Digital Modulation Tech

For successful transmission and reception of information by radio wave, two process modulation and demodulation are essential. The information (speech, music, data etc) are send to thousand of kilometers away by a radio transmitter. The scene in front of a television camera is also send many kilometers away to viewers.These signals can not be send directly to the air because the signal power are quite small.For transmission, the information signals are superimposed on a high frequency radio wave called carrier wave, and this process is called modulation.The recovery of the information signal from the carrier wave is called demodulation.

MODULATIONANALOG MODULATIONPULSE MODULATIONDIGITAL MODULATIONFMAMPMASKFSKPSKANALOG PULSE MODULATIONDIGITAL PULSE MODULATIONPCMDMADMPAMPWMPPMBPSKQPSKDPSKQAM

In traditional communication system the modulation technique are Amplitude Modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM). In analog modulation both the carrier and modulating signals are analog.

Pulse modulation is used to transmit analog information like continuous speech or data. In this system continuous wave form are sampled at regular intervals. Pulse modulation is divided in to two categories, analog and digital. Analog pulse modulation is classified into PAM, PPM and PWM.Digital pulse modulation is classified into PCM, DM and ADM.

In pulse amplitude modulation the amplitude of each pulse is made proportional to the instantaneous amplitude of the modulating signal. The largest pulse represents the greatest positive signal amplitude sample, while the smallest pulse represents the largest negative sample. The time duration of each pulse may be quite short and the time interval between pulses may be relatively long. If a radio frequency is pulse amplitude modulated instead of simply being amplitude modulated, much less power is required for the transmission of information because the transmitter is actually switched off between pulses. This is one advantage of pulse amplitude modulation. PULSE AMPLITUDE MODULATION

Modulating SignalClock PulsePulse Amplitude ModulationPULSE AMPLITUDE MODULATION WAVEFORM

Pulse duration modulation also termed pulse width modulation, the pulses have a constant amplitude and a variable time duration. The time elevation of each pulse is proportional to the instantaneous amplitude of the modulating signal. In this case the narrowest pulse represents the largest ve sample of the original signal, and the widest pulse represents the largest +ve sample. When PDM applied to radio transmission, the carrier frequency has a constant amplitude and the transmitter On-times is carefully controlled. In some circumstances, PDM can be more accurate than PAM.PULSE WIDTH MODULATION

PULSE WIDTH MODULATION WAVEFORM

PPM is more efficient than PDM or PPM for radio transmission. In PPM, all pulses have the same constant amplitude and narrow pulse width. The position in time of the pulses is made to vary in proportion to the amplitude of the modulating signal. The pulse near the right side of the sampling time period represent the largest positive signal sample, and those toward the left side correspond to the most ve sample of the original signal. PPM uses less power than PDM and essentially, have all advantages of PDM. One disadvantage of PPM is that the demodulation process to recover the original signal is more difficult than PPM.PULSE POSITION MODULATION

PULSE POSITION MODULATION WAVEFORM

Pulse code modulation is the most complicated type of pulse modulation. However it can be the most accurate and the most efficient of the other methods. In certain circumstances, it may be the only type of pulse modulation that can be employed. In PCM each amplitude sample of the signal is converted from an analog to a digital signal. The sample amplitude is then represented by a group of pulses, the presence of pulse indicating 1, and the absence of a pulse indicating 0. The accuracy can be improved by increasing the number of bits, employed. PULSE CODE MODULATION

PULSE CODE MODULATION PCM TRANSMITTER BLOCK DIAGRAMWAVE FORM OF PCM

THE OPERATION OF QUANTIZATION

DM is the simplest method for converting analog to digital form. It requires small bandwidth than PCM. This circuit operates on the principle that a binary output representing the most recent sampled amplitude levels. It is less complex and less costly than PCM.It does not required any more synchronization at the base of PCM. DELTA MODULATION

DELTA MODULATION

To over-cum the disadvantages of delta modulation Adaptive delta modulation is used.It is the modification of DM in which the step size is not kept constant. The over all performance of a delta modulator can be improved with out a significant increases of the bandwidth requirements. ADAPTIVE DELTA MODULATION

ADAPTIVE DELTA MODULATION

WAVEFORM OF ADM

In analog modulation both the modulating signal and carrier signals are analog. In digital modulation, the information signals, whether audio, video, or data, are all digital. In digital modulation technique, the digital information modulates an analog carrier. To differentiate between analog modulation and digital modulation, the terminology has been changed. Amplitude-shift keying (ASK) has been adopted instead of amplitude modulation; frequency-shift keying (FSK) instead of frequency modulation; and phase-shift keying (PSK) instead of phase modulation.

