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Digital D ata using Band-Limited (Analog) Signals

Digital D ata using Band-Limited (Analog) Signals. Based on Chapter 5 of William Stallings, Data and Computer Communication, 8 th Ed. Kevin Bolding Electrical Engineering Seattle Pacific University. Digital Data, Analog World. Two major ways to send data : Pulse Trains “Square” pulses

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Digital D ata using Band-Limited (Analog) Signals

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  1. Digital Data using Band-Limited (Analog) Signals Based on Chapter 5 of William Stallings, Data and Computer Communication, 8thEd. Kevin BoldingElectrical EngineeringSeattle Pacific University

  2. Digital Data, Analog World • Two major ways to send data: • Pulse Trains • “Square” pulses • Consumes entire bandwidth of medium • Simple • Band-Limited (Analog)signal • Uses AM/FM/PM techniques • Uses only a specified band of the medium • Gains benefits of analog signaling

  3. 0 1 0 1 1 1 0 1 0 0 0 1 0 1 Digital to Analog • Use techniques similar to analog/analog signaling • Modulate a digital signal onto an analog carrier • Similar to AM/FM • Difference: Data is now binary (discrete) instead of analog (continuous) • In general, this simplifies the modulation/demodulation process because there are a limited number of options

  4. Use AM techniques Amplitude of signal is proportional to data being sent But data can only be 1 or 0 0 1 0 1 1 1 0 1 0 0 0 1 0 1 A cos(2pfct), if d(t)=1 s(t)= 0, if d(t)=0 Amplitude Shift Keying • ASK • Easy to implement • Sudden (on/off) changes cause difficulty with electrical signals • Works very well for optical fiber • LEDs: on or off • Lasers: Low or High signal • ASK recovery is the same as AM recovery:Rectify and LPF.

  5. Use FM techniques Use two frequencies, one for 0, the other for 1 0 1 0 1 1 1 0 1 0 0 0 1 0 1 A cos(2pf1t), if d(t)=1 s(t)= A cos(2pf2t), if d(t)=0 Binary Frequency Shift Keying • BFSK • Easy to implement with either: • Two carriers and a voltage-controlled switch • A Voltage-controlled Oscillator (VCO) • f1 and f2 must be far enough apart to easily distinguish: The further apart they are, the more bandwidth BFSK takes • Demodulation involves bandpass filtering at f1 and f2

  6. 0 1 0 1 1 1 0 1 0 0 0 1 0 1 +1 0 p, if s(t) = 1 -1 F(t)= 0, if s(t) = 0 BPSK Recovery (Coherent) X BPSK Data out LPF Recovered Carrier Binary Phase Shift Keying • Use PM techniques • Use phase angles (usually 0 and p) BPSK signal Carrier BPSK Signal x Carrier LPF of this recovers signal • Coherent Recovery (BPSK):In-phase copy of carrier at receiver is multiplied by signal.

  7. Use more than just two phase angles Quadrature PSK (QPSK): p/4, 3p/4, 5p/4, 7p/4 Allows sending more than two bits per pulse QPSK sends two bits per baud (four bits per wave) 3p/4 p/4 1 1 01 11 p 0 7p/4 5p/4 BPSK 10 00 QPSK Multi-phase PSK Baud = Symbol = Signaling event Each baud/symbol/signaling event may contain more than one bit!

  8. 3p/4 p/4 1 01 11 7p/4 5p/4 10 00 QPSK QPSK Waveform Larger phase offset  Earlier in time (waveform offset to the left) 11 01 00 11 10 Dotted line is reference for 11 (p/4).

  9. Generally known as Quadrature Amplitude Modulation (QAM) Shift both phase and amplitude to generate multiple constellation points Combines ASK and PSK Multi-phase, Multi-amplitude PSK p/4 3p/4 011 111 Bit assignment= High: x coordinate Mid : y coordinate Low : 0-inner; 1-outer 010 110 100 000 001 101 7p/4 5p/4 8QAM - 3 bits/baud

  10. 8-QAM Waveform Bit assignment= High: x coordinate Mid : y coordinate Low : 0-inner; 1-outer p/4 3p/4 011 111 010 110 100 000 001 101 7p/4 5p/4 010 001 110 100 011 111 101 000

  11. QAMs go up to 512QAM 9 bits/baud Telephone modems use various forms of QAM to gain high rates Ex: 9 bits/baud x 3200 baud --> 28,800 bps Big QAMs p/4 3p/4 7p/4 5p/4 16QAM - 4-bits/baud

  12. 3p/4 p/4 1 1 01 11 p 0 7p/4 5p/4 BPSK 10 00 QPSK QAM and Noise • Different modulation schemes use approximately the same bandwidth • The closer together points are in the constellation, the more noise effects the signal p/4 3p/4 p/4 3p/4 011 111 010 110 7p/4 010 5p/4 000 001 011 16QAM - 4-bits/baud 7p/4 5p/4 • Higher order schemes send more bps, but require higher SNR 8QAM - 3 bits/baud

  13. Adaptive Modulation • Many channels have changing or variable noise characteristics • Worst-case design will pick a low-order QAM in order to give error-free transmission • Can’t make use of the times when noise level is low • Adaptive Modulation allows the system to change the QAM scheme to fit the transmission characteristics • Example: Start with a 256 QAM; if errors, try again with a 64QAM; if errors, try a 16QAM; if errors, use QPSK, etc.

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