第二章 无线通信基础

©2009 Guangming Song-Southeast UniversityOutline无线通信的基本概念模拟信号与数字信号带宽与通道容量信号编码与调制电磁信号电磁信号是一个时间函数电磁信号也可以表达为一个频率函数 信号由不同的频率分量组成时域模拟信号－信号强度随时间平滑变化 信号中无暂停与中断数字信号－信号强度在某一个时间段内维持一个常量，在下一个时间段又跳转到另一个常量时域时域周期信号－信号波形随时间周期重复变化 模拟或数字信号中最简单的一种s (t+T) = s (t) -∞< t< +∞T为信号周期非周期信号－信号波形不随时间周期重复变化时域时域Peak amplitude (A)-maximum value or strength of the signal over time; typically measured in voltsFrequency (f)-Rate, in cycles per second, or Hertz (Hz) at which the signal repeatsPeriod (T) -amount of time it takes for one repetition of the signalT= 1/fPhase (φ)-measure of the relative position in time within a single period of a signalWavelength (λ) -distance occupied by a single cycle of the signalOr, the distance between two points of corresponding phase of two consecutive cycles时域General sine waves(t) = A sin(2πft+ φ)Figure 2.3 shows the effect of varying each of the three parameters(a) A= 1, f= 1 Hz, φ= 0; thus T= 1s(b) Reduced peak amplitude; A=0.5(c) Increased frequency; f= 2, thus T= ½S(d) Phase shift; φ= π/4 radians (45 degrees)note: 2πradians = 360°= 1 period时域Fundamental frequency -when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequencySpectrum -range of frequencies that a signal containsAbsolute bandwidth -width of the spectrum ofa signalEffective bandwidth (or just bandwidth) -narrow band of frequencies that most of the signal’s energy is contained inAny electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phasesThe period of the total signal is equal to the period of the fundamental frequencyThe greater the bandwidth, the higher the information-carrying capacityConclusionsAny digital waveform will have infinite bandwidthBUT the transmission system will limit the bandwidth that can be transmittedAND, for any given medium, the greater the bandwidth transmitted, the greater the costHOWEVER, limiting the bandwidth creates distortionsData Communication TermsAnalog -continuousDigital -discreteData -entities that convey meaning, or informationSignals -electric or electromagnetic representations of dataTransmission -communication of data by the propagation and processing of signalsAnalogVideoAudio DigitalTextIntegersA continuously varying electromagnetic wavethat may be propagated over a variety of media, depending on frequencyExamples of media:Copper wire media (twisted pair and coaxial cable)Fiber optic cableAtmosphere or space propagationAnalog signals can propagate analog and digital dataA sequence of voltage pulses that may betransmitted over a copper wire mediumGenerally cheaper than analog signalingLess susceptible to noise interferenceSuffer more from attenuationDigital signals can propagate analog and digital dataDigital SignalsAnalog DataHow to Choose Data and Signal Combinations Digital data, digital signalEquipment for encoding is less complex and less expensive than digital-to-analog equipmentAnalog data, digital signalConversion permits use of modern digital transmission and switching equipmentDigital data, analog signalSome transmission media will only propagate analog signalsExamples include optical fiber and satelliteAnalog data, analog signalAnalog data easily converted to analog signalTransmit analog signals without regard to contentAttenuation limits length of transmission linkCascaded amplifiers boost signal’s energy for longer distances but cause distortionAnalog data can tolerate distortionIntroduces errors in digital dataConcerned with the content of the signalAttenuation endangers integrity of dataDigital SignalRepeaters achieve greater distanceRepeaters recover the signal and retransmitAnalog signal carrying digital dataRetransmission device recovers the digital data from analog signalGenerates new, clean analog signalChannel CapacityImpairments, such as noise, limit data rate that can be achievedFor digital data, to what extent do impairments limit data rate?Channel Capacity–the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditionsConcepts Related to Channel CapacityData rate -rate at which data can be communicated (bps)Bandwidth -the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz)Noise -average level of noise over the communications pathError rate -rate at which errors occurError = transmit 1 and receive 0; transmit 0 and receive 1Nyquist BandwidthFor binary signals (two voltage levels)C = 2BWith multilevel signalingC= 2B log2MM= number of discrete signal or voltage levelsSignal-to-Noise RatioShannon Capacity FormulaEquation:Represents theoretical maximum that can be achievedIn practice, only much lower rates achieved Formula assumes white noise (thermal noise)Impulse noise is not accounted forAttenuation distortion or delay distortion not accounted for()SNR 1log 2+=B CSpectrum of a channel between 3 MHz and 4 MHz ; SNR dB = 24 dBUsing Shannon’s formula()251SNR SNR log 10dB 24SNR MHz1MHz 3MHz 410dB ====−=B ()Mbps 88102511log 10626=×≈+×=CHow many signaling levels are required?