應用於WCDMA/WLAN異質網路之乏晰邏輯允諾控制

Abstract

藉由整合不同無線通訊網路所構成的異質網路（heterogeneous network）是有效提升整體服務容量與品質的方法之一。本篇論文所研究之寬頻分碼多重進接（WCDMA）系統與無線區域網路（WLAN）共存之異質網路，除兩者皆為現今使用最廣泛的通訊標準外，其個別之通訊特性更具有高度的互補性。WCDMA通訊覆蓋範圍大，支援高速移動通訊服務，並且具備完整之信令架構與核心網路，提供無線資源管理極佳的平台，唯其網路建置成本高，而且面對日益精緻之多媒體服務，但各通道之最高資料傳輸速率仍不足；雖有如多用戶偵測（multiuser detection）等進階機制可提昇系統容量，但所需的高運算量仍是實際必須考量之處。而WLAN則具備高傳輸速率以及網路建置成本低之優勢，但一般覆蓋範圍較小且多為區域性，再加上行動服務支援度低，因此無法有效提供行動用戶無縫式之寬頻服務。由此可知，在設計WCDMA/WLAN異質網路資源管理機制時，可針對彼此之特性截長補短，提供更優良的寬頻行動網路服務。

允諾控制（admission control）是WCDMA/WLAN異質網路資源管理中極為重要的管理機制之一。面對使用者所提出的連線與服務品質（QoS）要求，允諾控制必須能有效掌握各個網路之通訊品質狀態以及資源利用之程度，對於行動用戶經由換手（handoff）而產生的連線要求，更必須考慮該原有服務之連續性。因此本篇論文首先提出在WCDMA系統中應用多用戶偵測方法時的呼叫允諾控制器設計。利用序列式干擾消除（successive interference cancellation；SIC）而達成多用戶偵測的目的可大幅提昇系統容量，也由於接收訊號經過SIC再處理後特性已有改變，主要干擾源將變成來自於鄰近細胞。因此在我們所提出之呼叫允諾控制將鄰近細胞干擾的影響比例提高，並且引進乏晰邏輯技術，針對多變之系統狀態作出最佳之允諾決策。

其次，本篇論文也針對WLAN系統提出一結合允諾控制與排程之機制設計，其中針對訊號品質與服務要求，為使用者上下鏈路傳輸安排適當之傳輸機會（transmission opportunity）。另外鑑於WLAN中缺乏迅速有效之換手方式，因此我們也提出一套相容於IEEE 802.11e標準之快速換手協定（fast handoff protocol；FHP），以消除換手要求封包在競爭傳輸通道時的延遲不確定性，以利於換手預先動作（pro-active）啟動時機的選擇。

最後，本篇論文整合考慮WCDMA與WLAN共存異質網路中的允諾控制器設計。其中考慮兩系統個別之系統狀態、使用者QoS要求、以及使用者移動狀態估測等關鍵量測值，並利用具適應能力之類神經-乏晰推論系統（neural-fuzzy inference system）與Q-learning自我學習機制，決定新進使用者或換手使用者連線要求的允諾與否，以及允諾之最適合網路。故此一設計不但具備允諾控制功能，同時也能作為WCDMA/WLAN異質網路中的網路選擇（network selection）控制器。

The heterogeneous network is a type of the most direct and efficient infrastructure to extend the system capacity and service quality for the demanding multimedia environment. In this dissertation, two of the most popular systems, wideband code division multiple access (WCDMA) system and wireless local area network (WLAN) system, are considered to form the heterogeneous network. As the global cellular system, the WCDMA system has almost universal coverage in the world with high-mobility support and comprehensive core networks. But the cost of deployment and insufficient bandwidth for the growing multimedia services are its major disadvantages. WLAN system provides higher data rate to support multimedia services with lower cost, but the smaller service area and lack of complete handoff procedures restrict the mobility services. Hence, WCDMA and WLAN systems are highly complementary to each other. Basing on these features, we develop call admission control (CAC) schemes with fuzzy logic theorem for WCDMA and WLAN systems to achieve quality-of-service (QoS) guarantee and higher system utilization in the heterogeneous networks.

Multiuser detection (MUD) has been discussed and studied for a couple of years. Its impressive increase in capacity has attracted WCDMA systems to consider to adopt this technology. The capacity limit, however, still exists due to other cells’ multiple access interference (MAI) in a cellular system. As a result, a CAC scheme is essential to control the number of mobile users from the view of point of MUD. This dissertation proposes an outage-based fuzzy call admission controller with multiuser detection (OFCAC-MUD) for WCDMA systems. The successive interference cancellation (SIC) is used as MUD because it has lower complexity and more suitable for the fading channel with imperfect power control. The OFCAC-MUD determines the new call admission based on the uplink signal-to-interference ratios from home and adjacent cells and system outage probabilities. The OFCAC-MUD possesses both the effective reasoning capability of fuzzy logic system and the aggressive processing ability of MUD. Simulation results reveal that OFCAC-MUD without power control (PC) improves the system capacity by 70.5% as compared to an SIR-based CAC-RAKE with perfect PC. It also enhances the system capacity by 53.9% as compared to an OFCAC-RAKE with perfect PC, by 6.7% as compared to an SIR-based CAC-MUD without PC, and by 12.9% as compared to an OFCAC-MUD with perfect PC, given the same outage probability requirements. Moreover, OFCAC-MUD can prevent the violation of outage probability requirements in the hotspot environment, which is hardly achieved by SIR-based CAC.

For the WLAN systems, we propose an intuitive scheduling and admission control (ISAC) scheme based on hybrid coordination function (HCF) mode in IEEE 802.11e cellular WLAN systems. The ISAC scheme considers admission control, based on not only the quality of service (QoS) required by each application but also the link quality of air interface influenced by the co-channel interference from adjacent cells. Furthermore, we also propose a fast handoff protocol (FHP) for cellular IEEE 802.11e WLAN systems. The FHP, which is standard compatible, provides a controlled contention period (CCP) designated for handoff requests (HO-REQs), arranges these HO-REQs to contend sequentially in CCP, and proposes a fuzzy adjustment method (FAM) to determine a proper length for CCP. Simulation results reveal that the FHP can significantly decrease the forced termination rate of HO-REQ and enhance the system throughput of contention period for cellular IEEE 802.11e WLAN systems.

Finally, a fuzzy Q-learning admission control (FQAC) mechanism is proposed for WCDMA/WLAN heterogeneous networks in this dissertation. The FQAC consists of dwelling estimation and admissibility estimation to consider the mobility pattern and essential system measures. The dwelling estimation can assess the dwell time length for a mobile user in the reachable subnetworks and output dwelling costs. The admissibility estimation can judge which reachable subnetwork(s) can support the required QoS and output admissibility costs. With Q-learning method, the FQAC can adaptively adjust the actions to output the costs without the knowledge of system state transition probability. In order to minimize the expected maximal impact (cost) of the user’s admission request, the decision maker applies the Minimax criterion for these costs and decides the most suitable subnetwork or reject the user request. Simulation results show that FQAC can almost maintain the system QoS because it can appropriately admit or reject the users’ admission requests. The dwelling estimation can significantly reduce the number of handoffs, which makes FQAC to have lower handoff blocking probability in those real-time services.