Digital Signaling and Modulation Techniques: A Comprehensive Guide, Study notes of Digital Communication Systems

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Pass Band Transmission

Encoding and Modulation Techniques

Digital Signaling

• (^) • Digital data, digital signal Simplest encoding scheme: assign one voltage level to binary one
  • and another voltage level to binary zeroMore complex encoding schemes: are used to improve

performanceerrors). (reduce transmission bandwidth and minimize

• 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 facilityExample: Pulse Code Modulation (PCM), which involves

sampling the analog data periodically and quantizing the samples

Analog Signaling

• (^) • Digital data, Analog Signal A modem converts digital data to an analog signal so that it can
  • be transmitted over an analog lineThe digital data modulates the amplitude, frequency, or phase of a
  • carrier analog signalExamples: Amplitude Shift Keying (ASK), Frequency Shift
• Analog data, Analog Signal^ Keying (FSK), Phase Shift Keying (PSK)
  • Analog data, such as voice and video modulate the amplitude,frequency, or phase of a carrier signal to produce an analog signalin a different frequency band
  • Examples: Amplitude Modulation (AM), Frequency Modulation(FM), Phase Modulation (PM)

Periodic signals

  Data element:Signal element: a single binary 1 or 0 a voltage pulse of constant amplitude

  Unipolar:Polar: One logic state represented by positive voltage the other by negative All signal elements have the same sign

 voltage Data rate: Rate of data (R) transmission in bits per second

 Duration or length of a bit: (Tb=1/R) Time taken for transmitter to emit the bit

 Modulation rate: = signal elements per second. Depends on type of digital encoding used. Rate at which the signal level changes, measured in baud

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 BERBandwidth: increase bandwidth increase data rate
  • encoding scheme: mapping from data bits to signal elements

Modulation Techniques

Amplitude Shift Keying(ASK) Binary Frequency ShiftKeying (BFSK) Binary Phase Shift Keying (BPSK)

• In ASK, the two binary values are represented by to different^ Amplitude Shift Keying (ASK)
• amplitudes of the carrier frequencyThe resulting modulated signal for one bit time is
• • Susceptible to noiseInefficient modulation technique
• used for • • up to 1200bps on voice grade linesvery high speeds over optical fiber

s ( t ) (^)  0 A ,cos(^2  fct ), binarybinary^10

Full-Duplex BFSK Transmission ona 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 atthe same time

f 1 f 2 f 3 f 4

Multiple FSK (MFSK)

• • More than two frequencies (M frequencies) are usedMore bandwidth efficient compared to BFSK
• • More susceptible to noise compared to BFSKMFSK signal:

ML numbernumberofofbitsdifferentper signalsignalelementelements

ff thethecarrierdifferencefrequencyfrequency

wheref f i M f

s t A f t i M

d L ci c d

i i

( ) cos( 2  ), 1

Example

 With frequency assignment for each of the 8 possible 3-bit data combinations: fc=250KHz , fd=25KHz , and M=8 ( L=3 bits), we have the following

 This scheme can support a data rate of:

bandwidth W Mf KHz ff KHzKHz

ff KHzKHz

ff KHzKHz

ff KHzKHz s^2 d^400 111110 425375

87

65

43

21    

1 / Tb  2 Lfd  2 ( 3 bits )( 25 Hz )  150 Kbps

f (^) i  fc ( 2 i  1  M ) fd

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 thepossible 2-bit combinations transmitted as a different frequency.

cc dd

cc dd i c d ii ff ff f f

ii ff ff ff

f f i M f 1110 43 3

43

(^12)     

Differential PSK (DPSK)

• In DPSK, the phase shift is with reference to the previous bittransmitted rather than to some constant reference signal
• • Binary 0:signal burst with the same phase as the previous oneBinary 1:signal burst of opposite phase to the preceding one

Four-level PSK: Quadrature PSK (QPSK)

cos( 2 44 )) 1000

cos(cos( 22 334 )^01

cos( 2 4 ) 11

AA ff t t

s t AA ff tt

c c cc

• More efficient use of bandwidth if each signal element represents more than one bit
  • • eg. shifts ofeach signal element represents two bits /2 (90o)
  • split input data stream in two & modulate onto the phase of thecarrier
• 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