In mobile radio, a LOS between the mobile and the base station is a rare occurrence. WLL systems, however, typically need to be designed such that a LOS is more frequently achieved. That is because at the frequencies used for WLL, radio waves do not diffract well around obstacles. Hence, any obstacle tends to more or less block the signal resulting in inadequate signal strength for good reception. Further, WLL systems need to pro- vide a voice quality comparable with fixed systems, unlike cellular, which is able to provide an inferior voice quality. To obtain a high quality, a low bit error rate(BER) channel is required. Such channels typically require a LOS path to obtain an adequate SNR.
This is not too problematic for WLL, because the receiver units can be placed on building roofs, where a LOS is much more likely than would be the case for mobile radio. Establishing whether there is a LOS channel is straightforward—if the location of the subscriber antenna is visible (by eye) from the transmitter site, there is a LOS. (In practice, visual surveys of potential transmitter sites are time consuming, so computer models are used to help predict LOS paths.)
Despite earlier comments concerning the need for a LOS channel, signals do diffract to a limited extent, and reflections can often provide an adequate signal level. Figures 6.3 and 6.4 are examples of diffracted and reflected paths, respectively.
6.2.1 Diffraction
Diffraction is a phenomenon caused by the fact that electromagnetic waves propagate as if each point on a wavefront generates a new wave.
As the wave grazes the top of a building, wavelets are emitted in all directions, including away from the LOS path. The farther away from the LOS path, the weaker the signal. The extent of diffraction depends on two parameters, the diffraction angle (i.e., the angle through which the path needs to “bend” as it grazes the top of the building obscuring the receiver to arrive at the receiver) and the frequency of the carrier wave.
The greater the angle, the lower the received signal, and the higher the frequency the lower the received signal. Readers interested in a mathe- matical treatment of diffraction should consult texts on signal propaga-
Transmitter
Obstructing building Diffracted path
Angle of diffraction
Receiver
Figure 6.3 Example of a diffracted path.
Transmitter
Obstructing building Receiver
Point of reflection
Figure 6.4 Example of a reflected path.
Radio Propagation 77
tion. Figure 6.5 shows the variation of signal strength of a diffracted signal with parameterv, and Figure 6.6 shows how the signal strength for a given diffracted angle varies with frequency.
Loss(dB)
Frequency (MHz)
−6
−8
−9
−10
−11
−12
−13
−7 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000
Figure 6.6 Variation of diffraction loss for a particular obstruction with frequency.
5 0
0
−5
−10
−15
−20
−25
Loss(dB)
V
−3 −2 −1 1 2 3
Figure 6.5 Variation of diffraction loss with parameter v.
The parametervis given by
v=h√2(dλ ⋅1+d1dd22) (6.1)
whered1is the distance from the transmitter to the obstruction,d2is the distance from the obstruction to the receiver,his the height of the ob- struction, and λ is the wavelength of the transmitted frequency. As a guide, for a possible WLL deployment withd1=d2=500m, at 900 MHz the parameterv=0.16h, while at 3 GHz,v=0.28h. Using Figure 6.5, ifh=0 (i.e., the LOS path grazes the top of a building), the diffraction loss will be around 6 dB, while if the height is 5m (corresponding to an angle of 1.14 degrees), the loss will be 12.5 dB at 900 MHz and 16 dB at 3 GHz. Thus, diffraction angles greater than 1 degree are likely to result in insufficient signal strength at the frequencies of interest. As Figure 6.6 shows, the loss is frequency dependent, but because the loss already is severe, the frequency variation is unlikely to be the overriding issue.
6.2.2 Reflection
Reflection is caused when a wave strikes an object and is reflected back from it. It is, of course, the phenomenon by which we see the world around us. Different materials reflect to a different extent, the amount of absorption by the material being a key parameter in determining whether a sufficient signal strength will be achieved. In practice, concrete and glass buildings provide quite good reflections. Like light, the receiver needs to be in just the right place to see a reflected image of the transmitter (compare that with needing to be in the right place to see yourself in a mirror). Many building surfaces, however, are “rough,” so the reflection is scattered on hitting the building. That makes the zone where the reflections can be received larger but the signal strength received weaker.
The key difficulty for WLL, with its directional antennas at the subscriber equipment, is finding the reflections. An installer that is unable to obtain adequate signal strength when pointing the antenna toward the base station may need to turn the antenna through up to 360 degrees in Radio Propagation 79
an attempt to find suitable reflected paths. In many cases those will not exist.
Slightly more complexity is associated with the LOS path than has been explained so far. Readers may have been perplexed to notice that even when the diffraction angle is 0 degrees (i.e., the LOS path grazed an obstruction), there still is a significant loss. That is caused by a phenomenon known as the Fresnel effect. According to Huygens-Fresnel theory, the electromagnetic field at a receiver is due to the summation of the fields caused by re-radiation from some small incremental areas over a closed surface about a point source at the transmitter. The field at a constant distance from the transmitter, that is, on a spherical surface, has the same phase over the entire surface. That surface is called a wavefront.
If the distances from the wavefront to the receiver are considered, the contributions to the field at the receiver are seen to be made up of components that add vectorially in accordance with their relative phase difference. Any object blocking part of the wavefront affects the signal received.
It is possible to draw ellipsoids from the transmitter to the receiver, where the distances from the transmitter and from the receiver differ by multiples of a half wavelength. Each of those ellipsoids is known as a Fresnel zone. Broadly, if there is any obstruction in the first half of the first Fresnel zone, there is a significant reduction in signal strength. The radius of the first Fresnel zone can be approximated by
R=√λ ⋅d1+d1dd22 (6.2)
where the terms are as defined in Subsection 6.2.1. To avoid any loss of signal through interference with the first Fresnel zone, the closest ob- struction to a 1-km path must come no closer than 4.3m to the LOS path for a 1-GHz link. Readers who want to know more about Fresnel effects are referred to [2].
Using LOS, diffraction, and reflection, subscriber units can be in- stalled in such a manner that they receive sufficient signal strength.