【正文】 Tnermal noise consumer electronic applicationsVisible light400THz to900THz770nm to330nmLOSOptical municationGround wave propagationSky wave propagation (2 to 30 MHz)Lineofsight (LOS) propagation(above 30 MHz) Wireless Propagation ModesFigure Refraction of an Electromagnetic Wave [POOL98]Optical and Radio Line of Sight With no intervening obstacles, the optical line of sight can be expressed asWhere d is the distance between an antenna and the horizon in kilometers and h is the antenna height in meters. The effective, or radio, line of sight to the horizon is expressed as(Figure )：Figure Optical and Radio HorizonsWhere K is an adjustment factor to account for the refraction. A good rule of thumb is K=4/,the maximum distance between two antennas for LOS propagation is ,where and are the heights of the two antennas. LINEOFSIGHT TRANSMISSIONWith any munications system, the signal that is received will differ from the signal that is transmitted, due. o various transmission impairments. For analog signals, these impairments introduce various random modifications that degrade the signal quality. For digital errors are introduced：A binary I is transformed into a binary 0,and vice versa. In this section we examine the various impairments and ment on their effect on the informationcarrying capacity of a munications link. Our concern in this book is with LOS wireless transmission, and in this Context, the most Significant impairments are personal munications systemSHF(super high frequency)3GHz to30GHz10cm to1cmLOS。 Longdistance aircraft and ship municationVHF(very high frequency)30MHz to300MHz10m to1mLOS。 marine munication radio beaconsMF(medium frequency)300kHz to3000kHz1000m to100mGW。 earth’s surface (Figure ).with this propagation mode, a signal can be picked up thousands of kilometers from the transmitter.LineofSight PropagationAbove 30 MHz, neither ground wave nor sky wave propagation modes operate, and munication must be by line of sight (Figure ).For satellite munication, a signal above 30 MHz is not reflected by the ionosphere and therefore can be transmitted between an earth station and a satellite overhead that is not beyond the horizon. For ground—based munication, the transmitting and receiving antennas must be within an effective line of sight of each other. The term effective is used because microwaves are bent or refracted by the atmosphere. The amount and even the direction of the bend depends on conditions, but generally microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line of sight.Refraction Before proceeding, a brief discussion of refraction is warranted. Refraction occurs because the velocity of an electromagnetic wave is a function of the density of the medium through which it travels In a vacuum, an electromagnetic wave (such as light or a radio wave) travels at approximately 3m/s. This is the constant, c, monly referred to as the speed of light, but actually referring to the speed of light in a vacuum. In air, water, glass, and other transparent or partially transparent media, electromagnetic waves travel at speeds less than c.When an electromagnetic wave moves from a medium of one density to a medium of another density, its speed changes. The effect is to cause a onetime bending of the direction of the wave at the boundary between the two media. This is illustrated in moving from a less dense to a more dense medium, he wave will bend toward the more dense medium. This phenomenon IS easily observed by partially immersing a stick in water. The result will look much like Figure with the stick appearing shorter and bent.The index of refraction of one medium relative to another is the sine of the angle of incidence divided by the sine of the angle of refraction. The index of refraction is also equal to the ratio of the respective velocities in the two media. The absolute index of refraction of a medium is calculated in parison with that of a vacuum. Refractive index varies with wavelength, o that refractive effects differ for signals with different wavelengths.Although Figure shows an abrupt, onetime change in direction as a signal moves from one medium to another, a continuous, gradual bending of a signal will occur if it is moving through a medium in which the index of refraction gradually changes. Under normal propagation conditions, the refractive index of the atmosphere decreases with height so that radio waves travel more slowly near the ground than at higher altitudes. The result is a slight bending of the radio waves toward the earth.Table Frequency BandsBandFrequencyFreeSpace WavelengthRangePropagation CharacteristicsTypical UseELF(extremely low frequency)30Hz to300Hz10000km to1000kmGW。 submarine municationLF(low frequency)30kHz to300kHz10km to1kmGW。 AM broadcastingHF(high frequency)3MHz to30MHz100m to10mSW。 cellular telephone。 wireless local loopEHF(extremely high frequency)30GHz to300GHz10mm to1mmLOS。Multipath.Impulse noiseThermal noise is due to thermal agitation of electrons. It is present in all electronic devices and transmission media and is a function of temperature. Thermal noise is uniformly distributed across the frequency spectrum and hence is often referred to as white noise. Thermal noise cannot be eliminated and therefore places an upper bound on munications system performance. Because of the weakness of the signal received by satellite earth stations, thermal noise is particularly significant for satellite munication.The amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor isWhere=noise power density in watts per 1 Hz of bandwidthk=Boltzmann’s constant=J/KT=temperature, in kelvins (absolute temperature)The noise is assumed to be