異質性無線網路之干擾管理技術

Abstract

本論文主要在探討異質無線網路之干擾管理技術，包括正交分頻多重接取之毫微微細胞網路、機器型態通訊毫微微細胞網路以及裝置間通訊毫微微細胞網路。此外，本論文也提出了相對應的解決方案，用以確保對用戶的服務品質。本論文之主要研究課題如下所述。

在第一個部份中，我們在正交分頻多重接取之毫微微細胞系統下，提出了一個位置知曉機制，同時結合一個低成本、四區段的切換式波束方向性天線，以提高頻譜效率。這個切換式波束方向性天線安裝在毫微微細胞基地台上，用以減輕來自毫微微細胞的干擾。而且，我們提出利用調整使用的子載波數量來控制毫微微細胞與巨細胞之間的干擾，以確保所有用戶的鏈結可靠度。隨著知道戶外用戶的位置資訊，並藉著調整每個毫微微細胞所使用的正交分頻多重接取子載波數量，我們提出的位置知曉之毫微微細胞結合四區段的切換式波束方向性天線可以有效地避免毫微微細胞與巨細胞之間的干擾。此外，我們提供了一個設計原則，用以決定合適的頻譜分配方案以及毫微微細胞的接取方式。

在第二個部分中，我們在正交分頻多重接取之多用戶毫微微細胞系統下，提出一個低複雜度且分散式的穩定子通道分配演算法，使得在系統容量、用戶間公平性以及服務延遲等系統效能之間可以取得一個最佳的權衡。我們建議毫微微細胞可以結合低成本可重置的切換式多波束方向性天線，更進一步地降低干擾去改善系統容量。同時，我們發展一個天線輻射方位暨子通道分配法則，讓系統容量的最佳化、用戶間公平性之確保以及服務延遲效能之改善得以實現。以系統容量，用戶間公平性以及延遲效能為目標，相較於現有的方案，包括（1）通道導向的子通道分配方案、（2）使用者導向的子通道分配方案、（3）比例式公平的子通道分配方案、（4）指數法則的子通道分配方案、以及（5）排隊等候基準之指數法則的子通道分配方案，我們提出的穩定子通道分配結合切換式多波束方向性天線方案會是一個最佳的選擇。

在第三個部分中，我們在機器型態通訊毫微微細胞系統下，提出分群控時機制以改善因大量機器型態通訊裝置同時上傳資訊所造成的網路壅塞以及訊息延遲等問題。該分群控時機制將所有機器型態通訊裝置分成多個小群組，並指定每個群組一段可以上傳資訊的允傳時段。透過分群控時機制，就可以將大量機器型態通訊裝置所造成的巨大流量隨著時間而緩和其峰值流量。於是，該機制有能力同時減輕無線接取網路及核心網路過載的發生。然而，如果過度地分群會使得群組數量大量增加，於是每個群組在等候其允傳時段的時間就會過長，導致訊息傳遞所需要的時間大幅地增加，造成無法容忍的訊息延遲情況發生。因此，我們分析了機器型態通訊毫微微細胞系統下的網路過載機率以及訊息延遲的關係，並探討如何分群控時以達到同時解決網路壅塞並確保訊息延遲效能的最佳權衡。

在最後一個部分中，我們在裝置間通訊毫微微細胞系統下，提出了一個智慧型資源管理機制以改善系統容量與服務品質。藉由巨細胞基地台與毫微微細胞基地台所廣播之特定訊息，每對裝置間通訊用戶能夠有效地及自主地選擇合適的資源並調整適當的傳輸功率，以減輕複雜的三階層式之干擾。同時，我們建議透過調整正交分頻多重接取之毫微微細胞系統的資源使用率，以控制毫微微細胞系統對巨細胞系統及裝置間通訊系統的干擾，進一步保護所有用戶之鏈結可靠度。我們提供了一個設計原則，用以決定進入裝置間通訊模式之適合的距離，並選擇最佳的毫微微細胞系統資源使用率，以改善系統容量並確保鏈結可靠度。

