Hierarchical Architecture Management in Vertically Overlaid Orthogonal Frequency Division Multiple Access System for Uneven Traffic

Abstract

In the cellular system based on orthogonal frequency division multiple access (OFDMA), base stations allow multiple user to transmit simultaneously on different subcarriers during the same symbol period. The users in different location within a given cell experience different channel variation due to pathloss, multi-path fading, and shadowing. And intercell interference by adjacent cells that utilize same frequency bandwidth might have an effect on the users, thus interference power is increased. Since the radio resource is limited in a cellular system, it has been proved that the spectral efficiency could be improved by exploiting efficient resource allocation algorithm. Adaptive modulation is known as the method to maximize the system throughput usually. However, one of the problems of adaptive modulation is that users in a cell boundary may require more resources because they acquire worst channel gain, their subcarrier allocated by a BS are loaded on lower modulation, , therefore, the spectral efficiency is degraded. In such a system, it is important issue that the system throughput is not only sustained without degrading, but also the QoS fairness between each user must be guaranteed In this thesis, we consider the hierarchical architecture management (HAM) scheme in OFDMA system and use the decision function which estimate and decide when picocell base-station (BS) is operated based on OFDMA with adaptive modulation per subcarrier. For the purpose of providing service fairness for users, especially for hot-spot user traffic with high data-rate in a cell boundary, we propose the structure with overlaid picocells. In the conventional OFDMA, the resource allocation algorithm has been fit in a single cell system only with uniform distribution of users, and cochannel interference generated due to the traffic load in adjacent neighboring-cells has not been considered. That is, the problem in uneven user distribution is excluded. Even though there are limited but enough resources in the uniform distribution of a reference cell, the users may experience the resource deficiency temporarily due to high interference power or non-uniform distribution. Thus we must consider all parameters concerning with procedure of resource allocation to make the efficient resource allocation schemes possible ultimately. This thesis assumes that the proposed scheme is progressed in both a single cell and multicell environment with non-uniform user distribution. Provided uneven data traffic in a desired cell, the mechanism of hierarchical architecture management is influenced according to the traffic loads in near neighboring cells. In this condition, in order to satisfy users’ QoS requirement and also maximize the system throughput under users’ QoS guarantee, the macrocell-BS first calculate the decision function, which indicate if the macrocell-BS needs the hierarchical structure, in addition to which picocell BSs is chose. After decision, the macrocell BS must send control signal to run the picocell BS and assign serving users and transmission spectrum block or subcarrier index. By applying to a rule adaptively that decide whether picocell BS is operated (ON) or not, it is possible to provide service for not only user with good channel gain, but also the users who have been eliminated in balanced QoS guarantee. In this thesis we verify the performance of this hierarchical architecture management scheme considering the multicell OFDMA system as well as a single cell OFDMA system by computer simulation. After we generate hot-spot areas in a cell or change the ratio between various services, we present the system performance in OFDMA cellular system with adaptive resource allocation. Adopted the hierarchical architecture management algorithm, simulation results show that it is possible to improve the performance degraded due to suddenly increased hot-spot traffic or the fluctuation in various kinds of service classes.