无线射频技术基础_1

Slide Title, 2006 Cisco Systems, Inc. All rights reserved.,Cisco Confidential,Presentation_ID,28,Body Text,Second Level,Third Level,Fourth Level,Fifth Level,无线射频技术基础,Xiaogang Wu,xiawu,无线基本组件,馈线,发射器,天线,接收器,所有的无线设备,(,客户端和接入端,),分贝（,dB,）的定义,功率度量,Power,dBmdeciBels referenced to 1 mW,0 dBm = 1 mW,P,dBm,= 10 Log10 P,dBm,/1mW),dBWdeciBels referenced to 1 W,P,dBm,= 10 log10 (p,W,/1W),+ 30 dBm = 0 dBW,- 30 dBW = 0 dBm,Gain,dBideciBels referenced to gain of an isotropic antenna,dBddeciBels referenced to gain of a dipole antenna,dBdeciBel, Used to Express the Ratio of Two Values,Usually Gain or Loss as the Ratio Between Two Powers:,dB = 10 Log,10,P2/P1,分贝的值和对应的系数,Increase,Factor,Decrease,Factor,0 dB,1 x (same),0 dB,1 x (same),1 dB,1.25 x,-1 dB,0.8 x,3 dB,2 x,-3 dB,0.5 x,6 dB,4 x,-6 dB,0.25 x,10 dB,10 x,-10 dB,0.10 x,12 dB,16 x,-12 dB,0.06 x,20 dB,100 x,-20 dB,0.01 x,30 dB,1000 x,-30 dB,0.001 x,40 dB,10,000 x,-40 dB,0.0001 x,dBm,对应毫瓦,dBm,mW,dBm,mW,0,dBm,1,mW,0,dBm,1,mW,1,dBm,1.25,mW,-1,dBm,0.8,mW,3,dBm,2,mW,-3,dBm,0.5,mW,6,dBm,4,mW,-6,dBm,0.25,mW,7,dBm,5,mW,-7,dBm,0.20,mW,10,dBm,10,mW,-10,dBm,0.10,mW,12,dBm,16,mW,-12,dBm,0.06,mW,13,dBm,20,mW,-13,dBm,0.05,mW,15,dBm,32,mW,-15,dBm,0.03,mW,17,dBm,50,mW,-17,dBm,0.02 mw,20,dBm,100,mW,-20,dBm,0.01,mW,30,dBm,1000,mW,(1 W),-30,dBm,0.001,mW,40,dBm,10,000,mW,(10 W),-40,dBm,0.0001,mW,发射功率,Power that the transmitter applies to the transmission line,Also known as output power on some data sheets,Typically expressed in mW for 802.11 devices,Example: a wireless bridge has an output power of 30 mW; what power in dBm is applied to the transmission line?,Tx Pwr = 10 Log,10,(30) = 14.8 dBm,馈线 功率衰减,Directly proportional to the square root of the length of the cable,Inversely proportional to the diameter of the cable, assuming similar construction materials,Proportional to frequency: for a given length of cable, a higher frequency signal will always experience more loss than a lower frequency signal,Not depend upon which direction the signal travels through the cable (transmitted signals lose the same percentage of strength as received signals),Typically assume .25 dB of loss per connector,Cable Type,2.4 GHz,5.8 GHz,Loss (dB/100 ft.),Loss (dB/100 ft.),LMR400,6.7,10.8,LMR600,4.4,7.3,接收灵敏度,Example for 2.4GHz Direct Sequence,Indication of the ability of the receiver to decode the desired signal,The minimum receivedsignal level, in the absence of interference, at which the desired signal can be decoded with a particular Bit ErrorRate (BER),Typically expressed in dBm,The more negative the value,the better,Function of the data rate: the higher the data rate, the higher the receiver sensitivity required,Receiver Noise Floor(Will Vary for Each Environment),-92 dBm,Receiver Sensitivity 2 Mbps,Receiver Sensitivity 1 Mbps,-94 dBm,-98 dBm,-90 dBm,Receiver Sensitivity 5.5 Mbps,-87 dBm,Receiver Sensitivity 11 Mbps,接收灵敏度,Not all receivers are the designed the same,Noise can affect the quality of the signal,Signal/Noise ratio = Signal Noise Level,Example:,Current S/N = Current Signal Strength Current Noise Level,天线,Used to radiate transmitted signals and to capture received signals,Types:,Directional antennas,Radiate/capture RF energy predominantly in one direction,Yagi, Parabolic Dish, Sector, Patch,Omnidirectional (or “Omni”) antennas,Radiate/capture RF energy equally in all horizontal directions, 360 degrees,Characteristics:,Bandwidth,Beamwidth,Gain,Polarization,Cross polarization discrimination,天线的带宽,Antenna bandwidth,Band of frequencies over which the antenna is considered to perform acceptably,The wider the range of frequencies,the wider the bandwidth of the antenna,Antennas are provided pre-tuned bythe manufacturer for use in a specifiedband segment,Design trade-off: wider bandwidths generally result in a poorer performing antenna relative to a similar antenna optimized for narrower bandwidth,天线波束宽度,Antenna,Beamwidth,Total width in degrees of the main radiation lobe at the angle where the radiated power has fallen by 3 dB (half-power) below that on the center line of the lobe,In directional antennas the beamwidth is sometimes called half-power beamwidth,Azimuth and elevation,Half-Power (3 dB) Points,44 Degrees,天线增益,Antenna Gain,Antenna gain is a fundamental parameter in radio link engineering,Gain is an indication of the antennas concentration of radiated power in a given direction,Antenna