Communication Systems Fundamentals

1. DIGITAL MODULATION
TECHNIQUES

2. WHAT IS DIGITAL COMMUNICATION?
 Digital communications broadly refers to the
transmission of information using digital messages or
bit streams.
 There are notable advantages to transmitting data
using discrete messages.
 Errors caused by noise and interference can be
detected and corrected systematically.
 Digital communications also make the networking of
heterogeneous systems possible, with the Internet
being the most obvious such example.

3. DIGITAL COMMUNICATION

4. DIGITAL COMMNICATION
•Information Source and Input Transducer:
The source of information can be analog or digital, e.g. analog: audio or video signal,
digital: like teletype signal. In digital communication the signal produced by this source
is converted into digital signal consists of 1′s and 0′s. For this we need source encoder.
Source Encoder
In digital communication we convert the signal from source into digital signal as
mentioned above. The point to remember is we should like to use as few binary digits
as possible to represent the signal. In such a way this efficient representation of the
source output results in little or no redundancy. This sequence of binary digits is called
information sequence.
Source Encoding or Data Compression: the process of efficiently converting the output
of wither analog or digital source into a sequence of binary digits is known as source
encoding.

5. DIGITAL COMMNICATION
Channel Encoder:
The information sequence is passed through the channel encoder. The purpose of the
channel encoder is to introduced, in controlled manner, some redundancy in the
binary information sequence that can be used at the receiver to overcome the effects
of noise and interference encountered in the transmission on the signal through the
channel.
e.g. take k bits of the information sequence and map that k bits to unique n bit
sequence called code word. The amount of redundancy introduced is measured by the
ratio n/k and the reciprocal of this ratio (k/n) is known as rate of code or code rate.
Digital Modulator:
The binary sequence is passed to digital modulator which in turns convert the
sequence into electric signals so that we can transmit them on channel (we will see
channel later). The digital modulator maps the binary sequences into signal wave
forms , for example if we represent 1 by sin x and 0 by cos x then we will transmit sin x
for 1 and cos x for 0. ( a case similar to BPSK)

6. DIGITAL COMMNICATION
Channel:
The communication channel is the physical medium that is used for transmitting signals
from transmitter to receiver. In wireless system, this channel consists of atmosphere ,
for traditional telephony, this channel is wired , there are optical channels, under water
acoustic cahnenls etc.
Digital Demodulator:
The digital demodulator processes the channel corrupted transmitted waveform and
reduces the waveform to the sequence of numbers that represents estimates of the
transmitted data symbols.

7. DIGITAL COMMNICATION
Channel Decoder:
This sequence of numbers then passed through the channel decoder which attempts to
reconstruct the original information sequence from the knowledge of the code used by
the channel encoder and the redundancy contained in the received data
Source Decoder
At the end, if an analog signal is desired then source decoder tries to decode the
sequence from the knowledge of the encoding algorithm. And which results in the
approximate replica of the input at the transmitter end

8. DIGITAL COMMNICATION
Channel Decoder:
This sequence of numbers then passed through the channel decoder which attempts to
reconstruct the original information sequence from the knowledge of the code used by
the channel encoder and the redundancy contained in the received data
Source Decoder
At the end, if an analog signal is desired then source decoder tries to decode the
sequence from the knowledge of the encoding algorithm. And which results in the
approximate replica of the input at the transmitter end
Output Transducer:
Finally we get the desired signal in desired format analog or digital.

9. PULSE MODULATION

10. PULSE MODULATION

11. SAMPLING

12. SAMPLING THEOREM

13. 3 ANALOG PULSE MODULATION :

14. PULSE AMPLITUDE MODULATION (PAM)
Analog pulse
Sample pulse
 The amplitude of a constant width, constant position pulse is varied according to the
amplitude of the sample of the analog signal.
 the amplitude of a pulse coincides with the amplitude of the analog signal.

