A binary symmetric channel with p as transition probability, users repetition code ‘n’ times, with n = 2m + 1 being an odd integer. In a block of n bits, if the number of zeros exceed number of decoder decodes it as ‘0” otherwise as “ 1”. An error occurs when m + 1 or more transmissions out 3 are incorrect. What is the probability of error?
A binary symmetric channel with p as transition probability, users repetition code ‘n’ times, with n = 2m + 1 being an odd integer. In a block of n bits, if the number of zeros exceed number of decoder decodes it as ‘0” otherwise as “ 1”. An error occurs when m + 1 or more transmissions out 3 are incorrect. What is the probability of error? Correct Answer 3p<sup style="">2</sup> (1 –p) +p<sup style="">3</sup>
Given:
Probability of error when 0 is received = p
Probability of error when 1 is received = 1 - p
If the number of zeros exceeds the number, the decoder decodes it as ‘0” otherwise as “ 1”.
For n = 3 bits
|
Bits received by the decoder |
Probability of Error |
|
0 0 0 |
p3 |
|
0 0 1 |
p2(1 - p) |
|
0 1 0 |
p2(1 - p) |
|
0 1 1 |
x |
|
1 0 0 |
p2(1 - p) |
|
1 0 1 |
x |
|
1 1 0 |
x |
|
1 1 1 |
x |
Hence the probability of error is,
Pe = p3 + p2(1 - p) + p2(1 - p) + p2(1 - p)
Pe = p3 + 3p2(1 - p)