Digit_Modulat - 29/01/2003 Property of R. Struzak 1 Digital...

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Unformatted text preview: 29/01/2003 Property of R. Struzak 1 Digital Modulation Basics 29/01/2003 Property of R. Struzak 2 Outline Introduction to digital modulation Relevant modulation schemes Geometric representations Coherent & Non-Coherent Detection Modulation spectra 29/01/2003 Property of R. Struzak 3 Modulation & Demodulation Baseband Modulation Carrier Radio Channel Synchronization/Detection/ Decision Carrier Data in Data out 29/01/2003 Property of R. Struzak 4 Modulation Modulation - process (or result of the process) of translation the baseband message signal to bandpass (modulated carrier) signal at frequencies that are very high compared to the baseband frequencies. Demodulation is the process of extracting the baseband message back the modulated carrier. An information-bearing signal is non- deterministic, i.e. it changes in an unpredictable manner. 29/01/2003 Property of R. Struzak 5 Why Carrier? Effective radiation of EM waves requires antenna dimensions comparable with the wavelength: Antenna for 3 kHz would be ~100 km long Antenna for 3 GHz carrier is 10 cm long Sharing the access to the telecommunication channel resources 29/01/2003 Property of R. Struzak 6 Modulation Process Modulation implies varying one or more characteristics (modulation parameters a 1 , a 2 , a n ) of a carrier f in accordance with the information-bearing (modulating) baseband signal. Sinusoidal waves, pulse train, square wave, etc. can be used as carriers ( 29 1 2 3 1 2 3 , , ,... , (= carrier) , , ,... (= modulation parameters) (= time) n n f f a a a a t a a a a t = 29/01/2003 Property of R. Struzak 7 Continuous Carrier Carrier: A sin[ t + ] A = const = const = const Amplitude modulation (AM) A = A(t) carries information = const = const Frequency modulation (FM) A = const = (t) carries information = const Phase modulation (PM) A = const = const = (t) carries information 29/01/2003 Property of R. Struzak 8 Amplitude Shift Keying (ASK) Pulse shaping can be employed to remove spectral spreading ASK demonstrates poor performance, as it is heavily affected by noise, fading, and interference Baseband Data ASK modulated signal 1 1 Acos( t) Acos( t) 29/01/2003 Property of R. Struzak 9 Frequency Shift Keying (FSK) Example: The ITU-T V.21 modem standard uses FSK FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states Baseband Data BFSK modulated signal 1 1 where f 0 =Acos( c- )t and f 1 =Acos( c + )t f f f 1 f 1 29/01/2003 Property of R. Struzak 10 Phase Shift Keying (PSK) Major drawback rapid amplitude change between symbols due to phase discontinuity, which requires infinite bandwidth. Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and BFSK BPSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states Baseband Data BPSK modulated signal 1 1 where s 0 =-Acos( c t) and s 1 =Acos( c t) s s s 1 s 1 29/01/2003...
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This note was uploaded on 01/20/2012 for the course EE 301 taught by Professor Ahmetçakar during the Spring '11 term at Gazi Üniversitesi.

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Digit_Modulat - 29/01/2003 Property of R. Struzak 1 Digital...

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