Chapter 3 DIFFERENTIAL ENCODING Differential Encoding Eye Patterns

Chapter 3: DIFFERENTIAL ENCODING Ø Differential Encoding Ø Eye Patterns Ø Regenerative Receiver Ø Bit Synchronizer Ø Binary to Mary Conversion Huseyin Bilgekul Eeng 360 Communication Systems I Department of Electrical and Electronic Engineering Eastern Mediterranean University 1

Differential Coding System Ø Differential encoding removes the problem of Unintentional Signal Inversion. Ø Polarity of the differentially encoded signal may be inverted without affecting the decoded signal. Modulo-2 addition Exclusive OR I 1 I 2 Out 0 0 1 1 1 0 2

Example of Differential Coding Encoding Input sequence dn Encoded sequence en 1 1 1 0 0 1 0 1 1 0 0 0 1 1 1 0 0 1 1 1 0 1 0 0 1 Reference digit Decoding (with correct channel polarity) Receiver sequence 1 (Correct polarity) Decoded sequence Decoding (with inverted channel polarity) Received sequence 0 (Inverted polarity) Decoded sequence Ø Decoded sequence is same whethere is inversion or not. EEE 360 3

Eye patterns Ø The effects of channel filtering and channel noise can be seen by observing the received line code on an oscilloscope. Received Line Code Information from Eye Pattern • Timing error Eye opening • Sensitivity Slope of the open eye • Noise Margin height of the eye opening 4

Regenerative Repeater Ø Regenerate a noise-free digital signal. Amplify and clean-up the signal periodically Increases the amplitude Minimize the effect of channel noise & ISI Produces a sample value Produces a high level o/p if sample value>VT Generates a clocking signal 5

Synchronization Ø Synchronization signals are clock-type signals necessary within a receiver for detection of data from the corrupted input signal. Ø Digital communications need at least 3 types of synchronization signals. • Bit Synchronization (Bit Synch. ): To distinguish bit intervals. • Frame Synchronization (Frame Synch. ): To distinguish groups of bits. • Carrier Synchronization: For bandpass signals with coherent detection. Ø Sync signals are derived from • Corrupted input signal. • From a separate channel that transmits sync signals. 6

Bit Synchronizer for NRZ Signals Ø Derive the synch signal from the corrupted received signal. Ø Used for unipolar NRZ signals. Ø Synchronizer complexity depends on the line code used. Ø Synchronizarion of RZ signals is easier since PSD has delta at f=R=1/Tb. Ø Bit synchronizer for NRZ signals is given below. 7

Square-law Bit Synchronizer for NRZ Signals • Square Law Device converts polar NRZ signal to unipolar RZ format. • Unipolar RZ signals have delta in the PSD at f=R=1/Tb. • This frequency component can be obtained by filtering. • Filtered sinusoidal is converted to clock pulses using a comparator. 8

Binary-to-multilevel polar NRZ Signal Conversion Ø Binary to multilevel conversion is used to reduce the bandwidth required by the binary signaling. • Multiple bits (l number of bits) are converted into words having SYMBOL durations Ts=l. Tb where the Symbol Rate or the BAUD Rate D=1/Ts=1/l. Tb. • The symbols are converted to a L level (L=2 l ) multilevel signal using a l-bit DAC. • Note that now the Baud rate is reduced by l times the Bit rate R (D=R/l). • Thus the bandwidth required is reduced by l times. Ts: Symbol Duration L: Number of M ary levels Tb: Bit Duration l: Bits per Symbol L=2 l D=1/Ts=1/l. Tb=R/l Bnull=R/l 9

Power Spectra for Multilevel Polar NRZ Signals (c) L = 8 = 23 Level Polar NRZ Waveform Out 10

Spectral Efficiency Ø The Spectral efficiency of a digital signal is given by, where R is the data rate and B is the bandwidth required. • If limited BW is desired, then use a signaling technique that has high spectral efficiency. • Maximum spectral efficiency (which is limited by channel noise) is given by the Shannon’s Channel Capacity formula: Spectral efficiency for multilevel signaling is 11

PSD of a multilevel polar NRZ waveform I R (k ) = å (an an + k )i Pi i =1 PSD for a multilevel polar NRZ signal: Ps(f) Ø Multilevel signaling is used to reduce the BW of a digital signal 12