Time Slot Allocation in Dynamic TDD for Next Generation Mobile Communication Systems

Abstract

One of the important issues in the emerging wireless communication systems is how to cope with the asymmetric traffic for multimedia services. Since the existing FDD mode has limitations for supporting the asymmetric services, the TDD mode is considered as the key solution to solve the problem. With the TDD mode, however, the inter-BS/inter-MS interference becomes serious in the cross time slot (CTS) period and this interference seriously degrades the system performance. In this thesis, we propose a region and time partitioning dynamic TDD (RTP D-TDD) for OFDM systems. To mitigate the inter-BS/inter-MS interference in the CTS period, each time slot in the CTS period is split into minislots, and a cell is divided into several regions by the number of minislots in a time slot in the CTS period. Each minislot is assigned to the users in its corresponding region. We also propose a time slot allocation based on region and time partitioning (TRTP) under the proposed RTP D-TDD system. In the TRTP, the influence of the CCI in the CTS period is further reduced by assigning the minislots closer to the switching point to the users having higher link gains. In the minislot borrowing scheme (MBS) which is operated with the TRTP, users in the overloaded region are allocated to minislots besides the corresponding minislots to cope with the nonuniform traffic distribution in a cell. The BS in the reference cell periodically monitors the traffic load in each region. If the traffic load concentration ratio in a certain region exceeds the predefined system outage threshold, the MBS is operated. To minimize the total system performance degradation, the corresponding minislot of the adjacent region which is estimated with the lowest CCI is selected for the additional allocation in the overloaded region. By the computer simulation, it has been verified that the proposed TRTP outperforms conventional time slot allocation schemes with more than 0.33bps/Hz in view of the average bandwidth efficiency. The CINR outage probability of the TRTP is also lower than those of the conventional schemes. The MBS effectively utilizes the resources and improves the average bandwidth efficiency performance up to 0.5bps/Hz at the traffic concentration ratio of 0.65 when comparing to the conventional TSA scheme.