Inter-Cell Interference Coordination In Multi-Cell OFDMA Networks

Orthogonal frequency division multiplexing access(OFDMA) networks has no doubt of becoming one of the dominant strength in the future wideband wireless networks and mobile communication systems. It sufficiently takes advantage of the OFDM technology which is immune to frequency selective fading and quite suitable to be used for wideband communication. By using the frequency band in orthogonal subcarriers and time slots, OFDMA systems give a great freedom for their resource rescheduling to get large multi-user and multi-carrier gain. Many standardization communities have taken or would like to take OFDMA as part of their transmission technologies for their wireless communication standard, such as wireless metropolitan area networks (IEEE 802.16), 3GPP's long time evolution (LTE), and wireless RAN (IEEE 802.22).With the depth of OFDMA systems, researchers discover that intra-cell interference in OFDMA systems would be existed by taking advantage of the orthogonality between subcarriers and the main source of interference comes from neighboring cells instead of the current cell. On the other way, OFDMA technology is aim to be used in wide-band communication systems and the effectiveness of using spectrum resource becomes more important, so the traditional frequency reuse method for reducing the inter-cell interference is no more sufficient because of the strict requirement of wideband and high effectiveness. As a result, finding new methods for limiting inter-cell interference becomes one of the key problems for using OFDMA technology.Four kinds of technology are proposed to resolve the inter-cell interference currently, which are interference randomization, interference elimination, interference coordination and macro-diversity. Each technology has its own advantages and disadvantages, and among these technologies, interference coordination is considered the most important way to resolve the interference problem in the future mobile communication systems because it is simple and liable to be used with other technologies. Huawei, Alcatel and QUALCOMM proposed their interference coordination schemes. These methods proved to be effective. This thesis summarizes these proposals and finds out what influence their performance by analyzing the ideas under them and then proposes new inter-cell interference coordination schemes. To summary, the content of this thesis includes following aspects:Firstly, this thesis gives a detail analysis of the current environment and the development tendency of the 4th generation mobile communication systems. Then, the key techniques and challenges of the 4G systems are summarized, and principle and problems of OFDMA systems are detailed analyzed. After the analysis of interference in OFDMA systems, this thesis selects the interference coordination mechanism for OFDMA systems as the main research point.Then, a novel interference coordination method is proposed to solve the problem that the utilization of spectrum under traditional frequency reuse methods is low. The method actually is a novel frequency reuse architecture, which is called by overlapped fractional frequency reuse architecture. Through limiting the range of frequency used in different areas of the cell, this architecture could reduce the inter-cell interference greatly and then improve the system throughput. Based on this architecture, a relative carrier allocation algorithm is brought out. This architecture is also compared with existing architecture with simulation results.Based on the novel interference coordination method for OFDMA, this thesis then proposes an improved frequency reuse architecture, which is called by enhanced overlapped fractional frequency reuse. In overlapped fractional frequency reuse architecture, the limit of using frequency is simple, and therefore a specific interference would be brought. After analyzing the reason of this interference, a carrier avoiding mechanism is added to suppress it. Basic information about interference is required to be transmitted between cells in this mechanism, and this information must be simple and the period for transmitting it must be long enough to not bring much payload to the interface between BSs. Similarly, for the EOFFR architecture, a QoS guaranteed carrier allocation algorithm is proposed and simulation result is provided.A method is required for EOFFR to establish the carrier set used in the different areas of cell, and for this reason, this thesis proposed an adaptive sub-band avoiding mechanism. In this mechanism, interference factor is defined and used to transmitted between neighboring cells to establish how many and what carriers to be avoided. Simulation results prove that the mechanism could reduce the inter-cell interference and improve the channel condition of the users and finally the system throughput.In the end of this thesis, a summarization is presented and the potential work in this research area is pointed out.