2. Modulation Review

| University of Dar es Salaam | Department of Electronics and Telecommunications Engineering |

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Introduction to Wireless

Communications

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| TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 |

Communications

Modulation


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Outline

• Elements of digital communication systems

• Modulation

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• Modulation

– challenges

– tradeoffs

– schemes

– summary


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Digital Communication Systems

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Modulation

process of encoding information

from a message source in a

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Goal: transport message signal with the

best possible quality while occupying

the least amount of spectrum

from a message source in a

manner suitable for transmission


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Modulation Challenges

• High spectral efficiency

– How to achieve a large-capacity system using a limited spectrum

• High power efficiency

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– How to make small terminals

• High immunity to channel impairments

– E.g. immune to multipath fading for the mobile channel

• Good Quality of Service (QoS)

– Low BERs at low received SNRs

• Easy and cost-effective implementation


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Modulation Tradeoffs

• Existing modulation schemes do not simultaneously satisfy all of these requirements. Depending on the demands of the application, trade-offs are made when selecting a modulation scheme.

• Satellite and Deep Space Communication Systems

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• Satellite and Deep Space Communication Systems– Power is very expensive to generate in space and transmission

distances are enormous.

– So, satellite systems must be energy or power efficient.

• Mobile/Cellular Communication Systems– Power is costly (impacts battery life and size) but bandwidth is

also limited.

– Must be both bandwidth and power efficient.


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Analogue and Digital Modulation Schemes

• Analogue modulation– employed in first generation systems

• Digital modulation– proposed for use in present and future systems

– Advancements in VLSI and DSP have resulted in digital modulation

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– Advancements in VLSI and DSP have resulted in digital modulation

being more cost effective than analogue transmission systems

– Advantages:

• Greater noise immunity

• Robustness to channel impairments

• Easier multiplexing of various forms of information

• Greater security

• Two distinct categories – linear and nonlinear


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Linear Digital Modulation Schemes

• Amplitude of transmitted signal varies linearly with the modulating digital signal.

• Advantages:

– Bandwidth efficient (i.e. good spectral efficiency)

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– Bandwidth efficient (i.e. good spectral efficiency)

• Attractive for use in wireless communication systems where there is an increasing demand to accommodate more and more users within a limited spectrum

• Disadvantages:

– Generally, the envelope is not constant

• Need to transmit using linear RF amplifiers (which have poor power efficiency)

• E.g. QPSK


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Nonlinear or Constant Envelope Digital Modulation Schemes

• Amplitude of the carrier is constant, regardless of variation in the

modulating signal.

• Advantages:

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– Power efficient amplifiers can be used without introducing degradation

in the spectrum occupancy of the transmitted signal.

– Low out-of-band radiation is achievable.

– Limiter-discriminator detection can be used, simplifying receiver design.

• Disadvantages:

– Occupy larger bandwidth than linear modulation schemes.

• E.g. GMSK


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Combined Linear and Constant Envelope Digital Modulation Schemes

• Modern modulation techniques exploit the fact that digital baseband data may be sent by varying both the envelope and phase (or frequency) of an RF carrier.

• M-ary modulation maps baseband data into four or more possible RF carrier signals.

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• M-ary modulation maps baseband data into four or more possible RF carrier signals.

• Attractive for use in bandlimited channels, but limited in applicatins due to sensitivity to timing jitter.– Smaller distances between signals � poorer error performance.

• Better bandwidth efficiency at the expense of power efficiency.

• E.g. M-ary QAM


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Summary

• Modulation schemes fall into two distinct categories, those which

exhibit a constant power envelope and those that require some

degree of linear amplification.

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• If constant envelope amplification is required, then MSK or GMSK

represents an excellent compromise between power and spectral

efficiency.

• However, it is possible to receive MSK and GMSK as if they were

offset QPSK transmissions. This tends to blur the boundary

between these modulation schemes and demonstrates that MSK is

just a special case of QPSK exhibiting constant envelope.


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Summary

• If some degree of linearity is permitted in the transmitter then the

choice of modulation increases.

• At present the trend seems to be towards differentially or

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• At present the trend seems to be towards differentially or

discriminator detected root-raised cosine pi/4 QPSK. This is

currently being used in systems such as IS-54 and PHP.

• For satellite and microwave point to point communications, 16-QAM

is already very popular and there is much interest in applying these

ideas to the mobile radio market.


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Quiz

1)Draw the block diagram for generating a QPSK signal. Without any pulse filtering, sketch the resulting frequency domain.

2)Describe the various phase transients experienced in QPSK, why is pulse shaping (or filtering) so important for a practical QPSK transmission?

3)Explain the difference between offset and non-offset QPSK.

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4)How does pi/4 QPSK differ from standard QPSK, what advantages does the pi/4 constellation offer?

5)Draw the block diagram for a GMSK modulator, what is the role of the Gaussian filter?

6)Why is GMSK often preferred to QPSK?, what are the advantages and disadvantages of GMSK vs QPSK? Why should amplifier linearity be considered?

7)Why does the quiz focus on QPSK and GMSK?

2. Modulation Review

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