gain is mostly expressed in dBi which is gain over an isotropic antenna,Isotropic antenna is an ideal antenna which radiates in all directions and has a gain of one (0 dB) i.e., zero gain/zero loss,线性天线极化,Orientation of the electric field of the electromagnetic wavethrough space,Horizontally polarized antenna,electric field in the horizontal plane,Vertically polarized antenna,electric field in the vertical plane,Both antennas in a link must have the same polarization to avoid additional unwanted signal loss,Horizontal Electric Field,Vertical Electric Field,Horizontal Polarization,Vertical Polarization,Tx,Tx,Cross Polarization Discrimination,Defines how effectively an antenna discriminates between a signal with the correct polarization and the opposite polarization,2025 dB isolation is typical,Cross polarization can be used to great advantage when the two antennas belong to different links (such as at a hub), and you want to minimize any potential interference that one link might cause to the other,有效射频功率,Effective Isotropic Radiated Power (EIRP),EIRPthe energy radiated out of the antenna,Sum of the components,Regulators have specifications for point-to-point and point-to-multipoint,beamwidths,EIRPdBm = P,t,+ Gant,L,L,P,t,: output power of the transmitter in dBm,G,ant,: antenna gain in dBi,L,L,: transmission line loss in dB,有效射频功率,Effective Isotropic Radiated Power (EIRP),A wireless bridge has an output power of 100mW; the cable connecting this transmitter to its antenna has a loss of 4.6 dB and the antenna has a 13.5 dBi gain; What is the EIRP in dBm?,EIRPdBm = +20 dBm + 13.5 dBi 4.6 dB = +28.9,dBm,Must be under regulatory maximum value to operate,Note: (100,mW,= +20,dBm,),示例,:,Bridge or,Access Point,E.I.R.P.,Cable Loss (dB),Transmitter,Power (dBm),Antenna,Gain (dBi),有效射频功率,Effective Isotropic Radiated Power (EIRP),EIRP Is What Regulators Look at for Compliance,信号传播,衍射波,折射波,反射波,多路径衰减,The signal attenuation due to diffraction,refraction, reflection and absorption is knownas multipath fading,Also known as Selective fading as the attenuation varies with frequency or is frequency selective,A mobile receiver traveling through an urban area can receive rapid signal fluctuations caused by additions and cancellations of the direct signals at half wavelength intervals,Fading due to,multipath,environment is often referred to as Rayleigh fading,多径衰减和失真,Time,Received Signals,Combined Results,Time,Obstruction,Ceiling,TX,RX,Floor,Antenna “hears” more than one source when in reality there is only one,Diversity antennas help relieve multipath,Time,Received Signals,Combined Results,Time,Obstruction,Ceiling,TX,RX,Floor,Antenna “hears” more than one source when in reality there is only one,Diversity antennas help relieve multipath,多径衰减和失真,In a multipath environment, signal null points are located throughout the area,Moving the antenna slightly will allow you to move out of a null point and receive the signal correctly,RX1,RX2,TX,Ceiling,Obstacle,Dual Diversity antennas typically mean if one antenna is in a null, the other one will not be, therefore providing better performance in multipath environments,多径衰减和失真的查障,Troubleshooting tips for,multipath,effect,Indoor Environment,Moving client around to verify the null spots,Double-checking diversity settings on AP and clients,Checking environmental changes since last site survey,Checking status and stats change on ACU (if Cisco clients),Outdoor Environment,Realigning antennas,Observing environmental changes (new buildings, trees, etc.),干扰,2.4 GHz and 5 GHz are unlicensed bands,Interference can come from same sources (AP, BR) or different sources (other RF devices),2.4 GHz,频段的其他信号,2.4 GHz cordless phones,Baby monitors,Microwave ovens,Other vendors equipment,Bluetooth devices,Military use,Naturally occurring,5 GHz,的其他信号,5 GHz cordless phones,Other vendors equipment,Naturally occurring,Note: Because 2.4GHz spectrum is much more crowded than a few years ago, more and more consumer products are extending their RF spectrum to 5G unlicensed band.,无线干扰的查障,Difficult to troubleshoot,Trying different channels (only for narrow-band interference),Using a known good lab to do comparison test,Utilizing a spectrum analyzer (not affordable for all enterprise customers),Checking with the neighbors,Potential interference sources (commercial airport, military base, etc.) and the timing,