分時全雙工之正交分頻多用戶系統於非對稱服務環境下之跨時槽干擾分析

Abstract

在本篇論文中，我們在支援非對稱服務的環境裡，探討了部分頻率重覆使用 (Fractional Frequency Reuse, FFR) 機制在分時雙工且正交分頻多工存取 (Time Division Duplex-Orthogonal Frequency Division Multiple Access, TDD-OFDMA) 系統中所能獲得的好處。並討論如何應用部分頻率重覆使用機制來降低相鄰細胞間的同頻干擾 (Inter-Cell Interference) 並維持頻譜使用率 (Spectrum Efficiency)。然而，我們提出了一套分析的方式來分析跨時槽範圍 (Cross Time Slot Region, CTS Region) 與設計部分頻率重覆使用機制參數之間的關係。這套分析的方是主要是應用排隊理論原理 (Queuing Theorem) 並且考慮通訊傳輸系統中的每一位使用者 (Mobile Station, MS) 使用情形，來進一步計算出發生跨時槽範圍的機率。如此一來，能透過這套分析的方法來幫助我們在支援非對稱的環境中，適當地設計部分頻率重覆使用機制所使用的參數。 在無線資源管理 (Radio Resource Management) 的領域裡，一個好的干擾管理機制能夠減少相鄰細胞間同頻干擾的情形並且改善系統效能。舉例來說，在蜂巢式無線通訊系統中使用方向性天線 (Directional Antenna)、多輸入多輸出天線系統 (Multi-Input and Multi-Output Antenna, MIMO) 和部分頻率重覆使用機制…等等。據我們所瞭解，我們需要在未來的第四代 (Fourth Generation, 4G) 通訊系統中使用更多的頻寬來達到高傳輸率 (Transmission Rate) 的要求。然而，最有效換取更多頻寬的方式就是在每一個基地台 (Base Station, BS) 的覆蓋範圍 (Cell Coverage) 中重覆使用相同的頻譜。雖然，在每一個基地台重覆使用相同的頻率可以立即增加可使用頻譜，但是卻會嚴重影響傳輸訊雜比 (Signal to Interference Plus Noise Ratio, SINR)。然而，較差的訊雜比會影響到系統內使用者的連線品質 (Link Quality) 造就了干擾消除和頻譜使用率之間的難以取捨，也使得如何在重覆使用頻譜的環境下依然可以維持良好的連線品質變成了一個很有趣的議題。 為了維持良好的傳輸品質，我們必須謹慎的區分相鄰細胞間同頻干擾的情況。在無線通訊系統中，主要有兩種雙工的模式可以用來區分上傳 (Uplink) 和下載 (Downlink) 的傳輸，也就是分頻雙工 (Frequency Division Duplex) 和分時雙工 (Time Division Duplex)。在這兩種不同的雙工模式下，會產生不同的相鄰細胞間同頻干擾的情形，尤其是在分時雙工模式並且支援非對稱服務的環境下，更容易產生較強的相鄰細胞間干擾。這種特殊的干擾量主要發生在跨時槽範圍內，也就是細胞間同時執行不同的傳輸模式，而這種干擾量被稱為跨時槽干擾 (Cross-Slot Interference)。跨時槽干擾主要可區分成兩種，分別為基地台對基地台的跨時槽干擾 (BS-to-BS Cross-Slot Interference)和使用者對使用者的跨時槽干擾 (MS-to-MS Cross-Slot Interference)。其中，基地台對基地台的跨時槽干擾會對系統效能造成很嚴重的影響，特別是對於基地台較遠的使用者而言。 因此，為了改善細胞邊緣使用的傳輸品質，我們使用了部分頻率重覆使用機制來降低相鄰細胞間同頻干擾的情形，特別是在會產生跨時槽干擾的非對稱服務的環境中。設計部分頻率重覆使用機制中的參數，可以同時獲得提升頻譜使用率和降低相鄰細胞間干擾的好處。在本篇論文中，我們探討了如何配置頻寬給不同區域的使用者、設計細胞內圈的大小和細胞外圈的頻率重覆使用參數 (Frequency Reuse Factor)。此外，我們使用了一套分析的方法來分析跨時槽範圍的大小和設計部分頻率重覆使用機制參數之間的關係。更重要的是，我們可以應用這套分析的方法來找出最適當的參數設定並且隨時更新設定，藉此改善現有系統所需的繁複計算和長時間的模擬。最後，我們建立了一套應用部分頻率重覆使用機制在分時雙工且正交分頻多工存取系統並支援非對稱服務的模擬平台，藉由此平台來找出最適當的部分頻率重覆使用機制參數使系統傳輸量 (System Throughput) 最大並同時維持傳輸可靠度 (Link Reliability)。

