**Encoding and Modulating**

**Different Conversion Schemes**

**Digital to Digital Encoding**

Types of Digital to Digital Encoding

Types of Digital to Digital Encoding

Unipolar Encoding

Unipolar --->DC component (Average Amplitude is non zero)

--->Synchronization ( Due to unvarying signal)

**Bipolar Encoding Scheme**

Types of Bipolar Encoding

Types of Bipolar Encoding

Bipolar AMI Encoding

B8ZS Encoding

HDB3 Encoding

**Analog to Digital Encoding**

PAM

PAM

**Digital to Analog Encoding**

**Digital to Analog Modulation**

Digital-to-analog modulation: the process of changing one of the characteristics of an analog signal based on the information in a digital signal

“Don’t forget”: Characteristics of a sine wave are amplitude, frequency, phase.

**Carrier Signal**

Sender

Produce a high-frequency signal that acts as a basis for the information signal => carrier signal

Modulate the carrier signal to reflect the digital information. The information signal is called the modulating signal

Receiver

Tune in the carrier frequency to receive

Digital-Analog Modulation Schemes

Digital-Analog Modulation Schemes

ASK

FSK

PSK

Now we will study

ASK

FSK

PSK

QAM

Amplitude Shift Keying (ASK)

Amplitude Shift Keying (ASK)

Peak amplitude during each bit duration is constant

**Bit Rate vs. Baud Rate**

Bit rate is the number of bits per second

- More important in speaking of computer efficiency

Baud rate is the number of signal units per second that are required to represent those bits

More important in speaking of data transmission

Determine the bandwidth required to send the signal

Analogy in transportation: a baud is analogous to a car while a bit is analogous to a passenger (1: male, 0: female). The number of cars determines the traffic; that of passengers does not

Baud Rate Example

Baud Rate Example

An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and the bit rate

Baud rate = 1000 bauds per second (baud/s)

Bit rate = 1000 x 4 = 4000 bps

The bit rate of a signal is 3000. If each signal unit carries 6 bits, what is the baud rate?

Baud rate = 3000 / 6 = 500 baud/s

**ASK Disadvantage(2)**

**Disadvantage**

Highly susceptible to noise interference because ASK relies on amplitude to differentiate between 1 and 0

Need a great gap between amplitude values so that noise can be detected and removed

OOK (on/off keying)

A popular ASK technique

Zero voltage represent a bit value (e.g., 0)

Save energy in transmitting information

Used to transmit digital data over optical fiber

**ASK: Bandwidth Requirement**

Question: What is the min bw for an ASK signal transmitting at 2000 bps? The transmission mode is half-duplex.

**Frequency Shift Keying (FSK)**

FSK: Baud Rate and Bandwidth

Question: Find the maximum bit rates for an FSK signal if the bandwidth of the medium is 12,000 Hz and the difference between the two carriers is 2000 Hz. Transmission is in full-duplex mode.

**FSK vs. ASK**

FSK

Less susceptible to error

Commonly used for high-freq (3-30 MHz) radio

Also used at even high freq on LANs that use coaxial cable

Phase Shift Keying (PSK)

Phase Shift Keying (PSK)

2-PSK: only 2 phase values are used, each for 1 or 0

4-PSK

PSK: Baud Rate and Bandwidth

Same as in ASK, but can transmit more bps given same bandwidth

**PSK: Questions**

Given a bandwidth of 5000 Hz for an 8-PSK signal, what are the baud rate and bit rate? Transmission is in half-duplex mode.

For PSK the baud rate is the same as the bandwidth, which means the baud rate is 5000. But in 8-PSK the bit rate is 3 times the baud rate, so the bit rate is 15,000 bps.

**PSK: Drawback**

Modulation device is not able to distinguish small differences in phase => limit BitRate

Why not combine PSK and ASK: x variations in phase with y variations in amplitude result in xy variations => increase bit rate

Quadrature Amplitude Modulation

Quadrature Amplitude Modulation

A combination of ASK and PSK: both phase and amplitude varied

#amplitude shifts << #phase shifts

Lower susceptible to noise than ASK, higher bit rate than PSK

8-QAM

16-QAM

QAM Modulator

QAM: Bandwidth

Bandwidth requirement is the same as in ASK and PSK

QAM: BitRate vs. Baud Rate

QAM: BitRate vs. Baud Rate

ASK, FSK, 2-PSK Bit 1 N N

4-PSK, 4-QAM Dibit 2 N 2N

8-PSK, 8-QAM Tribit 3 N 3N

16-QAM Quadbit 4 N 4N

32-QAM Pentabit 5 N 5N

64-QAM Hexabit 6 N 6N

128-QAM Septabit 7 N 7N

256-QAM Octabit 8 N 8N

Analog to Analog Modulation

Analog to Analog Modulation

Amplitude Modulation: AM

Carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal

Freq, phase remain same

Example: AM

Example: AM

Let x(t) = cos(2fmt)

Derive an express for s(t)

Answer:

s(t) = cos(2fct) + (na/2)cos(2(fc-fm)t) + (na/2)cos(2(fc+fm)t)

**AM: Bandwidth**

Audio signal bandwidth is ~5Khz => an AM radio station needs a minimum 10Khz

**Frequency Modulation: FM**

Freq. of carrier signal is modified to reflect the changing amplitudes of the modulating signal

Amp., phase remain same

FM: Bandwidth

FM: Bandwidth

The bandwidth of a stereo audio signal is usually 15 KHz. Therefore, an FM station needs at least a bandwidth of 150 KHz. Let PTA requires the minimum bandwidth to be at least 200 KHz (0.2 MHz).

**Phase Modulation: PM**

Only phase is varied to reflect the change of amplitude in modulating signal

Require simpler hardware than FM

Use in some systems as an alternative to FM

Analog to Analog Modulation

Representation of analog information by an analog signal

Why do we need it? Analog is already analog!!!

Because we may have to use a band-pass channel

Think about radio…

Schemes

Amplitude modulation (AM)

Frequency modulation (FM)

Phase modulation (PM)

In Analog-to-Analog conversion why we use modulation technique in our transmission?

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please can you say me what is the default Kn value for 128 QAM modulation?

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