Chapter 8 COMMUNICATION SYSTEMS tín hiệu và hệ thống .pptx

COMMUNICATION SYSTEMS trong quyển tín hiệu và hệ thống

1. Chapter 8 COMMUNICATION SYSTEMS

2. 8.1 COMPLEX EXPONENTIAL AND SINUSOIDAL AMPLITUDE MODULATION Amplitude Modulation (AM) involves multiplying the baseband signal with the carrier : where : modulating signal, : carrier signal. Purpose of Modulation: Shift the signal to a frequency band suitable for radio transmission. Enable long-distance transmission in higher frequency bands (300 MHz to 300 GHz, satellite bands up to 40 GHz).  

3. 8.1.1 Amplitude Modulation with a Complex Exponential Carrier Carrier signal : Modulated signal: Demodulation: In the frequency domain (by the Fourier transform multiplication property):  The spectrum of is the spectrum of , shifted by .   a, the spectrum of modulating signal ) b, the spectrum of ) c, the spectrum of amplitude-modulated signal  

4. 8.1.2 Amplitude Modulation with a Sinusoidal Carrier Carrier: for : In the frequency domain: If , can be recovered from If , spectral overlap occurs, and exact recovery is impossible.   Effect in the frequency domain of amplitude modulation with a sinusoidal carrier: spectrum of modulating signal ; spectrum of carrier ; spectrum of amplitude-modulated signal.   Sinusoidal amplitude modulation with carrier for which : spectrum of modulating signal; spectrum of modulated signal.  

5. 8.2 DEMODULATION FOR SINUSOIDAL AM 8.2.1 Synchronous Demodulation At the receiver in a communication system, the information-bearing signal is recovered through demodulation If , spectra overlap, making accurate recovery impossible. For , for : Demodulation is performed by multiplying with and applying a low-pass filter (LPF): The LPF extracts the component, which is amplified by 2. This process is called synchronous demodulation, requiring phase synchronization.   Demodulation of an amplitude-modulated signal with a sinusoidal carrier: spectrum of modulated signal; spectrum of carrier signal; spectrum of modulated signal multiplied by the carrier. The dashed line indicates the frequency response of a lowpass filter used to extract the demodulated signal

6. 8.2.2 Asynchronous Demodulation (Envelope Demodulation) Asynchronous demodulation avoids the need for synchronization between the modulator and d emodulator, When , the envelope of approximates . Envelope detection can be done with a simple diode circuit. Conditions for envelope detection: require: (constant) varies much slower than .   (a) Amplitude-modulated signal for which the modulating signal is positive. The dashed curve represents the envelope of the modulated signal. waveforms associated with the envelope detector in (a): is the half-wave rectified signal, is the true envelope, and is the envelope obtained from the circuit in (a). The relationship between and has been exaggerated in (b) for purposes of illustration. In a practical asynchronous demodulation system, would typically be a much closer approximation to than depicted here  

7. 8.2.2 Asynchronous Demodulation (Envelope Demodulation) Modulation Index: Where is the maximum amplitude of and is the DC offset ensuring . (A must be greater than K so that ) The diagram illustrates cases where the modulation index is less than or greater than 1. Accurate recovery is possible only when .   Modulator for an asynchronous modulation-demodulation system. Output of the amplitude modulation system when: m = 0,5 (or m < 1) m = 1.0 (or m > 1)

8. 8.3 FREQUENCY-DIVISION MULTIPLEXING Many systems used for transmitting signals provide more bandwidth than is required for any one signal. If the individual signals, which are overlapping in frequency, have their frequency content shifted by means of sinusoidal amplitude modulation so that the spectra of the modulated signals no longer overlap  they can be transmitted simultaneously over a single wideband channel.  frequency-division multiplexing (FDM). Frequency-division multiplexing using sinusoidal amplitude modulation. Spectrum associated with the frequency-division multiplexing system

9. 8.3 FREQUENCY-DIVISION MULTIPLEXING To recover the individual channels in the demultiplexing process requires two basic steps: bandpass filtering to extract the modulated signal corresponding to a specific channel demodulation to recover the original signal Demultiplexing and demodulation for a frequency-division multiplexed signal

10. 8.4 SINGLE-SIDEBAND SINUSOIDAL AMPLITUDE MODULATION (SSB) In conventional sinusoidal AM, the modulated signal occupies twice the bandwidth of the original signal 𝑥(𝑡), since energy appears around both and  Using single-sideband modulation technique to remove the redundancy   Double- and single- sideband modulation: spectrum of modulating signal; spectrum after modulation with a sinusoidal carrier; spectrum with only the upper sidebands; spectrum with only the lower sidebands. System for single-sideband amplitude modulation, using a phase-shift network, in which only the lower sidebands are retained   System for retaining the upper sidebands using ideal highpass filtering

11. 8.5 AMPLITUDE MODULATION WITH A PULSE-TRAIN CARRIER Carier : Modulated signal:   Amplitude modulation of a pulse train.

12. 8.6 PULSE-AMPLITUDE MODULATION (PAM) Transmitted waveform for a single PAM channel. The dotted curve represents the signal   Transmitted waveform with three time-multiplexed PAM channels. The pulses associated with each channel are distinguished by shading, as well as by the channel number above each pulse. Here, the intersymbol spacing is .  

13. 8.7 SINUSOIDAL FREQUENCY MODULATION Angle modulation: Carrier: , with instantaneous phase: where is the frequency and the phase of the carrier  Angle modulation is to use the modulating signal to vary the phase so that the modulated signal takes the form Phase Modulation (PM): the phase is proportional to Frequency Modulation ( FM): The derivative of the phase is proportional to : Relationship between PM and FM:  

14. Phase modulation, frequency modulation, and their relationship: phase modulation with a ramp as the modulating signal; frequency modulation with a ramp as the modulating signal; frequency modulation with a step (the derivative of a ramp) as the modulating signal

15. 8.8 DISCRETE-TIME MODULATION Consider a discrete complex exponential carrier: Discrete-time AM modulation: Demodulation is done by multiplying with : In the frequency domain: With: Demodulation by multiplying by :   Discrete-time amplitude modulation Spectrum of ; spectrum of ; spectrum of .