Code Assignment Algorithm For 2-Dimensional Block Spread CDMA Downlink

In wireless broadband mobile communication systems, how to solve the problems of multiple-access interference(MAI) is the key point and difficulty in recent years. In CDMA systems, the MAI, which is caused by different fading channels between every user and the base station, can severely affect the performance of the receiver and lower the capacity of systems.Fumiyuki ADACHI from Tohoku University proposed 2-dimensional (2D) block spreading technology based on orthogonal variable spreading factor (OVSF) code and code scrambling in order to solve MAI. Meanwhile, the use of OVSF code as channel code can be used to support multi-rate data transmission, which is to assign OVSF codes according to the calling rate. However, in the code assignment of CDMA systems, the code blocking lowers the efficiency of code source and wastes system resource. This problem is more and more serious as there are more kinds of multi-media service.In order to use code tree efficiently and make a best code assignment of 2D block spreading CDMA systems, this thesis has a research on the principles and algorithms of OVSF code assignment and find the appropriate code assignment for 2D block spreading CDMA systems.This thesis firstly analyzes the model of 2D block spreading CDMA systems and the principle and structure of OVSF code tree. And then it proposes four single-code assignment algorithms for 2D block spreading CDMA systems based on existed code assignment algorithm and the angle of the orthogonality between two stages of spreading codes. Then we propose the multi-code assignment and finally compare all algorithms'performance as blocking probability and the affect of BER, calling rate and spreading factor on assignment, using the results of computer simulation.By comparing the performance of all algorithms, this thesis confirms the appropriate single-code and multi-code assignments for 2D block spreading CDMA systems, and prove that they can efficiently use limited code source and provide reliable multi-rate data transmission and multi-media service at the same time of solving MAI.