Research On Power Control Techniques In CDMA Systems

Code Division Multiple Access (CDMA) is one of the most important techniques for 3G mobile communication, which has advantages in anti-interference, high security, anti-multipath fading, soft capacity, ideal bandwidth efficiency, high data rate etc. CDMA systems are interference limited communication systems. With the increase of number of subscribers, interference from other users will increase, which causes error detection of signal and further loss in system capacity and performance. All the same, CDMA systems are self-interference limited communication systems.In 3G systems, multi-access interference, near-far effect and angle effect exist due to the fact that spread codes between different users are not completely orthogonal because of the fading of wireless channel and time-varying positions of mobile stations. Regular filter can not overcome these questions. Power control is one of key technologies to overcome above-mentioned problems. It is able to compensate signal fading, suppress interference and increase system capacity. In addition, power control can also minimize the mobile station transmitting power, thereby extending the battery life of mobile stations.In principle, power control is a process by which terminals and/or base stations are able to adjust their transmitting power based on the changing of channel properties. On the condition of guarantee the quality of service for every communication link, transmitting power is minimized to other users.This paper gives an overview on power control techniques, including its necessity, overall objectives, criterion, and categories. We also state propagation properties of wireless channel, enumerate several typical simulation models and then construct the simulation model of CDMA systems.A simulative study on power control algorithms was done based on the model above. Design and implementation of algorithms is the core of power control problem. We discuss SIR-balanced distributed algorithm, the main contributions are included as follows:Firstly, we discussed traditional distributed power control algorithms in which the iterated function is derived from numerical algebra. Based on the structure of these algorithms, we propose a general form of fully distributed and adaptive variable step size algorithms for time-varying link gain environments.Secondly, we proposed a new power control algorithm based on error estimation, which can be used in actual CDMA cellular radio systems.In the uplink, the Mobile Station (MS)estimate the Signal to Interference Ratio (SIR) by using power control commands sent by the Base Station (BS),which needs 1bit or 2 bits.The estimated SIR information is used to adjust the transmitting power for the mobile terminal by using the adaptive variable step size power control algorithm.The proposed algorithm has advantages in terms of quick response and strong robust, and is easy to be realized. Simulation results show that the algorithm can track the changes of link gains and is better than FSPC algorithm and DCPC algorithm for stochastic time-varying link gains.