无线通信原理与应用第三章

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9dBi antenna & 3dBi antenna
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Path Loss
• The path loss, which represents signal attenuation as a positive difference (in dB) between the effective transmitted power and the received power. • The path loss for the free space model when antenna gains are included is given by quantity measured in dB, is defined as the
df
2D 2

d f D and d f
• To be in the far-field region, d must satisfy
d df
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The Reference Distance
• It is clear that equation does not hold for d=0. For this reason, largescale propagation models use a known received power reference point. The received power, Pr(d), at any distance d>d0, may be related to Pr at d0.
Chapter 3: Mobile Radio Propagation: Large-Scale Path Loss
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3.1 Introduction to Radio wave Propagation Small-scale and large-scale fading
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• If Pr is in units of dBm or dBW, the received power is given by
d0 2 Pr (d ) Pr (d 0 )( ) d
d d0 d f
d0 Pr (d ) 10 log( Pr ( d 0 )) 20 log( ) d d 0 d f d
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• reflection（反射）at large obstacles, • Scattering (散射)at small obstacles, • diffraction (衍射)at edges
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EIRP&ERP
2.15dB
EIRP Pt Gt
PL(dB) PL(d0 ) 10n log(
d ) d0
n is the path loss exponent which indicates the rate at which the path loss increases with distance d0 is the close-in reference distance which is determined
Pt 2 PL(dB) 10log 10log[ ] 2 2 Pr (4) d
PL(dB) 32.44 20lg f 20lg d
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(f:MHz,d:km)
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The far-field region of a transmitting antenna
• The Friis free space model is only a valid predictor for Pr for values of d, which are in the far-field of the transmitting antenna. • The far-field of a transmitting antenna is defined as the region beyond the far-field distance df , which is related to the largest linear dimension of the transmitter antenna aperture and the carrier wavelength. The farfield distance is given by
Ri
R0
dA
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U(r) as a function of probability of signal above threshold on the cell boundary.
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Example 2
• A local average signal strength field measurements , the measured data fit a distant-dependent mean power law model having a log-normal distribution about the mean. Assume the mean power law was found to be Pr (d ) d 3.5 .If a signal of 1mW was received at d0=1m from the transmitter, and at a distance of 10m, 10% of the measurements were stronger than -25dBm, define the standard deviation, ,for the path loss model at d=10m.
d is the T-R separation distance
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Path-loss exponents
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Example 1
If a transmitter produces power:Pt=50w, receive sensitivity (minimum usable signal level)is 100dbm.Assume d0=100m, with a 900MHz carrier frequency, n=4,Gt=Gr=1; find the coverage distance d. Transmit Power: Pt=50W=47dBm Pr(d0)=-24.5dBm PL(dB)=40log(d/d0)=-24.5-(-100)=75.5db
• EIRP: Effective Isotropic Radiated Power Represents the maximum radiated power available from a transmitter in the direction of maximum antenna gain, as compared to an isotropic radiator. • ERP: Effective Radiated Power ERP is used instead of EIRP to denote the maximum radiated power as compared to a half-wave dipole antenna (instead of an isotropic antenna). In practice, antenna gains are given in units of dBi (dB gain with respect to an isotropic sourse) or dBd (dB gain with respect to a half-wave dipole)
Pt Gt Gr 2 PL(dB) 10log 10log[ ] 2 2 Pr (4 ) d
• When antenna gains are excluded, the antennas are assumed to have unity gain, and path loss is given by
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3.3 The three Basic Propagation Mechanisms
Reflection: occur from the surface of the earth and from
buildings and walls.
Diffraction:occurs when the radio path between the transmitter
and receiver is obstructed by a surface that has sharp irregularities (edges).
Scattering:occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large.
d PL(d )[dB] PL(dB) X PL(d 0 ) 10n log( ) X d0
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Log-normal Shadowing
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Determination of Percentage of Coverage Area
If n=4,log(d/d0)=75.5/40=1.8875,d=7718m
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Log-normal Shadowing
The model in Equation (3.11) does not consider the fact that the surrounding environmental clutter may be vastly different at two different locations having the same T-R separation. This leads to measured signals which are vastly different than the average value predicted by Equation (3.11).
PL(d ) 10log
Pt d PL(d0 ) 20log( ) Pr d0
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3.4 Link budge design using path loss model
Log-distance path loss model
Both theoretical and measurement-based propagation models indicate that average received signal power decreases logarithmically with distance, whether in outdoor or indoor channels. The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by using path loss exponent n.
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3.2 Free Space Propagation Model
In free space, the received power is predicted by Firiis Equ. 2
Pr ( d) P t Gt Gr
4 2 d2L
PБайду номын сангаас(d): Received power with a distance d between Tx and Rx Pt: Transmitted power Gt: Transmitting antenna gain Gr: Receive antenna gain : The wavelength in meters. d: distance in meters L: The miscellaneous losses L (L>=1) are usually due to transmission line attenuation, filter losses, and antenna losses in the communication system. L=1 indicates no loss in the system hardware.