總而言之，本論文之主要貢獻在於解決異質無線網路中所可能造成複雜的干擾問題以及網路壅塞情形。而且，本論文所提出的解決方案能顯著地改善系統容量及網路過載情況發生，並確保鏈結可靠度及延遲效能等服務品質。

In this dissertation, we investigate interference management techniques in heterogeneous wireless networks, including orthogonal frequency-division multiple access (OFDMA)-based femtocell networks, machine type communications (MTC) femtocell networks, and device-to-device (D2D) femtocell networks.

In the first part, we propose a location-aware mechanism combined with a low-cost four-sector switched-beam directional antenna to enhance the spectrum efficiency of OFDMA femtocell systems. The switched-beam directional antenna is installed for femtocell base stations to mitigate the interference from femtocells. Moreover, the subcarrier number adjustment method is developed to control the interference among femtocells and macrocells, and ensure all users’ link reliability. With the knowledge of the locations of outdoor users, the proposed four-sector switched-beam antenna in a femtocell can effectively avoid the interference among femtocells and macrocells by adjusting the number of OFDMA subcarriers used at each femtocell. Furthermore, we provide a design principle to decide the suitable spectrum allocation scheme and access method for femtocells.

In the second part, we propose a low-complexity distributed stable subchannel allocation algorithm to improve the tradeoff of capacity, users’ fairness, and head of line (HOL) delay for OFDMA-based multi-user femtocell systems. The stable subchannel allocation scheme can optimize the capacity by repeatedly rearranging the subchannel allocation, and guarantee the fairness and delay performance by limiting the maximum allowable number of subchannels for a user. In addition, we suggest applying the low-cost reconfigurable switched multi-beam directional antennas on femtocells to further reduce the two-tier interference and improve the system capacity. Furthermore, we develop a joint antenna pattern selection and subchannel allocation method to optimize the system capacity, guaranteeing the fairness among users, and improving the HOL delay. Compared to the existing method (i.e., the channel-oriented subchannel allocation scheme, user-oriented subchannel allocation scheme, proportional fair subchannel allocation scheme, exponential rule subchannel allocation scheme, and queue-based exponential rule subchannel allocation scheme), the proposed stable subchannel allocation scheme with the switched multi-beam antenna is the optimal option to achieve the highest femtocell capacity, while guaranteeing the fairness and HOL delay.

In the third part, we propose the group-based time control method to improve the network congestion and message delay caused by the massive concurrent data transmission from MTC devices in femtocell systems. By dividing the MTC devices into several groups and dedicating each group with a granted time interval for access, the heavy traffic load of massive MTC devices can be spread over the time to alleviate the traffic peak, which mitigates both the radio access network (RAN) and the core network (CN) overloads simultaneously. However, excessively dividing the groups may incur the intolerable message delay because of the immoderate increase on the cycle time. Accordingly, we analyze the overload probability and message delay of the MTC femtocell system to achieve the optimum tradeoff.

In the final part, we propose the intelligent resource management mechanism to enhance the system throughput and link reliability for D2D femtocell systems. Based on the specific information broadcast by the macrocell and femtocell base stations, the D2D pairs can jointly select the suitable resource blocks and adjust the appropriate power to alleviate the complicated three-tier interference in an effective manner. In addition to joint resource block selection and power adjustment for D2D communications, we suggest adjusting the utility rate of resource blocks for the OFDMA femtocells to guarantee the link reliability of all users. The design principle provided in this part can determine the suitable D2D distance to enter the D2D mode, and select the optimal resource utility rate of femtocells to improve the system throughput subject to the link reliability requirement.

In summary, the main contribution of this dissertation is to solve the complicated issues of interference and network congestion in the heterogeneous wireless networks. The proposed solutions significantly improve the system capacity and the network overload, while guaranteeing the quality of services.