102L2modul - Chapter 2 Wireless Systems: Wireless Systems:...

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2 - 1 October, 1997 Technical Introduction to Wireless -- 102 -- (c) 1997 Scott Baxter - V1.0 Wireless Systems: Modulation Schemes and Bandwidth Wireless Systems: Modulation Schemes and Bandwidth Chapter 2 f c f c Upper Sideband Lower Sideband f c f c I axis Q axis a b φ c QPSK I axis Q axis c a φ b p r v π /4 shifted DQPSK 1 0 1 0 1 0 1 0 1 0 1 0
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2 - 2 October, 1997 Technical Introduction to Wireless -- 102 -- (c) 1997 Scott Baxter - V1.0 Characteristics of a Radio Signal n The purpose of telecommunications is to send information from one place to another n Our civilization exploits the transmissible nature of radio signals, using them in a sense as our “carrier pigeons” n To convey information, some characteristic of the radio signal must be altered (I.e., ‘modulated’) to represent the information n The sender and receiver must have a consistent understanding of what the variations mean to each other n RF signal characteristics which can be varied for information transmission: Amplitude Frequency Phase SIGNAL CHARACTERISTICS S (t) = A cos [ ω c t + ϕ ] The complete, time- varying radio signal Amplitude (strength) of the signal Natural Frequency of the signal Phase of the signal Compare these Signals: Different Amplitudes Different Frequencies Different Phases
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2 - 3 October, 1997 Technical Introduction to Wireless -- 102 -- (c) 1997 Scott Baxter - V1.0 The Emergence of AM: A bit of History n The early radio pioneers first used binary transmission, turning their crude transmitters on and off to form the dots and dashes of Morse code. The first successful demonstrations of radio occurred during the mid-1890’s by experimenters in Italy, England, Kentucky, and elsewhere. n Amplitude modulation was the first method used to transmit voice over radio. The early experimenters couldn’t foresee other methods (FM, etc.), or today’s advanced digital devices and techniques. n Commercial AM broadcasting to the public began in the early 1920’s. n Despite its disadvantages and antiquity, AM is still alive: AM broadcasting continues today in 540-1600 KHz. AM modulation remains the international civil aviation standard, used by all commercial aircraft (108-132 MHz. band). AM modulation is used for the visual portion of commercial television signals (sound portion carried by FM modulation) Citizens Band (“CB”) radios use AM modulation Special variations of AM featuring single or independent sidebands, with carrier suppressed or attenuated, are used for marine, commercial, military, and amateur communications SSB LSB USB
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2 - 4 October, 1997 Technical Introduction to Wireless -- 102 -- (c) 1997 Scott Baxter - V1.0 Amplitude Modulation (“AM”) TIME-DOMAIN VIEW of AM MODULATOR x(t) = [1 + a m n (t)] cos ω c t where: a = modulation index (0 < a <= 1) m n (t) = modulating waveform ω c = 2 π f c , the radian carrier freq. Σ
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102L2modul - Chapter 2 Wireless Systems: Wireless Systems:...

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