Finals Exam Digital Communication
Name:
Section:
A.
1. Explain what is Information Capacity Theorem(1pt)
The term "Information Capacity Theorem" is not a widely recognized or standardized concept
in the field of information theory or related disciplines as of my last knowledge update in
January 2022. It's possible that the term could be used in a specific context or by a particular
author or researcher, but without additional context, it's challenging to provide a precise
explanation.In information theory, there are fundamental theorems and concepts such as
Shannon's Information Theory, Channel Capacity, and the Noisy Channel Coding Theorem.
2. Explain Bits, Bit rate and Baud (1pt)
1. **Bits:**
- A "bit" is the most basic unit of information in computing and digital communications.
- It can have one of two values: 0 or 1, representing binary digits.
- Information is often measured in bits, and larger units like bytes (8 bits), kilobits, megabits, etc., are
used to represent larger amounts of information.
2. **Bit Rate:**
- Bit rate, often expressed in bits per second (bps), is the number of bits transmitted or processed per
unit of time.
- It measures the speed or data transfer rate of a communication channel or a digital storage device.
- Higher bit rates generally allow for the transmission or processing of more information in a given
period.
3. **Baud:**
- Baud is a measure of signaling speed in communication systems.
- It represents the number of signal changes (or events) per second in a communication channel.
- Baud rate is often used interchangeably with bit rate, but they are not always the same. In simpler
systems, one symbol per bit is transmitted, and baud rate equals bit rate. However, in more complex
modulation schemes, multiple bits can be encoded in a single symbol, leading to a difference between
baud rate and bit rate.
3. Explain M-ary encoding (1pt)
M-ary encoding, also known as M-ary modulation, is a technique used in digital
communications to encode multiple bits into a single symbol or signal. The term "M-ary"
refers to the number of different symbols or levels used in the encoding scheme. M-ary
encoding allows for the transmission of multiple bits simultaneously, improving the efficiency
of data transmission.
In M-ary encoding, each symbol represents a group of bits, where the number of bits per
symbol is determined by the value of M. The relationship between the number of bits (b) and
the value of M is given by the equation:
\[ M = 2^b \]
This means that if you have, for example, 4 bits per symbol, you can represent \(2^4 = 16\)
different symbols. Each symbol can then carry a unique combination of 4 bits.
There are different modulation schemes under the M-ary encoding umbrella, such as:
1. **Binary Phase Shift Keying (BPSK):** M = 2, representing binary symbols (0 or 1).
2. **Quadrature Amplitude Modulation (QAM):** M = 2^n, where n is an integer. QAM uses
both amplitude and phase to encode multiple bits per symbol.
3. **Quadrature Phase Shift Keying (QPSK):** A specific case of QAM where M = 4,
representing four different symbols.
The advantage of M-ary encoding lies in its ability to transmit multiple bits simultaneously,
which can increase the data rate and spectral efficiency of a communication system. However,
it also requires more sophisticated modulation and demodulation techniques compared to
simple binary encoding.
B.
1.
2.
3.
4.
Explain how does ASK work give sample output? (4pts)
Explain how does FSK work give sample output? (4pts)
Explain how does PSK work give sample output? (4pts)
Explain how does QAM work and give simple output? (4pts)
1. Amplitude Shift Keying (ASK):
**Explanation:**
ASK is a digital modulation technique in which the amplitude of the carrier signal is varied to represent
binary data. A high amplitude might represent one binary state (e.g., 1), and a low amplitude might
represent the other binary state (e.g., 0).
**Sample Output:**
Suppose we want to transmit the binary sequence "10110" using ASK. Let's say high amplitude
(A_high) represents '1', and low amplitude (A_low) represents '0'. The carrier signal could be
modulated as follows:
\[ \text{Binary Data:} \quad 1 \quad 0 \quad 1 \quad 1 \quad 0 \]
\[ \text{ASK Modulation:} \quad A_{\text{high}} \quad A_{\text{low}} \quad A_{\text{high}} \quad
A_{\text{high}} \quad A_{\text{low}} \]
### 2. Frequency Shift Keying (FSK):
**Explanation:**
FSK involves varying the frequency of the carrier signal to represent different binary states. One
frequency might represent '1', while another represents '0'.
