Chpt 5 Bandpass - Chapter 5 Digital Bandpass Modulation and Demodulation Techniques 1 Motivations and Goals In a radio system the signals are often

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1 Chapter 5 Digital Bandpass Modulation and Demodulation Techniques
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2 Motivations and Goals In a radio system, the signals are often modulated to radio frequency (typically at very high frequency range). For this reason, we will study the modulation and demodulation principles related to bandpass systems. The following issues are important for bandpass systems. - Digital bandpass modulation - Receiver design and performance - Non-coherent detection and differential method - High order modulation techniques. f |S ( f )| 2 f |S ( f )| 2 A typical baseband spectrum: A typical bandpass or RF spectrum:
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3 We Need Two Signals for Each Bit 01
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4 Bandpass Binary Keying Methods 01 Binary phase shift keying (BPSK) Frequency shift keying (FSK) On-off amplitude shift keying (on-off ASK) Amplitude shift keying (ASK) (1) The frequency of the sin signal involved is called carrier frequency. (2) Which method is more prone to error here? (Just by looking.)
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5 Binary Phase Shift Keying (BPSK) A binary phase shift keying (BPSK) waveform is shown below. The two waveforms are sin(2 π f c t ) and –sin(2 f c t ) and f c is the carrier frequency. Baseband signal BPSK signal BPSK signal can be regarded as a special case of the bipolar system discussed earlier with s ( t )= sin( 2 f c t ). Therefore the same optimal receiver discussed earlier can be used here for BPSK. The error rate is given by τ t 10 1 00 t bs ( t ) reference s ( t ) 0 t s ( t ) decision device y bs 2 ( t ) z 0 2 BER ( ) b E Q N =
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6 Spectrum of a BPSK Signal A BPSK signal can be seen as a rectangular pulse multiplied by a cos signal. Therefore we can use the modulation property to derive its spectrum. Modulation property: s ( t )sin(2 π f c t ) [ S ( f - f c ) -S ( f+f c )]/(2 j ). The spectrum of the rectangular pulse is A τ sinc( f c t ). Thus the magnitude spectrum of the BPSK pulse is 0.5| A sinc( π( f+f c ) t )+ A sinc( f-f c ) t )|. 0 t s ( t ) = x t sin (2 f c t ) 0 t 1 A τ/2 f f c 2/τ -f c 2/τ
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7 Frequency Property of Modulation Time domain Frequency domain s ( t ) =m ( t )sin(2 π f c t ) |M ( f )| t t f t sin(2 f c t ) f 1/2 m ( t ) -f c -f c |S ( f )| f -f c c 1 1/2
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8 More General Modulation Property Time domain Frequency domain m(t) s ( t ) =m ( t )sin(2 π f c t ) |M ( f )| |S ( f )| t t f f t sin(2 f c t ) f 1/2 1 1/2
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9 On-off Amplitude-Shift Keying (ASK) On-off ASK is equivalent to multiplying an on-off baseband signal by a cos signal. We can use our knowledge on the on-off signal together with modulation property to find on-off ASK spectrum. The resultant amplitude-modulated waveform is shown below. Baseband signal On-off ASK signal Detection of on-off ASK can be carried out similar to that of BPSK , except that the decision threshold should be now a nonzero value similar to that of on-off signaling. The optimal performance for on-off ASK is the same as that for baseband on-off signaling, τ t 10 1 00 t 0 BER ( ) b E Q N =
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10 Bipolar and On-Off Signals Baseband signals Bandpass signals Bandpass magnitude spectrums before modulation after modulation after modulation f -f c -f c f -f c c f -f c c + 0.5 + = 0.5 0.5 0.5 = 1 1
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11 Frequency-Shift Keying (FSK) FSK is equivalent to FM with NRZ pulses as input. In FSK, the carrier
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This note was uploaded on 01/11/2011 for the course EE 3008 taught by Professor Pingli during the Fall '08 term at City University of Hong Kong.

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Chpt 5 Bandpass - Chapter 5 Digital Bandpass Modulation and Demodulation Techniques 1 Motivations and Goals In a radio system the signals are often

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