无线通信原理与应用第三章PPT课件

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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
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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
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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• 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 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.
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]
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.
• 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).
• 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
来自百度文库
(4)2d2
P L ( d B ) 3 2 .4 4 2 0 l g f 2 0 l g d(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.
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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
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)