Purpose: To improve the error performance in symbol transmission over noise channel
as shown in figure 1.
To overcome the problem
of noise in the system, we might consider adding redundancy during the encoding
process to overcome possible errors. Now describe a specific code and evaluate
their performance for additive white Gaussian noise channel, linear block code
and convolutional code.
ü In
binary PCM ,each sample of the signal is
represented by code word ,say, k bit .
ü In
the receiver recognized each code word in order to reconstruct the sample.
ü Error may occurred in transmit ion as a result of noise
mprovement the reliability of communication in
the presence of noise is to increase the signal to noise ratio. An alternative
is to add extra bit , at the expense of
Coding schemes
· Forward error
correction (FEC) where controlled redundant information is added to the symbol
stream. The redundant information can be utilized for error detection and error
correction . No return link required.
· Automatic
repeat-re quest(ARQ) where the symbol frame is retransmitted if symbol errors
are detected in the reception. Only error detection necessary . A return link
is required . introduce latency to the transmission.
Hybrid ARQ combines ARQ and FEC. Return
link necessary lover latency than pure
ARQ