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

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

Communication Systems, chapter 7 Matched Filter Communication Systems, chapter 7

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

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

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

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

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

Communication Systems, chapter 7 Eye Pattern Communication Systems, chapter 7