21. Which of the error detection is more efficient?
Parity check
Cyclic redundancy check
Parity & Cyclic redundancy check
None of the mentioned
Answer.2. Cyclic redundancy check
Explanation
A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. A cyclic redundancy check is more efficient than a parity check.
Parity check is the simplest technique for detecting and correcting errors. The MSB of an 8-bits word is used as the parity bit and the remaining 7 bits are used as data or message bits. The parity of the 8-bits transmitted word can be either even parity or odd parity.
22. To guarantee correction of upto 5 errors in all cases, the minimum Hamming distance in a block code must be ______.
11
6
5
2
Answer.1. 11
Explanation
To guarantee the correction of up to s errors in all cases, the hamming distance in a block code must be d ≥ 2s + 1
Let hamming distance be minimum
∴ dmin = 2s + 1
Calculation
number of errors = s = 5.
∴ dmin = 2(5) + 1 = 11
23. Which of the following channel coding scheme helps in correcting burst errors?
CRC
Convolution coding
Viterbi
Reed-Solomon
Answer.4. Reed-Solomon
Explanation
Reed–Solomoncodes:
1) These codes are particularly well suited to correcting burst errors (where a series of bits in the codeword are received in error).
2) Reed-Solomon algebraic decoding procedures can correct errors and erasures. An erasure occurs when the position of an erred symbol is known.
3) These codes are based on univariate polynomials over finite fields.
4) They are used to detect and correct multiple errors i.e. burst errors.
24. Which of the following can detect two-bit errors?
Parity check
Cyclic redundancy check
Parity & Cyclic redundancy check
None of the mentioned
Answer.2. Cyclic redundancy check
Explanation
A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. CRC is more powerful and it can detect various kinds of errors like 2-bit errors.
25. In a code-division multiple access (CDMA) system with N = 8 chips, the maximum number of users who can be assigned mutually orthogonal signature sequences is _______.
8
10
5
15
Answer.1. 8
Explanation
Spread factor (or) process gain determines to some extent the upper limit to the total number of users supported simultaneously by a station.
It is defined as:
SF = Chip Rate/Symbol Rate
Calculation:
If a single symbol is represented by a code of 8 chips, the chip rate will be:
80 × symbol rate
∴ The spreading factor (SF) will be:
SF = 8 × Symbol Rate/Symbol Rate
SF = 8
26. Consider a binary channel code in which each codeword has a fixed length of 5 bits. The Hamming distance between any pair of distinct codewords in this code is at least 2. The maximum number of codewords such a code can contain is ________.
10
16
8
4
Answer.2. 16
Explanation
Concept: Hamming distance gives minimum positions at which the corresponding symbols are different.
Calculation: Fixed length code n = 5
Minimum harming distance d min = 2
Hamming distance gives minimum positions at which the corresponding symbols are different.
The table shows all possible codewords having a minimum hamming distance of 2.
C4
C3
C2
C1
C0
0
0
0
0
1
0
0
0
1
0
0
0
1
0
0
0
0
1
1
1
0
1
0
0
0
0
1
0
1
1
0
1
1
0
1
0
1
1
1
0
1
0
0
0
0
1
0
0
1
1
1
0
1
0
1
1
0
1
1
0
1
1
0
0
1
1
1
0
1
0
1
1
1
0
0
1
1
1
1
1
Hence there are 16 such codes where minimum Hamming distance 2 is possible.
27. CRC or Cyclic Redundancy Check uses
Multiplication
Binary division
Multiplication & Binary division
None of the mentioned
Answer.3. Multiplication & Binary division
Explanation
CRC or Cyclic Redundancy Check is a method of detecting accidental changes/errors in the communication channel. CRC uses Generator Polynomial which is available on both sender and receiver sides. It also uses more math like multiplication and binary division.
28. In a mobile communication system, the handoff is a technique used to:
Reduce the effect of signal fading
Optimize power
Maintain an acceptable link quality when a mobile station traverse a cell boundary
Optimize the traffic
Answer.3. Maintain an acceptable link quality when a mobile station traverse a cell boundary
Explanation
Handoff is the process of changing the channel (frequency, time slot, spreading code, or combination of them) associated with the current connection while a call is in progress.
It is often initiated either by crossing a cell boundary or by a deterioration in the quality of the signal in the current channel.
In the mobile communication system, handoff is a technique used to maintain an acceptable link quality when a mobile station traverses a cell boundary.
