a hill). The system under consideration is a 2-hop DL, as shown schematically in Figure 2. A commercial 3.5 GHz panel antenna was used with a beamwidth of 90° in azimuth and 10° in elevation with 17 dBi gain. Since the BS antenna was not omnidirectional, its orientation and downtilt are important. Prior to commencing the main campaign, power measurements were taken at each location for downtilts between 0° and 10° following which during measurement the antenna was set to the downtilt which maximized the received power. The antenna was oriented approximately southwesterly for routes 1–5 and northeasterly for routes 6 and 7.

Figure 1: Map of Bristol, United Kingdom, showing the BS, the 7 MS routes, and the 5 RS locations.

Figure 2: Schematic of multihop measurement system. f1 = 2.58 GHz, f2 = 3.467 GHz.

The RS was on a portable pump-up mast that was left fixed in place once on location. Two dipoles were mounted on top of the mast at either end of a beam that could be rotated in a plane. One end was fixed to act as the RS–MS transmitter, and the other end rotated around it to act as the BS–RS receiver; this rotation was to permit measurements of the local variation of the BS–RS signal. The BS transmitted 20 W before antenna gain and after cable loss, etc., on 3.59 GHz and the RS 3.67 W on 3.467 GHz.
The MS was constructed on a trolley that was pulled or pushed along the selected routes. A laptop on the trolley provided “command and control.” The two wheels on one axle of the trolley were equipped with electronic pulse counters that incremented the counts on the laptop approximately every 4 mm. Also connected to the laptop was a spectrum analyzer which received the signal from the antenna. The antenna (mounted at 1.65 m above ground) was the same type of dipole as used at the RS. The mean length of a route was about 90 m, and each route was repeated with the RS at a selection of heights of {2,3,4,5} m.
Path lengths were determined by using survey grade GPS equipment to obtain positional fixes for the start and end of each route and the trolley’s distance pulses to estimate the route in between. Elevation was assumed to change linearly along a route. Path loss at a particular point was determined as the difference between the transmitted and received power after taking account of the system gains and losses in cables, amplifiers, etc. The effect of antenna patterns was approximated as being the gain in the LoS direction between the transmitter and receiver.
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