This presentation include the basic concept of communication, modulation techniques in analog and digital. ADC (Analog to Digital Conversion) and Demodulation schemes

1. SHRI RAM MURTI SMARAK COLLEGE OF ENGG. &
TECH. BAREILLY
PRESENTATION ON
FUNDAMENTALS OF COHERENT SYSTEMS
SUBMITTED TO- SUBMITTED BY-
MR. DESHRAJ SHAKYA FARHA ZEBA
ASSISTANT PROFESSOR M.TECH(MICROWAVE)
SRMSCET, BAREILLY BATCH- 2016

2. CONTENTS
 BASIC CONCEPT OF COMMUNICATION
 MODULATION
 CONVERSION OF ANALOG SIGNAL TO DIGITAL SIGNAL
 CHANNEL MULTIPLEXING
 MODULATION FORMAT
 DEMODULATION SCHEMES
 SYSTEM PERFORMANCE

3. 1. BASIC CONCEPT OF COMMUNICATION
 COMMUNICATION-
Transfer of information from one place to another. It should be efficient, reliable and secured.
 ELEMENTS OF COMMUNICATION SYSTEM-
Communication system consists of components or subsystems that act together to accomplish
information transfer.

4.  ANALOG AND DIGITAL SIGNALS-
 Analog Signal
 Continuous variation.
 Assumes all range of frequency or time.
 All information is transmitted.
 Digital signal
 Takes samples
 Non-continuous stream of on/off pulses
 Translates to 1s and 0s.

5. 2. MODULATION
Modulation is the addition of information to an electronic or optical
carrier signal by varying its amplitude, frequency and phase.
• Baseband Signal-
Base band signal is the modulating signal either in a digital or
analog form in communication system.
• Modulated Signal-
Modulated signal is baseband signal which its original frequency is
shifted to higher frequency to facilitate transmission purposes.
• Process of Modulation
 Continuous wave modulation such as AM, FM, PM.
 Sampling and coding such as PAM, PCM, PWM and PPM etc.
 Keying such as ASK, FSK,PSK.

6. AMPLITUDE MODULATION
 Amplitude of the carrier is changed in accordance with the
instantaneous value of the modulating signal.
 Carrier : c(t) = 𝑉𝐶 cos(2𝜋𝑓𝑐 𝑡)
Modulating signal : m(t) = 𝑉𝑚 cos( 2𝜋𝑓𝑚 𝑡)
 Information is contained in the envelope.
 Modulated signal : v(t) = 𝑉𝐶 cos( 2𝜋 𝑓𝑐 𝑡){1 + 𝐾𝑎 𝑉𝑚cos( 2𝜋𝑓𝑚 𝑡)}
FREQUENCY MODULATION
 Variation of d𝜃/𝑑𝑡 produces frequency modulation.
 𝑓𝑖(t) = 𝑓𝑐(t) + 𝐾𝑓m(t)
 𝜃𝑖(t) = 2𝜋𝑓𝑐(t) + 2𝜋 𝐾𝑓 0
𝑡
𝑚(𝑡) dt
 S(t) = 𝐴 𝑐 𝑐𝑜𝑠𝜃𝑖t
 S(t) = 𝐴 𝑐 cos[2𝜋𝑓𝑐t + 2𝜋 𝐾𝑓 0
𝑡
𝑚(𝑡) dt]
 =𝐴 𝑐 cos[𝜔𝑐t + ∆𝑓/𝑓 sin 𝜔 𝑚 𝑡]

7. PHASE MODULATION
 Phase modulation (PM) is a form of modulation that
represents information as variations in the
instantaneous phase of a carrier wave.
 PM changes the phase angle of the complex envelope
in direct proportion to the message signal.
 𝜃𝑖(t) = 2𝜋𝑓𝑐(t) + 𝐾 𝑝 𝑚(𝑡)
where 𝐾 𝑝 is sensitivity of phase modulation
 S(t) = 𝐴 𝑐 cos[2𝜋𝑓𝑐t + 𝐾 𝑝m(t)]

8. 3. CONVERSION OFANALOG SIGNAL TO DIGITAL SIGNAL


9. SAMPLING TECHNIQUES
 Quantized sample value can be converted into digital format by using suitable
conversion techniques such as
 PWM (Pulse Width Modulation)
 PPM (Pulse Position Modulation)
 PCM (Pulse Code Modulation)
PWM
In this type the amplitude is maintained constant but duration or width or length of each
pulse is varied in accordance with the instantaneous value of the analog signal.

