| 1. IntroductionIt is a very important problem in the designing of communication system that among lots kinds of wireless communication systems, especially the military communication systems, the information security particular in the channel transmitting should be sure that can't leak and be interpreted.In the witting spy system, whose aim is gaining the content of communication, the final goal to capture wireless signal is to gain the content of communication. Thus, capture has two steps which are both important: the first one is whether can capture the signals, and the second one is whether can translate the signals. If the enemy can only capture the signals but can't translate them to content, then the capturing is meaningless. For the enemy, if they want to translate the signals to content, they must have the right frequency segment to capture the signal correctly, and then, they have to identify the modulation method and demodulate synchronously. After gaining the encrypted data, they must to decrypt the data to gain the real content. From the procession of capturing and decrypting, it is found that if the sender uses one kind of modulation method, and the receiver uses other kind of modulation method, then the receiver will can't decrypt the signals to content and they will gain no more than nothing.The first step for enemy to decrypt signals is to identify the sender's modulation method effectively. It's meaning that if the enemy can't take the modulation method correctly or estimate the modulation method in time, they won't demodulate the captured signals, and won't take the information synchronization and decrypt. Thus, to the enemy, identifying modulation method is the precondition and basic. After years of studying on identify modulation, it has been able to identify the modulation method effectively by a mount of signals which are time continuous, have the same modulation method or the same methods. If the sender uses only one kind of modulation method during the processing of communication, it will give the enemy a good chance to gain a mount of time continuous and modulation continuous signals. Above all, it should be considered that during the processing of communication, one kind of modulation method should not be use for long time.Now days, there are a lot of techniques for information transmission security, but they all most be found above the one kind modulation communication system. So, we advance a new technique named modulation hopping for preventing the enemy from capturing the signals.2. Modulation hopping technique principleThe technique of modulation hopping is using two or more than two kinds of modulation methods to modulate data when the channel situation is permitting and not change too much. At the same time, we introduce the chaos theory, use the chaos code to arrange the modulation method to prevent the enemy from knowing the sequence of modulation below the same channel situation and enhance the security of information transmission. For the topic of ensuring information transmission, combining the modulation hopping technique and chaos theory to the application of communication system and communication counterwork, shortening the time of one kind of modulation method on the channel, using the modulation hopping to countermine the modulation identify. And then, the different kind of modulation will bring different bit numbers and different bit rate in the channel which will give the enemy more difficulty to decrypt the signals.Modulation hopping is the evolution of confirm modulation. If the channel situation is too bad, it is possible not to hop. But the communication will continue even there is only one kind of modulation can work. In the worst communication situation, the worst result is that our modulation hopping technique equate the one kind of modulation method communication system.Bring the modulation hopping to the ensuring information transmission security, is very suit to the important military communication and diplomatism communication. Because this kind of communication system usually including a lot of important secrets, so they should be protected more carefully. For this kind of communication system, if the security can't be sure, it is invaluable though it can provide data rate faster, communication distance longer, equipment cheaper and anti-jamming performance better.3. The actualize of modulation hopping techniqueThere are three sides of contents to actualize the modulation hopping technique.a) Sorting modulation methods In this article, we choice sever kinds of modulation methods which are QPSK,8DPSK,MSK,GMSK,8CPFSK,8QAM and 16QAM to actualize the modulation hopping technique. Because that by the situation of maintaining the same BER, every kind of modulation has its own request to the channel SNR., it is reasonable to divide the seven methods to three sorts by the relation between the BER and SNR.b) Choice modulation sortsAfter the sorting modulation methods, we want to choose one sort modulation to transmit the information. So one arithmetic has been introduced to the article to estimate the channel SNR.c) Sequence modulation methodsThere are two or more than two methods in every sort of modulation. The chaos theory has been introduced to the article to sequence every kind of method in every sort modulation. Due to the modulation identify technique has different performance following the channel SNR, we change the time of every modulation method following the channel, too. By this way, it is more effective to ensure the security of information transmission.The simulation result shows that the simulant simulation modulation hopping system we build in this article can estimate the channel SNR correctly, choose modulation methods sorts due to the SNR, and sequence modulation methods of every modulation sort by the chaos coding theory. When we output the BER of every modulation by every sort modulation time, we can see that all the BER are zero which can improve that the SNR estimation and modulation sorting are correct. |
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