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ENGR 4323/5323 Digital and Analog Communication Ch 7 Principles of Digital Data Transmission Engineering and Physics University of Central Oklahoma Dr. Mohamed Bingabr Chapter Outline • Digital Communication Systems • Line Coding • Pulse Shaping • Scrambling • Digital Receiver and Regenerative Repeaters • PAM: M-ARY Baseband Signaling for Higher Data Rate • Digital Carrier Systems • M-ARY Digital Carrier Modulation 2 Digital Communication Systems On-Off (RZ) Polar (RZ) Line Coding Bipolar (RZ) On-Off (NRZ) Polar (NRZ) 3 Digital Communication Systems Digital Carrier Modulation Multiplexer - Amplitude Modulation - Time Division - Frequency Modulation - Frequency Division - Phase Modulation - Code Division 4 Digital Communication Systems Regenerative Repeater - Used at regularly spaced interval. - Timing information extracted from the received signal. - Transparent line code does not effect the accuracy of the timing information. 5 Line Coding Property of Line Code - Transmission Bandwidth - Power Efficiency - Error Detection and Correction Capacity - Favorable Power Spectral Density - Adequate Timing Content - Transparency 6 PSD of Line Codes The PSD will depend on the line code pattern x(t) and the pulse shape p(t). = − = 2 () 7 PSD of Line Codes We can express the impulse as a pulse with narrow width and large amplitude such that the strength of the pulse is the same as the impulse. ℎ = 1 2 ℛ = lim 1 − →∞ 0 || ℛ = 1− 1 0 = lim →∞ 2 = 2 < 8 PSD of Line Codes 1 1 = lim →∞ 1 = lim →∞ +1 = +1 + = + To find ℛ , let ε0 ℛ 1 ℛ () = ∞ − =−∞ The PSD is the FT of ℛ 9 PSD of Line Codes 1 () = 0 + 2 ∞ 2 =1 = 2 () () () = 2 ∞ 0 + 2 2 =1 Again Rn is 1 = lim →∞ + = + 10 PSD of Polar Signaling 1 0 = lim →∞ 1 = lim →∞ 2 1 = lim →∞ + = 0 () () = 1=1 + = 1 or -1 with equal probability 2 2 For rectangular pulse shape = Π = 2 2 2 = 4 2 11 PSD of Polar Signaling 2 = 4 2 - Essential Bandwidth 2Rb Hz - No capability for error detection or correction - Nonzero PSD at dc ( f = 0) - For a given power, Polar signaling has the lowest error detection probability. - Transparent - Rectification of polar signal can help in extracting clock timing. 12 Constructing a DC Null in PSD by Pulse Shaping Split-phase (Manchester or twinned-binary) signal. Fig. a: Basic pulse p(t) for Manchester signaling. Fig. b: Transmitted waveform for binary data sequence using Manchester signaling. ∞ () −2 () = −∞ ∞ (0) = = 0 −∞ Read On-Off Signaling 13 PSD of Bipolar Signaling 1 0 = lim →∞ 2 Half the time aK equals 0 and the other half time equals either 1 or -1. 1 0 = 2 For R1, the combination of akak+1 = 11, 10, 01, 00. For bipolar rule the product is zero for the last three combination and -1 for the first combination. 1 3 1 = lim −1 + 0 →∞ 4 4 1 =− 4 = 0 for > 1 14 PSD of Bipolar Signaling () () = 2 () () = 2 2 ∞ 0 + 2 2 =1 1 − 2 () 2 () = 2 2 = 2 4 2 15 PSD of Bipolar Signaling 2 = 2 4 2 - Essential Bandwidth Rb Hz. - Single error detection capability. - Zero PSD at dc ( f =0). - Disadvantage require twice the power as a polar signal needs. - It is not transparent. 16 High-Density Bipolar (HDB) Signaling The HDB scheme is an ITU standard. In this scheme the problem of nontransparency in bipolar signaling is eliminated by adding pulses when the number of consecutive 0s exceeds N. (a) HDB3 signal and (b) its PSD. 17 Pulse Shaping The pulse shape p(t) effect the PSD Sy( f ) more than the choice of line code. Intersymbol Interference (ISI): Spreading of a pulse beyond its allocated time interval Tb will cause it to interfere with neighboring pulses. 18 Nyquist 1st criteria for Pulse Shaping Nyquist criteria for pulse shaping to eliminate ISI: Pulse shape that has a nonzero amplitude at its center and zero amplitudes at t = nTb (n =1, 2, 3, …) 1 = 0 =0 = ± 1 = 19 Nyquist 1st criteria for Pulse Shaping 1 1 − /2 = 1 − 2 2 0 < − 2 − < 2 > + 2 Nyquist 2nd criteria for Pulse Shaping Pulse broadening in the time domain leads to reduction of its bandwidth. Pulse satisfying second criteria is also knowing as the duobinary pulse. = 1 0 = 0, 1 for all other Information Sequence 1 1 0 1 1 0 0 0 1 0 1 1 1 Samples y(kTb) 1 2 0 0 2 0 -2 -2 0 0 0 2 2 Detected sequence 1 1 0 1 1 0 0 0 1 0 1 1 1 Nyquist 2nd criteria Duobinary Pulse = 1 − 2 = Π −/ The minimum bandwidth pulse that satisfies the duobinary pulse criterion and (b) its spectrum. Scrambling Scrambler tends to make the data more