Mobile Radio Propagation.pdf

Mobile Radio Propagation

Small scale fading • Small scale fading or simply fading is used to describe rapid fluctuations of the amplitudes, phase or multipath delays of radio signal over a short period of time or travel distances. • Fading is mainly caused by interference between two or more versions of the transmitted signal which arrive at the receiver at slightly different times. These waves are called multipath waves.

Small scale fading • Multipath in radio channel creates small scale fading effects. The three most important effects are: 1. Rapid changes in signal strength over a mall travel distance or time interval 2. Random frequency modulation due to varying Doppler shifts on different multipath signals. 3. Time dispersion (echoes) caused by multipath propagation delays.

Factors influencing small scale fading • Multipath propagation • Speed of mobile • Speed of surrounding objects. • The transmission bandwidth of the signal

Wireless losses

Wireless losses

Doppler Effect

• In 1842, JC Doppler gives the concept of doppler shift. • It is based on three parameters: wavelength, frequency & pitch. • Frequency (double)=wavelength (half) • High frequency=high pitch • Low frequency=lower pitch • High frequency=short wavelength=high pitch • Low frequency =longer wavelength=lower pitch

Defination: the apparent change in the frequency of a wave caused by relative motion between the source of the wave & the observer. Examples: Ambulance sound, train sound, galaxy Dopper shift is used to (1) Define the velocity of speeding car. (2) Depth of the water (3) Measure the speed of cars.

• Time dispersion: When a signal is transmitted, this signal can suffer a distortion caused by reflections and scattered propagation paths in the radio channel, and these phenomenon cause that an identical signal arrives at different times at its destination

• Doppler spread is a measure of the spectral broadening caused by the time rate of change of the mobile radio channel, and is defined as the range of frequencies over which the received Doppler spectrum is essentially nonzero.

• For an electromagnetic wave, the coherence time is the time over which a propagating wave (especially a laser or maser beam) may be considered coherent. In other words, it is the time interval within which its phase is, on average, predictable.

Fading

Types of small scale fading • Depending upon the relation between the signal parameters (such as signal BW, symbol period etc..) and the channel parameters (like delay spread and Doppler spread), different transmitted signals will undergo different types of fading. • Multipath delay spread leads to time dispersion and frequency selective fading. • Doppler spread leads to frequency dispersion and time selective fading. • These two propagation mechanisms are independent of one another

• If the mobile radio channel has a constant gain and linear phase response over a bandwidth which is greater than the bandwidth of signal, the received signal will undergo flat fading. • If the channel has a constant gain and linear phase response over a bandwidth that is smaller than the B/w of transmitted signal, then the channel creates frequency selective fading on a received signal.

• Coherence time is actually a statistical measure of the time duration over which the channel impulse response is essentially invariant, and qualifies the similarity of the channel response at the different times. • In other words, it is the time duration over which two received signal have a strong potential for amplitude correction.

• If the reciprocal bandwidth of the baseband signal is greater than the coherence time of the channel, then the channel will change during the transmission of the baseband signal/message, thus causing distortion at the receiver.

Raleigh Fading (NLOS) model It is used to describe statistical time varying nature of the received envelop of a flat fading signal.

σ

rms value of received voltage signal before envelop detection σ2 time average power of received signal before envelope detection

• Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system.

Okumura Model ⚫

It is one of the most widely used models for signal prediction in urban areas, and it is applicable for frequencies in the range 150 MHz to 1920 MHz

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Based totally on measurements (not analytical calculations)

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Applicable in the range: 150MHz to ~ 2000MHz, 1km to 100km T-R separation, Antenna heights of 30m to 100m

Okumura Model ⚫

The major disadvantage with the model is its low response to rapid changes in terrain, therefore the model is fairly good in urban areas, but not as good in rural areas.

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Common standard deviations between predicted and measured path loss values are around 10 to 14 dB.

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G(hre)：

 hte  G (hte ) = 20 log    200 

1000m  hte  30 m

 hre  G (hre ) = 10 log    3 

hre  3 m

 hre  G (hre ) = 20 log    3 

10m  hre  3 m

Okumura and Hata’s model ⚫

Example 4.10

Hata Model ⚫

Empirical formulation of the graphical data in the Okamura model. Valid 150MHz to 1500MHz, Used for cellular systems

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The following classification was used by Hata: ■Urban

area

■Suburban

area

■Open

LdB = A + B log d − E LdB = A + B log d − C LdB = A + B log d − D

area A = 69.55 + 26.16 log f − 13.82hb

B = 44.9 − 6.55 log hb C = 2(log( f / 28)) 2 + 5.4

D = 4.78 log( f / 28) 2 + 18.33 log f + 40.94

E = 3.2(log( 11.75hm )) 2 − 4.97

for large cities, f  300MHz

E = 8.29(log( 1.54hm )) 2 − 1.1

for large cities, f  300MHz

E = (1.11log f − 0.7)hm − (1.56 log f − 0.8) for medium to small cities

Indoor Propagation Models ⚫

The distances covered are much smaller

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The variability of the environment is much greater

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Key variables: layout of the building, construction materials, building type, where the antenna mounted, …etc.

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In general, indoor channels may be classified either as LOS or OBS with varying degree of clutter

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The losses between floors of a building are determined by the external dimensions and materials of the building, as well as the type of construction used to create the floors and the external surroundings.

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Floor attenuation factor (FAF)

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Log-distance Path Loss Model

• Tradeoff between simplicity and accuracy – Outdoor propagation models – Indoor propagation models