Past research in the field of cryptography has not given much consideration to arithmetic coding as a feasible encryption technique, with studies proving compression-specific arithmetic coding to be largely unsuitable for encryption. Nevertheless, adaptive modelling, which offers a huge model, variable in structure, and as completely as possible a function of the entire text that has been transmitted since the time the model was initialised, is a suitable candidate for a possible encryption-compression combine. The focus of the work presented in this paper has been to incorporate recent results of chaos theory, proven to be cryptographically secure, into arithmetic coding, to devise a convenient method to make the structure of the model unpredictable and variable in nature, and yet to retain, as far as is possible, statistical harmony, so that compression is possible. A chaos-based adaptive arithmetic coding-encryption technique has been designed, developed and tested and its implementation has been discussed. For typical text files, the proposed encoder gives compression between 67.5% and 70.5%, the zero-order compression suffering by about 6% due to encryption, and is not susceptible to previously carried out attacks on arithmetic coding algorithms.
In this paper we propose the use of Turbo Codes for M-ary pulse position modulated pulses over UWB communication channels with multipath. The transmitted power of the UWB pulses is required to be very small as they are used in the existing spectrum designated to other uses. This constraint requires that UWB signals have good performance at low SNR's. Turbo codes have been shown to perform well at low SNRs. We present the simulation results of using rate 1/2 and 1/3 turbo codes. Also, the effects of varying the symbols set size, the SNR of the channel and the interleaver size are presented. It has been shown in this paper that for a fixed SNR (4 dB), the rate 1/3 code shows an improvement of 2 orders of magnitude when M = 2 and 3 orders of magnitude when M = 4 or M = 8. For the same SNR, the rate 1/2 code shows an improvement of 1 to 2 orders of magnitude. To achieve a BER of 10-5, the rate 1/3 turbo code requires SNR = 6 dB when M = 4 and SNR = 3 dB when M = 8. Increase in the size of the interleaver from 29 to 214 results in a decrease in BER from 10-2 to 10-3 for SNR = 4 and M = 2.