In this thesis, we investigate the benefits of fractional frequency reuse (FFR) in a time division duplex (TDD) to orthogonal frequency division multiple access (TDD-OFDMA) systems for supporting asymmetric traffics. We consider FFR scheme to discuss how we can use FFR to reduce the effects of inter-cell interference (ICI) and maintain the spectrum efficiency. However, we provide an analytical approach to analyze the relationship between cross time slot (CTS) region and the design factors of FFR scheme. The analytical approach depends on queueing theory and considers each wireless access points to evaluate the probability of generating CTS region. Therefore, the analytical approach can help us to design FFR factors in asymmetric traffic environment with different uplink/downlink requirements. In the field of radio resource management (RRM), a good interference management scheme can mitigate the effects of inter-cell interference and improve system throughput, such as directional antenna, multi-input and multi-output antenna (MIMO), and FFR scheme while they implement in multi-cellular wireless communication systems. As we know, we need more and more frequency spectrums can be used in fourth generation (4G) wireless communication. The effective approach to increase frequency spectrums is reusing same bandwidth in multi-cellular system that means some cells may reuse same frequency spectrums in the meanwhile. Although this method can increase frequency spectrums immediately, it will degrade the received signal to interference plus noise ratio (SINR) seriously because of the effects of inter-cell interference comes from neighboring cells. The poor SINR levels will affect link quality while MS arrives in the system. Hence, how to reuse same bandwidth in multi-cellular system and also maintain link quality become an interesting issue in wireless network as the interference mitigation and spectrum efficiency become trade-off on the system performance. There are two options of division duplex mode may be used to separate uplink and downlink transmission, frequency division duplex (FDD) and time division duplex (TDD). In the different division duplex modes may generate different kinds of inter-cell interference from neighboring cells especially TDD mode. TDD mode with supporting asymmetric services may cause stronger inter-cell interference than symmetric services. The stronger inter-cell interference always occurs in the CTS region that presents observed cell and neighboring cells are executing different transmission mode, uplink or downlink transmission. This kind of inter-cell interference be called cross-slot interference in TDD mode with asymmetric traffics. For uplink transmission, base station (BS) may receive inter-cell interference from neighboring cell's mobile station (MS) because the neighboring MS is transmitting signal to the BS. Hence, the stronger inter-cell interference comes from neighboring BS, called BS-to-BS cross-slot interference. In other words, it will cause MS-to-MS cross-slot interference in downlink transmission while neighboring cell is executing uplink transmission. While the MS-to-MS cross-slot interference causes minimal degradation in downlink, the BS-to-BS cross-slot interference can severely decrease SINR in uplink, especially when the MS executes uplink mode and locates at cell boundary. Hence, using the FFR scheme can decrease the effects of inter-cell interference including cross-slot interference and non cross-slot interference especially it is surrounded in an asymmetric traffic environment. Different design for FFR scheme will get advantages on spectrum efficiency and interference mitigation, respectively. We develop FFR scheme that's main solution is considering interference mitigation at cell's outer region. In the thesis, we discuss how to allocate channel bandwidth, design the size of inner region and number of frequency reuse factor in outer region. Furthermore, we use an analytical method to analyze size of CTS and the design factors of FFR scheme in a TDD-OFDMA system with supporting asymmetric services. More importantly, using the analytical method can find optimal design factors of FFR scheme and updates adaptively according to the traffic requirements in practical systems due to the simulation-based adaptation requires huge complexity and processing time. Finally, we build a simulation platform for the FFR based TDD-OFDMA system to simulate different asymmetric traffic environments and find out the optimal FFR factors to maximize system throughput with guaranteeing link reliability.