Interleaving has heretofore been used as a technique for redundantly coding digital transmissions to avoid the effects of pulse interference by spreading the information bits over the duration of the message. One form of interleaving is to repeat the entire message a number of times in succession. When the message is received each bit is decoded in a conventional manner and the predominant state is selected as the correct state for the bit.
Redundantly coding the information bits in this manner increases the transmission bit rate over the case for non-coded bits for the same total message time length. At the higher transmission bit rate the signal-to-noise ratio for each bit is lower and consequently more errors will be made in determining the value for each bit. This transmission method, however, provides enhanced resistance to pulse interference having a short time period and an amplitude in excess of the amplitude of the signal.
The disadvantage of redundant coding is that each received bit has a very low signal-to-noise ratio. When each bit is decoded with a hard decision a substantial number of errors are produced which can lead to an unacceptably high error rate. Therefore, in order to better utilize the technique of interleaving there exists a need for a method and apparatus for decoding an interleaved signal to take maximum advantage of the redundant bits while avoiding hard errors due to erroneous decoding of individual bits.