Much work on propagation has been performed by those involved in cellular radio. Cellular propagation is complicated by the movement of the mobile, possibly into buildings and areas shadowed from the radio
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signal. WLL propagation is substantially less complicated than that.
However, some of the phenomena that cause such difficulty to mobile radio propagation have an impact on WLL systems and are discussed in this section.
Before examining WLL in more detail, it is worth reviewing briefly the mobile radio propagation environment and the phenomena that affect it. Mobile radio planners consider that the path loss experienced when a signal is transmitted through the channel is composed of three distinct phenomena:
■ Distance-related attenuation;
■ Slow fading;
■ Fast fading.
6.1.1 Distance-related attenuation
Distance-related attenuation simply expresses the fact that as the distance from the base station increases the signal strength decreases. That is entirely consistent with everyday experience: the farther one moves from someone who is talking, the weaker the signal. The drop in signal strength is caused by the fact that the signal spreads out from the source on the surface of a sphere. The area of the surface is proportional to the radius squared; hence, the signal strength is proportional to 1/d2, wheredis the distance from the transmitter.
Measurements of mobile radio channels have found that in practice, the signal strength decreases more quickly than 1/d2. Typical values often used in predicting mobile radio propagation are 1/d3.5or 1/d4, depending on the model used. The reasons for that more rapid reduction in signal are the following:
■ The presence of the ground interferes with the expansion of a spherical surface, resulting in only a hemisphere. The conductivity and reflectivity of the ground then determine to what extent propagation is affected.
■ Signals are attenuated by vegetation and buildings, and the loss associated with passing through or around those things tends to increase the propagation exponent.
In the case of WLL, most of the propagation is via directline-of-sight (LOS), as explained in Section 6.2. In this case, the second of the two reasons for the exponent deviating from 2 no longer applies, and the first effect is extremely weak, with the result that a path-loss law of 1/d2 typically is experienced.
6.1.2 Slow fading
Slow fading is a mobile radio phenomenon caused by the mobile passing behind a building. During the period the mobile is behind the building, the signal received is reduced. Driving along a road, the mobile will pass behind a sequence of buildings, causing the signal to reduce in strength, or fade, on a relatively slow basis (compared to fast fading). This phe- nomenon is not directly applicable to WLL, because the receiver does not move. However, receivers installed where the path to the transmitter is shadowed are, in effect, in a permanent slow fade. Understanding the loss of signal due to such a fade allows planners to assess whether subscribers who are shadowed are able to receive sufficient signal. Such an assessment is based on an understanding of diffraction and reflection.
If a signal is received via a reflection in a WLL system, it typically is about 15 dB weaker than if it is received via a direct path. If the use of reflections is needed in some areas, then that margin needs to be added into the path-loss budget.
6.1.3 Fast fading
Fast fading is another mobile radio phenomenon. It is caused by the signal arriving at the receiver via a number of paths. Imagine that two signals are received at the mobile. One passes directly from the base station to the mobile via a line-of-sight path. The other is reflected off a building be- hind the mobile and back into the mobile antenna, as shown in Figure 6.1.
The mobile then sees a signal that is the composite of those two signals.
The reflected signal has traveled a slightly longer path than the direct signal and thus is delayed slightly compared to the direct signal. The result of that delay is that the phase of the reflected signal differs from that of the transmitted signal. The phase difference is related to the difference in distance multiplied by the speed of light (giving the delay on the signal) multiplied by the frequency of transmission.
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Fast fading can be better understood by an example. Suppose the reflected signal travels an additional 10m before arriving at the mobile.
Light travels at 3×108m/s, so the additional distance causes a delay of around 30 ps (30×10−9sec). If the frequency of transmission is 3 GHz, one cycle of the carrier wave at that frequency takes 0.3 ps. Thus, the reflected wave is delayed by 100 cycles of the carrier wave exactly. Every additional 10-cm distance, in fact, delays the reflected wave by a further cycle of the carrier wave.
When the reflected wave is delayed by an exact multiple of a cycle of the carrier wave, the two received signals are said to be in phase. The two signals sum, such that the total received signal has twice the strength of the direct signal. When the reflected wave is delayed by an exact multiple plus exactly a further half-cycle, the two signals are said to be in antiphase and cancel each other exactly. That is to say, at a certain point, the mobile loses all received signal. By moving a distance of only half a wavelength then, some 5 cm, the mobile moves from a position where the signal strength is doubled to one where there is no received signal. That phenomenon repeats continuously as the mobile moves, so the signal fades rapidly, giving the effect its name.
Of course, in real life, there are many more that just a single reflected signal path, and the reflected signals are not all equal strength, so the prospect of an exact cancellation is somewhat reduced. Nevertheless, fading is a severe problem. Figure 6.2 shows a typical fading waveform, often termed Rayleigh fading, after the mathematician who developed the statistics that can be used to describe such a waveform. It can be seen that in a period of 1 sec a number of fades, some as deep as 40 dB, are experienced. This book is not the place to discuss the mathematics of
Line-of-sight path Reflected path
Figure 6.1 Multipath propagation.
Rayleigh fading; those interested in learning more can consult almost any book that discusses mobile radio propagation, for example, [1].
A further difficulty may be caused by fast fading. Imagine that instead of the reflection coming from a nearby building, it comes from some faraway mountain. The delay of the reflected signal may now be quite large. If the delay is greater than the time taken to transmit a bit of information, then when the reflected signal finally arrives, it is carrying different information to the direct signal. The result is that the previous bit transmitted, or symbol, interferes with the current symbol, generat- ing a phenomenon known asintersymbol interference(ISI). The problem is akin to listening to a public address system when there are a number of loud speakers: the signal from the farthest loudspeakers is so delayed that a syllable arrives during the time when the next syllable is being heard from the nearest loud speaker, making comprehension difficult.
Fast fading is less relevant to WLL. Typically, where there is a LOS path, there is little fast fading because any reflected signals tend to be so much weaker than the LOS signal that the cancelation effect is minimal.
Only where the main path is shadowed is fast fading likely to be a problem.
Signal level (dB)
10 5 0
−5
−10
−15
−20
−25
−30
−35
−40
Time (0-1s)
Figure 6.2 Rayleigh fading waveform for a mobile moving at walking speed over 1 sec.
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During installation of the receiver equipment, it is important to mount the equipment such that it is not in a fade (which tends to be more or less stationary in space); small movements of around 5 cm of the receiver on its mountings might be required to receive the strongest possible signal.
If, during a call, a bus drives by, for example, further reflections off the bus may be generated. Those reflections will temporarily change the reflection pattern, possibly causing fading to occur, each fade happening as the bus moves around 10 cm. Hence, WLL equipment, especially that installed in shadowed areas, needs to be tolerant to fading occurring from time to time. The implications of this on system design are described in Chapter 7.