无线通信原理与应用第三章

: 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.
• When antenna gains are excluded, the antennas are assumed to have
unity gain, and path loss is given by
PL(dB)10logP t 10log[ 2 ]
P r
(4)2d2
P L ( d B ) 3 2 .4 4 2 0 l g f 2 0 l g d(f:MHz,d:km)
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)d3.5 .If a signal of 1mW was received at d0=1m from the transm itter, 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.
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)
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 If n=4,log(d/d0)=75.5/40=1.8875,d=7718m
scale 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.
P r(d )P r(d 0)d d (0)2
2D2
df
df Danddf
• 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=0wk.baidu.com For this reason, large-
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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).
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.
PL(dB)PL(d0)10nlog(dd0)
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
P (d ) L d [] B P (d L ) B X P (d 0 L ) 1n l0 o d d 0 ) g X (
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Log-normal Shadowing
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Determination of Percentage of Coverage Area
无线通信原理与应用第三章ppt
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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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
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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.
• 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).
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3.2 Free Space Propagation Model
In free space, the received power is predicted by Firiis Equ.
PtGtGr2 Pr(d)
42d2L
Pr(d): Received power with a distance d between Tx and Rx Pt: Transmitted power Gt: Transmitting antenna gain Gr: Receive antenna gain
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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
EIRPPtGt
• 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.
• 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
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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.
dd 0df
• If Pr is in units of dBm or dBW, the received power is given by
P r(d ) 1 0 lo g (P r(d 0 )) 2 0 lo g (d d 0 ) d d 0 d f P L (d)10logP P r t P L (d0)20log(d d 0)
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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
P(d L)B 1l0oP P r tg1l0o(G 4 g tG )[r 2 d22]