WIRELESS COMMUNICATION - MULTIPLE ACCESS TECHNIQUES

EC3501 WIRELESS COMMUNICATION UNIT III: MODULATION TECHNIQUES AND EQUALIZATION AND DIVERSITY


UNIT- III

MODULATION TECHNIQUES AND EQUALIZATION AND DIVERSITY

Digital Modulation – An Overview: Factors That Influence The Choice Of Digital Modulation, Linear
Modulation Techniques: Minimum Shift Keying (MSK), Gaussian Minimum Shift Keying(GMSK),
Spread Spectrum Modulation Techniques: Pseudo- Noise (PN) Sequences, Direct Sequence Spread
Spectrum (DS-SS)- Modulation Performance In Fading And Multipath Channels- Equalization,
Diversity And Channel Coding: Introduction-Fundamentals Of Equalization- Diversity Techniques:
Practical Space Diversity Considerations, Polarization Diversity, Frequency Diversity, Time Diversity.

3.1 Digital Modulation – An Overview
3.1.1 Factors That Influence The Choice Of Digital Modulation
3.2 Linear Modulation Techniques
3.2.1 Binary Phase Shift Keying (BPSK)
3.2.2 Differential Phase Shift Keying (DPSK)
3.2.3 Quadrature Phase Shift Keying (QPSK)
3.2.4 QPSK Transmission and Detection Techniques
3.2.5 Offset QPSK
3.2.6  / 4 QPSK
3.2.7  / 4 QPSK Transmission Techniques
3.2.8  / 4 QPSK Detection Techniques
3.3 Minimum Shift Keying (MSK)
3.4 Gaussian Minimum Shift Keying (GMSK)
3.5 Spread Spectrum Modulation Techniques
3.5.1 Pseudo- Noise (PN) Sequences
3.5.2 Direct Sequence Spread Spectrum (DS-SS)
3.6 Modulation Performance In Fading And Multipath Channels
3.7 Equalization, Diversity and Channel Coding
3.7.1 Introduction
3.7.2 Fundamentals Of Equalization
3.8 Diversity Techniques
3.8.1 Practical Space Diversity Considerations
3.8.1.1 Selection Diversity
3.8.1.2 Feedback or Scanning Diversity
3.8.1.3 Maximal Ratio Combining
3.8.1.4 Equal Gain Combining
3.8.2 Polarization Diversity
3.8.3 Frequency Diversity
3.8.4 Time Diversity




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, EC3501 WIRELESS COMMUNICATION UNIT III: MODULATION TECHNIQUES AND EQUALIZATION AND DIVERSITY


3.1 Digital Modulation – An Overview
 Modern mobile communication systems use digital modulation techniques.
 Digital modulation more cost effective than analog transmission systems.
 Some advantages are
o greater noise immunity and robustness to channel impairments
o easier multiplexing of various forms of information (e.g., voice, data, and video), and
o greater security.
 Digital transmissions accommodate
o digital error-control codes which detect and or correct transmission errors, and
o support complex signal conditioning and processing techniques (source coding, encryption, and
quantization).
 The modulating signal (e.g., the message) may be represented as a time sequence of symbols or pulses.
 Each symbol represents n bits of information, where n = log2m, bits/symbol.


3.1.1 Factors that influence the choice of Digital Modulation:
Explain the factors that influence the choice of Digital Modulation.
 Several factors influence the choice of a digital modulation scheme.
 Some modulation schemes are better in terms of the bit error rate performance, while others are better
in terms of bandwidth efficiency.

 The performance of a modulation scheme is often measured in terms of its power efficiency and
bandwidth efficiency.
 Power efficiency describes the ability of a modulation technique to preserve the fidelity of the digital
message at low power levels.
 The power efficiency,  p (sometimes called energy efficiency) of a digital modulation scheme is a
measure of how favorably this tradeoff between fidelity and signal power is made.
 It is expressed as the ratio of the signal energy per bit to noise power spectral density (Eb/N0) required
at the receiver input.

