디지털통신 Bandpass Modulation 1 임 민 중 동국대학교 정보통신공학과

Digital Bandpass Modulation Baseband Modulation Bandpass Modulation Signal Mapping TX Filter RX Filter Decision TX Data RX Data binary discrete pulse pulse discrete binary Noise Analog version > 0 → 1 < 0 → 0 1 → 1 0 → -1 Signal Mapping TX Filter RF Modulation RF Demod ulation RX Filter Decision TX Data RX Data binary discrete pulse wave wave pulse discrete binary Noise Analog version > 0 → 1 < 0 → 0 1 → 1 0 → -1

Digital Bandpass Modulation

Bandpass Modulation - 1 Digital modulation Baseband modulation The process by which digital symbols are transformed into waveforms that are compatible with the characteristics of the channel Baseband modulation Waveforms are pulses Bandpass modulation Information signal modulates a sinusoid called a carrier For radio transmission the carrier is converted to an electromagnetic field for propagation to the desired destination Digital symbol: a sinusoid of duration T Why is it necessary to use a carrier? antenna size, frequency division Carrier wave Amplitude, Frequency, Phase

Amplitude Shift Keying Frequency Shift Keying Bandpass Modulation - 2 Bandpass modulation Amplitude Shift Keying (ASK) t T T T ASK is similar to DSB-SC FSK is similar to FM PSK is similar to PM Frequency Shift Keying (FSK) t T T T Phase Shift Keying (PSK) t T T T

Bandpass Modulation - 3 ASK, PSK, FSK Signals 2ASK (= 2PSK) 4ASK t t

Bandpass Modulation - 4 Bandpass demodulation the reverse of the modulation process Coherent demodulation The receiver exploits knowledge of the carrier’s phase to detect the signals Optimal method Noncoherent demodulation The receiver does not utilize phase reference information Phase estimation is not required Suboptimal method RF Modulation RF Demodulation cos(2fct) cos(2fct) RF Modulation RF Demodulation cos(2fct) cos(2fct+) Coherent demodulation is more difficult but produces better results Modulation for Coherent Receiver Coherent Demodulation Some transmission methods require coherent demodulation and will be called coherent method Other transmission methods allow both coherent and noncoherent demodulation and will be called noncoherent method Modulation for Noncoherent Receiver Noncoherent Demodulation

Quadrature Amplitude Modulation Bandpass Modulation - 5 Linear Modulation ASK, PSK, QAM Baseband signal (M-ary PAM signal)  Carrier Bandwidth efficient high data rate transmission High cost Large peak-to-average power ratio Constant Envelope Modulation FSK Power efficient Low cost Low peak-to-average power ratio Quadrature Amplitude Modulation

Modulation/Demodulation Procedures 0 1 0 1 1 → 1 0 → -1 Signal Mapping Pulse Shaping Filter RF Modulation Power Amp binary data discrete symbol baseband pulse sinusoidal wave Decision Matched Filter RF Demodulation Low Noise Amp > 0 → 1 < 0 → 0 0 1 0 1

Signal Space Diagram - 1 Signal Space Diagram (Constellation) for ASK, PSK, QAM (not for FSK) sine and cosine waves are orthogonal Assuming that carrier frequency is not changing and we only consider amplitude and phase sin(2fct) Quadrature (Amplitude, Phase) Constellation sin cos(2fct) -cos cos 2cos In-phase (I,Q) For example, -2  -2 cos(2fct) j  cos(2fct - /2) = sin(2fct) 1 + j  sqrt(2) cos(2fct - /4)

Signal Space Diagram - 2 Example

Amplitude Shift Keying ASK, ASK-SSB, OOK ASK-SSB  M-ASK = M-PAM x Carrier OOK  ASK  unipolar  QAM  bandwidth efficient 

Amplitude Shift Keying - 1 Transmitted signal waveforms sm(t) = Amcos2fct, m = 1,2,…,M Am = amplitude signal =-M, -M+2, …-1, 1, …, M-2, M similar to DSB-SC Baseband signal Am Bandpass signal Am cos2fct 2ASK 4ASK 8ASK 2PAM 4PAM 8PAM 2ASK = 2PAM x Cosine Carrier 4ASK = 4PAM x Cosine Carrier 8ASK = 8PAM x Cosine Carrier Carrier cos2fct 1 1 2ASK t t 11 01 10 00 4ASK t t

