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Communication Systems. Chapter 7 Baseband Pulse Transmission. Baseband Pulse Transmission. The transmission of digital data over a baseband channel Digital data have a broad spectrum with a significant low-frequency content
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Communication Systems Chapter 7 Baseband Pulse Transmission
Baseband Pulse Transmission • The transmission of digital data over a baseband channel • Digital data have a broad spectrum with a significant low-frequency content • Requires the use of low-pass channel with a bandwidth large enough to accommodate the essential frequency content of the data stream Communication Systems, chapter 7
Matched Filter • Problem of detecting a pulse transmitted over a noisy channel x(t) = g(t) + ω(t) 0≤ t ≤ T • Where : • x(t) is the filter input • g(t) is the pulse signal • ω(t) is the noise y(t) = g0(t) + n(t) • y(t) is the filter output • g0(t) is the output signal component • n(t) is the output noise component Communication Systems, chapter 7
Matched Filter Communication Systems, chapter 7
Error rate due to noise • PCM system with NRZ signalling • Two possible errors • Symbol 1 is chosen when 0 was transmitted • Symbol 0 is chosen when 1 was transmitted Communication Systems, chapter 7
Intersymbol Interference • ISI arises when the communication channel is dispersive • Major source of bit errors in the reconstructed data stream at the receiver • To correct for it, pulse shaping has to be done in the system • ISI and noise causes errors in the decision device at the receivers output • Transmit and receive filters should minimize the effects of noise and ISI and deliver the digital data with the smallest error rate possible Communication Systems, chapter 7
Nyquist criterion • For distortionless baseband transmission in the absence of noise • The frequency function P(f) eliminates intersymbol interference for samples taken at intervals Tb provided that it satisfies Eq. (7.53) Eq. 7.53 Communication Systems, chapter 7
Nyquist criterion • The special value of the bit rate Rb = 2W is called the Nyquist rate • W is itself called the Nyquist bandwidth • The ideal baseband pulse transmission system in the frequency domain or in the time domain is called the ideal Nyquist channel Communication Systems, chapter 7
Correlative-Level Coding • Since ISI introduced into the transmitted signal is known, its effect can be interpreted at the receiver in a deterministic way • The basic idea is duobinary signalling • Doubling of the transmission capacity a straight binary system • Also called class I partial response Communication Systems, chapter 7
Duobinary signalling • When two-level (0/1) sequence of short pulses is applied into a duobinary encoder it is converted into a three-level output, -2,0,+2 Communication Systems, chapter 7
Modified Duobinary Signalling • In Duobinary signalling the power spectral density of transmitted pulse is nonzero at the origin • By using modified duobinary technique with correlation span of two binary digits we can correct this undesirable feature Communication Systems, chapter 7
Adaptive Equalization • Equalizer adjust itself continuously and automatically by operating on the input signal • Prechannel equalization (at transmitter) • Postchannel equalization (at receiver) Communication Systems, chapter 7
Eye Pattern • An experimental tool to collect information of combined effect of impairments on overall system performance in an operational environment • Provides great deal of useful information about the performance of a data transmission system • Time interval over which the received signal can be sampled without error from ISI • Sensitivity of the system to timing errors • Noise margin of the system Communication Systems, chapter 7
Eye Pattern Communication Systems, chapter 7
