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Problem 10.29. One of the simplest forms of forward error correction code is the 
repetition code. With an N-repetition code, the same bit is sent N times, and the decoder 
decides in favor of the bit that is detected on the majority of trials (assuming N is odd). 
For a BPSK transmission scheme, determine the BER performance of a 3-repetition code. 
 
Solution 
With 3-repetition code, the decoder will output the correct bit if there are one or fewer 
errors in the 3-bit code. Thus, assuming bit errors are independent, the bit error rate is 
 
3 2
2
3
(1 ) (1 )
1
(1 ) (1 2 )
coded
b e e
e e
P P P
P P
⎛ ⎞= − + −⎜ ⎟⎝ ⎠
= − +
eP , 
 
where Pe is the bit error rate of channel bit. With BPSK, the formula for bit error 
probability is 
 
0
0
2
2
3
c
e
b
EP Q
N
EQ
N
⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠
⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠
, 
 
since ratio of channel bit energy to information bit energy is given by Ec = 1/3Eb. 
Therefore, the bit error probability of the 3-repetition code is 
 
2
0 0
2 21 1 2
3 3
coded b b
b
E EP Q Q
N N
⎛ ⎞ ⎛⎛ ⎞ ⎛ ⎞= − +⎜ ⎟ ⎜⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝
⎞⎟⎟⎠
 
 
 
 
 
 
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