Amplitude Shift Keying - 2 2ASK signal 4ASK signal

Amplitude Shift Keying - 3 Received signal r(t) = Amcos2fct + n(t), 0  t  T n(t) = bandpass noise process ASK Receiver coherent receiver is required, in other words, knowledge of carrier phase is required at ASK receiver cos2fct Since the average of cosine or sine is zero, the integration of high-frequency cosine or sine is zero

Amplitude Shift Keying - 4 ASK modulation and demodulation Message signal x(t) Carrier at transmitter 2cos2fct After RF modulation (ASK signal) x(t) 2cos2fct Carrier at receiver 2cos2fct After RF demodulation low pass filter can extract the baseband signal x(t) 2cos2fct 2cos2fct

Amplitude Shift Keying - 5 Symbol Decision Boundaries 2-ASK 4-ASK 8-ASK

Amplitude Shift Keying - 6 Gray Coding and Bit Decision Boundaries adjacent binary values differ in only one bit 2-ASK 1 4-ASK 11 10 00 01 Left bit Right bit 8-ASK 110 111 101 100 000 001 011 010 Left bit Right bit Middle bit Due to Gray coding, one symbol error implies one bit error out of log2M bits in M-ary modulation Hence, M-ASK BER = M-ASK SER / log2M 2ASK SER = 1 x 2ASK BER 4ASK SER = 2 x 4ASK BER 8ASK SER = 3 x 8ASK BER

Amplitude Shift Keying - 7 With fixed average symbol energy distance between two adjacent points = immunity to noise When symbol energy = 1 2-ASK 1 most robust (-A A) symbol energy = A2 = 1  A = 1 -A A 4-ASK 11 10 00 01 -3B -B B 3B (-3B -B B 3B) symbol energy = (B2 + (3B)2)/2 = 5B2= 1  B = 1/sqrt(5) 8-ASK 110 111 101 100 000 001 011 010 high data rate vulnerable to noise high SNR is required -7C -5C -3C -C C 3C 5C 7C (-7C -5C -3C -C C 3C 5C 7C) symbol energy = (C2 + (3C)2 + (5C)2 + (7C)2)/4 = 21C2 = 1  C = 1/sqrt(21)

(rotation version of TX signal + noise) On-Off Keying - 1 On-Off Keying (OOK) Unipolar Noncoherent method Envelope detection or energy detection can be used 2ASK ~ bipolar OOK ~ unipolar 1 t T T T 2ASK requires coherent receivers OOK allows coherent or noncoherent receivers (coherent demodulation produces better performance) TX signal (no transmission) RX signal (noise) Advantage: - Simple receiver - If 0 occurs with high probability, this method can save energy Disadvantage: - When 0 and 1 occur with equal probability, this method wastes power RX signal (rotation version of TX signal + noise) TX signal

(rotated TX signal + noise) On-Off Keying - 2 Coherent receiver Noncoherent receiver TX signal 1  1 0  0 > T/4  1 < T/4  0 cos2fct RX signal (TX signal + noise) cos2fct The performance is affected by real noise TX signal 1  1 0  0 | |2 > T/4  1 < T/4  0 cos2fct cos(2fct+) RX signal (rotated TX signal + noise) The carrier phase is not known The performance is affected by complex noise sin(2fct+)

On-Off Keying - 3 Performance 2ASK > OOK with coherent receiver > OOK with noncoherent receiver Bipolar vs. Unipolar Coherent vs. Noncoherent Bipolar -1 1 Unipolar

Carrier at transmitter ASK-SSB - 1 SSB Transmission Message signal Carrier at transmitter ASK wastes bandwidth Double-sideband ideal bandpass filter SSB filter Transmitted signal single sideband

ASK-SSB - 2 SSB Reception Received signal Carrier at receiver

Bandwidth and Symbol Rate Baseband Passband SSB Example 8PAM, 10 MHz, r = 0 data rate = 3 bit/symbol  2 symbol/Hz  10 MHz = 60 Mbit/sec 8ASK, 10 MHz, r = 0 data rate = 3 bit/symbol  1 symbol/Hz  10 MHz = 30 Mbit/sec 8ASK-SSB, 10 MHz, r = 0 PAM B: bandwidth (Hz) Rs: symbol rate (symbol/sec) r: roll-off factor Rb: data rate (bit/sec) = Rs log2M for M-ary modulation bandwidth ASK bandwidth ASK-SSB bandwidth