Base Band pulse signaling

Wollo University
KIoT
School of Electrical and Computer Engineering

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Overview
 Sampling Theorem
 Pulse Analog Modulation Techniques
 Pulse Amplitude Modulation (PAM)
 Pulse Width Modulation (PWM)
 Pulse Position Modulation (PPM)
 Pulse Digital Modulation Techniques
 Sampling, Quantizing and Encoding
 Pulse Code Modulation (PCM)
 Delta Modulation (DM)
 Differential PCM (DPCM)
 Line Coding Techniques: RZ, NRZ, BRZ, Split-Phase
 Digital Modulation Techniques: ASK, PSK, FSK & QAM

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Sampling an analog signal.

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Sampling theorem illustration

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Sampling theorem illustration

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Effect of aliasing

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Anti-aliasing filter

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Aliasing phenomena

The aliased spectrum shown by the solid curve in Fig. (b) pertains to an
“undersampled” version of the message signal represented by the
spectrum of Fig. (a).

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Cont..
 To combat the effects of aliasing in practice, we may use
two corrective measures:
1. Prior to sampling, a low-pass anti-alias filter is used to
attenuate those high-frequency components of a message
signal that are not essential to the information being
conveyed by the signal.
2. The filtered signal is sampled at a rate slightly higher than
the Nyquist rate.
 The use of a sampling rate higher than the Nyquist rate
also has the beneficial effect of easing the design of the
synthesis filter used to recover the original signal from its
sampled version.

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PAM
 The amplitudes of regularly spaced pulses are varied in
proportion to the corresponding sample values of a
continuous message signal; the pulses can be of a
rectangular form or some other appropriate shape.
 PAM as defined here is somewhat similar to natural
sampling, where the message signal is multiplied by a
periodic train of rectangular pulses. In natural sampling,
however, the top of each modulated rectangular pulse is
permitted to vary with the message signal, whereas in PAM
it is maintained flat.

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PAM waveforms

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Waveforms of PAM detection

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Pulse Width Modulation
 Illustration of PWM, (a) trailing edge, (b) leading edge and (c) both edges.

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(a) Modulating wave. (b) Pulse carrier. (c) PWM wave. (d) PPM wave.

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Generation of PWM Signal

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Practical PWM Generator Circuits

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Demodulation of PWM Signal

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Generation of PPM Signal

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Demodulation of PPM signal

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Quantization process
• Amplitude quantization is defined as the process of
transforming the sample amplitude 𝑚(𝑛𝑇𝑠) of a baseband
signal 𝑚(𝑡) at time 𝑡=𝑛𝑇𝑠 into a discrete amplitude 𝑣(𝑛𝑇𝑠)
taken from a finite set of possible levels.
• Quantizers can be of a uniform or non-uniform type. In a uniform
quantizer, the representation levels are uniformly spaced; otherwise,
the quantizer is non-uniform. The quantizers considered in this section
are of the uniform variety.

Fig. 4.19: Description of a memoryless quantizer.

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Quantization process
 Two types of quantization: (a) mid-tread and (b) mid-rise.

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PCM transmission

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Delta modulation: staircase approximation

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DM system: (a) Transmitter and (b) receiver.

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Quantization Errors
 Illustration of quantization errors, slope-overload distortion
and granular noise, in delta modulation.

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 Differential Pulse Code Modulation
 DPCM system: (a) Transmitter and (b) receiver (c) prediction filter

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Line Coding techniques

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Digital communication systems
 Digital Modulation uses
 Sinusoidal carrier signal like analog modulations
 Digital signals as a message
• Why digital modulation?
As compared to analog modulation, digital modulation
offers many advantages:
Greater noise immunity
Robustness to channel impairments
Greater security (encryption)
More flexibility
Easier multiplexing
More information capacity

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Types of digital modulation techniques
 Digital modulation techniques can be classified in to the
following broad categories

 Two types
 Binary or M-ary signaling

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 Binary modulation
 In binary modulation, the modulator produces one of two
distinct signals in response to one bit of source data at a time.

• In all cases,
 One bit is sent per symbol

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 Amplitude Shift Keying (ASK)

 Representation: 𝑠 𝑡 = 𝑚(𝑡)𝑉𝑐 cos(2𝜋𝑓𝑐 𝑡)

 ASK modulation wave form example

 Demodulation: only the presence or absence of a sinusoid in

a given time interval needs to be determined
 Advantage: simplicity
 Disadvantage: ASK is susceptible to noise and interference
 Application: is used to transmit digital data over optical fiber

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 Phase Shift Keying (PSK)
 Representation: 𝑠 𝑡 = 𝑉𝑐 cos(2𝜋𝑓𝑐 𝑡 + ∆𝑝𝑚(𝑡))

 Demodulation: demodulator must determine the phase of

received sinusoid with respect to some reference phase
 Advantage: PSK is less susceptible to errors than ASK and
has more efficient bandwidth utilization compared to FSK
 Disadvantage: more complex signal detection PSK
modulation wave form example
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 Frequency Shift Keying (FSK)
 FSK modulation waveform example
𝑉𝑐 cos 2𝜋𝑓1 𝑡 + 𝜃1 𝑓𝑜𝑟 𝑏𝑖𝑛𝑎𝑟𝑦 1
 Representation: 𝑠 𝑡 =
𝑉𝑐 cos(2𝜋𝑓2 𝑡 + 𝜃2 ) 𝑓𝑜𝑟 𝑏𝑖𝑛𝑎𝑟𝑦 0

