Encoding and Modulating

Encoding and Modulating

Different Conversion Schemes

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

Bipolar AMI Encoding
B8ZS Encoding
HDB3 Encoding


Analog to Digital Encoding

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

ASK
FSK
PSK

Now we will study

ASK
FSK
PSK
QAM

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

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)

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

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

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

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

Let x(t) = cos(2fmt)
Derive an express for s(t)

Answer:
s(t) = cos(2fct) + (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

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)