次世代無線網路下頻譜共享通訊協定之設計

Abstract

根據近期的研究，授權給特定使用者的頻譜有大部分的時間是處於空閒的狀態，而可以與其它系統共享以提高頻譜利用率的靈活性。滿足不斷增長的移動通訊資料需求，頻譜共享成為一個有前途的解決方案。基於不同的頻譜使用的原則，可以分為基於競爭的頻譜共享(contention-based spectrum sharing)，基於經營管理的頻譜共享(broker-based spectrum sharing)，以及感測為基礎的頻譜共享(sensing-based spectrum sharing)。根據美國聯邦通信委員會(FCC) 的提議，原本是分配給智能交通系統(ITS) 專用來進行短程通信(DSRC) 的5.9 GHz 頻段，現在應共享給無照的設備(unlicensed devices)。基於競爭的通道存取機制是IEEE 802.11p 標準的媒體存取控制(MAC) 協議並且由IEEE 1609.4 標準的多通道操作來增進。DSRC 設備和無照的設備之間的共存會導致DSRC 設備吞吐量的退化。我們提出了賦予優先權的最佳通道分配(POCA) 計劃，以最大限度地提高無照設備的 吞吐量，並且同時保證了DSRC 設備的服務質量(QoS)。在每個通道上無照設備的最佳數量可以根據分散式或集中式網絡來進行設計。POCA 方案可以顯著提高系統的性能，並且優於現有的其他方法。在經營管理為基礎的頻譜共享網絡，頻譜經紀人(spectrum broker)或稱為獲得授權/許可的共享訪問(ASA/ LSA) 控制器是一個集中的頻譜管理器，可以允許被許可的授權者(例如，行動網路運營商) 在得到現任用戶的同意下來存取未充分利用的頻譜。然而，不同的運營商可以共享頻譜經紀人提供的頻譜，導致在共享頻譜上無法預測的干擾。我們提出了一個細胞(cell) 間的隔空通信(OTA) 機制，來加強屬於不同的運營商細胞之間的訊息交換機制。基於各細胞之間的訓息交換得到的干擾協調和全局通道狀態信息(CSI) 中，我們提出了俱有能源效益(energy-efficient) 的細胞選擇和資源分配(EECR)以最大化系統能源效益。能源效率可以通過切斷未充分利用的基站(base station) 來進一步改善。此外，基於不同的地理定位而有未充分利用的許可頻譜，也可以共享給其他系統使用。基於感知無線電(CR) 的概念，次要用戶(SU) 可以動態地存取和利用主要用戶(PU)沒有使用的頻譜。我們利用部分可觀測馬爾科夫決策過程(POMDP) 來檢測頻率載波並估計通道佔用信息(COI)。對於先進長期演進技術(LTE-A) 系統中，載波聚合(CA) 允許LTE基地台(eNB) 和用戶設備(UE) 能夠存取不同連續和不連續的分量載波(CC)，讓頻譜共享成為可行的。我們提出了基於POMDP 的載波選擇和資源分配(POCR) 方案，根據部分可觀察的通道佔用信息、通道狀態信息(CSI)、以及競爭細胞的數目(NOC)，證明出可以達到最佳目標載波和最佳的資源分配。數值結果表明，本文所提出的協議可以有效地提高共享頻譜系統性能。

According to recent research, a large portion of auctioned frequency spectrum remains idle at specific time and location and can be shared with other systems to improve the flexibility of spectrum utilization. Spectrum sharing becomes a promising solution to serve continuously growing traffic demands. Based on different spectrum usage principles, spectrum sharing can be categorized into contention-based spectrum sharing, broker-based spectrum sharing, sensing-based spectrum sharing. According to the proposal of Federal Communications Commission (FCC), the 5:9 GHz band, which is originally allocated for dedicated short-range communications (DSRC) to be used by intelligent transportation system (ITS), should be shared with unlicensed devices. The contention-based channel access mechanism is the media access control (MAC) protocol of IEEE 802:11p and is enhanced by IEEE 1609:4 standard for multi-channel operations. The coexistence between DSRC devices and unlicensed devices will cause the degradation of throughput of DSRC devices. The prioritized optimal channel allocation (POCA) schemes are proposed to maximize the throughput of unlicensed devices with guaranteeing quality of service (QoS) of DSRC devices. The optimal number of unlicensed devices on each channel can be derived in either distributed or centralized networks. The POCA schemes can significantly improve the system performance and outperforms other existing works. In the broker-based spectrum sharing network, the spectrum broker or called Authorized/ Licensed Shared Access (ASA/LSA) controller is a centralized spectrum manager which allows licensees (e.g., mobile operators) to access the underutilized spectrum with the agreement with the incumbent user. Different operators can share the spectrum provided by spectrum broker and the interference will become unpredictable on the shared spectrum. An over-the-air (OTA) inter-cell communication mechanism is proposed to enhance the conventional message exchange mechanism between cells belonging to different operators. Based on the interference coordination and global channel state information (CSI) derived by the information exchange between each cells, a energy-efficient cell selection and resource allocation (EECR) is proposed to maximize the system energy efficiency. The energy efficiency can be further improved by switching off the base stations if it is underutilized. Besides, an licensed spectrum which is underutilized based on different geolocation can also be shared by other systems. The concept of cognitive radio (CR) is therefore adopted for secondary user (SU) to dynamically access and utilize opportunistic spectrums not occupied by primary user (PU). The partially observable Markov decision process (POMDP) is applied to estimate the channel occupancy information (COI) by partially sensing the frequency carriers. A POMDP-based spectrum handoff (POSH) scheme is proposed to select the feasible spectrum to access for SUs. By adopting the policies resulted from the POSH scheme for target channel selection, minimal waiting time for spectrum handoff can be achieved. Moreover, for long term evolution-advanced (LTE-A) system, carrier aggregation (CA) allows LTE evolved Node B (eNB) and user equipment (UE) to access different continuous and discontinuous component carriers (CCs) which becomes feasible to adopt spectrum sharing. A POMDP-based CC selection and resource allocation (POCR) scheme is proposed and proved to achieve optimal target carrier and optimal resource allocation according to the partially observable COI, channel state information (CSI), and number of contending cells (NOC) in the network. Numerical results illustrate that the proposed protocols can effectively improved system performance on shared spectrum in this dissertation.