Extended synchronizing codewords for binary prefix codes

Synchronizing codewords (SC's) have been previously studied as a means to stop error propagation in variable-length codes. However, SC's retain one disadvantage: the symbols after the SC may be put in the wrong positions since the number of decoded symbols before the SC can be different from the original number due to channel errors. Thus we propose the idea of extended synchronizing codewords (ESC's) which can overcome the drawback of SC's. After the decoder receives an ESC, the decoder correctly knows it is in synchronization, regardless of the preceding slippage. We derive some of the essential properties of ESC's and provide several upper bounds on the amount of overhead needed in designing a code with an ESC.