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1/45 Chapter 5 – Signal Encoding and Modulation Techniques.

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1 1/45 Chapter 5 – Signal Encoding and Modulation Techniques

2 2/45 Encoding and Modulation Techniques

3 3/45 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

4 4/45 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

5 5/45 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)

6 6/45 Digital Data, Digital Signal  Digital signal  discrete, discontinuous voltage pulses  each pulse is a signal element  binary data encoded into signal elements

7 Periodic signals 7/45  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 (T b =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.

8 8/45 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

9 9/45 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

10 10/45 Encoding Schemes Positive level (+5V) Negative level (-5V) Positive level (+5V) No line signal (0V) Negative level (-5V)

11 11/45 Encoding Schemes

12 12/45 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

13 13/45 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

14 14/45 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

15 15/45 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

16 16/45 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

17 17/45 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 log 2 3 = 1.58 bits  requires approx. 3dB more signal power for same probability of bit error

18 18/45 Theoretical Bit Error Rate (BER) For Various Encoding Schemes

19 19/45 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 (Ethernet) LAN standard

20 20/45 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 token ring LAN

21 21/45 Advantages and disadvantages of Manchester Encoding  Disadvantages  at least one transition per bit time and possibly two  maximum modulation rate is twice NRZ  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)

22 22/45 Modulation Rate versus Data Rate  Data rate (expressed in bps)  Data rate or bit rate R=1/T b =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/(0.5T b )=2/1μs=2Mbaud

23 23/45 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

24 24/45 B8ZS and HDB3

25 25/45 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+–

26 26/45 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

27 27/45 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 f c, or phase θ of a carrier signal  Modulation techniques  Amplitude Shift Keying (ASK)  Frequency Shift Keying (FSK)  Phase Shift Keying (PSK)

28 28/45 Modulation Techniques Amplitude Shift Keying (ASK) Binary Frequency Shift Keying (BFSK) Binary Phase Shift Keying (BPSK)

29 29/45 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  Susceptible to noise  Inefficient modulation technique  used for  up to 1200bps on voice grade lines  very high speeds over optical fiber

30 30/45 Binary Frequency Shift Keying (BFSK)  The most common form of FSK is Binary FSK (BFSK)  Two binary values represented by two different frequencies ( f 1 and f 2 )  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 f2f2 f2f2 f1f1 f1f1 f2f2 f1f1 f2f2 f2f2 f2f2 f1f1 f2f2

31 31/45 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 f1f1 f 3 f4f4 f2f2

32 32/45 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:

33 33/45 Multiple FSK (MFSK)  MFSK signal:  Period of signal element  Minimum frequency separation  MFSK signal bandwidth:

34 34/45 Example  With f c =250KHz, f d =25KHz, and M=8 ( L=3 bits), we have the following frequency assignment for each of the 8 possible 3-bit data combinations:  This scheme can support a data rate of:

35 35/45 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.

36 36/45 Phase Shift Keying (PSK)  Phase of carrier signal is shifted to represent data  Binary PSK (BPSK): two phases represent two binary digits ππ00π0πππ0π

37 37/45 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

38 38/45 Four-level PSK: Quadrature PSK (QPSK)  More efficient use of bandwidth if each signal element represents more than one bit  eg. shifts of  /2 (90 o )  each signal element represents two bits  split input data stream in two & modulate onto the phase of the carrier  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

39 39/45 QPSK and Offset QPSK (OQPSK) Modulators

40 40/45 Example of QPSK and OQPSK Waveforms

41 41/45 Performance of ASK, FSK, MFSK, PSK and MPSK  Bandwidth Efficiency  ASK/PSK:  MPSK:  MFSK:  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

42 42/45 Performance of MFSK and MPSK  MFSK: increasing M decreases BER and decreases bandwidth Efficiency  MPSK: Increasing M increases BER and increases bandwidth efficiency

43 43/45 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

44 44/45 QAM modulator

45 45/45 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


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