Analog Pulse Amplitude Modulation.ppt

Analog Pulse Amplitude Modulation 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 structure of an analog communication system is shown below 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. In a digital communications system, we might add redundancy to the message bit stream m[n] to assist in error detection (and possibly correction) in the receiver Transmitter Carrier circuits Convert baseband signal into a frequency band appropriate for the channel Uses analog and/or digital modulation 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 Wireline 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 different 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) Wireline Channel Impairments Analog transmission over communication channels Spreading in time domain due to convolution of transmitted waveform channel impulse response Wireless Channel Impairments Same as wireline channel impairments plus others Fading: multiplica