1. CHAOTIC FREQUENCY AND ORTHOGONAL CODE HOPPING FOR CODE DIVISION MULTIPLE ACCESS Yılmaz AKIN, 2004

Keywords : Spread spectrum, CDMA, Frequency Hopping, Code Hopping, Chaotic Sequence, Chaotic Frequency Hopping and Chaotic Orthogonal Code Hopping.

The use of chaotic sequences for code division multiple access (CDMA) communication systems has received considerable attention in the near past. Chaotic signals and systems possess several suitable properties for spread spectrum (SS) communications and although chaotic signals are random-like, they are produced by deterministic systems and can thus be reproduced. Chaotic sequences have a behavior similar to white Gaussian noise, making it really difficult for an unauthorized receptor to intercept them. Chaotic systems are extremely dependent on their initial conditions. Even the smallest change in the initial condition results in a totally different sequence, thus allowing the generation of many sequences with very little effort and with small cross correlation. This feature also allows simple addition of new codes or users to a system already in operation.

Channels in digital mobile communication systems are usually synchronous. Orthogonality (signature waveforms) are valuable for synchronous channels in CDMA since they yield a natural cancellation of interference. A novel chaotic based frequency (orthogonal code) hopping algorithm for code division multiple access systems with frequency (code) hopping is proposed in this thesis. Making use of chaotic sequences for multiple users, the interference between users is kept at a low level while periodicity during frequency (code) hopping is avoided and hence a more secure system against eavesdropping is obtained.

In the case of synchronous frequency (code) hopping with system control, all users can use the same chaotic sequence, with each user being assigned a different binary access code (v), to enable communication without code congestion. Hence the orthogonality feature of the system is preserved. The system will assign an unused code to a new user that joins the system, and when a user moves out of the system, the corresponding code is assigned as available. The amount of access codes, thus the length of v, will determine the number of users supported by the system. The length of the sequence v is typically determined by the number of available user codes. For synchronous frequency (code) hopping, the system runs the chaotic hopping pattern generator from a certain initial value and new users are informed about the state of the chaotic system as well as their user code v. As a result of this synchronization, a transmission without frequency (code) collision can be provided for synchronous hopping.

Chaotic orthogonal code hopping provides increased security against unauthorized interception, but has a same bit error rate (BER) performance with the CDMA system.