PD-无线射频技术基础

1、无线射频技术基础Xiaogang Wuxiawu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PowerdBmdeciBels referenced to 1 mW0 dBm = 1 mWPdBm = 10 Log10 PdBm/1mW)dBWdeciBels referenced to 1 WPdBm = 10 log10 (pW/1W) + 30 dBm = 0 dBW- 30 dBW = 0 dBmGaindBideciBels referenced to gain of an isotropic antennadBddeciBels ref

2、erenced to gain of a dipole antennadBdeciBel, Used to Express the Ratio of Two Values,Usually Gain or Loss as the Ratio Between Two Powers:dB = 10 Log10 P2/P1分贝的值和对应的系数IncreaseFactorDecreaseFactor0 dB1 x (same)0 dB1 x (same)1 dB1.25 x-1 dB0.8 x3 dB2 x-3 dB0.5 x6 dB4 x-6 dB0.25 x10 dB10 x-10 dB0.10 x

3、12 dB16 x-12 dB0.06 x20 dB100 x-20 dB0.01 x30 dB1000 x-30 dB0.001 x40 dB10,000 x-40 dB0.0001 xdBm对应毫瓦dBmmWdBmmW0 dBm1 mW0 dBm1 mW1 dBm1.25 mW-1 dBm0.8 mW3 dBm2 mW-3 dBm0.5 mW6 dBm4 mW-6 dBm0.25 mW7 dBm5 mW-7 dBm0.20 mW10 dBm10 mW-10 dBm0.10 mW12 dBm16 mW-12 dBm0.06 mW13 dBm20 mW-13 dBm0.05 mW15 dBm3

4、2 mW-15 dBm0.03 mW17 dBm50 mW-17 dBm0.02 mw20 dBm100 mW-20 dBm0.01 mW30 dBm1000 mW (1 W)-30 dBm0.001 mW40 dBm10,000 mW (10 W)-40 dBm0.0001 mW发射功率Power that the transmitter applies to the transmission lineAlso known as output power on some data sheetsTypically expressed in mW for 802.11 devicesExampl

5、e: a wireless bridge has an output power of 30 mW; what power in dBm is applied to the transmission line?Tx Pwr = 10 Log10 (30) = 14.8 dBm馈线 功率衰减Directly proportional to the square root of the length of the cableInversely proportional to the diameter of the cable, assuming similar construction mater

6、ialsProportional to frequency: for a given length of cable, a higher frequency signal will always experience more loss than a lower frequency signalNot depend upon which direction the signal travels through the cable (transmitted signals lose the same percentage of strength as received signals)Typic

7、ally assume .25 dB of loss per connectorCable Type2.4 GHz5.8 GHzLoss (dB/100 ft.)Loss (dB/100 ft.)LMR4006.710.8LMR6004.47.3接收灵敏度Example for 2.4GHz Direct SequenceIndication of the ability of the receiver to decode the desired signalThe minimum receivedsignal level, in the absence of interference, at

8、 which the desired signal can be decoded with a particular Bit ErrorRate (BER)Typically expressed in dBmThe more negative the value,the betterFunction of the data rate: the higher the data rate, the higher the receiver sensitivity requiredReceiver Noise Floor(Will Vary for Each Environment)-92 dBmRe

9、ceiver Sensitivity 2 MbpsReceiver Sensitivity 1 Mbps-94 dBm-98 dBm-90 dBmReceiver Sensitivity 5.5 Mbps-87 dBmReceiver Sensitivity 11 Mbps接收灵敏度Not all receivers are the designed the sameNoise can affect the quality of the signalSignal/Noise ratio = Signal Noise LevelExample:Current S/N = Current Sign

10、al Strength Current Noise Level天线Used to radiate transmitted signals and to capture received signalsTypes:Directional antennas Radiate/capture RF energy predominantly in one directionYagi, Parabolic Dish, Sector, PatchOmnidirectional (or “Omni”) antennasRadiate/capture RF energy equally in all horiz

11、ontal directions, 360 degreesCharacteristics:BandwidthBeamwidthGainPolarizationCross polarization discrimination天线的带宽 Antenna bandwidthBand of frequencies over which the antenna is considered to perform acceptablyThe wider the range of frequencies the wider the bandwidth of the antennaAntennas are p

12、rovided 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 BeamwidthTotal width in degrees of the main radiation lobe at the an

13、gle where the radiated power has fallen by 3 dB (half-power) below that on the center line of the lobeIn directional antennas the beamwidth is sometimes called half-power beamwidthAzimuth and elevationHalf-Power (3 dB) Points44 Degrees天线增益 Antenna GainAntenna gain is a fundamental parameter in radio

14、 link engineeringGain is an indication of the antennas concentration of radiated power in a given directionAntenna 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 ga

15、in/zero loss线性天线极化Orientation of the electric field of the electromagnetic wavethrough spaceHorizontally polarized antenna electric field in the horizontal planeVertically polarized antenna electric field in the vertical planeBoth antennas in a link must have the same polarization to avoid additiona

16、l unwanted signal lossHorizontal Electric FieldVertical Electric FieldHorizontal PolarizationVertical PolarizationTxTxCross Polarization DiscriminationDefines how effectively an antenna discriminates between a signal with the correct polarization and the opposite polarization2025 dB isolation is typ

17、icalCross 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 antenn

18、aSum of the componentsRegulators have specifications for point-to-point and point-to-multipoint beamwidthsEIRPdBm = Pt + Gant LLPt: output power of the transmitter in dBmGant: antenna gain in dBiLL: transmission line loss in dB有效射频功率Effective Isotropic Radiated Power (EIRP)A wireless bridge has an o

19、utput 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 dBmMust be under regulatory maximum value to operateNote: (100 mW = +20 dBm)示例:Bridge orAccess Poi

20、ntE.I.R.P.Cable Loss (dB)TransmitterPower (dBm)AntennaGain (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 fadingAlso known as Sele

21、ctive fading as the attenuation varies with frequency or is frequency selectiveA mobile receiver traveling through an urban area can receive rapid signal fluctuations caused by additions and cancellations of the direct signals at half wavelength intervalsFading due to multipath environment is often

22、referred to as Rayleigh fading 多径衰减和失真TimeReceived SignalsCombined ResultsTimeObstructionCeilingTXRXFloorAntenna “hears” more than one source when in reality there is only oneDiversity antennas help relieve multipathTimeReceived SignalsCombined ResultsTimeObstructionCeilingTXRXFloorAntenna “hears” m

23、ore than one source when in reality there is only oneDiversity antennas help relieve multipath多径衰减和失真In a multipath environment, signal null points are located throughout the areaMoving the antenna slightly will allow you to move out of a null point and receive the signal correctlyRX1RX2TXCeilingObs

24、tacleDual 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 EnvironmentMoving client around to verify the null spotsDouble-checking diversity settings on AP and clientsChecking environmental changes since last site surveyChecking status and stats change on ACU (if Cisco clients)Outdoor EnvironmentRealigning antennasObserving environmental changes (new buildings, trees, etc.)干扰