15. PULSE WIDTH MODULATION (PWM)
 A constant amplitude pulse is varied proportional to the amplitude of the
analog signal at the time the signal is sampled.
 The maximum analog signal amplitude produces the widest pulse, and the
minimum analog signal amplitude produces the narrowest pulse.
 All pulses have the same amplitude.

16. PULSE POSITION MODULATION (PPM)
 The position of a constant-width pulse within prescribed time slot is varied
according to the amplitude of the sample of the analog signal.
 The higher the amplitude of the sample, the farther to the right the pulse
is positioned within the prescribed time slot.
 The highest amplitude sample produces a pulse to the far right, and the
lowest amplitude sample produces a pulse to the far left.

17. DIGITAL PULSE MODULATION (DPM)
• In DPM, a code used to represent the amplitude of
the samples that has been divided into various levels.
• There are 2 types of DPM:
– Pulse Code Modulation
– Delta Modulation

18. PULSE CODE MODULATION (PCM)
• PCM is a form of modulation, which uses coded group of pulses to represent certain values of
the information signal.
• The analog signal is sampled and then converted to a serial n-bit binary code for
transmission.
• Each code has the same number of bits and requires the same length of time or
transmission.

19. PCM BLOCK DIAGRAM
Analogue Low Pass Low Pass Analogue
Signal Filter Filter Signal
Quantiser Encoder Decoder Expander
Sampler

20. PCM BLOCK DIAGRAM
• PCM is a form of modulation, which uses coded group of
pulses to represent certain values of the information signal.
• The information signal is limited to a certain maximum
freq and sampled and changed to PAM.
• The PAM signal is then quantise by the quantiser and
then changed into the binary code by the encoder.
• Then the PCM signal is sent through the cable.
• PCM has superior signal to signal characteristics for a
given bandwidth.

21. PCM BLOCK DIAGRAM

22. PCM TRANSMITTER

23. PCM RECEIVER

24. PULSE CODE MODULATION (PCM)
The 3 main processes in PCM:
1) Sampling
2) Quantization
3) Encoding

25. PCM - SAMPLING
Process of taking samples of the information signals at
Nyquist Rate :
fs ≥ 2fmax
fs – frequency sampling
fm – modulating frequency
Minimum freq sampling, fs = 2fm

26. PCM - QUANTIZATION
• The amplitude of the samples are then divided into
respective levels. The number of levels for the samples
depend on the number of bits used to code the signal.
• The relationship between the number of bits (B) is given
B
by the equation: M= 2
M- Number of levels
B – Bits/ samples
•The more levels used means that an analogue signal can be
describe more accurate.

27. PCM - ENCODING
• In this process, the samples that has been divided into
various levels is coded into respective codes where the
samples that have the same number of level are coded into
the same code.
• The number of bits depends on the number of level used
to quantise the samples.
B = log2 M

28. PCM

29. DELTA MODULATION

30. DELTA MODULATION
• Next form of pulse modulation
• Transmits information only to indicate whether the
analog signal that is being encoded goes up or goes
down
• The Encoder Outputs are highs or lows that
“instruct” whether to go up or down, respectively
• DM takes advantage of the fact that voice signals
do not change abruptly

31. DELTA MODULATION

32. DELTA MODULATION
There are two problems associated with delta modulation that do
not occur with conventional PCM: slope overload and
granular noise.

33. DELTA MODULATION (Quantization
Errors)

34. SLOPE OVERLOAD
• When the analog input signal changes at a
faster rate than the DAC can maintain.
• The slope of the analog signal is greater than
the delta modulator can maintain and is called slope
overload.
• Increasing the clock frequency reduces the
probability of slope overload occurring.
• Another way to prevent slope overload is to
increase the magnitude of the minimum step size.

35. SLOPE OVERLOAD

36. GRANULAR NOISE
• When the original analog input signal has a
relatively constant amplitude, the reconstructed
signal has variations that were not present in the
original signal.

