Advanced Hybrid OFDM system with Multi-Layer TDD Architecture for the 4th Generation Mobile Communication Systems

Abstract

As the expected amount of the multimedia service traffic increases drastically towards the 4th generation (4G) cellular mobile communication systems, the efficiency of accommodations for these emerging traffic has become one of the most important issues. Accordingly, there have been several researches regarding hybrid duplexing (HD) technologies which are known to be very effective to serve asymmetric multimedia traffic. Many of them apply time division duplexing (TDD) technology in the inner region and frequency division duplexing (FDD) technology in the outer region of each cell. One concern for such arrangement would be the increase of intercell interference. The HD technology is quite efficient for the throughput and bit error rate (BER) performance, but it does not improve the other QoS aspects enough. In TDD mode, there exists an inevitable time gap between the uplink (UL) and downlink (DL) time slots, so the quality of service (QoS) parameter in the users’ requirements regarding the round-off response time can be deteriorated. Since a larger frame size results in higher utilization of bandwidth, the frame size in the conventional TDD system tends to be designed as large as possible, which also causes a longer system response time. The time gap in a TDD frame results mainly from the propagation time from the base station (BS) to the farthest mobile station (MS) in the TDD region, which limits the reduction of the time gap. Such drawbacks bring the system to be vulnerable to the interactive real-time traffic in the future mobile communication systems. In this thesis, we propose a hybrid duplexing OFDM system with multi-layer (ML) TDD architecture. Combining the advantages of FDD and TDD, the proposed HD can accommodate asymmetric high data rate traffic with TDD, and rather symmetric high-speed mobile users in FDD. In the proposed ML-TDD architecture, several concentric TDD layers are defined within the TDD area, having different radii, so with different frame sizes. The concentric TDD layers have an overlapped structure so that the BS can allocate the multimedia traffic sources flexibly among the multiple TDD layers. In fact, a flexible operation of selecting an appropriate TDD layer according to the allowable time delay in each multimedia service is possible. Since the inner TDD layer has a shorter radius, the time gap between the UL and DL time slots can be shortened because of the reduced distance between the BS and the farthest MS within the inner TDD layer. That is, in the proposed ML-TDD architecture the time gap can be controlled in each TDD layer separately, so the length of the frame and thus the system response time in each TDD layer can be optimized according to various traffic demands. Accordingly, the proposed hybrid duplexing OFDM system effectively improves the drawbacks of the TDD scheme in order to efficiently support the real-time multimedia traffic in the 4G mobile communication systems. By the computer simulation, it has been verified that the proposed hybrid duplexing OFDM system with ML-TDD architecture improves the users’ QoS parameters regarding the system response time and the degree of satisfaction for the required services. Especially, the proposed scheme has been proved to be suited for the multimedia traffic requiring a real-time response without any loss in the system performance.