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Chapter 4. 4.1 : Digital Modulation. 4.2 : Digital Transmission. 4.3 : Multiple Access Methods. 4.1 Digital Modulation. Outlines Introduction Information capacity, Bits, Bit Rate, Baud, M-ary Encoding Digital Modulation Techniques - ASK, FSK, PSK, QAM.

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chapter 4

Chapter 4

4.1 : Digital Modulation

4.2 : Digital Transmission

4.3 : Multiple Access Methods

4 1 digital modulation
4.1 Digital Modulation

Outlines

  • Introduction
  • Information capacity, Bits, Bit Rate, Baud,

M-ary Encoding

  • Digital Modulation Techniques

- ASK, FSK, PSK, QAM

slide3

Introduction to Digital Modulation

  • Digital modulation
    • Is the transmittal of digitally modulated analog signals between to or more points in a communications system.
    • Can be propagated through Earth’s atmosphere and used in wireless communication system - digital radio.
    • Offer several outstanding advantages over traditional analog system.
      • Ease of processing
      • Ease of multiplexing
      • Noise immunity
cont d
Cont’d...
  • Applications:
      • Low speed voice band data comm. modems
      • High speed data transmission systems
      • Digital microwave & satellite comm. systems
      • PCS (personal communication systems) telephone
why digital modulation
Why digital modulation?
  • The modulation of digital signals with analogue carriers allows an improvement in signal to noise ratio as compared to analogue modulating schemes.
important criteria
Important Criteria
  • High spectral efficiency

2. High power efficiency

3. Robust to multipath

4. Low cost and ease of implementation

5. Low carrier-to-co channel interference ratio

6. Low out-of-band radiation

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Cont’d…

7. Constant or near constant envelop

8. Bandwidth Efficiency

  • Ability to accommodate data within a limited bandwidth
  • Tradeoff between data rate and pulse width

9. Power Efficiency

  • To preserve the fidelity of the digital message at low power levels.
  • Can increase noise immunity by increasing signal power
forms of digital modulation
Forms of Digital Modulation
  • If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK)
  • If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)
cont d2
Cont’d…
  • If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK)
  • If both the amplitude and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)
example 1
Example 1

For the digital message 1101 1100 1010, sketch the waveform for the following:

a. ASK

b. FSK

c. PSK

d. QAM

block diagram
Block Diagram

Simplified block diagram of a digital modulation system

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Cont’d…
  • Precoder performs level conversion & encodes incoming data into group of bits that modulate an analog carrier.
  • Modulated carrier filtered, amplified & transmitted through transmission medium to Rx.
  • In Rx, the incoming signals filtered, amplified & applied to the demodulator and decoder circuits which extracts the original source information from modulated carrier.
slide14

Information Capacity, Bits,

Bit Rate, Baud, M-ary Encoding

  • Information capacity, Bits & Bit Rate
    • Represents the number of independent symbols that can be carried through a system in a given unit of time.
    • Basic digital symbol is the binary digit or bit.
    • Express the information capacity as a bit rate.
hartley s law
Hartley’s Law

Where

I = information capacity (bps)

B = bandwidth (Hz)

t = transmission time (s)

From the equation, Information capacity is a linear function of bandwidth and transmission time and directly proportional to both.

shannon s formula
Shannon’s Formula

Where

I = information capacity (bps)

B = bandwidth (Hz)

= signal to noise power ratio (unitless)

The higher S/N the better the performance and the higher the information capacity

example 2
Example 2

By using the Shannon’s Formula, calculate the information capacity if S/N = 30 dB and B = 2.7 kHz.

nyquist sampling rate
Nyquist Sampling Rate
  • fs is equal or greater than 2fm

fs >= 2fm

fs = minimum Nyquist sample rate (Hz)

fm = maximum analog input frequency (Hz)

example 3
Example 3

Determine the Nyquist sample rate for a maximum analog input frequency 7.5 kHz.

m ary encoding
M-ary Encoding
  • It is often advantageous to encode at a level higher than binary where there are more then two conditions possible.
  • The number of bits necessary to produce a given number of conditions is expressed mathematically as

Where N = number of bits necessary

M = number of conditions, level or combinations possible with N bits.

cont d5
Cont’d…
  • Each symbol represents n bits, and has M signal states, where M = 2N.
example 4
Example 4

Find the number of voltage levels which can represent an analog signal with

a. 8 bits per sample

b. 12 bits per sample

baud minimum bw
Baud & Minimum BW
  • Baud refers to the rate of change of a signal on the transmission medium after encoding and modulation have occurred.

Where

baud = symbol rate (symbol per second)

ts = time of one signaling element @ symbol (seconds)

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Cont’d…
  • Minimum Bandwidth
    • Using multilevel signaling, the Nyquist formulation for channel capacity

Where fb= channel capacity (bps)

B = minimum Nyquist bandwidth (Hz)

M = number of discrete signal or voltage levels

cont d7
Cont’d…

For B necessary to pass M-ary digitally modulated carriers

Where N is the number of bits encoded into each signaling element.

slide26

Digital Modulation Techniques

  • Amplitude Shift Keying (ASK)
  • Frequency Shift Keying (FSK)
  • Phase Shift Keying (PSK)
  • Quadrature Amplitude Modulation (QAM)
amplitude shift keying ask
Amplitude Shift Keying (ASK)
  • A binary information signal directly modulates the amplitude of an analog carrier.
  • Sometimes calledDigital Amplitude Modulation (DAM)

Where vask (t) = amplitude shift keying wave

vm(t) = digital information signal (volt)

A/2 = unmodulated carrier amplitude (volt)

ωc = analog carrier radian frequency (rad/s)

cont d8
Cont’d...

Digital Amplitude Modulation

frequency shift keying fsk
Frequency Shift Keying (FSK)
  • Called as Binary Frequency Shift Keying (BFSK)
  • The phase shift in carrier frequency (∆f) is proportional to the amplitude of the binary input signal (vm(t)) and the direction of the shift is determined by the polarity

Where vfsk(t) = binary FSK waveform

Vc = peak anlog carrier amplitude (volt)

fc = analog carrier center frequency (Hz)

∆f = peak shift in analog carrier frequency (Hz)

vm(t) = binary input signal (volt)

phase shift keying psk
Phase Shift Keying (PSK)
  • Another form of angle-modulated, constant amplitude digital modulation.
  • Binary digital signal input & limited number of output phases possible.
  • M-ary digital modulation scheme with the number of output phases defined by M.
  • The simplest PSK is Binary Phase-Shift Keying (BPSK)
    • N= 1, M=2
    • Two phases possible for carrier with one phase for logic 1 and another phase for logic 0
    • The output carrier shifts between two angles separated by 180°
cont d9
Cont’d...

a) Truth Table b) Phasor Diagram c) Constellation Diagram

cont d10
Cont’d...

BPSK Transmitter

cont d11
Cont’d...

BPSK Receiver

bandwidth efficiency
Bandwidth Efficiency
  • Used to compare the performance of one digital modulation technique to another.

Bη = Transmission bit rate (bps)

Minimum bandwidth (Hz)

example 5
Example 5

For 16-PSK system, operating with an information bit rate of 32 kbps, determine:

a. Baud

b. Minimum bandwidth

c. Bandwidth efficiency

conclusion
CONCLUSION
  • To decide which modulation method should be used , we need to make considerations of

a) Bandwidth

b) Speed of Modulation

c) Complexity of Hardware