Radio Propagation Phenomena

In the wireless telecommunication, if the signal propagates as a straight line from T-R, we call it line-of-sight (LOS). Moreover, all signal propagations are LOS in the theoretical model. But in reality, some signal propagations don’t tour as LOS so non-line-of-sight (NLOS) is presented. NLOS describes that the signal from the transmitter doesn’t travel to the receiver with the straight line but some other paths. Generally, most reasons of NLOS are caused by obstacles. Because of some physical characteristics, the signal may be absorbed or its direction may be changed. Especially in the extreme hostile atmosphere, NLOS is taken place commonly. The diagram of LOS and NLOS phenomena is showed in Figure 1.

Figure 1: LOS and NLOS phenomena

Free space path loss is the signal strength attenuation process from the isotropic transmitter to the receiver through LOS. And it needs that the radio space is not with the confused physical state and no obstacle or the unstable atmosphere causes the interference.

By free space path loss, the signal strength attenuation is in approximate theory and easily derived because its propagation is like a sphere. The most used free space path loss model is Friis transmission equation which is showed in equation (1) where the variables P_t is the

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transmitted power in the transmitter, Pr is RSS in the receiver, Gt and Gr are antenna gains, λ is the signal’s wavelength, and d is the T-R distance. The equation describes that signal strength and the square of the distance are the inverse proportion with each other when others variables are fixed. This relationship of inverse proportion also exists between the signal strength and the square of the signal’s wavelength when others variables are fixed.

P_r

Pt = G_t× G_r × _{4× π× d}^{λ 2} (1)

2.1.2 Multipath

In the radio propagation, the multipath effect is the phenomenon that the signal from the transmitter reaches the receiver by two or more paths. Due to this effect, the receiver gets the incorrect signal information and the failure distance calculation by the RSS

measurement may be occurred. Reasons of multipath include atmospheric disturb, obstacles’

reflection, refraction, and scattering, and etc. Figure 2 shows the transmission of multipath in the space.

Figure 2: Multipath propagation

The effects of the multipath may be constructive or destructive on the RSS information.

Some signals aid the direct path and constructively reinforce the signal, and others subtract from the direct path and destructively interfere with the signal. In other words, when there

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are many paths from one signal transmission to the target, the actual RSS on the target is the sum of all signals’ vector from those paths.

The multipath effect causes some errors and impacts on the quality of communications.

Therefore, some methods were proposed to solve this problem such as Equalizers.

Orthogonal frequency division modulation (OFDM) and Rake receivers are often be used.

Although multipath is one deep impact of the radio propagation, it may be one way to transmit the signal. When the situation is the hard object lies in the straightway from T-R so that the signal can’t pass through it, the signal just can travel to the receiver by the multipath effect. This is what we mention about NLOS in the above issue.

And in the positioning system, multipath effect also has the deep impact on the time-based method. Because of multiple paths from the source to the destination, the receiver may not determine the path which is the straight one. On the other hand, multipath affects the RSS-based positioning, too. When one signal arrives the receiver at the same time with some different signals arrive by multipath, RSS may be stronger or weaker in terms of the construction or the destruction of two signal phases.

2.1.3 Fading

In the wireless communication, the fading effect is the signal strength attenuation process which occurs when there are objects on the line between the transmitter and the receiver. Indeed, when the signal experiences some obstacles such as solid or liquid media in particular, the fading effect appears because of physical characteristics like absorption or reflection. The fading effect can be seen as the attenuation rate and it varies with the

position and the time in the space. And on the other hand, the radio frequency of the signal also decides the fading effect. In general, the signal with the higher frequency comes with the faster signal strength attenuation in general. However, the real fading progress is hard to

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estimate and the fading model is usually the random process with a specific attenuate rate.

So it usually uses large scale RSS data from one transmitted signal with fixed power to derive the proper model or the equation in the current environment.

The fading effect is the most impact on the RSS-based positioning system. Due to its imprecise quantification and random characteristic, it always affects the RSS measurement between every T-R link. Generally, fading can be divided into some typical types by specific factors. For example, if Doppler spread or coherence time of the channel is considered, there are slow fading and fast fading (time-selective fading) are discussed. And when delay spread or coherence bandwidth of the channel is considered, frequency-selective fading and flat fading are discussed. The following paragraphs introduce these fading types in detail.

Slow fading occurs when coherence time is huge relative to delay constraint through the channel. In this situation, the radio amplitude (signal strength) and the phase change imposed by the channel. And they also can be considered approximately a constant over the period. Fast fading arises when coherence time is small relative delay constraint through the channel. The signal strength and the phase change imposed by the channel in this situation, and they vary very fast like multipath over the period. In other words, fast fading refers to that the fluctuations of signal strengths are violent in the short distance.

Flat fading means that coherence bandwidth is huge relative to bandwidth through the channel. In this situation, signal strengths with different frequencies have the same attenuate rate. On the other hand, frequency-selective fading is that coherence bandwidth is small relative bandwidth through the channel. And when encountering frequency-selective fading, signal strengths with different frequencies experience irrelevant attenuates.

2.1.4 Doppler Effect

The Doppler Effect, which is also called Doppler shift, is the phenomenon that the

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frequency or the wavelength of the signal changes when the transmitter moves and emits the signals simultaneously. This effect is commonly occurred when the vehicle (transmitter) sounds a siren and approaches, passes, and recedes from the observer (receiver). And comparing to the source frequency, the frequency of the received signal will be increased during the approach, identical at the moment of the pass, and decreased during the recession.

Because the frequency of the signal is the consideration of the RSS-to-distance

calculation, the Doppler Effect may influence it. But when we just locate the static target in the positioning system, the effect is not considered necessarily. However, it is better to reference Doppler Effect for doing preferable positioning in the tracking system because the target moves actively.