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Chapter 5 – Signal Encoding and Modulation Techniques 1/45 Encoding and Modulation Techniques 2/45 Digital Signaling Versus Analog Signaling Digital signaling Digital or analog data is encoded into a digital signal Encoding may be chosen to conserve bandwidth or to minimize error Analog Signaling Digital or analog data modulates analog carrier signal The frequency of the carrier fc is chosen to be compatible with the transmission medium used Modulation: the amplitude, frequency or phase of the carrier signal is varied in accordance with the modulating data signal by using different carrier frequencies, multiple data signals (users) can share the same transmission medium 3/45 Digital Signaling Digital data, digital signal Simplest encoding scheme: assign one voltage level to binary one and another voltage level to binary zero More complex encoding schemes: are used to improve performance (reduce transmission bandwidth and minimize errors). Examples are NRZ-L, NRZI, Manchester, etc. Analog data, Digital signal Analog data, such as voice and video Often digitized to be able to use digital transmission facility Example: Pulse Code Modulation (PCM), which involves sampling the analog data periodically and quantizing the samples 4/45 Analog Signaling Digital data, Analog Signal A modem converts digital data to an analog signal so that it can be transmitted over an analog line The digital data modulates the amplitude, frequency, or phase of a carrier analog signal Examples: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK) Analog data, Analog Signal Analog data, such as voice and video modulate the amplitude, frequency, or phase of a carrier signal to produce an analog signal in a different frequency band Examples: Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM) 5/45 Digital Data, Digital Signal Digital signal discrete, discontinuous voltage pulses each pulse is a signal element binary data encoded into signal elements 6/45 Periodic signals Data element: a single binary 1 or 0 Signal element: a voltage pulse of constant amplitude Unipolar: All signal elements have the same sign Polar: One logic state represented by positive voltage the other by negative voltage Data rate: Rate of data (R) transmission in bits per second Duration or length of a bit: Time taken for transmitter to emit the bit (Tb=1/R) Modulation rate: Rate at which the signal level changes, measured in baud = signal elements per second. Depends on type of digital encoding used. 7/45 Interpreting Signals Need to know timing of bits: when they start and end signal levels: high or low factors affecting signal interpretation Data rate: increase data rate increases Bit Error Rate (BER) Signal to Noise Ratio (SNR): increase SNR decrease BER Bandwidth: increase bandwidth increase data rate encoding scheme: mapping from data bits to signal elements 8/45 Comparison of Encoding Schemes signal spectrum Lack of high frequencies reduces required bandwidth, lack of dc component allows ac coupling via transformer, providing isolation, should concentrate power in the middle of the bandwidth Clocking synchronizing transmitter and receiver with a sync mechanism based on suitable encoding error detection useful if can be built in to signal encoding signal interference and noise immunity cost and complexity: increases when increases data rate 9/45 Encoding Schemes Positive level (+5V) Negative level (-5V) Positive level (+5V) No line signal (0V) Negative level (-5V) 10/45 Encoding Schemes 11/45 NonReturn to Zero-Level (NRZ-L) Two different voltages for 0 and 1 bits Voltage constant during bit interval no transition, i.e. no return to zero voltage more often, negative voltage for binary one and positive voltage for binary zero 12/45 NonReturn to Zero INVERTED (NRZI) Nonreturn to zero inverted on ones Constant voltage pulse for duration of bit Data encoded as presence or absence of signal transition at beginning of bit time transition (low to high or high to low) denotes binary 1 no transition denotes binary 0 Example of differential encoding since have – data represented by changes rather than levels – more reliable detection of transition rather than level 13/45 Advantages and disadvantages of NRZ-L, NRZI Advantages easy to engineer good use of bandwidth Disadvantages dc component lack of synchronization capability Unattractive for signal transmission applications 14/45 Multilevel Binary Bipolar Alternate Mark Inversion (AMI) Use more than two levels (three levels, positive, negative and no line signal) Bipolar-AMI zero represented by no line signal one represented by positive or negative pulse one pulses alternate in polarity no loss of sync if a long string of ones long runs of zeros still a problem no net dc component lower bandwidth easy error detection 15/45 Multilevel Binary Pseudoternary Binary one represented by absence of line signal Binary zero represented by alternating positive and negative pulses No advantage or disadvantage over bipolar-AMI Each used in some applications 16/45 Multilevel Binary Issues Advantages: No loss of synchronization if a long string of 1’s occurs, each introduce a transition, and the receiver can resynchronize on that transition No net dc component, as the 1 signal alternate in voltage from negative to positive Less bandwidth than NRZ Pulse alternating provides a simple mean for error detection Disadvantages receiver distinguishes between three levels: +A, -A, 0 a 3 level system could represent log23 = 1.58 bits requires approx. 3dB more signal power for same probability of bit error 17/45 Theoretical Bit Error Rate (BER) For Various Encoding Schemes 18/45 Manchester Encoding has transition in middle of each bit period low to high represents binary one transition serves as clock and data high to low represents binary zero used by IEEE 802.3 (Ethernet) LAN standard 19/45 Differential Manchester Encoding midbit transition is clocking only transition at start of bit period representing binary 0 no transition at start of bit period representing binary 1 used by IEEE 802.5 token ring LAN 20/45 Advantages and disadvantages of Manchester Encoding Disadvantages at least one transition per bit time and possibly two maximum modulation rate is twice NRZ R L D : Modulation rate, [baud ] R : Data Rate, [bps ] L : number of bits per signal elements D requires more bandwidth Advantages synchronization on mid bit transition (self clocking codes) has no dc component has error detection capability (the absence of an expected transition can be used to detect errors) 21/45 Modulation Rate versus Data Rate Data rate (expressed in bps) Data rate or bit rate R=1/Tb=1/1μs=1Mbps Modulation Rate (expressed in baud) is the rate at which signal elements are generated Maximum modulation rate for Manchester is D=1/(0.5Tb)=2/1μs=2Mbaud 22/45 Scrambling Use scrambling to replace sequences that would produce constant voltage These filling sequences must produce enough transitions to maintain synchronization be recognized by receiver & replaced with original be same length as original Design goals have no dc component have no long sequences of zero level line signal have no reduction in data rate give error detection capability 23/45 B8ZS and HDB3 24/45 Bipolar with 8-Zero Substitution (B8ZS) To overcome the drawback of the AMI code that a long string of zeros may result in loss of synchronization, the encoding is amended with the following rules: If 8 zeros occurs and the last voltage pulse was positive, then the 8 zeros are encoded as 000+–0–+ If zeros occurs and the last voltage pulse was negative, then the 8 zeros are encoded as 000–+0+– 25/45 High Density Bipolar-3 zeros (HDB3) The scheme replaces strings with 4 zeros by sequences containing one or two pulses In each case, the fourth zero is replaced with a code violation (V) successive violations are of alternate polarity 26/45 Digital Data, Analog Signal Main use is public telephone system has freq range of 300Hz to 3400Hz use modem (modulator-demodulator) The digital data modulates the amplitude A, frequency fc , or phase θ of a carrier signal A cos( 2f ct ) Modulation techniques Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) 27/45 Modulation Techniques Amplitude Shift Keying (ASK) Binary Frequency Shift Keying (BFSK) Binary Phase Shift Keying (BPSK) 28/45 Amplitude Shift Keying (ASK) In ASK, the two binary values are represented by to different amplitudes of the carrier frequency The resulting modulated signal for one bit time is A cos( 2f ct ), binary 1 s(t ) binary 0 0, Susceptible to noise Inefficient modulation technique used for up to 1200bps on voice grade lines very high speeds over optical fiber 29/45 Binary Frequency Shift Keying (BFSK) The most common form of FSK is Binary FSK (BFSK) Two binary values represented by two different frequencies ( f1 and f2 ) A cos( 2f1t ), binary 1 s(t ) A cos( 2f 2t ), binary 0 0 0 1 1 0 1 0 0 0 1 0 f2 f2 f1 f1 f2 f1 f2 f2 f2 f1 f2 less