DIGITAL MODULATION Line coding

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DIGITAL MODULATION Line coding

• CHAPTER 4

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Group Activity

• Draw line codes for 10110100
• Unipolar NRZ, polar RZ, NRZ-I, AMI, Manchester,
  Def. Manchester, bipolar

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By the end of this lesson, you should be able to

• Explain the basic concept of line coding and the
  different types of line coding available.
• Determine the line coding for any binary codes.

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Group Activity

• Draw line codes for 101011100
• Unipolar NRZ, polar NRZ, NRZ-I, AMI, Manchester,
  Def. Manchester

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Block diagram for digital transmission system
ASK, FSK, PSK
Sampling Quantization Coding
RZ, NRZ, AMI
Digital transmission
Analog
ADC
Line coding
Block diagram for digital transmission system
Before any digital signal can be transmitted (via
transmission medium or wireless) it must undergo
line coding. What is line coding??
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PCM
We are here..
Line Coding
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PCM Procedures
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What happens after PCM?

• What do we do with the digital data that we have?
• How can we send the digital data through the
  cables?
• How do we convert it into electrical signals?

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What is line coding??
Line coding consists of representing the digital
signal to be transported by an amplitude- and
time- discrete signal that is optimally tuned for
the specific properties of the physical channel
(and of the receiving equipment).

• Mapping Conversion of binary information
  sequence into the digital signal that enters the
  channel
• Ex. 1 maps to A square pulse 0 to A pulse

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Why we need Line Coding ??????
The purpose of a line code is to match the output
signal to the channel for baseband transmission.
Reasons for line coding

• Synchronization
• Error detection
• Error correction

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Lack of synchronization
Process that allows the clock of the receiver in
a digital system to operate at the same frequency
(and phase) as the clock used to transmit the
bits Ensuring this criteria is called
Synchronization
Error
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A few things that you need to know before
learning line coding
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Signal element versus data element
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Signal level versus data level
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DC component
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Six main properties of line coding

• Transmission bandwidth it should be as small as
  possible
• Power efficiency transmitted power should be as
  small as possible
• Error detection and correction capability it
  should be possible to detect , and preferably
  correct, detection errors.
• Transmission voltage and DC component
• Adequate timing content it should be possible
  to extract timing or clock information from the
  signal
• Transparency it should be possible to transmit a
  digital signal correctly regardless of the
  pattern of 1s and 0s.

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Line coding schemes
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UNIPOLAR ENCODING
Unipolar encoding uses only one voltage level.

• A positive voltage represents a binary 1
• Zero volts indicates a binary 0.
• It is the simplest line code

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Unipolar encoding

• drawbacks are that it is not self-clocking
• (Receiver Setting the clock matching the senders
  )

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POLAR ENCODING
Polar encoding uses two voltage levels (positive
and negative).
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Types of polar encoding
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NRZ vs. RZ

• NRZ
• No return to zero during a portion of bit duration

• RZ
• Signal return to zero at the middle of bit
  duration
• the signal drops (returns) to zero between each
  pulse.
• This takes place even if a number of consecutive
  0's or 1's occur in the signal. The signal is
  self-clocking.
• This means that a separate clock does not need to
  be sent alongside the signal, but suffers from
  using twice the bandwidth to achieve the same
  data-rate as compared to non-return-to-zero
  format.

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RZ encoding
Signal level returns to zero at every cycle
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Polar Non-Return-to-Zero (NRZ)

• Polar NRZ
• Popular method
• easy
• signal never returns to zero, and the voltage
  during a bit transmission is level (1 or 0)
• No synchronization. Can use start bit for
  synchronization purposes

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Unipolar Polar Non-Return-to-Zero (NRZ)
Unipolar NRZ
Polar NRZ
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Note
In NRZ-I the signal is inverted if a 1 is
encountered.
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"Zero" has no transition/follow previous signal
element.
NRZ-I encoding
Transition because next bit is 1
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NRZ-I encoding
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NRZ-I encoding
This means that the encoding is different for the
same binary pattern depending on the voltage
starting point.
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Note
A good encoded digital signal must contain a
provision for synchronization.
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Manchester Encoding

• Manchester encoding is a special type of unipolar
  signaling in which the signal is changed from a
  high to low (0) or low to high (1) in the middle
  of the signal.
• More reliable detection of transition rather than
  level
• Transitions still detectable even if polarity
  reversed
• Manchester encoding is commonly used in local
  area networks (ethernet, token ring).

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Manchester encoding
Manchester Coding 1 gt transition from LO to HI
at the middle of the interval 0 gt
transition from HI to LO at the middle of the
interval used in Ethernet IEEE 802.3
standard in LAN
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Note
In Manchester encoding, the transition at the
middle of the bit is used for both
synchronization and bit representation.

• Timing recovery easy
• Uses double the minimum bandwidth

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Differential Manchester encoding
Differential line coding provides robustness to
this type of error 1 mapped into no transition
in signal level 0 mapped into transition in
signal level
Key feature transition at the beginning only
for zero.
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Differential Manchester encoding
Key feature Mapping binary information into
transitions at the beginning of each interval
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Note
In differential Manchester encoding, the
transition at the middle of the bit is used only
for synchronization. The bit representation is
defined by the inversion or noninversion at the
beginning of the bit.
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Note
In bipolar encoding, we use three levels
positive, zero, and negative.
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Bipolar AMI encoding
6. RZ (AMI Alternately Mark Inversion) 1
gt Alternately ve and -ve 0 gt Low level
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Baseband Transmission
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Note

• For long distance transmission
• bandwidth efficiency is important
• bipolar coding is used
• For short distance transmission like LAN,
• bandwidth efficiency is less important than cost
  per station
• in Ethernet LAN Manchester encoding is used

The complexity and cost of line code
implementations depends on the application
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Example 1
A signal has two data levels with a pulse
duration of 1 ms. We calculate the pulse rate and
bit rate as follows
Pulse Rate 1/ 10-3 1000 pulses/s Bit Rate
Pulse Rate x log2 L 1000 x log2 2 1000 bps
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Example 2
A signal has four data levels with a pulse
duration of 1 ms. We calculate the pulse rate and
bit rate as follows
Pulse Rate 1000 pulses/s Bit Rate
PulseRate x log2 L 1000 x log2 4 2000 bps
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Example 3
In a digital transmission, the receiver clock is
0.1 percent faster than the sender clock. How
many extra bits per second does the receiver
receive if the data rate is 1 Kbps? How many if
the data rate is 1 Mbps?
Solution
At 1 Kbps 1000 bits sent ?1001 bits received?1
extra bps At 1 Mbps 1,000,000 bits sent
?1,001,000 bits received?1000 extra bps
Higher the bit rate, higher the criticality of
clock synchronization mechanism
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Example

• Draw line codes for 101011100
• Unipolar NRZ, polar NRZ, NRZ-I, AMI, Manchester,
  Def. Manchester