UNIT 2- Digital Communication
Introduction:
Information:
 Knowledge or intelligence that is communicated between two or more points.
Digital Modulation:
 High frequency analog signal is modulated by relatively low frequency digital
information signal.
 Also called as digital radio.
Advantages:
 Ease of processing
 Ease of multiplexing
 Noise immunity
Digital transmission systems:
 Transport information in digital form - requires a physical medium
 Eg., metallic wire, coaxial cable or optical fiber cable.
Major difference between analog and digital communication systems:
 Analog - Both the information and the carrier signal is analog
 Digital - The carrier is analog, but the information signal is digital.
Various forms of digital modulations:
1. Amplitude Shift Keying (ASK)
2. Frequency Shift Keying (FSK)
3. Phase Shift Keying (PSK)
4. Quadrature Amplitude Modulation (QAM)
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Amplitude Shift Keying (ASK)
o Amplitude of the carrier is varied with respect to the binary information
signal.
Frequency Shift Keying (FSK)
o Frequency of the carrier is varied with respect to the binary information
signal.
Phase Shift Keying (PSK)
o Phase of the carrier is varied with respect to the binary information signal.
Quadrature Amplitude Modulation (QAM)
o Both the amplitude and phase of the carrier is varied with respect to the binary
information signal.
Applications of digital Modulation:
 Relatively low speed voice data communication modems
 High speed data transmission systems
 Digital microwave and satellite communication systems
 Cellular telephone
Digital Modulation Systems:
Block diagram of digital modulation system
Transmitter:
 Binary information is given to precoder - performs level conversion.
 Encodes the incoming binary data into groups of bits - modulate the analog carrier.
 Modulated carrier is given to the Band Pass Filter (BPF).
 Filtered signal is amplified by power amplifier.
Transmission Medium:
 Amplified signal is transmitted to the receiver.
 Transmission medium can be metallic cable, optical fiber cable or wireless systems.
Receiver:
 Received signals are filtered, amplified and the applied to the demodulator and
decoder.
 Extracts the original source information from the modulated carrier.
Carrier and clock recovery circuits:
 Recover the analog carrier and digital timing signals
 Required to perform the demodulation process.
Information Capacity, Bit rate and Baud:
Information Capacity:
 Measure of amount of information can be propagated through a communication
system.
 Function of bandwidth and transmission time.

Bit:

