1、Analog Pulse Amplitude Modulation,12 - 2,Communication Systems,Information sources Message signal m(t) is the information source to be sent Possible information sources include voice, music, images, video, and data, which are baseband signals Baseband signals have power concentrated near DC Basic st
2、ructure of an analog communication system is shown below,Review,12 - 3,Transmitter,Signal processing Conditions the message signal Lowpass filtering to make sure that the message signal occupies a specific bandwidth, e.g. in AM and FM radio, each station is assigned a slot in the frequency domain. I
3、n a digital communications system, we might add redundancy to the message bit stream mn to assist in error detection (and possibly correction) in the receiver,Review,12 - 4,Transmitter,Carrier circuits Convert baseband signal into a frequency band appropriate for the channel Uses analog and/or digit
4、al modulation,Review,12 - 5,Channel,Transmission medium Wireline (twisted pair, coaxial, fiber optics) Wireless (indoor/air, outdoor/air, underwater, space) Propagating signals experience a gradual degradation over distance Boosting improves signal and reduces noise, e.g. repeaters,Review,12 - 6,Wir
5、eline Channel Impairments,Linear time-invariant effects Attenuation: dependent on channel frequency response Spreading: finite extent of each transmitted pulse increases, i.e. pulse widens (see next slide) Linear time-varying effects Phase jitter: sinusoid at same fixed frequency experiences differe
6、nt phase shifts when passing through channel (i.e. time-varying phase response) Nonlinear effects Additive noise: arises from many sources in transmitter, channel, and receiver (e.g. thermal noise) Harmonics: due to squaring & other nonlinear transformations (e.g. in power amplifiers),12 - 7,Wirelin
7、e Channel Impairments,Analog transmission over communication channels Spreading in time domain due to convolution of transmitted waveform & channel impulse response,Th,t,1,Tb,t,A,Tb,-A,Model channel as LTI system with impulse response h(t),t,-A Th,t,Th+Tb,Th,Assume that Th Tb,t,Th+Tb,Th,A Th,12 - 8,
8、Wireless Channel Impairments,Same as wireline channel impairments plus others Fading: multiplicative noise Example: talking on a cellular phone while driving a car when the reception fades in and out,12 - 9,Receiver and Information Sinks,Receiver Carrier circuits undo effects of carrier circuits in
9、transmitter, e.g. demodulate from a bandpass signal to a baseband signal Signal processing subsystem extracts and enhances the baseband signal Information sinks Output devices, e.g. computer screens, speakers, TV screens,Review,12 - 10,Hybrid Communication Systems,Mixed analog and digital signal pro
10、cessing in the transmitter and receiver Example: message signal is digital but broadcast over an analog channel (compressed speech in digital cell phones) Signal processing in the transmitterSignal processing in the receiver,m(t),A/D Converter,Error Correcting Codes,Digital Signaling,Decoder,Wavefor
11、m Generator,Equalizer,Detection,digital sequence,digital sequence,code,baseband signal,D/A Converter,A/D,D/A,12 - 11,Single-Carrier Modulation Methods,Analog communication Transmit and receive analog waveforms Amplitude Modulation (AM) Phase Modulation (PM) Freq. Modulation (FM) Quadrature Amplitude
12、 Modulation (QAM) Pulse Amplitude Modulation (PAM),Digital communication Same but treat transmission and reception as digitized Amplitude Shift Keying (ASK) Phase Shift Keying (PSK) Freq. Shift Keying (FSK) QAM PAM,12 - 12,Pulse Amplitude Modulation (PAM),Amplitude of periodic pulse train is varied
13、with a sampled message signal m Digital PAM: coded pulses of the sampled and quantized message signal are transmitted (lectures 12 and 13) Analog PAM: periodic pulse train with period Ts is the carrier (below),m(t),s(t) = p(t) m(t),12 - 13,Analog PAM,Pulse amplitude varied with amplitude of sampled
14、message Sample message every Ts Hold sample for T seconds (T Ts) Bandwidth 1/T,Transmitted signalh(t) is a rectangular pulse of duration T units,t,m(t),Ts,T,T+Ts,2Ts,s(t),m(0),m(Ts),sample,hold,12 - 14,Analog PAM,Transmitted signalFourier transform,Equalization of sample and hold distortion added in
15、 transmitter H(f) causes amplitude distortion and delay of T/2 Equalize amplitude distortion by post-filtering with magnitude responseNegligible distortion (less than 0.5%) if,msampled(t),12 - 15,Analog PAM,Requires transmitted pulses to Not be significantly corrupted in amplitude Experience roughly
16、 uniform delay Useful in time-division multiplexing public switched telephone network T1 (E1) line time-division multiplexes 24 (32) voice channels Bit rate of 1.544 (2.048) Mbps for duty cycle 10% Other analog pulse modulation methods Pulse-duration modulation (PDM), a.k.a. pulse width modulation (PWM) Pulse-position modulation (PPM): used in some optical pulse modulation systems.,