susceptible to noise than ASK used for up to 1200bps on voice grade lines high frequency radio (3 to 30MHz) even higher frequency on LANs using coaxial cable 30/45 Full-Duplex BFSK Transmission on a Voice-Grade line Voice grade lines will pass voice frequencies in the range 300 to 3400Hz Full duplex means that signals are transmitted in both directions at the same time f1 f2 f3 f4 31/45 Multiple FSK (MFSK) More than two frequencies (M frequencies) are used More bandwidth efficient compared to BFSK More susceptible to noise compared to BFSK MFSK signal: si (t ) A cos( 2f i t ), 1 i M where f i f c (2i 1 M ) f d f c the carrier frequency f d the difference frequency M number of different signal elements 2 L L number of bits per signal element 32/45 Multiple FSK (MFSK) MFSK signal: si (t ) A cos( 2f i t ), 1 i M where f i f c ( 2i 1 M ) f d M number of different signal elements 2 L L number of bits per signal element Period of signal element Ts LTb , Ts : signal element period Tb :bit period Minimum frequency separation 1 / Ts 2 f d 1 /( LTb ) 2 f d 1 / Tb 2 Lf d (bit rate) MFSK signal bandwidth: Wd M (2 f d ) 2Mf d 33/45 Example With fc=250KHz, fd=25KHz, and M=8 (L=3 bits), we have the following frequency assignment for each of the 8 possible 3-bit f i f c (2i 1 M ) f d data combinations: 000 001 010 011 100 101 110 111 f1 75 KHz f 2 125 KHz f 3 175 KHz f 4 225 KHz bandwidth Ws 2 Mf d 400 KHz f 5 275 KHz f 6 325 KHz f 7 375 KHz f 8 425 KHz This scheme can support a data rate of: 1 / Tb 2 Lf d 2(3bits)( 25Hz ) 150Kbps 34/45 Example The following figure shows an example of MFSK with M=4. An input bit stream of 20 bits is encoded 2bits at a time, with each of the possible 2-bit combinations transmitted as a different frequency. f i f c (2i 1 M ) f d 00 i 1 f1 f c 3 f d 01 i2 f2 fc fd 10 i3 f3 fc f d 11 i4 f4 fc 3 fd 35/45 Phase Shift Keying (PSK) Phase of carrier signal is shifted to represent data Binary PSK (BPSK): two phases represent two binary digits binary 1 A cos( 2f c t ), s (t ) A cos( 2f c t ), binary 0 binary 1 A cos( 2f c t ), A cos( 2f c t ), binary 0 Ad (t ) cos( 2f c t ), d (t ) 1 0 0 1 1 0 1 0 0 0 1 0 π π 0 0 π 0 π π π 0 π 36/45 Differential PSK (DPSK) In DPSK, the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal Binary 0:signal burst with the same phase as the previous one Binary 1:signal burst of opposite phase to the preceding one 37/45 Four-level PSK: Quadrature PSK (QPSK) More efficient use of bandwidth if each signal element represents more than one bit eg. shifts of /2 (90o) each signal element represents two bits split input data stream in two & modulate onto the phase of the carrier A cos( 2 f t ) c 4 3 A cos( 2f c t ) 4 s (t ) 3 A cos( 2f c t ) 4 A cos( 2f c t ) 4 11 01 00 10 can use 8 phase angles & more than one amplitude 9600bps modem uses 12 phase angles, four of which have two amplitudes: this gives a total of 16 different signal elements 38/45 QPSK and Offset QPSK (OQPSK) Modulators QPSK : OQPSK : 1 1 I (t ) cos( 2f c t ) Q(t ) sin( 2f c t ) 2 2 1 1 s (t ) I (t ) cos( 2f c t ) Q(t Tb ) sin( 2f c t ) 2 2 s (t ) 39/45 Example of QPSK and OQPSK Waveforms for QPSK : 1 11 1 4 3 4 3 0 0 1 1 4 1 0 1 1 4 0 1 1 1 40/45 Performance of ASK, FSK, MFSK, PSK and MPSK Bandwidth Efficiency ASK/PSK: MPSK: data rate R 1 , transmissi on bandwidth BT 1 r R log 2 M , BT 1 r 0 r 1 M : number of different signal elements R log 2 M MFSK: BT (1 r ) M Bit Error Rate (BER) bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK (see Fig. 5.4) for MFSK & MPSK have tradeoff between bandwidth efficiency and error performance 41/45 Performance of MFSK and MPSK MFSK: increasing M decreases BER and decreases bandwidth Efficiency MPSK: Increasing M increases BER and increases bandwidth efficiency 42/45 Quadrature Amplitude Modulation (QAM) QAM used on asymmetric digital subscriber line (ADSL) and some wireless standards combination of ASK and PSK logical extension of QPSK send two different signals simultaneously on same carrier frequency use two copies of carrier, one shifted by 90° each carrier is ASK modulated 43/45 QAM modulator QAM : s (t ) d1 (t ) cos( 2f c t ) d 2 (t ) sin( 2f c t ) ASK ASK 44/45 QAM Variants Two level ASK (two different amplitude levels) each of two streams in one of two states four state system essentially QPSK Four level ASK (four different amplitude levels) combined stream in one of 16 states Have 64 and 256 state systems Improved data rate for given bandwidth but increased potential error rate 45/45