Represents the number of independent symbols that can be carried through a system
in a given unit of time.
Basic digital symbol used to represent the information.
Hartley’s Law:
 Relationship among bandwidth, transmission time and information capacity.
 States that
o I α B*t
Where I = information capacity (bps)
 B = bandwidth (Hz)
 t = transmission time (sec)
 Information capacity - directly proportion to both bandwidth and transmission time.
 If either B or t changes, a change occurs in Information capacity (I).
Shannon’s information capacity theorem:
 Information capacity - related to bandwidth and signal to noise ratio (SNR).
 The higher the SNR, the better the performance and the higher the information
capacity.
 Shannon’s limit for information capacity is,
I= B log2(1+(S/N))
Or
I= 3.32B log10(1+(S/N))
Where I= information capacity
B= bandwidth
S/N= signal to noise ratio
Bit Rate and Baud:
Bit Rate
Baud
Rate of change of digital information in Rate of change of signal on the transmission
binary form.
medium after the encoding and modulation
process
Number of bits per second
Number of signal units per second.
Rate of change of input to the modulator.
Rate of change of output of the modulator.
Denoted as bits per second (bps)
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Denoted as symbols per second.
Baud is less than or equal to the bit rate.
Reciprocal of the time of one output signaling element.
A signaling element can represent several information bits.
Baud = 1/ts
Where ts = time of one signaling element (sec)
Minimum Nyquist Bandwidth or Minimum Nyquist Frequency:
 Minimum bandwidth required to propagate a signal is called the minimum Nyquist
bandwidth or frequency.
 fb= 2B
where fb = bit rate in bps
B = ideal Nyquist bandwidth
 Nyquist formula for channel capacity is
 fb= 2B log2 M
where fb = channel capacity in bps
B = minimum Nyquist bandwidth
M = number of discrete signal
 Minimum bandwidth required to pass the modulated carriers
 B = fb / log2 M
 If N is substituted for log2 M, then
 Baud= fb / N
Where N = number of bits encoded into each signaling element
Amplitude Shift Keying (ASK):
 Amplitude of the carrier is varied in accordance with the binary information signal.
 Two output amplitudes possible.
 Also called as Digital Amplitude Modulation (DAM).
 ASK is mathematically expressed as,
VASK(t) = [1+Vm(t)][(A/2)cosωct]……………..(1)
Where VASK(t) = ASK signal
Vm(t) = Digital Modulating signal
A/2 = Carrier amplitude
ωct = Analog carrier in frequency
 Modulating signal Vm(t) is a binary waveform,
where +1V = logic 1
-1V = logic 0
 For logic 1 input,
The Eqn (1) becomes,
 VASK(t) =[(A/2)cosωct]
 For logic 0 input,
The Eqn (1) becomes,
 VASK(t) = 0
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Carrier is either “ON” or “OFF”. Also called as On-Off Keying (OOK).
Time of one bit in binary signal (tb) is equal to the time of one analog signaling
element
(ts) in ASK signal.
 Rate of change of the ASK waveform is same as the rate of change of binary input
(bps). Hence Bit rate is equal to baud rate for ASK.
 For ASK, the bit rate also equal to the minimum Nyquist bandwidth and by setting
N=1, then
Bandwidth B= fb/N
Baud = fb
Advantages:
 Simplicity
 Used to transmit digital data over optical fiber
Disadvantages:
 Susceptible to noise
 Used as low bit rate upto 100 bps.
Comparison of AM and ASK:
Parameter
AM
Continuous variation in the
Variation in the carrier
carrier amplitude in accordance
amplitude
with the modulating signal.
Nature of modulating signal Analog
2
Number of sideband
Poor
Noise immunity
Radio and broadcasting
Application
ASK
Carrier ON or OFF depending
on whether a 1 or 0 is to be
transmitted.
Digital
2
Poor
Data transmission at low bit rate
Frequency Shift Keying (FSK):
 Frequency of the carrier is varied in accordance with the binary information signal.
 Also called as Binary FSK (BFSK).
 BFSK is mathematically expressed as,
VFSK(t) = Vccos (2Π[fc+Vm(t)∆f]t……………..(1)
Where VFSK(t) = binary FSK signal
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Vm(t) = Digital Modulating signal
Vc = Carrier amplitude
fc = Analog carrier frequency
∆f = Peak change in carrier frequency
If the symbol “1” is transmitted Vm(t) =+1V, then the carrier frequency will be fc+∆f
If the symbol “0” is transmitted Vm(t) = -1V, then the carrier frequency will be fc- ∆f
Thus VFSK(t) = Vccos (2Π[fc+∆f]t for symbol “1”
VFSK(t) = Vccos (2Π[fc - ∆f]t for symbol “0”
As the binary input changes from logic “0” to logic “1” or vice versa, the output
frequency shifts between two frequencies.
1. A mark or logic 1 frequency (fm)
2. A space or logic 0 frequency (fs)
Truth table for FSK:
Binary i/p Frequency o/p
0
Space (fs)
1
Mark (fm)