Amplitude-shift keying (ASK) refers to the digital modulation technique.In digital information alternates the amplitude of the carrier between two distinct levels. The digital modulation method is also referred to as ON-OFF keying, (OOK). The ON state represents binary 1; the OFF state represents binary 0. In ASK output signal is the product of the two-level unipolar information signal and the carrier signal. Amplitude-shift keying is perhaps the simplest of all digital modulation schemes. The advantage is a reduction in the amount of energy required to transmit information.AMPLITUDE-SHIFT KEYING

Information SignalDigitalModulatorCarriercos ctModulated OutputONONOFFAMPLITUDE-SHIFT KEYING

AMPLITUDE-SHIFT KEYING WAVEFORM

Frequency-shift keying (FSK) refers to the digital modulation technique.In digital information alternates the frequency of the carrier between two distinct levels, amplitude and phase remain constant.The frequency of the signal during each bit duration is constant, and its value depends on the bit (0 or 1). In frequency-shift keying modulation incorporates two carrier frequencies for the transmission of two bits of binary data. In FSK Carrier frequency f1 corresponds to bit 1, and carrier frequency f2 corresponds to bit 0. FREQUENCY-SHIFT KEYING

FREQUENCY-SHIFT KEYING

FREQUENCY-SHIFT KEYING WAVEFORM

FSK BANDWIDTHfcf2f11/1/Bw (FSK)

In PSK modulation scheme, the phase of the carrier is altered in accordance with the input binary-coded information. PSK is the most commonly used digital communications systems. In reference to the number of carrier phase levels are used in PSK. PSK modulation schemes are further subdivided into BPSK, QPSK, and DPSK.In PSK the system must maintain a fixed bit error rate (BER). Bit error rate is a very important element in system performance evaluation. In PSK modulation schemes are characterized by their higher bandwidth efficiency. They can transmit more digital information per given channel bandwidth at the expense of a higher SNR. PSK is not susceptible to the noise degradation that effects ASK or to the bandwidth limitations of FSK.PHASE-SHIFT KEYING

PHASE-SHIFT KEYING

BINARY PHASE-SHIFT KEYING In BPSK modulation the output carrier switches between two phase in accordance with the input binary information signal. If the input binary information is one, the output-modulation carrier is in phase with the oscillator frequency. If the input binary information is 0, the output modulated carrier is out of phase with the carrier oscillator by 180o. In BPSK phase shift of the local oscillator frequency between 0 and 180o.

BINARY PHASE-SHIFT KEYING (BPSK) MODULATION

(BPSK) PHASOR DIAGRAM (BPSK) CONSTELLATION DISPLAY DIAGRAM

BFSK OUTPUT WAVEFORM00Data InBPSK Signal Output

In QPSK the modulated output signal is shifted by four phases in accordance with the input binary data. QPSK digital modulation is one level above the BPSK scheme. In QPSK there are four phase shifted at the output of the modulated signal, the QPSK method requires two input bits for each phase shift.QPSK modulation exhibits better spectral efficiency but at the expense of more complex circuitry and more critical performance requirements. QUADRATURE PHASE-SHIFT KEYING

Q-PSK MODULATOR BLOCK DIAGRAM

QPSK PHASOR DIAGRAM

Q-PSK MODULATOR TRUTH TABLE

Binary InputOutput (V)1+10-1

TRUTH TABLEIQ100201310411

1I Q0 -1 0 -1Io = - sin ct Qo = - cos ct 3 Quadrant2I Q0 -1 1 +1Io = - sin ct Qo = cos ct 2 Quadrant3I Q1 +1 0 -1Io = sin ct Qo = - cos ct 4 Quadrant4I Q1 +1 1 +1Io = sin ct Qo = cos ct 1 Quadrant

Input BitsPhase Shift0 0- 135o0 1+ 135o1 0 45o1 1+ 45o

1 11 00 00 1QPSK CONSTELLATION DISPLAY DIAGRAM

Q-PSK SIGNAL BANDWIDTH2fbfb = binary input stream (b/S)u(t) = sin ct x sin mt = cos(c m)t cos(c + m)tSin A x Sin B = Cos(A B) Cos(A + B)BW = fc - fm - (fc + fm) = fc - fm - fc - fm = 2fmfm = 4fbBW = 2 = 4 fb )fb2 BW = 2 fb

DPSK modulation scheme is use to over-cum the smallest phase difference between the receiver and transmitter carrier frequencies.In DPSK clock recovery circuit is used to activate absolute synchronization between the transmitter carrier frequency and the receiver recovery carrier frequency. In a DPSK scheme, the state of the output data will be determined by comparing the bit applied at the input of the modulator with the next bit.If the phase of the two bits is zero, then the detected bit is a zero. Likewise, if the phase of the two bits is not zero, then the detected bit is a one. DIFFERENTIAL PHASE-SHIFT KEYING

DPSK MODULATOR BLOCK DIAGRAM

TRUTH TABLEaby100100001111

Input data1 1 0 1Output data0 0 1 1

DIFFERENTIAL DATA PHASORQ 2/40 I/43/44/45/46/47/4

QAM is a combination of ASK and PSK. In a QAM system, the digital information is carried in both the phase and the amplitude of the carrier signal. In other words, with QAM the amplitude and the phase of the carrier are simultaneously varied in accordance with the input digital information.This modulation technique exhibits certain definite advantages over the PSK scheme. QUADRATURE AMPLITUDE MODULATION

