06/01/20101

Modulation Techniques

Wireless CommunicationsPrinciples and Practice

Theodore S. Rappaport SESE

Dr. Abdel-Rahman al-Qawasmi

MSK, GMSK and Spread Spectrum MSK, GMSK and Spread Spectrum TechniquesTechniques

06/01/20102

Digital Modulation Techniques for Mobile Radio

Digital Modulation Techniques Digital Modulation Techniques for Mobile Radiofor Mobile Radio

Digital ModulationDigital Modulation:

1. Provides low bit errors at low received SNRs.

2. Performs well in multipath and fading.

3. Occupies a minimum of bandwidth.

4. Is easy and cost-effective to implement.

The Performance of Digital Modulation Schemes is often measured in terms:

••Power efficiencyPower efficiency and

••Bandwidth efficiencyBandwidth efficiency.

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Digital Modulation Techniques for Mobile Radio

Digital Modulation Techniques Digital Modulation Techniques for Mobile Radiofor Mobile Radio

Power efficiency is expressed as the ratio of the signal energy per bit and to noise power spectral density required at the receiver input for a certain probability of error.

0NEb

p =η

Bandwidth efficiency is the ability of modulation technique to accommodate data within a limited bandwidth.

HzbpsBR

B /=η

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Digital Modulation Techniques for Mobile Radio

Digital Modulation Techniques Digital Modulation Techniques for Mobile Radiofor Mobile Radio

+==

NS

BC

B 1log2η

The Shannon's Channel Capacity (C) for AWGN non-Fading

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Minimum Shift Keying (MSK)Minimum Shift Keying (MSK)Minimum Shift Keying (MSK)•Is a special type of continuous phase-frequency shift keying. The peak frequency modulation is equal to ¼ the bit rate.

•The modulation index kFSK equals to ½.

bFSK R

fk ∆=

2Peak DeviationPeak Deviation

A modulation index of ½ corresponds to the minimum frequency spacing that allows to FSK signals to be coherently orthogonal, and the name minimum shift keying implies the minimum frequency separation that allows orthogonal detection

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Minimum Shift Keying (MSK)Minimum Shift Keying (MSK)Minimum Shift Keying (MSK)

•MSK is a spectrally efficient modulation scheme and is particularly attractive for use in Mobile radio communications systems.

•MSK has a constant envelope, spectral efficiency, good BER performance, and self-synchronizing capability.

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MSK Power SpectrumMSK Power SpectrumMSK Power SpectrumThe normalized power spectral density for MSK

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MSK TransmitterMSK TransmitterMSK Transmitter

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MSK ReceiverMSK ReceiverMSK Receiver

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Gaussian Minimum Shift KeyingGaussian Minimum Shift KeyingGaussian Minimum Shift KeyingGMSK is a binary modulation scheme by which the sidelobe levels of the spectrum are further reduced by passing the modulating NRZ data waveform through a premodulation Gaussian pulse-shaping filter.

Baseband Gaussian pulse shaping smoothes the phase trajectory of the MSK signal and hence stabilize the instantaneous frequency variations over time.

The GMSK premodulation filter has an impulse response

And transfer function

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Gaussian Minimum Shift KeyingGaussian Minimum Shift KeyingGaussian Minimum Shift KeyingThe parameter α is related to B

The 3-dB bandwidth-bit duration product (BT) of the filter is greater than 0.5.

The GMSK filter may be completely defined from B and baseband symbol duration.

Reducing BT increases the error rate produced by the lowpass filter due to ISI.

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Gaussian Minimum Shift KeyingGaussian Minimum Shift KeyingGaussian Minimum Shift Keying

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GMSK Bit Error RateGMSK Bit Error RateGMSK Bit Error Rate

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GMSK TransmitterGMSK TransmitterGMSK TransmitterThe simplest way to generate a GMSK signal is to pass a NRZ message bit stream through a Gaussian baseband filter having an impulse response

This modulation technique is used in GSM systems

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GMSK ReceiverGMSK ReceiverGMSK Receiver

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Gaussian Minimum Shift KeyingGaussian Minimum Shift KeyingGaussian Minimum Shift Keying

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Spread Spectrum Modulation Techniques

Spread Spectrum Modulation Spread Spectrum Modulation TechniquesTechniques

•This system is bandwidth inefficient system for a single user but using this technique, many users can use the same bandwidth at the same time without interference in multiple access environment.

• The spreading wave form is controlled by a pseudo-noise (PN) code, which is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers.

•Spread spectrum signals are demodulated at the receiver through cross-correlation with a locally-generated version of the pseudorandom carrier to restore the modulated narrowband signal.

06/01/201019

Spread Spectrum Modulation Techniques

Spread Spectrum Modulation Spread Spectrum Modulation TechniquesTechniques

Spread spectrum well-suited for use in the mobile environment because:

1.Its inherent interference rejection in the mobile environment.

2.Resistance to multipath fading.

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Pseudo-Noise (PN) Sequences PseudoPseudo--Noise (PN) Sequences Noise (PN) Sequences

1. PN sequence is a binary sequence that has a very low correlation between shifted versions of the sequence and very low cross-correlation between any two sequences.

2. In a feedback shifted register, binary sequences are shifted through the shift registers in response to clock pulses, and the output of the various stages are logically combined and fed back as the input to the first stage.

06/01/201021

Direct Sequence Spread Spectrum (DS-SS)

Direct Sequence Spread Direct Sequence Spread Spectrum (DSSpectrum (DS--SS) SS)

DS-SS system spreads the baseband data by directly multiplying the baseband data pulses with a PN sequence that is produced by PN code generator.

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Direct Sequence Spread Spectrum (DS-SS)

Direct Sequence Spread Direct Sequence Spread Spectrum (DSSpectrum (DS--SS) SS)

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Frequency Hopped Spread Spectrum (FH-SS)

Frequency Hopped Spread Frequency Hopped Spread Spectrum (FHSpectrum (FH--SS) SS)

•A frequency hopping signalfrequency hopping signal is a sequence of modulated data bursts with time-varying, pseudorandom carrier frequencies.

••HopsetHopset is the set of possible carrier frequency.

•The bandwidth of a channel used in a hopset is called instantaneous instantaneous bandwidthbandwidth.

•The bandwidth of the spectrum over which the hopping occurs is called the total hopping bandwidthtotal hopping bandwidth.

•Data is sent by hopping the transmitter carrier to seemingly random channels which known only to the desired receiver.

•Frequency hopping may be classified as FastFast or SlowSlow.

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Frequency Hopped Spread Spectrum Transmitter

Frequency Hopped Spread Frequency Hopped Spread Spectrum Transmitter Spectrum Transmitter

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Frequency Hopped Spread Spectrum Receiver

Frequency Hopped Spread Spectrum Frequency Hopped Spread Spectrum Receiver Receiver

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Performance of DS-SSPerformance of DSPerformance of DS--SSSS

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Performance of DS-SSPerformance of DSPerformance of DS--SSSSA DS-SS systems with kk multiple access users (as shown in Figures) assuming that each user has a PN sequence with NN chips per message symbol period TTsuch that NTNTcc=T=T. Where the transmitted signal of the kthkth user

The average probability of bit error given by

And for single user k=1 is (BPSK)

=

0

2NEQP b

e

For the interference limited case and thermal noise=0

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Performance of FH-SSPerformance of FHPerformance of FH--SSSS

For BFSK

For M possible hoping channels (slots)

For thermal noise=0