COMMUNICATION SYSTEM EEEB453Chapter 5 (Part IV Additional)

DIGITAL TRANSMISSION

Differential PCM

In conventional PCM, there are often successive samples taken in which there is little difference between the amplitudes.

Thus there is a redundancy of transmitting identical PCM codes.

With DPCM, the difference in the amplitude of two successive samples is transmitted rather than the actual sample.

Thus, fewer bits are required for DPCM than PCM.

Pulse Transmission

All digital carrier systems involve the Tx of rectangular pulses through a medium with a finite BW.

The output frequency spectrum of the rect pulse (by Fourier series) is sinc function i.e a damped sine wave in which successive peak is smaller than the preceding one.

Output waveform from bandlimited channel when a narrow pulse is applied to its rect input.

Bandlimiting causes the energy to spread over.

90% of signal power is contained within the first lobe. Therefore, the signal can be limited to a BW, B = 1/T.

But theoretically, only amplitude at the middle of each pulse interval needs to be preserved, i.e B = 1/2T

By Nyquist Theorem, the maximum signaling rate,R is:R = 2B

Intersymbol Interference (ISI)

Due to imperfect LPF, at the sampling instant (centre of the pulse), the signal doesn’t always achieve the max. value.

Other lobes may overlapped and interfere with major lobe of other pulse.

This is called an ISI

Distortion of received signal which are spreading and consequent overlapping of individual pulses to the degree that the Rx cannot reliably distinguish between changes of state.

When pulses form more than one source are multiplexed together, ISI may cause crosstalk. Overcome – use equalizer.

ISI is an important consideration in the Tx of pulse over circuits with a limited BW and a nonlinear phase response.

Four primary cause of ISI are: Timing inaccuracy Insufficient BW Amplitude Distortion Phase Distortion

Intersymbol Interference (ISI)

Eye Pattern

The ISI and other signal degradation can be studied/displayed on an oscilloscope through what is known as the eye diagram.

The oscilloscope shows the superposition of several traces of input signal cut up every interval of 1 pulse, Tb.

a. Ideal channel, infinite BW, pulse receive without distortion.

b. Distortion channel, finite BW, received signal will rounded and spread out.- full opening at mid-pt – No ISI

c. Noise channel, ISI is not zero, the eye close partially at the mid-pt

Eye Pattern

The width of the eye opening defines the time interval over which the received signal can be sampled without error from ISI. ( preferred = eye open widest)

The sensitivity of the system to timing errors is determine by the rate of closure of the eye as the sampling time is varied

The height of the eye opening, at a specified sampling time, defines the noise margin of the system.