TRELLIS HOPPING TURBO CODING

CASTIÑEIRA MOREIRA, J.; PETRUZZI, D. M.

PROCEEDINGS VIII ISCTA

HW COMMUNICATIONS LTD.

Lugar: Ambleside, UK; Año: 2005 vol. 1 p. 432 - 437

Abstract—This paper presents a turbo coding scheme where the constituent codes are designed usingconvolutional encoders with time varying coefficients. The constituent encoders are finite statesequence machines operating over the Galois Field GF(q), whose coefficients are changedperiodically in time, during transmission. This provides the scheme with an encryption procedure inwhich these coefficients are varying trough a key word generated by other process. In this paper thistime varying rule is generated randomly.These schemes perform equally better than classic binary turbo coding schemes with the additionaladvantage of providing the system with a high level of encryption. The variation of the coefficients ofthe constituent encoders (finite state sequence machines designed over the Galois Field GF(q)) makesthe whole trellis of the system be constituted from sub-trellises corresponding to different codes.Thus, the transmission hops from one trellis to another, following a random sequence. It is shownthat these schemes perform slightly better than classic binary turbo coding schemes using timeinvariant coefficients in turbo coding schemes with medium size block interleavers, and performslightly worse than classic binary turbo coding schemes using convolutional encoders with timeinvariant coefficients and large random interleavers. The difference in BER performance is in bothcases of around 0.2 to 0.3 dB. This shows that turbo coding and encryption techniques are not in astrong trade off, providing the whole system with both very good encryption characteristics andexcellent BER performance. The proposed turbo coding schemes perform equally better thantraditional equivalent turbo coding schemes, with the additional advantage of providing the systemwith levels of encryption.