10. PPM
In this type, the sampled waveform has fixed amplitude
and width whereas the position of each pulse is varied as
per instantaneous value of the analog signal.
PWM
PPM
PAM
PWM
PPM

11. PCM
Analog signal is converted into digital signal by using a digital code. Each
quantized level is represented by a code which is converted into binary code
before transmission.
Relation between Quantized level and no. of bits-
M = 2 𝑚 or m = 𝑙𝑜𝑔2 𝑀
Bit rate is given by B = m𝑓𝑠 ≥ 2∆𝑓𝑙𝑜𝑔2 𝑀
Where, 𝑓𝑠 ≥ 2∆𝑓
B >(∆𝑓/3)SNR

12. 4. CHANNEL MULTIPLEXING
 Most fiber-optic communication systems are capable
of transmitting at a rate of more than 1 Gb/s.
 Many channels are transmitted simultaneously
through multiplexing.
 Two types of multiplexing are discussed i.e. FDM
and TDM.
FDM
FDM is an analog multiplexing technique that combines
analog signals. It uses the concept of modulation.

13. TDM
TDM is a digital multiplexing technique for combining several low-rate digital
channels into one high-rate one.
 Time division multiplexing
 Synchronous Time division multiplexing

14. Example:
 The bit slot is about 15 μs for a single voice channel
operating at 64 kb/s.
 Five such channels can be multiplexed and delayed by 3
μs.
 In the case of FDM, the channels are spaced apart in the
frequency domain.
 Carrier frequencies are spaced more than the channel
bandwidth to avoid overlapping.

15.  In North America and Japan, the first level corresponds to
multiplexing of 24 voice channels with a composite bit rate of 1.544
Mb/s (hierarchy DS-1).
 The second-level hierarchy is obtained by multiplexing 4 DS-1 TDM
channels. This results in a bit rate of 6.312 Mb/s (hierarchy DS-2) for
North America or Japan and 8.448 Mb/s for Europe.
 The lack of an international standard led to the advent of a new
standard, first called the synchronous optical network (SONET) and
later termed as synchronous digital hierarchy or SDH.

16. 5. MODULATION FORMAT
 There are two choices for the modulation format for
converting into optical bit stream.
1. Return-to-zero (RZ)- Amplitude of bit 1 returns to zero
before the bit duration is over.
2. Non return-to-zero (NRZ)- Amplitude does not drop to zero
between two or more successive 1 bits.
 An advantage of the NRZ format is that the bandwidth
associated with the bit stream is smaller than that of the RZ
format by about a factor of 2.
 The RZ format is provided by the dispersion-managed soliton
systems where a chirped pulse propagates inside the fiber link
in a periodic fashion known as CRZ(Chirped Return to Zero).

17.  The optical carrier wave before modulation
is of the form
E(t) = ˆeAcos(ωt +φ ), where E is the
electric field vector, ˆe is the polarization unit
vector.
 The modulation techniques in the digital
case and are called amplitude-shift keying
(ASK), frequency-shift keying (FSK), and
phase shift keying (PSK).
 The simplest technique consists of simply
changing the signal power between two
levels, one of which is set to zero, and is
often called on–off keying (OOK).

18. 6. Demodulation Schemes


19.  The receiver noise is included through
 The resulting baseband signal is
 Synchronous demodulation requires recovery of the microwave carrier at
the intermediate frequency 𝑤𝐼𝐹.

20. 1.

21. 7. SYSTEM PERFORMANCE
 This section focuses on the system performance issues while reviewing the
state of the art of coherent lightwave systems.
 Asynchronous Heterodyne Systems-
 Experiments have been performed with the ASK, FSK, and DPSK modulation
formats.
 An ASK experiment showed a baseline sensitivity (without the fiber) of 175
photons/bit at 4 Gb/s.
 The sensitivity degraded by only 1 dB when the signal was transmitted through
160 km.
 With the DPSK format using a LiNbO3 phase modulator the baseline receiver
sensitivity is at 4Gb/s was 209 photons/bit and degraded by 1.8 dB

22.  Synchronous Heterodyne Systems-
 They are also more difficult to implement as the microwave carrier must be
recovered from the received data for synchronous demodulation.
 Since the sensitivity advantage is minimal for ASK and FSK formats.
 Experiments have focused on the PSK format for which the receiver sensitivity
is only 18 photons/bit.
 Homodyne Systems-
 Homodyne systems with the PSK format offer the best receiver sensitivity as
they require, in principle, only 9 photons/bit.
 PSK homodyne systems the receiver sensitivity achieved in these experiments
depends on the bit rate at 140 Mb/s.

23. THANK YOU