| Summary: | 博士 === 國立交通大學 === 電信工程研究所 === 100 === Thanks to its effectiveness of improving spectral efficiency and its capability of combating frequency selective fading, orthogonal frequency division multiple access (OFDMA) has been widely adopted in the next generation (i.e. 4th generation (4G)) mobile communication systems as downlink transmission scheme. Considering an OFDMA downlink system, signals originating from the same cell are orthogonal, while those from different cells interfere with each other. As a consequence, inter-cell interference (ICI) becomes a major performance degradation factor, especially on cell borders. Nevertheless, for developing next generation mobile communication systems, a more homogeneous distribution of user data rate over the coverage area is highly desirable. To meet this end, ICI must be effectively managed. In this dissertation, we have studied ICI mitigation schemes in OFDMA systems and especially, we focus on the downlink side.
The objective of ICI mitigation is to provide better service to cell edge users without sacrificing cell throughput. In emerging 4G cellular systems, inter-cell interference coordination (ICIC) is considered as a promising technique to deal with the ICI. Among the variety of ICIC strategies, the soft frequency reuse (SFR) scheme and the parital frequency reuse (PFR) scheme are widely accepted. In the first part of this research, we review and compare the throughput performance of PFR and SFR in a multi-cell OFDMA downlink system and especially, this work is done by using the signal strength difference based (SSD-based) user grouping method, which is recommended by Long Term Evolution (LTE) standard. We show that both PFR and SFR are very effective ways to cope with ICI in an OFDMA downlink system, but PFR is a more appropriate one to achieve data-rate fairness among users with having an acceptable system capacity.
It is well-known that soft handover is a key technique to extend the cell coverage and to increase the cell edge user data rate in 3G cellular communication systems. In the second part of this research, we deliver a hybrid ICI mitigation scheme which combines PFR and soft handover. Its basic principle is to dynamically choose between a partial frequency reuse scheme (with a reuse factor of 3) and a soft handover scheme to provide better signal quality for cell edge users. Simulation results show that this hybrid scheme yields a significant cell edge throughput gain over the standard PFR scheme. Furthermore, considering data rate fairness among users, the proposed hybrid method also outperforms the standard PFR scheme in total cell throughput.
Traditionally, mobile cellular networks are typically deployed as homogeneous networks in which only high-power macro base stations are contained. Recently, heterogeneous networks (HetNets) or multi-layered network, in which low-power nodes (LPNs) are deployed within macrocell layout, has attracted a lot of interest as a way to maximize system capacity per unit area. Moreover, in order to extent the coverage region of open access LPNs and hence offload more traffics from macrocells, cell range expansion (CRE) strategy is suggested to apply in HetNets. However, assuming a co-channel macro-pico HetNet, the total network throughput could actually decrease due to CRE if the inter-layer interference couldn’t be effectively managed. The third part of this research presents an inter-layer interference coordination (ILIC) scheme for an OFDMA co-channel macro-pico HetNet that carries out CRE technique. Our simulation results confirm that the proposed ILIC scheme can lead to a significant improvement in link quality for those users in the extended region and thus reduce user outage rate in the system; and further, it can provide a substantial total area throughput gain over the conventional reuse-1 scheme.
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