Encoding and Modulation

Baud ratePulse encoding (digital to digital)Modulation (digital to analog)Pulse code modulation

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CSE3318 slide 2 Module 5

Encoding

There are four types:Digital information, digital signalAnalog information, digital signalDigital information, analog signalAnalog information, analog signal

Modulation - data onto analog signalEncoding - data onto digital signal

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CSE3318 slide 3 Module 5

Encoding vs Modulation

Encoder

g(t)digital

oranalog

x(t)

digital

g(t)Decoder

(a) Encoding onto a digital signal

x(t)

t

Modulator

m(t)digital

oranalog

s(t)

analog

m(t)Demodulator

(a) Modulation onto an analog signal

S(f)

tfc

fc

CSE3318 slide 4 Module 5

Digital Data, Digital Signals

Categories of this encoding are:Unipolar - one voltage level used.Polar - two voltage levels are use. Examples NRZ, NRZ-L, NRZ-I, RZ and Manchester encodingBipolar - ones are represented by alternating positive and negative voltages: examples include AMI, B8ZS, HDB3.

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CSE3318 slide 5 Module 5

Baud rate

Also known as Signalling rate or modulation rate.

Signal elements per second (baud).The rate at which signal elements are transmitted. bit rate = baud rate x M where M is the number of bits per signal elementfor two-level signalling (binary), bit rate is equal to the baud rate.

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CSE3318 slide 6 Module 5

Criteria for Digital Encoding Formats

Various techniques are compared in terms of the following:

Reduced bandwidth.Ease of synchronization.No zero frequency component (DC). Possible error detection.Reduced cost and complexity.Immunity to noise and interference.

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CSE3318 slide 7 Module 5

Unipolar

Binary 1 is encoded as a positive value; Binary 0 as zero voltage, or an idle line. Unipolar encoding is simple and primitive. The average amplitude of a unipolar signal is nonzero. This creates a DC component. Some transmission media cannot handle that. When a signal is not varying (e.g. long runs of 1s or 0s), the receiver cannot determine the beginning and ending of each bit.

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CSE3318 slide 8 Module 5

Non-return to zero (NRZ)

In NRZ-L, the level of signal depends on the type of bit it represents. A positive voltage represents binary 1, and a negative voltage represent binary 0. In NRZ-I, the transition between a positive and a negative voltage represents a 1 bit. A 0 bit is represented by no change. An advantage of NRZ-I over NRZ-L is that signal changes every time a 1 bit is transmitted, it enables synchronization.

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CSE3318 slide 9 Module 5

Manchester encoding

In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.

Binary 0 = positive-to-negative transitionBinary 1 = negative-to-positive transition

In Differential Manchester encoding, the transition at the middle of the bit is used only for synchronization.

Always a transition in middle of interval.Binary 0 = transition at beginning of interval.Binary 1 = no transition at beginning of interval.

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CSE3318 slide 10 Module 5

Bipolar encoding

Bipolar encoding uses three voltage levels: positive, negative and zero.

Bipolar-AMI0 bit = no line signal1bit = positive or negative level, alternating for successive ones. This encoding achieves two things: first, the DC component is zero, and second, a long run of 1s stays synchronized.

Pseudoternary0 bit = positive or negative level, alternating for successive zeros1 bit = no line signal

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CSE3318 slide 11 Module 5

Digital Data, Analog Signals

Modem - to produce signals in the voice frequency range (300-3400Hz).Carrier signal is a sine wave.Modulation - to superimpose digital data on a carrier signal. One or more characteristics of carrier is changed

Amplitude, Frequency or Phase.

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CSE3318 slide 12 Module 5

Amplitude Shift Keying (ASK)

In ASK, the amplitude of the carrier signal is varied to represent binary 1 or 0. Two binary values are represented by two different amplitudes the carrier.

Binary 1 = Acos(2pi f_c t).Binary 0 = 0.Where f_c is the carrier frequency.

Inefficient and susceptible to noise. Optical fiber channels.

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CSE3318 slide 13 Module 5

Frequency-shift-keying (FSK)

In FSK, the frequency of the carrier signal is varied to represent binary 1 or 0. The two binary values are represented by two different frequencies.

Binary 1 = Acos(2 pi f_1 t).Binary 2 = Acos(2 pi f_2 t).

Full-duplex operation over voice grade lines.High-frequency operation.Used in some local area networks.

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CSE3318 slide 14 Module 5

Phase-shift-keying (PSK)

In ASK, the phase of the signal is varied to represent binary 1 or 0. The phase of the carrier signal is shifted to represent data. In the binary case:

Binary 1 = A cos(2 pi f_c t + pi).Binary 0 = Acos(2 pi f_c t).

4,8,16 levels of signalling possible.High efficiency.High speed modems.

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CSE3318 slide 15 Module 5

QPSK

4PSK; the phase of the carrier signal is shifted to represent data.

Binary 00 = A cos(2 pi f_c t ).Binary 01 = Acos(2 pi f_c t+pi/2)Binary 11 = Acos(2 pi f_c t+pi)Binary 10 = Acos(2 pi f_c t + 3pi/2)

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CSE3318 slide 16 Module 5

Quadrature amplitude modulation

In QAM, both the phase and amplitude of the carrier signal vary. QAM enables a higher data transmission than other modulation methods.

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CSE3318 slide 17 Module 5

Analog-to-digital encoding

This is called pulse code modulation (PCM). PCM involves sampling, quantizing each sample to a set number of bits, and then assigning voltage levels to the bits. The term sampling means measuring the signal at regular intervals.

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CSE3318 slide 18 Module 5

Pulse amplitude modulation (PAM)

The first step of PCM is called PAM. This method takes analog information, samples and generates a series of pulses based on the results of the sampling. According to the Nyquist theorem, the sampling rate must be at least two times the highest frequency.

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CSE3318 slide 19 Module 5

Voice Digitization

Normal voice signal bandwidth 4kHz. Sampling rate 8000/sec. 8 - bit encoding (256 levels)64 kbps - PCM signal

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CSE3318 slide 20 Module 5

PCM

Analogvoice signal

Sampling

PAM signal

Quantizerand compander

PCM signal

CSE3318 slide 21 Module 5

Quantization

n-bit encoding, there are only 2^n - levelsSignal level x approximated by the nearest quantization level. SNR due to this noise is given by

SNR = 6n , approximately

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CSE3318 slide 22 Module 5

Companding

Lower amplitudes are more affected by the quantization noise. Uniform quantizing is not effectiveNon uniform quantizingMore gain to weak signals

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