Frequency deviation – half the difference between mark (fm) and space (fs)
frequency.
Frequency deviation = ∆f = (|fm-fs|)/2
FSK bit rate, Baud and Bandwidth:
 Time of one bit is same as the time of mark or space frequency.
 Bit rate is equal to baud.
 Baud= fb / N
 Minimum bandwidth required for BFSK:
B= |(fs+fb) – (fm-fb)|
= |fs-fm| + 2fb
Since |fs-fm| = 2∆f , then
B = 2(∆f+ fb)
FSK Transmitter:
 Similar to FM modulator and is a voltage controlled oscillator (VCO)
 Logic 1 input shifts the VCO output to the mark frequency.
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Logic 0 input shifts the VCO output to the space frequency.
Works in sweep mode, where peak frequency deviation (∆f) is the product of binary
input voltage and deviation sensitivity of VCO, and can be expressed as,
∆f = Vm(t) kl
∆f = peak frequency deviation
Vm(t)= binary modulating signal
kl = deviation sensitivity
Amplitude of the input signal can be either logic 0 or 1. So the peak frequency
deviation is constant and always at its maximum.
FSK Receiver:
 FSK input signal is applied to the inputs of BPF through the power splitter.
 Corresponding filter passes only the mark or space frequency to its corresponding
envelope detector.
 Envelope detectors indicate the total power in each sideband.
 Comparator provides output which is largest of two powers.
 FSK detection is not coherent, because there is no frequency involved in the
demodulation process (i.e., synchronized either in frequency, phase or both with the
FSK signal.
Coherent FSK Receiver:
 Received FSK signal is given to the power splitter.
 Output of power splitter is multiplied by the carrier signal.
 Coherent FSK detection is used.
PLL FSK Demodulator:
 Main blocks of PLL are a loop filter, VCO and a phase detector.
 Feedback system – output is compared with the i/p.
 The feedback energy is phase.
 VCO – adjust its own frequency until it is equal to the i/p frequency, with the result,
the phase and frequency of the two signals are in synchronism.
 VCO output is given to the phase detectors - it tracks the fedback signal.
 Phase lock is obtained by comparing the phase of the output signal with that of the
reference.
 Phase difference is converted to an error correction voltage.
 If the signal feedback is not equal to the i/p signal, the difference error will change
the signal feedback until it is close to the i/p signal.
 As the PLL with an FSK locks into either 0 or 1, the loop output voltage shifts to high
or low voltage.
 Voltages are detected, amplified and then converted into logic levels.
Advantages:
 Implementation is simple and inexpensive.
 Affected by less noise.
Disadvantages:
 Probability of error is higher than BPSK.
 More bandwidth is required.
 Error rate is more.
Applications:
 Used in low performance, low cost data modems.
Phase Shift Keying (PSK) or Binary PSK (BPSK):
 Angle modulated constant amplitude digital modulation scheme.
 Before modulation, the i/p binary signal encoded into group of bits.
 Bits ranges from 1 to 12 or more.
 Number of output phases (M) is based on the number of bits in the group.
Binary PSK:
 Two phases are possible – one phase denotes logic ‘1’ and other phase denotes logic
‘0’.
 When the i/p changes from logic 0 to 1 or vice versa, the phase of the o/p carrier
shifts between two angles differed by 180 degree.
 Also known as Phase Reversal Keying (PRK) or Biphase Modulation.
 Form of square wave modulation of continuous wave signals.
BPSK Transmitter:
 Act as a phase reversing switch.
 Based on the logic condition of the input, the carrier is transferred to the output either
in phase or 180 degree out of phase with the reference carrier oscillator.
Balanced Ring Modulator:
 Has two i/p’s, a carrier that is in-phase with the reference oscillator and binary digital.
 For proper operation, the digital i/p voltage must be greater than the peak carrier
voltage.
 Digital i/p controls the ON/OFF states of the diode.
 When the binary i/p is logic 1 (positive voltage), diodes D1 and D2 are forward
biased and ON, D3 and D4 are reverse biased and OFF.
 The carrier voltage is developed across transformer T2 is in-phase with the carrier
voltage across T1. Hence the o/p signal is in-phase with the i/p signal.
 When the binary i/p is logic 0 (negative voltage), diodes D1 and D2 are reverse
biased and OFF, D3 and D4 are forward biased and ON.
 The carrier voltage is developed across transformer T2 is 180 degree out of phase
with the carrier voltage across T1. Hence the o/p signal is 180 degree phase with the
i/p signal.
BPSK waveform:
BPSK Truth Table, Phasor Diagram and Constellation diagram:
Bandwidth consideration of BPSK, Bit rate and Baud:
 Binary i/p signal and the reference carrier are given to the balanced modulator –
where the carrier is multiplied by the binary data.
 The i/p carrier Sinωct is multiplied by either +1 or -1.
 Output signal is either +1Sinωct or -1 Sinωct
 In BPSK, the rate of change of output is equal to the rate of change of input.
 For altering 1’s and 0’s sequence, the widest output bandwidth occurs.
 Frequency (fa) of binary sequence is equal to half the bit rate. (fb/2).
fa= fb/2
 BPSK output = [sin (2πfat)]* [sin (2πfct)]
Where fa= frequency of binary i/p
fc= frequency of carrier
 By solving the trignometry identity for the product of 2 sine functions
½ cos [2π(fc-fa)t] - ½ cos [2π(fc+fa)t]
 Minimum Nyquist bandwidth (B) is, (fc+fa) - (fc-fa) = 2fa
B= 2fa
= 2fb/2
= fb
BPSK receiver:
 The i/p signal is - Sinωct or + Sinωct.
 Signal is detected by coherent carrier recovery circuit and it regenerates the carrier
signal which is both frequency and phase coherent with the original transmitter carrier
signal.
 Balanced modulator multiplies the carrier signal by received signal.
 Then it is given to the LPF which filters the recovered binary data from the
demodulator signal.
For logic 1 or + Sinωct, the o/p of the balanced modulator is,
o/p = (Sinωct) (Sinωct)
= Sin2ωct …………………………………………. (1)
= ½ (1 – cos2ωct)
= ½ - 1/2 cos2ωct (filtered out) ……………………(2)
=+½ V
= logic 1 ……………………………………………(3)
 The o/p of the balanced modulator has the positive voltage and cosine wave at twice
the carrier frequency 2ωct.
 In eqn (2) second harmonic of the carrier is filtered out by LPF and passes only the
positive constant component which is a demodulated logic 1.
For logic 0 or - Sinωct, the o/p of the balanced modulator is,
o/p = (- Sinωct) ( Sinωct)
= - Sin2ωct …………………………………………. (4)
= ½ - (1 – cos2ωct)
= - ½ + 1/2 cos2ωct (filtered out) ……………………(5)
=-½ V
= logic 0 ……………………………………………...(6)
 The o/p of the balanced modulator consists of negative voltage and cosine wave at
twice the carrier frequency 2ωct.
 In eqn (5) second harmonic of the carrier is filtered out by LPF and passes only the
positive constant component which is a demodulated logic 0.
Advantages of PSK:
 Requires less bandwidth.
 Gives minimum probability of error.
 Good noise immunity.
Disadvantages of PSK:
 Generation and detection is difficult.
Applications:
 Used for higher bit rates (1800 bps).
 Due to less bandwidth, BPSK modems are preferred over FSK at high operating
speed.
Quadrature Phase Shift Keying (QPSK):
 Also called as Quaternary phase shift keying.
 Constant amplitude digital modulation scheme.
 Also called as M-ary encoding scheme, where M (no of phases) = 2^n, where n= no of
bits.
 Four possible outputs for a single carrier frequency.
 Binary i/p is combined into group of two bits. (00, 01, 10, 11) called dibits.
 Each dibit code generates one of four possible o/p phases.
o (+45○, +135○, -45○, -135○)
QPSK Transmitter:
 Each dibit produces one out of phase change. Hence rate of change at o/p (baud) is
one half the bit rate.
 Two bits are clocked into bit splitter.
 Both i/p’s are serially given to the i/p and they are sent out in parallel.
 One bit is passed to the I channel and the other bit is passed to the Q channel.
 I bit modulates the carrier signal is in phase with the reference oscillator & Q
bit modulates the carrier signal in 90○ out of phase or in quadrature with the
reference.
 Basically QPSK modulator is two BPSK modulators combined in parallel.
 Two phases are occurs at the o/p of I balanced modulator (+Sinωct, -Sinωct)
similarly two phases occurs at the o/p of the Q balanced modulator (+Cosωct,
- Cosωct).
 Linear summer adds the two quadrature signals, resulting in four phasors.
 i.e., (+Sinωct +Cosωct, +Sinωct - Cosωct, -Sinωct + Cosωct, -Sinωct - Cosωct)
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Angular separation between any 2 adjacent phasors in QPSK in 90○.
Hence a QPSK signal can undergo almost +45○ or -45○ shift in phase during
transmission.
QPSK Waveform:
Bandwidth consideration of QPSK:
 Bit rate in either I or Q channel is equal to half the i/p rate (fb/2).
 QPSK o/p signal doesn’t change its phase until two bits have been clocked into the bit
splitter, the o/p rate of change (baud) is equal to the half the i/p bit rate.
 The binary i/p have a 1100 repetitive pattern which is given to the I or Q balanced
modulators.
 The highest transition in the I or Q channel takes the same time as four i/p data bits.
 The highest fundamental frequency at the i/p and fastest rate of change at the o/p of
the balanced modulators is equal to one fourth of binary i/p bit rate (fb/4).
 The o/p of the balanced modulator is

