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V.29, V.32, V.32 bis...

V.29, V.32, V.32 bis. THE INS AND OUTS OF MODEMS. BUT FIRST. NONCOHERENT DETECTION. Handling Phase Sync Problems through Differential Encoding. PHASE SYNCHRONISM. Coherent detection requires precise phase sync between the received signal and local oscillator. LPF. phase error.

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V.29, V.32, V.32 bis...

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  1. V.29, V.32, V.32 bis... THE INS AND OUTS OF MODEMS

  2. BUT FIRST...

  3. NONCOHERENT DETECTION Handling Phase Sync Problems through Differential Encoding

  4. PHASE SYNCHRONISM • Coherent detection requires precise phase sync between the received signal and local oscillator. LPF phase error

  5. DIFFICULTY ESTABLISHING ABSOLUTE PHASE REFERENCE • What is the phase of the RF pulse seen on the scope?

  6. 01 11 00 0 10 CONVENTIONAL ENCODING • Take a 4-phase modulation, known as Quadrature Phase Shift Keying(QPSK) 90 180 270

  7. DIFFERENTIAL ENCODING • Although the absolute phase of the incoming signal varies, by as much as 50o, phase transitions across symbols are very stable • Instead of mapping symbols into absolute phases, they are encoded by phase transitions

  8. MAPPING DATA TO PHASE CHANGES: BINARY EXAMPLE • Data is encoded such that a digit 1 causes a 180 phase shift relative to the previous phase but 0 causes no phase shift • Receiver will in turn look for phase changes rather than absolute phases

  9. 1 0 1 1 0 1 DPSK       DIFFERENTIAL ENCODING FOR BPSK • Encode 1 0 1 1 0 1 in BPSK and differential PSK (DPSK) 1 0 1 1 0 1 PSK       phase phase(reference 

  10. symbol       phase change Data 1 0 1 1 0 1 DECODING 1 0 1 1 0 1 DPSK       phase

  11. QPSK WITH GRAY CODING 00 01 11 10 00->0 01->90 11->180 10->270

  12. DQPSK bits change 00 0 01 90 11 180 10 270 270 0 180 90 90 11 01 11 10 arbitrary ref.

  13. HOW DO MODEMS WORK?A look into modulation methods

  14. A BIT OF HISTORY • Early modems designed in the 1950’s used Frequency Shift Keying(FSK) at 300 bps over public switched phone lines • In the 1960’s equalized bandwidth increased to 2400Hz thus allowing bit rates of 2400 bps.

  15. FIRST MILESTONE:V.26 • The first 2400 bps modem was made available in 1962 using 4-PSK (actually, differential PSK, DPSK) • This was followed in 1967by 8-DPSK at 4800 bps • Note that to achieve higher rates, one has to go to larger constellation sizes: BM-ary=Bbinary/logM=4800/3=1600 Hz

  16. BAUD vs. BITS • Baud is the number of symbols/sec. • Bauds and bits are related by logM, where M is the number of modulation levels • R=Rb/logM • Commercial modems are rated by bits/sec even though the box says “baud”

  17. 9600 bits/sec: V.29/V.32 • The first 9600 bps modem was introduced in 1976 using 16-QAM constellation • Baudrate=bandwidth=2400 Hz • Requires adaptive channel equalization to reduce intersymbol interference

  18. QAM MODEMS • All modern modulation techniques used in modems are based on the Quadrature Amplitude Modulation (QAM) method 16-QAM

  19. 1101 1001 0001 0101 1100 1000 0000 0100 1110 1010 0010 0110 0011 0111 1111 1011 NON-DIFFERENTIAL GRAY CODING

  20. L-fold AMBIGUITY • An L-fold rotationally symmetric constellation maps onto itself for a rotation of +/-K.(2pi/L) • The receiver is unable to resolve a number of possible carrier lock positions 01 01 00 11 11 00 10 10

  21. DIFFERENTIAL QAM • Divide the signal space into L equal pie-shaped sectors • For 16-QAM, L=4, differentially Gray encode sectors by 00, 01, 11, 10 • Gray encode the remaining 2 bits within each sector.

  22. DQAM CONSTELLATION 0111 0101 0011 0001 0110 0100 0010 0000 1111 1101 1011 1001 1110 1100 1010 1000

  23. V.29

  24. DATA MAPPING

  25. ENCODING THE REMAINING 3 BITS • The remaining 3 bits are differentially Gray coded: • Q2 Q3 Q4 Phase Change 0 0 1 0 0 0 0 45 0 1 0 90 0 1 1 135 1 1 1 180 1 1 0 225 1 0 0 270 1 0 1 315

  26. FALL BACK RATE • If conditions demand, transmission falls back to 8-QAM at 7200 bps. Bit Q1 is permanently set to zero limiting the amplitudes. 90 45 135 3 3 180 0 225 315    3 270

  27. V.33 • V.33 is a transmission standard at 14,400 bps. • Modulation is 128-QAM constellation • Carrier frequency is 1800 Hz. • Signaling rate is 2400 baud.

  28. V.34 • Adopted in September 94. • Signaling at 28.8 kbps. • Baud rate at 3200 symbols/sec • Constellation size: 768 points

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