37. DELTA MODULATION (Quantization
Errors)

38. DELTA SIGMA MODULATION
The modulation which has an integrator can relieve the draw back of
delta modulation (differentiator)
Beneficial effects of using integrator:
1. Pre-emphasize the low-frequency content
2. Increase correlation between adjacent samples
(reduce the variance of the error signal at the quantizer input )
3. Simplify receiver design
Because the transmitter has an integrator , the receiver consists simply
of a low-pass filter. (The differentiator in the conventional DM receiver is
cancelled by the integrator )

39. DELTA SIGMA MODULATION

40. DELTA SIGMA MODULATION

41. Signal-to-Quantization-Noise Ratio
(SQNR or SNqR)
• is a measurement of the effect of quantization errors introduced by
analog-to-digital conversion at the ADC.
 Refer to page 421 - 422

42. MODEM (Modulation & Demodulation)

43. MODEM CONNECTION
PC PC
MODEM
MODEM
110011
110011
DCE DCE
DTE DTE

44. MODEM BLOCK DIAGRAM

45. MODEM BLOCK DIAGRAM

46. MODEM BLOCK DIAGRAM

47. COMMON MODEM USED:

48. MULTIPLEXING

49. MULTIPLEXING (MUX)
General multiplex scheme: the ν input lines-channels are multiplexed into a single fast
line. The demultiplexer receives the multiplexed data stream and extracts the original
channels to be transferred

50. MULTIPLEXING (MUX)

51. MULTIPLEXING (MUX)

52. FREQUENCY DIVISION MULTIPLEXING
FDM

53. TIME DIVISION MULTIPLEXING
TDM

54. WAVELENGTH DIVISION MULTIPLEXING

55. WDM

56. CDMA (CODE DIVISION MULTIPLE ACCESS)

57. CDMA

58. COMPARE BETWEEN TDM & FDM

59. COMPARE BETWEEN TDM & FDM
 The primary difference between FDM and TDM is how they divide the
channel. FDM divides the channel into two or more frequency ranges that do
not overlap, while TDM divides and allocates certain time periods to each
channel in an alternating manner.
 Due to this fact, we can say that for TDM, each signal uses all of the
bandwidth some of the time, while for FDM, each signal uses a small portion of
the bandwidth all of the time.
 TDM provides greater flexibility and efficiency, by dynamically allocating
more time periods to the signals that need more of the bandwidth, while
reducing the time periods to those signals that do not need it. FDM lacks this
type of flexibility, as it cannot dynamically change the width of the allocated
frequency.

60. COMPARE BETWEEN TDM & FDM
 The advantage of FDM over TDM is in latency. Latency is the time it
takes for the data to reach its destination.
 As TDM allocates time periods, only one channel can transmit at a given
time, and some data would often be delayed, though it’s often only in
milliseconds. Since channels in FDM can transmit at any time, their latencies
would be much lower compared to TDM.
 FDM is often used in applications where latency is of utmost priority, such
as those that require real-time information.
FDM and TDM are often used in tandem, to create even more channels in a
given frequency range. The common practice is to divide the channel with
FDM, so that you have a dedicated channel with a smaller frequency range.
Each of the FDM channels is then occupied by multiple channels that are
multiplexed using TDM. This is what telecoms do to allow a huge number of
users to use a certain frequency band.

61. COMPARE BETWEEN MUX & MULTIPLE
ACCESS

62. INFORMATION CAPACITY
•Is a measure of how much information can be propagated
through a communications system and is a function of
bandwidth and transmission time.
•Information capacity represents the number of independent
symbols that can carried through a system in a given unit of
time.
•The most basic digital symbol used to represent information
is the bit.

63. BIT, BIT RATE, BAUD, BANDWIDTH

64. BANDWIDTH

65. SHANNON’S LIMIT & M-ary ENCODING

66. DIGITAL MODULATION TECHNIQUES

67. Amplitude Shift Keying (ASK)

68. Frequency Shift Keying (FSK)

69. Phase Shift Keying (PSK)