Test: Fundamentals of Error Correction - Electronics and Communication Engineering (ECE) MCQ

The "G" in cell phone standards, such as 1G, 2G, 3 G, 4G, and 5G, stands for "generation." It represents the different generations used in mobile networks.

• 1G (First Generation): Introduced in the 1980s, 1G represented the first analog cellular network with basic voice calling capabilities.
• 2G (Second Generation): Introduced in the late 1980s and early 1990s, 2G brought digital communication and introduced features like text messaging (SMS) and picture messaging (MMS).
• 3G (Third Generation): Introduced in the early 2000s, 3G brought faster data rates, allowing for mobile internet access, video calling, and more advanced mobile services.
• 4G (Fourth Generation): Introduced in the late 2000s, 4G brought significant improvements in data rates, capacity, and network efficiency, enabling high-quality video streaming, online gaming, and other data-intensive applications.
• 5G (Fifth Generation): Introduced in the 2010s, 5G represents the latest and most advanced cellular network technology. It offers even faster data rates, lower latency, higher capacity, and supports advanced technologies like Internet of Things (IoT), augmented reality (AR), and virtual reality (VR).

To represent the 4-bit code 1101 into a 7-bit, even parity Hamming code, we first need to add 3 parity bits to the 4 data bits. The parity bits are chosen so that the total number of 1's in each group of bits (including the parity bit) is even.
Hamming Code:

Given data 1101
i.e. d1 = 1, d2 = 1, d3 = 0, d4 = 1
We can write:
P1 = d1 ⊕ d2 ⊕ d4 = 1 ⊕ 1 ⊕ 1 = 1
P2 = d1 ⊕ d4 ⊕ d3 = 1 ⊕ 1 ⊕ 0 = 0
P3 = d2 ⊕ d4 ⊕ d3 = 1 ⊕ 1 ⊕ 0 = 0
Then transmitted final code is P1 P2 d1 P4 d2 d3 d4 which is 1010101

• Convolution codes are used in those applications where the performance requirement is good and the implementation cost is low
• For the purpose of decoding convolution code, the Viterbi algorithm is used
• Viterbi algorithm is based on Maximum likelihood decoding
• Viterbi algorithm is used in most of the problems in digital communication systems
• This algorithm cannot detect an error but it sometimes corrects the error by finding the path with the minimum metric value
• The maximum likelihood decoding of convolutional encoders with the Viterbi algorithm is an efficient forward error correction method
• This algorithm is best suitable for single and double-bit error correction

Hence the correct option is 1

Convolution codes

• Convolution codes are generally error-detecting codes 
• They are comprised of parity bits with the message bits
• In convolution coding, k number of message bits are encoded continuously with m number of parity bits to have an overall sequence of n bits
• Linear matrix is not used to represent convolution code

The convolution code is generally represented by

• State diagram
• Logic tables
• Tree diagram
• Trellis diagram
• Generator polynomial 
• Generator matrix

Hence the correct option is 4

General Packet Radio Service

• GPRS, or General Packet Radio Service, is a best-effort packet-switching communications protocol for cellular networks.  
• GPRS was one of the first widely used data transfer protocols on cellular networks.
• GPRS is a third-generation step toward internet access.
• GPRS is also known as GSM-IP that is a Global-System Mobile Communications Internet Protocol as it keeps the users of this system online, allows them to make voice calls, and access the internet on-the-go.

Hamming codes are linear block codes. The family of (n, k) hamming codes for q ≥ 3 is defined by the following expressions:

1. Number of Block diagrams (n) = 2^q – 1
2. Number of message bits (k) = 2^q – q – 1
3. Number of Parity bits (q) = (n – k)
Hamming code is a set of error-correction codes that can be used to detect and correct '1' bit errors that can occur when the bitstream is sent through the channel. It can be employed in both burst and signal error correction. 

The minimum bits change between any valid Hamming codes is 3.
Analysis:

• The change of bits between the given code: 0001011 and option C code: 0011010 is two bits change only.
• So 0011010 can’t be a valid Hamming code of the same group.

Full Duplex:

• In Full-duplex mode, both stations can transmit and receive at the same time.
• The channel capacity is shared by both communicating devices at all times.
• Example: Mobile phones

Simplex:

• In Simplex, communication is unidirectional. Only one of the devices sends the data and the other one only receives the data.
• The channel capacity is used by only one device all the time.
  • Example: A CPU sends data while a monitor only receives data.

Half Duplex:

• In half-duplex both the stations can transmit as well as receive but not at the same time. When one device is sending others can only receive and vice-versa
• The channel capacity is shared by both the devices but at a time only once the device uses the full capacity.
  • Example: A walkie-talkie

CSMA stands for Carrier Sense Multiple Access. 

• It is a Media Access Control (MAC) protocol that is used to control the flow of data in a transmission media so that packets do not get lost and data integrity is maintained.
• There are two modifications to CSMA: 1) CSMA CD (Collision Detection) and 2) CSMA CA (Collision Avoidance)

CDMA stands for Code Division Multiple Access.

• It is a channel access method used by various radio communication technologies.
• The principle of the spread spectrum is used to work with CDMA.
• CDMA is not frequency specific to each user, instead, every channel uses the full available spectrum.
• Each user in a CDMA system uses a different code to modulate their signal.

FDMA stands for Frequency Division Multiple Access.

• FDMA is a channel access method used in some multiple-access protocols.
• FDMA allows multiple users to send data through a single communication channel, such as a coaxial cable or microwave beam, by dividing the bandwidth of the channel into separate non-overlapping frequency sub-channels and allocating each sub-channel to a separate user.
• Users can send data through a sub-channel by modulating it on a carrier wave at the sub-channels frequency.
• FDMA is not vulnerable to timing errors.

Time-division multiple access (TDMA):

• This is a channel access method for shared-medium networks.
• It allows several users to share the same frequency channel by dividing the signal into different time slots.
• The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity.
• TDMA uses different time slots for transmission and reception thus duplexers are not required.

As TDMA allows multiple users to use the channel, so option(3) is the correct answer.

Frequency division multiple access (FDMA):

• FDMA is a channel access method used in some multiple-access protocols.
• FDMA allows multiple users to send data through a single communication channel, such as a coaxial cable or microwave beam, by dividing the bandwidth of the channel into separate non-overlapping frequency sub-channels and allocating each sub-channel to a separate user.
• Users can send data through a sub-channel by modulating it on a carrier wave at the sub-channels frequency.

Code division multiple access (CDMA):

• CDMA stands for Code Division Multiple Access.
• CDMA (Code Division Multiple Access) uses a digital modulation technique called Direct Sequence spread spectrum which spreads voice data over a very wide spectrum using a user or cell-specific pseudo-random codes.
• It is a wireless technology used in the transmission of signals from places with high security and Noise reduction.
• The principle of the spread spectrum is used to work with CDMA.
• CDMA is not frequency specific to each user, instead, every channel uses the full available spectrum.
• Each user in a CDMA system uses a different code to modulate their signal.