正交分頻多工接取下鏈系統之細胞間干擾抑制技術研究

Abstract

因為能有效提升頻譜使用效率及可提供優異頻率選擇衰落(frequency selective fading)對抗能力，正交分頻多工接取(OFDMA)技術被下一代(4G)行動通信系統廣泛採用，舉凡LTE/LTE-A及Mobile WiMAX/WiMAX 2.0皆選擇OFDMA為下鏈(downlink)傳輸技術。在OFDMA下鏈系統中，由於細胞內傳輸具有正交特性因此沒有細胞內干擾(intra-cell interference)，其主要干擾源係來自細胞間干擾(inter-cell interference)，此細胞間干擾問題使得系統效能下降，尤其在細胞邊緣影響更大。然而，考量設計4G行動通信系統時，於涵蓋區提供更一致化的用戶體驗(資料速率)是主要發展需求之一，因此細胞間干擾需有效的處理應付。本論文主要探討應用於OFDMA下鏈系統之細胞間干擾抑制技術。 細胞間干擾抑制技術主要發展目標為提升細胞邊緣用戶資料速率，而且細胞流通量(throughput)亦要適當維持。近年來，細胞間干擾協調(ICIC)技術被視為是減輕細胞間干擾的有效方法，其中部分頻率重用(PFR)及軟頻率重用(SFR)機制已被下一代行動通信系統支援實現。本論文第一部份即在探討並比較部分頻率重用及軟頻率重用機制之效能，比較特別的是，此研究係基於LTE建議使用之信號強度差額(SSD-based)用戶分群法。我們的研究結果顯示，部分頻率重用及軟頻率重用皆是處理細胞間干擾有效方法，但若考慮用戶資料速率公平性因素，部分頻率重用則可提供較佳之系統容量。 對3G行動通信系統而言，軟交遞(soft handover)是用以延伸細胞涵蓋及提升細胞邊緣用戶資料速率的關鍵技術。本論文第二部份提出一創新之混合型細胞間干擾抑制方法，此方法結合部分頻率重用及軟交遞概念，其基本運作原理為利用在部分頻率重用及軟交遞機制之間實行動態選擇(切換)，以提供細胞邊緣用戶更佳之信號品質。根據我們的模擬評估結果顯示，相較於傳統部分頻率重用機制，此混合型細胞間干擾抑制方法的確可大幅改善細胞邊緣流通量，而且在考量用戶資料速率公平性情況下，亦能進一步提升整體細胞流通量。 傳統行動通信網路佈建係以同質網路(homogeneous network)為主，亦即所有節點都是高功率宏基站(macro BS)。然而，為使單位面積系統容量最大化，藉由在宏細胞(macrocell)涵蓋區內佈放多個低功率節點之異質網路(heterogeneous network)或多層次網路佈建方式近來引起極大關注與討論。而且，為增加開放取用(open access)低功率節點卸載宏細胞訊務(traffic)效果，細胞涵蓋擴展(CRE)技術被建議應運用於異質網路佈建。在一個宏細胞搭配特微細胞(picocell)之同頻異質網路佈建(以下簡稱macro-pico同頻異質網路)情境下，倘若宏細胞層與特微細胞層之層間干擾無法妥善處理，引入細胞涵蓋擴展技術將可能導致整體網路容量下降。因此，本研究第三部份即提出一種有效之層間干擾協調機制，其適合應用於一個實現細胞涵蓋擴展技術之macro-pico同頻異質網路。我們的模擬評估結果說明，此方法確實可顯著改善位於涵蓋擴展區域用戶之信號品質，因而使得系統用戶停運率(outage rate)得以下降；除此，此方法亦能提供相當優異之總體地區流通量增益。

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.