The o/p frequency spectrum extends from fc-(fb/4) or fc+(fb/4) and the minimum
bandwidth (fN) is,
QPSK Receiver:

Received QPSK signal is given to the power splitter which directs the i/p signal to the
and Q product detectors and carrier recovery circuit.
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Function of carrier recovery is to reproduce the original transmit carrier signal.

Recovered carrier is frequency and phase coherent with the transmit reference carrier.

I and Q product detectors, demodulate the QPSK signal and generates I and Q data
bits.

O/p of the product detectors given to the combining circuit, where they are converted
from parallel I and Q data channels to a single binary o/p data stream.

Received QPSK signal may be any one of the four possible o/p phases.

Let, -Sinωct + Cosωct be the received QPSK signal which is given as the i/p tp the I
product detector. The other i/p is recovered carrier Sinωct.

The o/p of I product detector is,

Next, -Sinωct + Cosωct be the received QPSK signal which is given as the i/p tp
the Q product detector. The other o/p is 90○ phase shifted recovered carrier cosωct.

The o/p of Q product detector is,
QPSK Truth Table, Phasor diagram and Constellation diagram:
Advantages of QPSK:
 Due to reduced bandwidth, the information transmission rate of QPSK is high.
 Amplitude remains constant, hence the carrier power also remains constant.
 Bit error rate s reduced as half as compared to BPSK.
Disadvantages of QPSK:
 Generation and detection is complex.
Quadrature Amplitude Modulation: (QAM)
 Both the amplitude and the phase of the carrier are varied with respect to the binary
information signal.
 Combination of FSK and PSK.
 Digital information is contained in both the amplitude and the phase of the
transmitted carrier.
 In a QAM signal, there are two carriers, each having the same frequency but differing
in phase by 90 degrees.
 One is called I signal and the other is called Q signal.
 One signal can be represented by a sine wave and the other by a cosine wave.
 Two modulated carriers are combined at the source for transmission.
 At the destination, the carriers are separated, the data is extracted from each, and then
the data is combined into the original modulating signal.
8 QAM:
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
8-QAM is an M-ary encoding technique where M = 8.
Unlike 8- PSK, the output signal from an 8-QAM modulator is not a constantamplitude signal.
8-QAM transmitter:
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In 8-PSK, the incoming data are divided into groups of three bits (tribits): the I, Q,
and C bit streams,
Each with a bit rate equal to one-third of the incoming data rate.
The I and Q bits determine the polarity of the PAM signal at the output of the 2-to-4level converters, and the C channel determines the magnitude.
Because the C bit is fed uninverted to both the I and the Q channel 2 to-4-level
converters. The magnitudes of the I and Q PAM signals are always equal.
Their polarities depend on the logic condition of the I and Q bits and, therefore, may
be different.
The truth table for the I and Q channel 2-to-4-level converters; they are identical
8 QAM Truth Table, Phasor diagram and Constellation Diagram:
8 QAM Waveform:
8 QAM Receiver:
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Power splitter directs the input 8-QAM signal to the I and Q product detectors and the
carrier recovery circuit.
Carrier recovery circuit reproduces the original reference oscillator signal.
Incoming 8-PSK signal is mixed with the recovered carrier in the I product detector
and with a quadrature carrier in the Q product detector.
Outputs of the product detectors are 4-level PAM signals that are fed to the 4-to-2level analog-to-digital converters (ADCs).
Outputs from the I channel 4-to-2-level converter are the I and C bits.
Outputs from the Q channel 4-to-2-level converter are the Q and C^ bits.
Parallel-to-serial logic circuit converts the I/C and Q/C^ bit pairs to serial I, Q, and C
output data streams.
Bandwidth Efficiency
Bandwidth efficiency (sometimes called information density or spectral efficiency, often used
to compare the performance of one digital modulation technique to another.
Mathematical bandwidth efficiency is
Carrier Recovery:
 Process of extracting a phase coherent reference carrier from a receiver signal.
 Also called as phase referencing or absolute phase encoding.
Mechanism:
 To correctly demodulate the data, a phase coherent carrier was recovered and
compared with the received carrier in a product detector.
 To determine absolute phase of the carrier, it is necessary to produce a carrier at the
receiver that is phase coherent with the transmit reference oscillator.
 With PSK ad QAM, the carrier is suppressed in the balanced modulators and therefore, is
not transmitter.
 To overcome this problem, sophisticated methods of carrier recovery are required such as
squaring loop, costas loop or remodulator.
Squaring Loop:
 Common method for achieving carrier recovery for BPSK.
 Received BPSK signal is filtered and then squared.
 Filtering reduces the spectral width of the received noise.
 Squaring circuit removes the modulation and generates the second harmonic of the
carrier frequency.
 Harmonic is phase tracked by the PLL.
 VCO o/p frequency from the PLL then is divided by 2 and used as the phase
reference for the product detectors.
 With BPSK only 2 output phases are possible: +sinωct and -sinωct
For a received signal of +sinωct, the o/p of the squaring circuit is
o/p = (Sinωct) (Sinωct)
= Sin2ωct
= ½ (1 – cos2ωct)
= ½ - 1/2 cos2ωct (filtered out)
=+½ V
For a received signal of -sinωct, the o/p of the squaring circuit is
o/p = (- Sinωct) ( Sinωct)
= - Sin2ωct
= ½ - (1 – cos2ωct)
= - ½ + 1/2 cos2ωct (filtered out)
=-½ V
Costas Loop:
 Another method of carrier recovery is costas or quadrature loop.
 It produces the same results as a squaring circuit followed by an ordinary PLL in the
place of the BPF.
 This recovery scheme uses two parallel tracking loops (I and Q) simultaneously to
derive the product of the I and Q components of the signal that drives the VCO.
 The in-phase (I) loop uses the VCO as in a PLL, and the quadrature (Q) loop uses a
90○ shifted VCO signal.
 Once the frequency of the VCO is equal to the suppressed carrier frequency, the
product of the I and Q signals will produce an error voltage proportional to any phase
error in the VCO.
 The error voltage controls the phase and thus the frequency of the VCO.
Differential Phase-Shift Keying (DPSK):
 In DPSK the binary input information is contained in the difference between two
successive signaling elements rather than the absolute phase.
 Changes phase each time a 1 bit is transmitted and does not change phase when a 0
bit is transmitted.
 Combination of two operations:
 Differential encoding
 Phase shift keying
 No need to recover a phase coherent carrier.
DBPSK transmitter:
 Incoming information bit is XNORed with the preceding bit prior to entering the
BPSK modulator.
 For the first data bit, there is no preceding bit with which to compare it.
 Therefore, an initial reference bit is assumed.




