Research Of The CDMA System Code Application Algorithm

Third-generation (3G) Wireless Cellular systems are recognized as systems that can achieve high-date-rate transmission beyond those currently provided by second-generation (2G) wireless cellular systems, and are capable of delivering multiple services to users at different locations under diverse channel conditions. As we all knows, there are three multiple access (MA) technologies, frequency-division MA (FDMA), time-division MA (TDMA), code-division MA (CDMA). CDMA with its proven capacity enhancement over TDMA and FDMA, has been chosen as the main MA scheme for 3G mobile cellular system. So the"code"of the CDMA technology is the most important in 3G mobile cellular system. In the CDMA system, there are two important codes: PN code and Walsh code. In order to support application with different bandwidth requirements, the Walsh code is extended to the OVSF code in 3G mobile cellular system. The research on PN code and OVSF code is critical in 3G mobile cellular system.The phase of PN code is used to identify the base stations in CDMA system. Before communication, the receiver terminal must get the PN phase of transmit terminal. The process is called code acquisition. In the nineteen nineties, the research of PN acquisition is matured in direct sequence spread-spectrum (DS/SS) systems. With the development of CDMA system, the research environment on PN acquisition is transfer to CDAM system. In the late of nineteen nineties, an auxiliary sequence is used to develop a novel search method for PN acquisition. The auxiliary sequence is correlated with the received PN sequence to yield a value that minimizes the phase searching location. But the research is base on DS/SS system, so the challenge is to use the auxiliary sequence in CDMA system.The OVSF code is allocated to different users to support different bandwidth requirements. The OVSF code placement problem has significant impact on code utilization and code blocking probability. So the scheme how to place the OVSF code is very important to the system performance. The past works are concerned on the decreasing the code blocking probability, and take the code blocking probability as the uniquely standard of the system performance. According to the feature of the OVSF code tree, the higher rate calls are blocked by the system more easily. In some special case, the system code block probability is zero, but the higher rate calls are totally blocked, it is unfairness to the higher rate calls. So it is insufficiency that system takes the code blocking probability as the uniquely standard. Now the challenge is decrease the code blocking probability and concerned about the fairness.To solve the above challenge problems, the content of this thesis includes three issues:(1) use the auxiliary sequence in the CDMA system; (2) efficient allocate the OVSF codes;(3) concern the fairness of allocating OVSF codes. The works in this thesis has been supported by the National Science Foundation of China"Investigation of Wireless Multimedia Techniques Based on Multimedia Transmission Property"(No.60202005).The contributions of this thesis include:1. The improved auxiliary sequence: the auxiliary used in DS/SS system is out of control at some special case when the phase of the auxiliary sequence is the same with the received PN sequence. The improved auxiliary sequence is working well.2. Extended the improved auxiliary sequence used in BPSK-CDMA system: the DS/SS system is similar with BPSK-CDMA system. But the period of the PN sequence not the same. So the improved auxiliary sequence must be extended.3. Code match scheme: the existing OVSF code placement strategies were based on code replacement which would increase the access delay and interrupt the existing call services. In order to avoid this problem, a new code match strategy is proposed in this paper. This new code match strategy is simple in operation for not using the code replacement. And the simulation results also show that the new strategy can decrease the code blocking probability and the access delay.4. The fairness research of OVSF codes allocation: According to the feature of the OVSF code tree, the higher rate calls are blocked by the system more easily. In some special case, the system code block probability is zero, but the higher rate calls are totally blocked, it is unfairness to the higher rate calls. So it is insufficiency that system takes the code blocking probability as the uniquely standard. We take the fairness of different rate calls as the system performance standard and propose a fairness scheme. After employing the scheme, the system can assure the fairness in different rate calls.