()16log 4log 102108log 222662==××=×=M MMMB CWhat limits bandwidth?Sounds like higher bandwidth signals are a good idea.So ... Why not transmit at the highest bandwidth possible?Answer: The FCC!Bandwidth AllocationIn the U.S., the FCC is responsible for allocating radio frequencies.Why allocate the radio spectrum?Prevent interference between different devicesIt would be unfortunate if the local TV station interfered with police radio...Generally, any transmitter is limited to a certain bandwidthe.g., a single 802.11 channel is 30 MHz “wide”FCC also regulates the power and placement of transmittersConsumer devices generally limited to transmitting < 1 W of powerCan't have two TV stations on channel 5 next to each otherIn China, 信息产业部无线电管理局Classifications of Transmission MediaTransmission Medium-Physical path between transmitter and receiverGuided MediaWaves are guided along a solid mediumE.g., copper twisted pair, copper coaxial cable, opticalfiberUnguided MediaProvides a means of transmission but does not guide electromagnetic signalsUsually referred to as wireless transmissionE.g., atmosphere, outer spaceUnguided MediaTransmission and reception are achieved by means of an antennaConfigurations for wireless transmission DirectionalOmnidirectionalRadio frequency range30 MHz to 1 GHzSuitable for omnidirectional applicationsMicrowave frequency range1 GHz to 40 GHzDirectional beams possibleSuitable for point-to-point transmissionUsed for satellite communicationsInfrared frequency rangeRoughly, 3x1011to 2x1014HzUseful in local point-to-point multipoint applications within confined areasTerrestrial MicrowaveDescription of common microwave antenna Parabolic "dish", typical size: 3 m in diameterFixed rigidly and focuses a narrow beamAchieves line-of-sight transmission to receiving antennaLocated at substantial heights above ground levelApplicationsLong haul telecommunications serviceShort point-to-point links between buildingsSatellite MicrowaveDescription of communication satelliteMicrowave relay stationUsed to link two or more ground-based microwave transmitter/receiversReceives transmissions on one frequency band (uplink), amplifies or repeats the signal, andtransmits it on another frequency (downlink)ApplicationsTelevision distributionLong-distance telephone transmissionPrivate business networksBroadcast RadioDescription of broadcast radio antennas OmnidirectionalAntennas not required to be dish-shapedAntennas need not be rigidly mounted to a precise alignmentApplicationsBroadcast radioVHF and part of the UHF band; 30 MHZ to 1GHzCovers FM radio and UHF and VHF televisionCarrier WaveHow do we send information in a radio signal?Carrier waveAn RF signal –usually a sinusoid –that carries informationCarrier is usually a much higher frequency than the information itself!Example: 2.4 GHz 802.11b networks carry a lot less than2.4 GBit/sec of data....Rather, carry up to 11 MBit/sec of informationWhy use a carrier?Easier to generate a sinusoid signal, and it will travel further. Carrier wave frequencyThe frequency of a radio transmission is the center frequency of the carrierActual frequency of the carrier changes over time, e.g., with FM transmissionHow do we encode information in a carrier wave?An information signal must be modulated onto the carrier waveThat is, we must modify the carrier wave in some way...Receiver must demodulate the carrier to get back the original signalAmplitude Modulation (AM)Modify the amplitude of the carrier with respect to the amplitude of the signalFrequency Modulation (FM)Modify the frequency of the carrier with respect to the amplitude of the signalInformation SignalAmplitude Modulation(AM)Information SignalFrequency Modulation(FM)How do we modulate digital signals?Amplitude shift keying (ASK)“0”bit is the absence of the carrier (flat signal)“1”bit is the presence of the carrier with some fixed amplitude Frequency shift keying (FSK)“0”bit is carrier at frequency f0“1”bit is carrier at frequency f1Other modulation techniquesLots of other modulation schemes are used in practiceEach has different properties in terms of resiliency to noise, interference, multipath effects, etc.Gaussian Frequency Shift Keying (GFSK)Binary 1 is a positive frequency shift from base frequencyBinary 0 is negative frequency shift from base frequencyUsed in BluetoothPhase shift keying (PSK)The phase of the carrier is used to represent data“Differential quadrature phase shift keying”(DQPSK) used by 802.11b networksFour phase levels representing 00, 01, 10, and 11 bit sequencesQuadrature Amplitude Modulation (QAM)Combination of AM + PSKUse two amplitudes and 4 phase levels to represent each sequence of 3 bitsDecibels and Signal StrengthThere will be a loss, or attenuation in signal transmissionAmplifiers will impart a gain to compensateRatio between two signal power levels is often measured in decibels (dB):gain(dB) = 10log10(P out/ P in)loss(dB) = –10log10(P out/ P in) = 10 log10(P in/P out)Where P out is the output power level, and P in is the input power levelExampleSignal with power level 10mW transmitted over a wireless channel.Receiver gets a signal of 2mW.Loss = 10 lg(10/2) = 10(0.699) = 6.99 dB。