If the initial reference bit is assumed a logic 1, the output from the XNOR circuit is
simply the complement of that shown.
The first data bit is XNORed with the reference bit.
If they are the same, the XNOR output is a logic 1; if they are different, the XNOR
output is a logic 0.
The balanced modulator operates the same as a conventional BPSK modulator; a
logic I produces +sinωct at the output, and a logic 0 produces -sinωct at the output.
XNOR operation
First Bit Reference Bit
1
1
0
0.
0
1
1
0
Output
1
1
0
0
DPSK Output
Output DPSK Output
1
-Sinωct
0
+ Sinωct
DBPSK receiver:
 The received signal is delayed by one bit time, then compared with the next signaling
element in the balanced modulator.
 If they are the same, a logic 1(+ voltage) is generated.
 If they are different, a logic 0 (- voltage) is generated.
 If the reference phase is incorrectly assumed, only the first demodulated bit is in
error.
Advantages:
 Simple to implement.
 No carrier recovery circuit is needed.
Disadvantages:
 Requires between 1 dB and 3 dB more signal-to-noise ratio to achieve the same
bit error rate as that of absolute PSK.
Bandwidth of DPSK signal:



In DPSk, two bits form a symbol
Symbol duration Ts = 2Tb
Tb- duration of a single bit
Minimum bandwidth required for DPSK is,
B= 2/Ts
= 2/Tb
= 1/Tb
=fb
Comparison of BPSK and DPSK:
Parameter
Variable
Characteristics
Effect of Noise
Bandwidth
Error Probability
Complexity
Need of synchronous
carrier
Detection Method
Bit determination at
receiver
BPSK
DPSK
Phase
Phase
Low
Higher than DPSK
Low
Lower than DPSK
Need
Higher than BPSK
Lower than BPSK
Higher than BPSK
Higher than BPSK
No need
Synchronous
Based on single bit
interval
Synchronous
Based on signal
received in 2
successive bit intervals
Comparison of Digital Modulation Systems:
Parameter
Variable
Parameter
ASK
BFSK
Amplitude Frequency
BPSK
Phase
QPSK
Phase
No. of bits per
symbol
No. of possible
symbol M=2^n
Demodulation
Method
Symbol
duration
Minimum
Bandwidth
1
1
1
2
2
2
2
4
Tb
M=
2^N
Non-coherent Coherent Coherent Coher
ent
Tb
Tb
2 Tb
N Tb
2fb
4fb/N
2fb
fb
Coherent
QAM
Amplit
ude &
Phase
N
2fb/N