15-01-2014, 04:57 PM
CELLULAR AND MOBILE COMMUNICATIONS Questions & Answers
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1. Explain ground incident angle, elevation angle, ground reflection and reflection point.
Answer:
The ground incident angle and the ground elevation angle over a communication link are described as
follows. The ground incident angle 0 is the angle of wave arrival incidently pointing to the ground as
shown in Fig. 1.1. The ground elevation angle is the angle of wave arrival
at the mobile unit as shown in Fig. 1.1
2. Write about the phase difference between the direct path and the ground reflected path.
Answer:
Based on a direct path and a ground reflected path, the equation
indicates a two-wave model which is used to understand the path-loss phenomenon in a mobile radio
environment. It is not the model for analyzing the multipath fading phenomenon. In a mobile environment
av = - 1 because of the small incident angle of the ground wave caused by a relatively low cell-site
antenna height.
3. Why there is a constant standard deviation along a path-loss curve.
Answer:
When plotting signal strengths at any given radio-path distance, the deviation from predicted value. is
approximately 8 dB.1012 This standard deviation of 8 dB is roughly true in many different areas. The
explanation is as follows. When a line-of-sight path exists, both the direct wave path and reflected wave
path are created and are strong. When an out-of-sight path exists, both the direct wave path and the
reflected wave path are weak. In either case, according to the theoretical model, the 40-dB/dec path-loss
slope applies. The difference between these two conditions is the 1-mi intercept (or 1-km intercept) point.
It can be seen that in the open area, the 1-mi intercept is high. In the urban area, the 1-mi intercept is low.
The standard deviation obtained from the measured data remains the same along the different path-loss
curves regardless of environment.
Support for the above argument can also be found from the observation that the standard deviation
obtained from the measured data along the predicted path-loss curve is approximately 8 dB. The
explanation is that at a distance from the cell site, some mobile unit radio paths are line-of-sight, some
are partial line-of-sight, and some are out-of-sight. Thus the received signals are strong, normal, and
weak, respectively. At any distance, the above situations prevail. If the standard deviation is 8 dB at one
radio-path distance, the same 8dB will be found at any distance. Therefore a standard deviation of 8 dB is
always found along the radio path as shown in Fig.3. The standard deviation of 8 dB from the measured
data near the cell site is due mainly to the close-in buildings around the cell site. The same standard
deviation from the measured data at a distant location is due to the great variation along different radio
paths.
4. Discuss the merits of point-to-point model.
Answer:
The area-to-area model usually only provides an accuracy of prediction with a standard deviation of 8
dB, which means that 68 percent of the actual path-loss data are within the ± 8 dB of the predicted value.
The uncertainty range is too large. The point-to-point model reduces the uncertainty range by including
the detailed terrain contour information in the path-loss predictions.
5. Explain about foliage loss
Answer:
Foliage loss is a very complicated topic that has many parameters and variations. The sizes of leaves,
branches, and trunks, the density and distribution of leaves, branches, and trunks, and the height of the
trees relative to the antenna heights all be considered. An illustration of this prv1em is shown in Fig. 5.1.
There are three levels: trunks, branches, and leaves. In each level, there is a distribution of sizes of trunks,
branches, and leaves and also of the density and spacing between adjacent trunks, branches, and leaves.
The texture and thickness of the leaves also count. This unique problem can become very complicated
and is beyond the scope of this book. For a system design, the estimate of the signal reception due to
foliage loss does not need any degree of accuracy.