可認知服務品質之多載波OFDMA網路排程算法

Abstract

在近幾年間，隨著無線網路技術的進步，越來越多人使用無線上網，各種在 無線環境的應用也越來越多。也正因如此，當眾多使用者加入無線網路中，便伴隨著頻寬的不足，尤其當使用各種多媒體服務(包含即時性與非即時性)，如何利用有效率的封包排程確保各使用者的QoS(Quality of service)並能兼顧使用者間的 公平性是個重要的議題。 根據 ITU 的要求，在未來4G 通訊系統(IMT-Advanced systems)中，高速移的使用者必須能還能維持連線並保持100 Mbps 的傳輸速率，而固定或游牧式必須能達到1 G bps，要達到如此速率，需要用更大的頻譜來傳輸，然而在現行系統下無法一次供給如此大的連續頻譜，必須使用多個載波以達到此頻寬需求。因此為達到IMT-Advanced 的要求，3GGP 和IEEE 802.16 兩個組織分別針對此議題做出解決方法，其共同概念即以多個整合多個載波來傳輸，其提出的系統標準分別為 LTE-A(Long Term Evolution-Advanced ) 與802.16m。早期封包排程研究中，多針對單一載波是以OFDMA(Orthogonal frequency-division multiple access)作傳輸技術，而OFDMA 有許多好處，包括提高頻譜使用效率、減少多重路徑干擾等。因此上述此兩系統(LTE-A、802.16m)均被期望設計以OFDMA 作傳輸技術。 本論文提及了出一個在多載波OFDMA 系統下之QoS-aware 並以 PF(Proportional Fairness)為基礎的封包排程演算法。不同於大多數PF 的演算法是考慮實體層上的效能，並不以上層實際封包來做排程，因此我們提出的算法著重在以封包為考量的單位(Packer layer)，並考慮即時性服務對於封包延遲的要求，在多載波的環境下分配使用者資源，如子通道以及時槽，並提供使用者間的公平性。 利用 Matlab 模擬軟體，研究對不同QoS 的封包做排程，並分析模擬結果。模 擬結果顯示，所提出之演算法能有效提升系統資源的使用效率，特別當系統附載高時，並能保障即時性服務封包之延遲時間。故能滿足使用者之服務品質需求。

In recent years, due to the fast development of the wireless technology, more and more people access a lot of applications via wireless networks. However, when people access wireless networks, it often leads to a short of frequency resources; thus, how to design an efficient packet scheduling algorithm to guarantee the QoS (throughput, delay, etc.)of the user’s real-time traffic and non-real time traffic and also to maintain the fairness among users have become important issues. According to ITU’s requirements, in future cellular systems (IMT-Advanced systems), it must support data transmissions in multi-user environments with a target peak rate up to approximately 100 Mbps for high mobility (such as mobile access) and up to approximately 1 Gbps for low mobility (such as nomadic/local wireless access). To achieve such high speed data transmissions, it must use much larger frequency bands to transmit; however, the existing systems cannot provide such large consecutive frequency band. To overcome this problem, one must employ multiple carriers to fulfill the bandwidth requirement. Thus, both the 3GPP and IEEE 802.16 working groups have become candidates for IMT-Advanced standard, and both have adopted the concept “carrier aggregation,” which integrates multicarrier for transmissions to achieve the speed requirement of the IMT-Advanced. The proposed systems from respective working groups are LTE-Advanced and 802.16m. Moreover, in the recent studies of packet scheduling algorithms for a single carrier transmission, the OFDMA scheme is often assumed, since the OFDMA technology has many advantages, including high spectrum efficiency, resistance against multipath interference and etc. Therefore, the multicarrier systems discussed above are also expected to be designed with an underlying OFDMA technology. In this thesis, we propose a QoS-aware proportional fairness based packet scheduling algorithm for multicarrier OFDMA systems considering the downlink environment. The proposed scheduling approach is different from most previous scheduler algorithms, which usually did not consider the supported applications; instead, they almost all focused on the physical layer. Thus, the main scheduling algorithm we proposed covers the packet layer, considering user’s QoS requirements (such as packets delay for real-time applications) and the fairness issue in multicarrier OFDMA system. We use Matlab as the simulation tool to set up the simulation environment in which packets can be simulated according to different QoS requirements. We then examine and analyze the obtained simulation results. Based on the simulation results, the scheduling algorithm we proposed can be shown to provide improvement of the system transmission capacity, especially when the system load is high. We also show that the proposed algorithm does guarantee the packet delay for real-time service. Therefore the proposed algorithm can achieve all users’ QoS requirements as expected.