无线通信基础(双语)_教学课件_9.ppt

《无线通信基础(双语)_教学课件_9.ppt》由会员分享，可在线阅读，更多相关《无线通信基础(双语)_教学课件_9.ppt（52页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、This chapter is concerned with method to mitigate the channel impairments identified and characterized in Chapter 2.The focus is:lDigital modulation schemelThe probability of transmission error in the presence of Rayleigh fading and AWGNlSpread Spectrum2 2、Signal Space and Decision RegionsSignal Spa

2、ce and Decision Regions1 1、IntroductionIntroduction3 3、QPSK and 2FSKQPSK and 2FSKl Elements of a digital communication systeml Bandpass Transmission A/DconverterSourceencoderChannelencoderModulatorPhysicalWaveformchannelDemodulatorSourcedecoderChanneldecoderD/AconverterBinarysequenceAnalogWaveformAn

3、alogWaveformBinarysequenceSignal design and receiver optimization together have the ability to combat fading and multipath effect:l Modulation-signal designl Demodulation optimized receiverl Efficient radiationl Facilitates channel assignment and reduces interference from other transmissionsmodulato

4、rModulating signalModulated signalCarriermodulatorModulating signalModulated signalCarrierThe modulation can be classified into two categories:l Linear modulationl Nonlinear modulationFor wireless communications,the criteria commonly used to evaluate the suitability of a modulation scheme include:l

5、Compact power spectral densityl Good transmission performance l Small(or no)envelope fluctuations after band pass filtering The amplitude of the carrier is constant,regardless of the variation in the message signal.It is the phase that changes.Benefits of Constant Envelope:l Power efficientl Low out

6、-of-band radiationl Simpler receiver design can be usedl High immunity against random AWGN and Rayleigh fadingl Vector-Space Representation of M-ary Signall Signal Detection and Optimal Receiver To understand digital modulation,it is important to know that a signal can be equivalently represented bo

7、th in the time domain and in the signal space domain.Each of the waveforms can represented as a point in an N-dimensional signal space over the interval 0 Ts.The digital source generates digital symbols for transmission at a rate of Rs symbols per second,the symbols are taken from an alphabet of siz

8、e M.Each symbol is represented by a unique baseband waveformOver each time interval of Ts=1/Rs,one of the M waveforms is selected for transmission Each of the waveforms can be represented as a point in an N-dimensional signal space defined by orthonormal basis function over the interval Thus,in the

9、signal space defined by the ,vm(t)ban be represented by the vector IQDefinition 3.1 The correlation coefficient between realvalued signals xm(t)and xk(t)isThe squared Euclidean distance between xk(t)and xm(t)is defined as:0The receiver can be divided into two components:l The demodulator l Decision

10、devicen(t)channelTransmitted signal x(t)r(t)DemodulatorDecisiondeviceReceiverThe signal space representation leads itself conveniently to decision making.The received signal is The received signal can be represented by a vector ,where Given the demodulator output ,the decision device is to perform a

11、 mapping from to an estimate of the transmitted symbol in a way that will minimize the probability of error in the decision making process.ML Decision Rule:Set if for all and In other word,the decision rule is to choose the message point closest to the received signal point,which is intuitively sati

12、sfyiing.The receiver consisting of the matched-filter demodulator and the decision device using the ML decision rule is commonly referred to as matched filter receiver.The receiver is optimum in the sense that it minimizes the probability of transmission error in an AWGN channel.Only the basis funct

13、ion,is needed to represent the BPSK signals.The most commonly used digital modulation techniques are M-ary phase shift keying(MPSK)such as differential quadrature phase shift keying(DQPSK),and minimum shift keying(MSK).In the second generation cellular system,North American IS-54 and Japanese PDC us

14、e/4 shifted DQPSK(/4-DQPSK),European GSM uses Gaussian filtered MSK(GMSK).In M-ary signaling,the mth waveform is represented aswhereorAs each waveform represents l binary digits,we have Es=lEb and Ts=lTb,where Eb and Tb are the bit energy and bit duration,respectivelyWith QPSK signaling,the symbols

15、can take on four possible values,each corresponding to two bits.For ,the modulated signal can be represented as:LettingThusIn the two-dimensional signal space defined by the orthogonal set ,the QPSK signal can be represented by a pointThe signal constellation of QPSK for is shownWhere 00011110iqThe

16、boundaries of the decision regions are the That is,the optimum decision regions for the equally likely symbols are just the four quadrants.The decision rule is In order to minimize transmission bit error rate,the binary digits are assigned to the symbols in such a way that the adjacent symbols diffe

17、r by only one binary digit.-Gray encoding00101101iqGray mapping for QPSK with7/4 5/43/4/40t +1 -1 -1 -1 +1 +1 +1 -1 -1 +1+1-1-1 +1QI+1-1+1+1-1-1-1+1-1+1iq-1 +1+1-1IQS/PS/P TransmitterNRZanQPSK SignalMFMF并/串anDDDDCombinedr(t)Carrier recoveryabReceiverIQS/PS/P TransmitterNRZanQPSK SignalswitchDataData

18、switchreversef1f22FSK SignalVCO FSK SignalansssBPF1BPF2Envelope DetectorEnvelope DetectorTiming RecoverJudgeData2FSKtttcdBPF1BPF2LPFLPFTimingJudgeData2FSKLocal Carrier1Local Carrier2tt+=ttf2f2-fsf1+fs Pc()Pc()Pc()f1+fsf2f2-fsf1+fsf2f2-fs2B2B f2 2、Signal Space and Decision RegionsSignal Space and Decision Regions1 1、IntroductionIntroduction3 3、QPSK and 2FSKQPSK and 2FSKl P3-1 and P3-2.