2. Encoding and Modulation

Encoding Techniques

• Digital Data, Digital Signal
• Analog Data, Digital Signal
• Digital Data, Analog Signal
• Analog Data, Analog Signal

Digital Data, Digital Signal

• Discrete, Discontinuous voltage pulses
• Binary data encoded into signal elements
• Each pulse is a signal element

Common Terms

• Unipolar
  • All signal elements have the same sign
• Polar
  • One logic state by positive and other by negative
• Data rate
  • Rate of transmission bits per second
• Duration / Length of bit
  • Time taken for transmitter to emit the bit
• Encoding
  • Take a digital signal and convert it into a suitable form for sending on a wire
• Modulation
  • Take a signal and modifies a carrier signal with it
• Modulation Rate
  • Rate at which signal level change
  • Measured in baud
• Mark | Space
  • Binary 1 | Binary 0

Baseband vs Broadband

• Baseband
  • Digital Signals: Entire medium carries only one signal at a time
  • Analog Signals: Original Frequency range of an analog signal before it’s modulated
• Broadband
  • Carries two or more data in separate channels

Encoding Schemes

• Nonreturn to Zero-Level (NRZ-L)
• Nonreturn to Zero Inverted (NRZI)
• Bipolar -AMI
• Pseudoternary
• Manchester
• Differential Manchester
• B8ZS
• HDB3

Nonreturn to Zero-Level (NRZ-L)

• Two different voltages for 0 and 1
• Voltage constant during bit interval
• Zero voltage for 0, Positive voltage for 1
• Sometimes negative voltage for zero

Nonreturn to Zero Inverted

• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration bit
• Data encoded as presence or absence of signal transition at beginning of bit time
• Transition denotes a 1
• No transition denote a 0
• An example of Differential Encoding

Differential Encoding

• Data represented by changes
• Advantages
  • More reliable detection of transition
• Disadvantages
  • In complex layouts it’s easy to lose sense of polarity

NRZ pros and cons

• Pros
  • Easy to engineer
  • Make good use of bandwidth
• Cons
  • DC component
  • Lack of Synchronization
• Used for magnetic recording
• Not often used for signal transmission

Multilevel Binary

• Use more than two levels
• Bipolar AMI (Alternate Mark Inversion)
  • 0 No signal
  • 1 Positive or negative pulse
  • Pulse alternate in polarity
  • No loss of sync if a long string of ones, (not good for long zeroes)
  • No net DC component
  • Low bandwidth
  • Easy error detection

Pseudoternary

• 1 No signal
• 0 Alternating positive and negative
• No advantage or disadvantage over bipolar-AMI

Trade Off for multilevel binary

• Not as efficient as NRZ
  • Each signal only represent one bit
  • In 3 level system could represent bits
  • Receiver must distinguish between three levels
  • Requires 3dB more signal power for same probability of bit error

Biphase

Manchester

• Transition in middle of each bit period
• Transition serves as clock and data
• LOW HIGH 1
• HIGH LOW 0

Differential Manchester

• Midbit transition is clocking only
• Transition at start of a bit 0
• No transition at start of bit 1
• This is a differential encoding scheme

Biphase Pros and Cons

• Con
  • At least one transition per bit time or two
  • Max modulation rate is twice NRZ
  • Require more bandwidth
• Pros
  • Synchronization on mid bit transition
  • No dc component
  • Error detection

Scrambling

• Use scrambling to replace sequences that produce constant voltage
• Filling sequence
  • Must produce enough transitions to sync
  • Must be recognized by receiver and replace with original
  • Same length as original
• No dc component
• No long sequences of zero level line signal
• No reduction in data rate
• Error detection capability

B8ZS

• Bipolar with 8 zeros substitution
• Based on bipolar-AMI (Alternate Mark Inversion)
• Solves the problem where
• If 8 continuous zeroes and previous pulse is positive, encode as 000+-0-+
  • If a pulse is + and there are 8 zeroes after that, then encode it as 000+-0-+ this causes an AMI violation because the 4th item is + and the previous pulse is also + the second AMI violation occurs at position 7
• If 8 continuous zeros and previous pulse is negative, encode as 000-+0+-
  • If a pulse is - and there are 8 zeroes after that, then encode is as 000-+0+- this causes an AMI violation because the 4th item is - and the previous pulse is also - the second AMI violation occurs at position 7
• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all zeros

HDB3

• High Density Bipolar 3 Zeros
• Based on Bipolar-AMI
• String of four zeroes replaced with one or two pulses

Digital Data, Analog Signal

• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PK)

Amplitude Shift Keying

• Values represented by different amplitudes of the carrier wave
• One amplitude is zero
• Susceptible to sudden gain changes
• Inefficient
• Up to 1200bps on voice grade lines
• Used over optical fiber

Binary Frequency Shift Keying

• Two binary values represented by two different frequencies
• Less susceptible to error
• Upto 1200bps on voice grade lines
• High frequency radio
• Even higher frequency on LANs using co-ax

Phase Shift Keying

• Phase of carrier signal is shifted to represent data
• Binary PSK
  • Two phases represent two binary digits
• Differential PSK
  • Phase shifted relative to previous transmission rather than some reference signal

BPSK	DPSK

Quadrature PSK

• More efficient
• Each element represent more than one bit
  • Eg: Shifts of can represent two bits
  • Can use 8 phase angles and have more than one amplitude
  • 9600bps modems use 12 angles, four of which have two amplitudes

Performance of Digital to Analog Modulation Schemes

• Bandwidth
  • for ASK and PSK bandwidth is directly related to bit rate
  • FSK bandwidth related to
    • Data rate for lower freq
    • Offset of modulated freq from carrier at high freq

Quadrature Amplitude Modulation

• Used on ADSL and some wireless
• Combination of ASK and PSK
• Logical extension of QPSK
• Send two different signals simultaneously on same frequency
  • Use two copies of career, one shifted
  • Each carrier is ASK modulated
  • Two independent signals over the same medium
  • Demodulate and combine for original binary input

Analog Data, Digital Signal

• Digitization
  • Conversion of analog data into digital data

Pulse Code Modulation (PCM)

• Signal is sampled at regular intervals, each sample assigned a digital value
  • Ex: 4 Bit signal gives 16 levels
• Quantized
  • Quantizing error or noise
  • Approximation : Can’t recover original exactly
• Ex: 8000 samples per second of 8 bits gives 64kbps

Nonlinear Encoding

• Quantization levels not evenly spaced
• Reduce overall signal distortion
• Can also be done by companding

Delta Modulation

• Analog output is approximated by a staircase function
• Move up or down one level at each sample interval
• Binary behavior
  • Function moves up or down at each sample interval
• Good Voice reproduction
  • PCM - 128 levels (7bit)
  • Voice bandwidth 4KHz
  • = 56kbps
• Data compression can improve on this
  • Eg: Interframe coding techniques for video

Analog Data, Analog Signals

• Why modulate analog signals?
  • Higher frequency can give efficient transmission
  • Permit frequency division multiplexing
• Types of modulation
  • Amplitude
  • Frequency
  • Phase

Amplitude Modulation (AM)

• Amplitude of a carrier signal is altered
• Frequency of the carrier is usually greater than the highest frequency of the input signal*