ECE 4710: Lecture #27 1 QPSK & MPSK  QPSK and MPSK  if baseband m(t) is rectangular pulse then envelope of RF signal is  constant (excluding bit transitions)  QPSK signal constellation   Signal points located on circle of constant radius = A c “00” “11” I Q “01” “10”

ECE 4710: Lecture #27 2 QAM  Quadrature Amplitude Modulation = QAM  Signal constellation NOT restricted to having signal points on circle of constant radius = A c »Envelope of RF signal will vary in amplitude  Quadrature  phase information required for symbol detection  Amplitude  amplitude information also required for symbol detection  QPSK is special case of QAM when M = 4  QAM is most general form of combined AM and PM digital modulation method

ECE 4710: Lecture #27 3 QAM  General QAM signal  I & Q time domain waveforms

ECE 4710: Lecture #27 4 QAM Generation R bits/sec

ECE 4710: Lecture #27 5 QAM Signal Constellation  M = 16 Symbol Constellation  16-QAM » x i & y i are each M =4 multi-level signals  4 bits/symbol  Rectangular constellation »Amplitude modulation!!  QPSK has circular constellation »Constant envelope  16-QAM used for 2400 bps V.22 Modem standard I Q xixi yiyi

ECE 4710: Lecture #27 6 FSK & QAM Modem Standards

ECE 4710: Lecture #27 7 Other QAM Dial Up Modems Type Data Rate Modulation V.32 9, QAM V.32bis 14, QAM V.34 28, QAM V.90 56,000 PCM

ECE 4710: Lecture #27 8 V.32 Modem Standard Option 1: 32 QAM or QPSK Option 2: 16 QAM or QPSK Option 1 Data Rate Modulation Option 2 Data Rate Modulation Error Coding!! 4 data bits + 1 coding bit = 5 bits/symbol 4 data bits/symbol

ECE 4710: Lecture #27 9 V.33 with 128 QAM 6 data bits + 1 coding bit = 7 bits/symbol

ECE 4710: Lecture #27 10 QAM Modems  Most dial-up modems transmitted at 2400 sps  VF spectrum on phone line limited to 300 to 3600 Hz »Lower frequencies are noisy from 60 Hz AC power supply + harmonics  Signal BW  1 / T s  2.4 kHz < 3.3 kHz phone line BW  As # of QAM levels M increases the data rate increases for same symbol rate and signal BW  Why not increase M indefinitely to get higher data rates on phone lines??  As constellation diagram becomes more dense, the spacing between signal points becomes smaller thereby increasing likelihood of noise corrupting data »Required S/N increases for same BER »BER increases for same S/N

ECE 4710: Lecture #27 11 Channel Capacity  Shannon’s formula   Available phone line BW 3300 Hz (e.g – 300) S/N (dB) C (kbps) C (kbps) Ideal Realistic

ECE 4710: Lecture #27 12 Channel Capacity  S/N ’s of dB required for reasonable data rates over VF phone lines  Phone line S/N varies with time on single line and between different lines and circuits  Average (typical) U.S. phone line can support kbps  Modern modems negotiate data rate based upon S/N  Modem drops to “fallback” lower data rate if S/N cannot support maximum data rate offered by modem  S/N monitored throughout connection to adjust modem speed as necessary

ECE 4710: Lecture #27 13 Channel Capacity  High density QAM signal constellations are only appropriate for high S/N communication systems  Normally this implies it is a wired network »Twisted pair, T1, T3, fiber optic, etc.  dB S/N needed for high density QAM »128 QAM  V.32bis »960 QAM  V.34 »1664 QAM  V.34+

ECE 4710: Lecture #27 14 Channel Capacity  Wireless Communication Channels  Attenuation in channel is much larger than wired channel for same distance  S/N ratios are much lower due to wireless channel loss  Typical S/N ratios for wireless cellular mobile phones are 8-12 dB  Low S/N ratios cannot support high density signal constellations  Typical signal constellations used for wireless mobile »QPSK, OQPSK,  /4 DQPSK  all are M = 4!!