**Sample Output:**
Suppose we use two frequencies, \(f_1\) for '0' and \(f_2\) for '1'. Transmitting the binary sequence
"10110" using FSK might look like:
\[ \text{Binary Data:} \quad 1 \quad 0 \quad 1 \quad 1 \quad 0 \]
\[ \text{FSK Modulation:} \quad f_2 \quad f_1 \quad f_2 \quad f_2 \quad f_1 \]
### 3. Phase Shift Keying (PSK):
**Explanation:**
PSK involves changing the phase of the carrier signal to represent different binary states. Commonly, a
phase shift of 180 degrees is used for binary '0' and no phase shift for '1'.
**Sample Output:**
Assuming a 180-degree phase shift for '0' and no phase shift for '1', transmitting "10110" using PSK
might look like:
\[ \text{Binary Data:} \quad 1 \quad 0 \quad 1 \quad 1 \quad 0 \]
\[ \text{PSK Modulation:} \quad \text{No Phase Shift} \quad 180^\circ \quad \text{No Phase Shift}
\quad \text{No Phase Shift} \quad 180^\circ \]
### 4. Quadrature Amplitude Modulation (QAM):
**Explanation:**
QAM is a combination of ASK and PSK. It involves varying both the amplitude and phase of the carrier
signal to represent multiple bits.
**Sample Output:**
Let's consider 16-QAM, where four different amplitudes and four different phases are used.
Transmitting the binary sequence "10110" using 16-QAM might look like:
\[ \text{Binary Data:} \quad 1 \quad 0 \quad 1 \quad 1 \quad 0 \]
\[ \text{QAM Modulation:} \quad \text{Amplitude 3, Phase 90°} \quad \text{Amplitude 1, Phase 180°}
\quad \text{Amplitude 3, Phase 0°} \quad \text{Amplitude 3, Phase 90°} \quad \text{Amplitude 1,
Phase 180°} \]
C.
1. Draw QPSK Transmitter and Receiver and explain every part of it and also show the
output(20pts)
2. Draw 8PSK Transmitter and Receiver and explain every part of it and also show the output(20pts)
3. Draw 8QAM Transmitter and Receiver and explain every part of it and also show the
output(20pts)
4. Draw 16QAM Transmitter and Receiver and explain every part of it and also show the
output(20pts)
QPSK (Quadrature Phase Shift Keying):
Transmitter:
Binary Data Input: The input is a stream of binary data (0s and 1s).
Serial to Parallel Conversion: The binary stream is converted into two parallel streams (I and
Q).
Mapping to Symbols: Each pair of bits is mapped to a specific phase shift in the I-Q plane.
Modulation: The I and Q signals modulate the in-phase and quadrature components of the
carrier signal.
RF Signal Output: The modulated signals are combined to produce the QPSK signal.
Receiver:
RF Signal Input: The received QPSK signal is fed into the receiver.
Demodulation: The incoming signal is demodulated to extract the in-phase and quadrature
components.
Symbol Detection: The phase of the received signal is compared to predefined phase values to
determine the transmitted symbol.
Parallel to Serial Conversion: The detected symbols are converted back into a serial binary
stream.
2. 8PSK (8-Phase Shift Keying):
The structure is similar to QPSK, but with eight different phase shifts corresponding to 3 bits
per symbol.
3. 8QAM (8-Quadrature Amplitude Modulation):
Transmitter:
Binary Data Input: Input binary stream.
Serial to Parallel Conversion: Split the stream into three parts (I1, I2, and Q).
Mapping to Symbols: Each triplet of bits maps to a specific combination of amplitude and
phase.
Modulation: Amplitude and phase modulate the carrier signal.
RF Signal Output: Combined modulated signals.
Receiver:
RF Signal Input: Received signal.
Demodulation: Extract amplitude and phase information.
Symbol Detection: Determine the closest predefined symbol.
Parallel to Serial Conversion: Convert symbols back to binary.
4. 16QAM (16-Quadrature Amplitude Modulation):
The structure is similar to 8QAM but with a more extensive constellation, allowing for four bits
per symbol.