encoders are used to provide error correction and detection. Each
transmitted symbol is dependant on both the present and past inputs.
This has the effect of restricting the number of possible future
The decoder examines the symbols as they arrive
and compares their values over a period of time. Once it has successfully
locked on to the data, it can anticipate that the next decoded symbol
will be one of only two possibilities. If the decoded output is
not one of these two states, an error is deemed to have occurred
and corrective action can be taken.
Simple Convolution Encoder
= number of shifts before adding (normally = 1)
= number of adders
(or L) = constraint length (equal to the number of shift
registers if k = 1)
rate = k/n (in this case = Ĺ, i.e. there are 2 output bits
for every input bit)
Increasing the output bit rate seems counterproductive.
However, since each serial output value is dependent on a combination
of three consecutive input values, it should be clear that although
the input sequence may be random, the output sequence isnít. Therefore,
the decoder can anticipate the possible next value and detect an
The following SystemView file illustrates a
simple convolution encoder/decoder pair.
The following SystemView example shows the
difference in BER when using hard and soft decision thresholds in
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