©1997 BG Mobasseri110/27/2015 V.29, V.32, V.32 bis... THE INS AND OUTS OF MODEMS

©1997 BG Mobasseri210/27/2015 BUT FIRST...

©1997 BG Mobasseri310/27/2015 NONCOHERENT DETECTION Handling Phase Sync Problems through Differential Encoding

©1997 BG Mobasseri410/27/2015 PHASE SYNCHRONISM Coherent detection requires precise phase sync between the received signal and local oscillator. LPF phase error

©1997 BG Mobasseri510/27/2015 DIFFICULTY ESTABLISHING ABSOLUTE PHASE REFERENCE What is the phase of the RF pulse seen on the scope?

©1997 BG Mobasseri610/27/2015 CONVENTIONAL ENCODING Take a 4-phase modulation, known as Quadrature Phase Shift Keying(QPSK)

©1997 BG Mobasseri710/27/2015 DIFFERENTIAL ENCODING Although the absolute phase of the incoming signal varies, by as much as 50 o, phase transitions across symbols are very stable Instead of mapping symbols into absolute phases, they are encoded by phase transitions

©1997 BG Mobasseri810/27/2015 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

©1997 BG Mobasseri910/27/2015 DIFFERENTIAL ENCODING FOR BPSK Encode in BPSK and differential PSK (DPSK)   PSK phase phase( reference      DPSK   

©1997 BG Mobasseri1010/27/2015 DECODING  phase change symbol Data    DPSK    phase

©1997 BG Mobasseri1110/27/2015 QPSK WITH GRAY CODING 00->0 01->90 11-> >

©1997 BG Mobasseri1210/27/2015 DQPSK bits change arbitrary ref

©1997 BG Mobasseri1310/27/2015 HOW DO MODEMS WORK? A look into modulation methods

©1997 BG Mobasseri1410/27/2015 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.

©1997 BG Mobasseri1510/27/2015 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: B M-ary =B binary /logM=4800/3=1600 Hz

©1997 BG Mobasseri1610/27/2015 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”

©1997 BG Mobasseri1710/27/ 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

©1997 BG Mobasseri1810/27/2015 QAM MODEMS All modern modulation techniques used in modems are based on the Quadrature Amplitude Modulation (QAM) method 16-QAM

©1997 BG Mobasseri1910/27/2015 NON-DIFFERENTIAL GRAY CODING

©1997 BG Mobasseri2010/27/2015 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

©1997 BG Mobasseri2110/27/2015 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.

©1997 BG Mobasseri2210/27/2015 DQAM CONSTELLATION

©1997 BG Mobasseri2310/27/2015 V.29

©1997 BG Mobasseri2410/27/2015 DATA MAPPING

©1997 BG Mobasseri2510/27/2015 ENCODING THE REMAINING 3 BITS The remaining 3 bits are differentially Gray coded: Q2 Q3 Q4 Phase Change

©1997 BG Mobasseri2610/27/2015 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   

©1997 BG Mobasseri2710/27/2015 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.

©1997 BG Mobasseri2810/27/2015 V.34 Adopted in September 94. Signaling at 28.8 kbps. Baud rate at 3200 symbols/sec Constellation size: 768 points