A study on access control scheme for multimedia CDMA ALOHA system

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The object of this research is to study on various access control schemes for CDMA ALOHA that can provide high efficiency under practical environments. The CDMA ALOHA is considered promising for simple and efficient multimedia mobile communications.We have developed a model with which the system performance can be evaluated, and studied on the access control schemes capable of maintaining high efficiency even in the presence of fading and near-far problems. As a result, It was shown that access controlling in accordance with the distance from a base station with a rake receiver and simple transmit power control can provide high efficiency.We have also developed a method for evaluating the performance of multimedia CDMA systems in which there coexist voice and data channels having different required quality of service, and proposed an access control scheme such that the traffic is controlled with a threshold determined by the total numbers of users for telephone service and data service, and with another threshold determined only by the number of data users. We have also proposed another scheme that can improve the throughput of data packet without degradation in the performance of telephone service, utilizing the random access feature of data packet service.To reduce the amount of multiple access interference, we have studied the application of the quasi-synchronous code sequence to CDMA slotted ALOHA, and clarify its effectiveness using the binary quasi-synchronous code sequence that can be easily generated. As another mean to reduce the effect of multiple access interference, we have analyzed the relationship between cross correlation and noise enhancement accompanied by a decorrelating detector with respect to a synchronous CDMA system, and proposed a interference canceller based on MMSE criterion, applied to the CDMA unslotted ALOHA system.