random by removing long strings of 1s and 0s. Removing long 0s or 1s help in timing extraction. However, the main purpose of scrambling is to prevent unauthorized access to the data. = ⨁3 ⨁5 = ⨁(3 ⨁5 ) Scrambling Example The data stream 101010100000111 is fed to the scrambler. Find the scrambler output T, assuming the initial content of the registers to be zero. Scrambling Example The data stream 101010100000111 is fed to the scrambler. Find the scrambler output T, assuming the initial content of the registers to be zero. S 1 2 3 4 5 T 1 0 0 0 0 0 1 0 1 0 0 0 0 0 1 0 1 0 0 0 1 0 1 0 1 0 0 1 1 1 1 0 1 0 1 0 1 1 1 0 1 0 1 0 1 1 1 0 0 0 0 0 1 1 1 0 0 0 0 0 1 1 1 0 1 0 0 0 1 1 0 1 1 0 0 0 0 T=101110001101001 0 0 1 1 0 0 1 Digital Receivers and Regenerative Repeaters Tasks of Receivers or repeaters: 1. Reshaping incoming pulses by means of an equalizer. 2. Extracting the timing information required to sample incoming pulses. 3. Making symbol detection decisions based on the pulse samples. Time Extraction Three general methods of synchronization 1- Derivation from a primary or a secondary standard (transmitter and receiver slaved to a master timing source). 2- Transmitting a separate synchronizing signal (pilot clock) 3- Self-synchronization, where the timing information is extracted from the received signal itself. Eye Diagrams: An Important Tool Three general methods of synchronization Eye diagrams of a polar signaling system using a raised cosine pulse with roll-off factor 0.5: over 2 symbol periods 2Tb with a time shift Tb/2; PAM: M-ARY Baseband Signaling for Higher Data Rate The information IM transmitted by an M-ary symbol is = log 2 bits The transmitted power increases as M2. Example Determine the PSD of the quaternary (4-ary) baseband signaling when the message bits 1 and 0 are equally likely. Digital Carrier Systems In transmitting and receiving digital carrier signals, we need a modulator and demodulator to transmit and receive data. The two devices, modulator and demodulator are usually packaged in one unit called a modem for two-way (duplex) communication. Amplitude Shift Keying (ASK) (a) The carrier cos ωct. (b) The modulating signal m(t). (c) ASK: the modulated signal m(t) cos ωct. Digital Carrier Systems (Modulator) Phase Shift Keying (PSK) Frequency Shift Keying (FSK) Spectrum of Modulated Digital Signals PSD of ASK PSD of PSK PSD of FSK Digital Carrier Systems (Demodulator) Noncoherent detection of FSK Coherent detection of FSK Coherent binary PSK detector Differential PSK (DPSK) DPSK allows noncoherent demodulation at the receiver. The transmitter encodes the information data into the phase difference θk - θk-1. For example a phase difference of zero represent 0 whereas a phase difference of signifies 1. Transmitter Encoding Receiver Decoding Differential PSK (DPSK) Transmitter Encoding Receiver Decoding M-Ary Digital Carrier Modulation Higher bit rate transmission can be achieved by either reducing Tb or by applying M-ary signaling; the first option requires more bandwidth; the second requires more power to keep the error bit rate within acceptable level. M-ary shift keying can send Log2 M bits each time by transmitting any one of M signals. M-ary ASK and noncoherent Detection = 0, , 2 , … , M − 1 M-ary FSK and noncoherent Detection = 2 where = 1 + ( − 1) and = 1, 2, … , Choice of the Frequencies for FSK The choice of will determine the performance and bandwidth of the FSK modulation. − 1 1 ∆ = = − 1 2 2 Large leads to bandwidth waste, whereas small is prone to detection error due to transmission noise interference. To minimize error detection the choice of should be large enough to make the FSK modulating signals 2 orthogonal over the period Tb. 0 2 2 = 0 1 = 2 Comparison between ASK and FSK FSK does not require increase in power but the bandwidth increase linearly with M (compared with binary FSK or M-ary ASK). ASK does not require increase in bandwidth but the power increase linearly with M. M-ary PSK = + 2 = 0 + −1 0 = 180 = 1, 2, … , 2 0 = 0 = 90 0 = 45 M-ary PSK symbols in the orthogonal signal space: (a) M = 2; (b) M = 4; (c) M = 8. M-ary PSK = 1 = 2 2 + 2 2 = 0 ≤ < 2 = 1 + 2 M-ary PSK symbols in the orthogonal signal space: (a) M = 2; (b) M = 4; (c) M = 8. Quadrature Amplitude Modulation (QAM) = () + () 0 ≤ < = ( − ) = 2 + 2 = −1 p(t) is a properly shaped baseband pulse. A simple choice is a rectangular. 16-point QAM (M = 16). QAM or Multiplexing