 Bandwidth efficiency describes the ability of a modulation scheme to accommodate data within a
limited bandwidth.
 In general, increasing the data rate implies decreasing the pulse width of a digital symbol which
increases the bandwidth of the signal.
 Thus, there is an unavoidable relationship between data rate and bandwidth occupancy.
 However, some modulation schemes perform better than the others in making this tradeoff.
 Bandwidth efficiency reflects how efficiently the allocated bandwidth is utilized.
 Bandwidth efficiency is defined as the ratio of the throughput data rate per Hertz in a given bandwidth.



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, EC3501 WIRELESS COMMUNICATION UNIT III: MODULATION TECHNIQUES AND EQUALIZATION AND DIVERSITY


If R is the data rate in bits per second, and B is the bandwidth occupied by the modulated 1W signal,
then bandwidth efficiency  B is expressed as
R
B  bps / Hz
B

 The system capacity of a digital mobile communication system is directly related to the bandwidth
efficiency of the modulation scheme.

 By Shannon's channel coding theorem, the channel capacity formula.
C  S
 B max   log 2 1  
B  N

where C is the channel capacity (in bps), B is the RF bandwidth, and S/N is the signal-to-noise ratio.

 In the design of a digital communication system, very often there is a tradeoff between bandwidth
efficiency and power efficiency.

 Sensitivity to detection of timing jitter, caused by time-varying channels, is also an important
consideration in choosing a particular modulation scheme.
3.2 Linear Modulation Techniques
 Digital modulation techniques may be broadly classified as linear and non-linear.
 In linear modulation techniques, the amplitude of the transmitted signal, s(t), varies linearly with the
modula6ng digital signal, m(t).
 Linear modulation techniques are bandwidth efficient.

 In a linear modulation scheme, the transmitted signal s(t) can be expressed as
s(t )  Re[ A m(t ) exp( j 2fct )]
 A [mR (t ) cos(2fct )  mI (t ) sin( 2fct )]
where A is the amplitude, fc is the carrier frequency, and m(t )  mR (t )  jmI (t ) is a complex envelope
representation of the modulated signal.

 From the equation, the amplitude of the carrier varies linearly with the modulating signal.
 Linear modulation schemes, in general, do not have a constant envelope.
 The most popular linear modulation techniques include pulse-shaped QPSK, OQPSK, and n/4 QPSK,
which are discussed subsequently.

3.2.1 Binary Phase Shift Keying (BPSK)
Explain in detail about Binary Phase Shift Keying (BPSK).
 In binary phase shift keying (BPSK),
o the phase of a constant amplitude carrier signal is switched between two values according to the
two possible signals m1 and m2, corresponding to binary 1 and 0, respectively.

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, EC3501 WIRELESS COMMUNICATION UNIT III: MODULATION TECHNIQUES AND EQUALIZATION AND DIVERSITY



 Normally, the two phases are separated by 180 .
1 2
 If the sinusoidal carrier has an amplitude Ac and energy per bit Eb  Ac Tb , then the transmitted
2
BPSK signal is either

2 Eb
S BPSK (t )  cos(2f c t   c ) 0  t  Tb (binary 1)
Tb

Or

2 Eb
S BPSK (t )  cos(2f c t     c )
Tb

2 Eb
 cos(2f c t   c ) 0  t  Tb (binary 0)
Tb

 It is often convenient to generalize m1 and m2 as a binary data signal m(t), which takes on one of two
possible pulse shapes.
 Then the transmitted signal may be represented as

2 Eb
S BPSK (t )  m(t ) cos(2f c t   c )
Tb

 The BPSK signal is equivalent to a double sideband suppressed carrier amplitude modulated waveform,
where cos(2f c t ) is applied as the carrier, and the data signal m(t) is applied as the modulating
waveform.
 Hence a BPSK signal can be generated using a balanced modulator.


Spectrum and Bandwidth of BPSK
 The BPSK signal using a polar baseband data waveform m(t) can be expressed in complex envelope
form as

S BPSK  Re g BPSK (t ) exp( j 2f ct ) 
where g BPSK (t ) is the complex envelope of the signal given by
2 Eb
g BPSK (t )  m(t )e j c
Tb
 The power spectral density (PSD) of the complex envelope can be shown to be
2
 sin fTb 
Pg BPSK ( f )  2 Eb  
 fTb 
 The PSD of a BPSK signal at 1W is given by




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