Data InputData 3 DividerTwo-To-Four- Level Converter Two-To-Four- Level Converter ControlI ChannelQ Channel Carrier OscillatorBalanced Modulator # 1Balanced Modulator # 2QAM outputBPFLPFLPFfb3fb3fb3~~~8-QAM MODULATOR BLOCK DIAGRAM~

TIME DOMAIN FOR AN 8-QAM SIGNAL

8-LEVEL QAM MODULATOR STRCTURE

TRUTH TABLEIQCA000B001C010D011E100F101G110H111

I = 0 - sin ct I = 1 + sin ct PolaritiesQ = 0 - cos ct Q = 1 + cos ct PolaritiesC = 0 0.4142 V C = 1 1 VMagnitude


AI Q C0 - sin ct 0 - cos ct 0 0.4142 V

A = - 0.4142 Sin ct 0.4142 Cos ctMagnitudeVA= 0.41422 + ( 0.4142)2 = 0.586 VPhase=tan -1(-0.4142/-0.4142)= 45o=-180o + 45o= -135o


BI Q C0 - sin ct 0 - cos ct 1 1 V

B = -Sin ct Cos ctMagnitudeVB= 12 + 12= 1.414 VPhase=tan -1(-1/-1)= 45o=-180o + 45o= -135o


CI Q C0 - sin ct 1 +cos ct 0 0.4142 V

C = - 0.4142 Sin ct + 0.4142 ctMagnitudeVC= 0.41422 + 0.41422 = 0.586 VPhase=tan -1(-1/1)= - 45o=180o - 45o= 135o


DI Q C0 - sin ct 1 +cos ct 1 1 V

D = -Sin ct + Cos ctMagnitudeVD= 12 + 12= 1.414 VPhase=tan -1(-1/1)= - 45o=180o - 45o= 135o


EI Q C1 +sin ct 0 - cos ct 0 0.4142 V

E = 0-.4142 Sin ct -0.4142 Cos ctMagnitudeVE= 0.41422 + 0.41422 = 0.586 VPhase=tan -1(-0.4142/0.4142)= - 45o


FI Q C1 +sin ct 0 - cos ct 1 1 V

F = Sin ct - Cos ctMagnitudeVF= 12 + 12 = 1.414 VPhase=tan -1(1/-1)= - 45o


GI Q C1 +sin ct 1 +cos ct 0 0.4142 V

G = 0.4142 Sin ct + 0.4142 Cos ctMagnitudeVG= 0.41422 + 0.41422 = 0.586 VPhase=tan -1(0.4142/0.4142)= 45o


HI Q C1 +sin ct 1 +cos ct 1 1 V

H = Sin ct + Cos ctMagnitudeVH= 12 + 12 = 1.414 VPhase=tan -1(1/1)= 45o

PHASOR DIAGRAM OF 8-QAM1 0 1- sin ct+ sin ct+ cos ct- cos ct0 0 11 1 10 1 11 0 00 0 00 1 01 1 0-135o0.586 V1.414 V

8-QAM CONSTELLATION DISPLAY

16-QAM CONSTELLATION DISPLAY

16-LEVEL QAM TRUTH TABLE

TRUTH TABLEA0000B0001C0010D0011E0100F0101G0110H0111J1000K1001L1010M1011N1100O1101P1110Q1111

BIT AND BAUD RATEBitDibitTribitQuadbitBaud rate = NBit rate = NBaud rate = NBit rate = 2NBaud rate = NBit rate = 3NBaud rate = NBit rate = 4N

0010100010101010

0 0 1 01 00 01 01 01 11 0

0 0 10 1 00 0 10 1 01 1 10

0 0 1 01 0 0 01 0 1 01 1 1 0

BIT AND BAUD RATE COMPARISON

Modulation UnitsBits/BaudBaud RateBit RateASK, FSK, 2-PSKBit1NN4-PSK, 4-QAMDibit2N2 N8-PSK, 8-QAMTribit3N3 N16-QAMQuadbit4N4 N32-QAMPentabit5N5 N64-QAMHexabit6N6 N128-QAMSeptabit7N7 N256-QAMOctabit8N8 N

Modulation constitutes a significant amount of communication which involves transmitting signal (analog and digital) to an appreciable distance without interference. Analog signals are liable to change in its amplitude, phase and frequency arbitrarily, hence the carrier signals are made to adopt these changes in itself to perform analog modulation.The limitations observed in analog modulation are successfully overcome in digital modulation. In which successive samples of signals are taken and superimposed on carrier signal to achieve high quality noiseless communication. A considerable trend towards updating the modulating technique will add new means and values to the exiting communication.

Digital Modulation Tech

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