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FSK Modulation

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FSK demodulation

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 M-ary modulation
 In binary modulation, we send only one of two possible
signals during each bit interval 𝑇𝑏
 In M-ary modulation, we can send one of 𝑀 possible signals
during each signaling interval 𝑇
 In almost all applications, 𝑀 = 2𝑛 and 𝑇 = 𝑛𝑇𝑏 , where 𝑛 is
an integer
 Each of the M signals is called a symbol
 These signals are generated by changing the amplitude,
phase, frequency, or combined forms of a carrier in 𝑀
discrete steps.
 Thus, we have:

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 M-ASK
 Amplitude of a carrier takes M different values
 For example, if M = 4, the ASK refers to four different
Amplitudes in which the carrier is sent.
 As 4 states are possible, two bits can be encoded per symbol
 In general, if number of possible states 𝑀 > 2, each symbol
can carry log 2 𝑀 bits.
𝑀 = 4, log 2 4 = 2 → 2 bits symbol
𝐴1 cos 2𝜋𝑓𝑐 (𝑡) 𝐴1 = 0: 00
𝐴2 cos 2𝜋𝑓𝑐 (𝑡) 𝐴2 = 1: 01
𝑆 𝑡 =
𝐴3 cos 2𝜋𝑓𝑐 (𝑡) 𝐴3 = 2: 10
𝐴4 cos 2𝜋𝑓𝑐 (𝑡) 𝐴4 = 3: 11

 This scheme is therefore more bandwidth efficient than BASK.

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Cont. …
 Constellation plots of M-ASK

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M-FSK

• The frequency of the carrier takes on M possible values:

• For example, if M = 4, the FSK refers to four different
frequencies in which the carrier is modulated and sent.
• As 4 states are possible, two bits can be encoded per
symbol.
• The following diagram shows an example of frequency
modulated signal waveform

𝐴 cos 2𝜋𝑓1 (𝑡) 00

𝐴 cos 2𝜋𝑓2 (𝑡) 01
𝑆 𝑡 =
𝐴 cos 2𝜋𝑓3 (𝑡) 10
𝐴 cos 2𝜋𝑓4 (𝑡) 11

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 M-PSK
The phase of the carrier takes on M possible values:
 When 𝑀 = 4,the modulation scheme is called QPSK
 Since 2 bits are allocated to each symbol, QPSK can achieve
twice the data rate of a comparable BPSK scheme for a given
bandwidth.

𝐴 cos 2𝜋𝑓𝑐 𝑡 + 𝜑1
𝜑1 =00 𝜑1 = 450 00
𝐴 cos 2𝜋𝑓𝑐 𝑡 + 𝜑2 𝑂𝑟 𝜑1 = 1350 01
𝑆 𝑡 = 𝜑1 = 900
𝐴 cos 2𝜋𝑓𝑐 𝑡 + 𝜑3 𝜑3 = 2250 10
𝜑3 = 1800
𝐴 cos 2𝜋𝑓𝑐 𝑡 + 𝜑4 𝜑4 = 3150 11
𝜑4 = 2700

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Cont. …
 Constellation plots of M-ary PSK

 Thus, As M increases, the bandwidth efficiency increases

but the waveform energy (i.e., the transmission power used
to send the symbol) must be increased to keep the BER at
a certain level.

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 M-QAM
 The principle of M-QAM is to have X possible variations in
phase and Y possible variations of amplitude
 X • Y possible variations
 Example: 8-QAM
 2 different Amplitudes (𝐴1 = 1; 𝐴2 = 2)
 4 different Phases (0𝑜 , 90𝑜 , 180𝑜 , 270𝑜 )
 3 bits per symbol
• We can have numerous possible variations of phase shifts
and amplitude shifts
• However the number of phase shifts should be selected to
be greater than number of amplitude shifts.
 16-QAM
 There are sixteen QAM symbols 4 bits per symbol

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QAM Constellation examples

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 Performance measures
Two key performance measures of a modulation scheme are;
 Power efficiency: Describes the ability of a modulation
technique to preserve the fidelity of the digital message at low
power levels
 Fidelity: an acceptable bit error probability
 Often expressed as the ratio of the signal energy per bit (𝐸𝑏 ) to
the noise PSD (𝑁0 ) required to achieve a given probability of
error (say 10−5 ):

 is a measure of how favorably the tradeoff between fidelity and

signal power is made

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 Cont. …
 Bandwidth efficiency: the ability of a modulation scheme to
accommodate data within a limited bandwidth
 It is defined as the ratio of the data rate R to the required RF
bandwidth B:

 Channel capacity gives an upper bound of achievable

bandwidth efficiency:

 It reflects how efficiently the allocated bandwidth is utilized

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Bandwidth and power efficiency comparisons

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Selection of modulation systems

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