Propagation Characteristic of Mobile Radio Channel Ideal radio
Propagation Characteristic of Mobile Radio Channel Ideal radio channel : Original Signal->Transmitter -> Channel -> Receiver -> Original Signal. In ideal radio channel we receive what is transmitted
Propagation Characteristic of Mobile Radio Channel Propagation means - Behavior of radio waves when they are propagate from Transmitter to Receiver.
The propagation of a radio wave is complicated and less predictable process that is governed by reflection, diffraction , and scattering whose intensity varies with different environments at different instances.
• Reflection : - is the physical phenomenon that occurs when a propagating EM wave impinges upon an object with very large dimensions compared to the wavelength. • Example – Surface of earth, Building. • Diffraction : - It is when the radio path between the transmitter and receiver is obstructed by surface with sharp irregularities or small openings. • Scattering : - Is the physical phenomenon that forces the radiation of an EM wave to deviate from a straight path by one or more local obstacles with small dimensions compared to the wavelength.
A unique characteristic in a wireless channel is a phenomenon called Fading is variation of the signal amplitude over time and frequency. In addition to additive noise fading is most common source of signal degradation that is characterized as a non–additive signal disturbance in wireless channel.
• Fading may be due to – multipath propagation, referred to as multi-path fading – Shadowing from obstacles that affect the propagation of a radio wave, referred to as shadow fading.
Coherence bandwidth is a measurement of the range of frequencies over which the channel can be considered "flat", In other words the approximate maximum bandwidth or frequency over which two frequencies of a signal are likely to experience comparable or correlated amplitude fading. • If the multipath time delay spread equals D seconds, then the coherence bandwidth in rad/s is given approximately by the equation: • Wc = 2 /D rad/sec. OR Bc = 1/D Hz If the coherence bandwidth is defined as the bandwidthover which the frequency correlation function is above 0. 9, then the coherence bandwidth is approximately
Delay spread It can be interpreted as the difference between the time of arrival of the earliest significant multipath component (typically the line-of-sight component) and the time of arrival of the latest multipath components Doppler Shift The Doppler effect (or Doppler shift) is the change in frequency of a wave for an observer moving relative to the source of the wave.
• Fading phenomenon can be broadly classified into two different types – Large scale fading – Small scale fading • Large scale fading : - Occurs as the mobile moves through a large distance, (cell size). – It is caused by path loss of signal as a function of distance and shadowing by large objects. (Buildings) – Shadowing is a slow fading process characterized by variation of median path loss between transmitter and receiver in fixed locations. (Large scale fading is is average path loss and shadowing) • Small scale fading : - is rapid variation of signal levels due to the constructive and destructive interference of multiple signal paths ( Multi path) when mobile units moves small distance. – Depending on the time variation in a channel due to mobile speed (Doppler spread) short term fading is classified as either fast fading or slow fading
• Physical factors influence small-scale fading in the radio propagation channel: • (1) Multipath propagation – Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. The effects of multipath include constructive and destructive interference, and phase shifting of the signal. • (2) Speed of the mobile – The relative motion between the base station and the mobile results in random frequency modulation due to different doppler shifts on each of the multipath components. • (3) Speed of surrounding objects – If objects in the radio channel are in motion, they induce a time varying Doppler shift on multipath components. If the surrounding objects move at a greater rate than the mobile, then this effect dominates fading. • (4) Transmission Bandwidth of the signal – If the transmitted radio signal bandwidth is greater than the “bandwidth” of the multipath channel (quanti- fied by coherence bandwidth), the received signal will be distorted
• Flat Fading Such types of fading occurs when the bandwidth of the transmitted signal is less than the coherence bandwidth of the channel. So we can say that flat fading occurs when BS << BC where BS is the signal bandwidth and BC is the coherence bandwidth. Equivalently if the symbol period of the signal is more than the rms delay spread of the channel, then the fading is flat fading. TS >> στ where TS is the symbol period and στ is the rms delay spread. And in such a case, mobile channel has a constant gain and linear phase response over its bandwidth.
• In flat fading, the multipath structure of the channel is such that the spectral characteristics of the transmitted signal are preserved at the receiver. • However the strength of the received signal changes with time. • Due to fluctuations in the gain of the channel caused by multipath • Channel gain changes over time • A change of amplitude occurs in the received signal • Spectrum of the transmission is preserved
• Frequency Selective Fading Frequency selective fading occurs when the signal bandwidth is more than the coherence bandwidth of the mobile radio channel equivalently the symbols duration of the signal is less than the rms delay spread. BS >> BC and TS << στ
For frequency selective fading, The spectrum S(f) of the transmitted signal has a bandwidth which is greater than the coherence bandwidth Bc of the channel. Viewed in the frequency domain, the channel becomes frequency selective, where the gain is different for different frequency components. Frequency selective fading is caused by multipath delays which approach or exceed the symbol period of the transmitted symbol. Frequency selective fading channels are also known as wideband channels
Example 4. 1 Consider a transmitter which radiates a sinusoidal carrier frequency of 1850 MHz. For a vehicle moving 60 mph, compute the received carrier frequency if the mobile is moving (a) directly towards the transmitter, (b) directly away from the transmitter, (c) in a direction which is perpendicular to the direction of arrival of the transmitted signal.
Fading Effects due to Doppler Spread Fast Fading In a fast fading channel, the channel impulse response changes rapidly within the symbol duration of the signal. Due to Doppler spreading, signal undergoes frequency dispersion leading to distortion. Therefore a signal undergoes fast fading – if TS >> TC • where TC is the coherence time and • BS >> BD where BD is the Doppler spread. Transmission involving very low data rates suffer from fast fading. Slow Fading In such a channel, the rate of the change of the channel impulse response is much less than the transmitted signal. We can consider a slow faded channel a channel in which channel is almost constant over atleast one symbol duration. Hence TS TC (5. 8) and BS BD (5. 9) We observe that the velocity of the user plays an important role in deciding whether the signal experiences fast or slow fading. 7
The level crossing rate (LCR) is defined as the expected rate at which the Rayleigh fading envelope, normalized to the local rms signal level, crosses a specified level in a positive-going direction. The number of level crossings per second is given by where f is the time derivative of r (t ) (i. e. , the slope), p(R, r) is the joint density function of r and r at r = R , fm is the maximum Doppler frequency and p = R/Rrms is the value of the specified level R, normalized to the local rms amplitude of the fading envelope
The average fade duration is defined as the average period of time for which the received signal is below a specified level R. For a Rayleigh fading signal, this is given by where Pr [r <R] is the probability that the received signal r is less than R and is given by where i is the duration of the fade and T is the observation interval of the fading signal.
• Attenuated : Drop of the signal power when it is transmitted from one point to other. This is due to any object in between Xt. And Xr. • When there is obstruction In between Xt and Xr like Hills or buildings then it is Called as shadowing • Multipath Effect : - This gives multiple transmission paths up to the receiver. The relative phases of the multiple reflected signal causes destructive or constructive interference at the receiver. This is normally experienced for very short distances (typically at half of the wavelength distances), thus is given the term - fast fading. • The Rayleigh distribution which is commonly used to describe the statistical time varying nature of the received signal power describes the probability of the signal level being received • due to the fading. – Delay Spread – Frequency Selective fading – Doppler Effect
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