if i change convolution encoder to 3/4 or 7/8 code rate can u tell BER shd improve wid comp to1/2 ?

i have implemented MODEM which has Convolution encoder. when i change 1/2 to 3/4 or 7/8 . My BER for 3/4 or 7/8 comes out to be zero .....with AWGN as 1 db for Eb/No .is it right ?plese give ur suggestion

Answer 1

[charan]

Hi Shailee,

In general when the code rate is increased from 1/2 to 3/4 or 7/8, the BER is reduced due to increased redundancy in the data.Higher code rates give better error correction capability but the overall throughput is reduced. 1dB is a low snr value to check the correctness of the code.Testing over a range of Eb/No values can give a better idea.

Hope this helps!

[Paulius]

Indeed, it is quite opposite. The largest redundancy is at 1/2 rate – for every bit you generate 2 bits – it is 50% redundancy. At rate 3/4 you get only 25% redundancy: for every 3 bits you generate 4 bits as output. The 7/8 rate has the lowest redundancy. So speaking about BER at the same noise level in the channel, it starts increasing as the code rate increases: the BER will be worse at 3/4 than at 1/2.