Attacks In The Reliable Communication

This paper introduces a new method of constructing low rate codes those decoding is base on correlation attack method. We use the initial state of the m sequence as the information bits, a length N truncated m sequence as codeword, and decode the codeword passed through the channel with correlation attack algorithm. This method has a simple coding structure and low decoding complexity. If we use the truncate position as a secret key, this communication scheme is secure. Much more, this method uses a hard decision decoding, so it has less requirements on demodulator than the LDPC code of using soft decision decoding.We give some analysis and simulations to three kinds of correlation attack: B algorithm, CT algorithm and Reconstruct algorithm, and compare them by horizontal relations (one correlation attack algorithm to another correlation attack algorithm) and vertical relations (correlation attack algorithm to LDPC code decoding algorithm).B algorithm [5] is the simplest one among the correlation attack algorithms, simulations show that when the code rate is 31/2000, SNR in AWGN channel is lower than -7.2dB, the BER of B algorithm after decoding is lower than thaot of the random LDPC code with the same code rate. But when the SNR are higher than -7.2dB, random LDPC code has better performance.CT algorithm [13] is more complex than B algorithm, but has stronger error-correcting ability. Simulation shows that under AWGN channel and same code rate, CT algorithm has 3.5dB better performance. With the same low code rate, 31/2000 random LDPC code gets 4dB SNR gains when BER are lower than 10?3 , and gets 5dB when code rate drops down to 31/4000.LDPC Codes have good performance when the code rate is high (>0.5). We repeat the rate 31/200 LDPC code for 10 times and get a rate 31/2000 low rate code. Comparing with our low rate code with CT algorithm decoding, the simulation shows it has 2dB better performance. The reason is CT algorithm is a hard decision decoding algorithm while LDPC code decoding use a soft decoding algorithm. We derive a soft decision CT algorithm from the original one, the simulation shows that the modified algorithm almost has the same performance with the low rate LDPC code derived from high rate LDPC code through repetition.Reconstruct algorithm [12] has lower performance than CT algorithm while its complexity is higher, but it suits intermittent reception. This property makes it useful in reliable communication. Analyze also shows this algorithm are flexible and has steady performance with different LFSR generated polynomials. To show a way to promote the performance of such intermittent reception suitable algorithm, we introduce a correlation attack base on irregular LDPC code.At last this paper discuses the correlation attack on combined sequence. We focus on the binary additive combined LFSR sequence, point out we can rescue the information bits through correlation attack when the member LFSR has co primitive generated polynomials, then gives an example on how to do it.