行動寬頻無線網路之適應性通訊與省電機制

Abstract

近幾年來，隨著移動式網際網路與無線多媒體應用的需求增長，刺激著寬頻無線存取技術的研究發展。在各式各樣的寬頻無線存取技術發展之際，以IEEE 802.16標準為基礎的移動式全球互通微波存取（Mobile WiMAX）是目前已經開始提供服務的領先技術。此篇論文論及一些在以IEEE 802.16為基礎的移動式寬頻無線網路之問題。更具體地說，四個議題在此篇論文中被探討：應用於IEEE 802.16點對多點（Point-to-multipoint, PMP）網路使用者（Subscriber Station, SS）間的適應性直接通訊方法；針對每對欲採用直接通訊方式的移動式使用者（Mobile Station, MS），以預測移動與干擾為基礎的排程演算法；在IEEE 802.16m睡眠模式運作中的傾聽視窗（Listening Window）調整機制；以及在該模式中以統計方式控制睡眠視窗（Sleep Window）的方法。 在第一個議題中，一個有彈性且免競爭的通訊方式被提出以提供IEEE 802.16 點對多點網路使用者間進行直接通訊。基地台（Base Station, BS）控制和安排特定的時段給兩個正在進行封包傳送的使用者。在所提出的方法中，封包傳送的運作方式將依據基地台和使用者間的頻道狀態（Channel Condition）主動地在直接與間接通訊間切換。這樣的機制將會增加頻寬的利用率進而提升使用者生產量（User Throughput）。 以上述所提及之直接通訊為基礎，對於每對預期採用直接通訊的移動式使用者之排程演算法在第二個議題中被探討。藉由分析與估測每對欲通訊的移動式使用者的干擾範圍（Interference Region）和可實行範圍（Feasible Region），提出的演算法將適當地把該對移動式使用者進行直接通訊的時期安排在下載子框架（Downlink Subframe）或上傳子框架（Uplink Subframe），亦或者將其通訊方式轉換成原本的間接通訊方式，即經由基地台傳送。此外，合併該排程演算法與提出的預測使用者移動之機制，更新移動式使用者位置資訊所帶來的控制花費（Control Overhead）將被有效率地降低。 另一方面，為了改進移動式使用者在睡眠模式中具有即時性傳輸資料量（Real-time Traffic）時的能量保存效能，一個有彈性的傾聽視窗調整機制將在第三個探討議題中被提出。不同於IEEE 802.16e-2005標準中所規範的以視窗為基礎之運作方式，在提出的機制中將以週期為運作之基礎。該機制根據儲存的封包數目、欲重新傳送的封包數目以及可容忍的延遲限制（Delay Constraint），動態地調整每個睡眠周期（Sleep Cycle）中傾聽視窗與睡眠視窗的比例，以達到節省能量之目的。值得一提的是作者於IEEE 802.16m國際標準制訂會議中提出該機制，其概念順利被採納於目前的IEEE 802.16m標準草稿中。 至於改善移動性使用者在睡眠模式中具有非即時性傳輸資料量（Non-real-time Traffic）時的能量保存效能，一個以IEEE 802.16m睡眠模式運作的概念為基礎，利用統計方式控制睡眠視窗的方法則是在第四個議題中被提出探討。該機制建構一個不連續時間的馬可夫控制的卜瓦松程序（Discrete-time Markov-modulated Poisson process, dMMPP）以表達非即時性傳輸資料的狀態。此外，利用部分環境可知馬可夫決策程序（Partially Observable Markov Decision Process, POMDP）來推測目前傳輸資料量的狀態。藉由該猜測的狀態加上可容忍的延遲限制和佇列大小（Queue Size）的考量，兩個次佳的策略在此方法中被提出，以達到改善能量保存之目的。

The growing demand for mobile Internet and wireless multimedia applications has motivated the development of broadband wireless-access technologies in recent years. While various broadband wireless-access technologies are under development at this time, mobile WiMAX based on IEEE 802.16 standards is the leading technology already in service today. This dissertation deals with some problems regarding IEEE 802.16-based mobile broadband wireless networks. Specifically, four topics are investigated: an adaptively direct communication approach for subscriber stations (SSs) in an IEEE 802.16 point-to-multipoint (PMP) network, a predictive motion and interference-based scheduling algorithm for mobile stations (MSs) with direct communication, a flexible listening window adjustment approach, and a statistical sleep window control approach for IEEE 802.16m sleep mode operations. A flexible and contention-free communication approach is proposed to support direct communication between SSs in an IEEE 802.16 PMP network. The base station (BS) is coordinating and arranging specific time periods for the two SSs that are actively involved in packet transmission. According to the channel conditions among the BS and SSs, the packet transmission operation is switched between the direct link and indirect links in the proposed approach, which improves the bandwidth utilization and consequently increases the user throughput. Based on the procedures mentioned above, a scheduling algorithm for each pair of MSs that is expected to conduct direct communication is presented. Both the interference region and feasible region for the pair of MSs to perform direct communication are studied and calculated. Based on these two types of information, the algorithm properly arranges the MSs to conduct direct communication in either downlink subframe or uplink subframe, or to communicate with the conventional communication manner, i.e., via the BS. Furthermore, by incorporating the motion prediction mechanism with the scheduling algorithm, the control overheads regarding the updates of MSs positions can be efficiently reduced. On the other hand, in order to enhance the efficiency of energy conservation, a flexible listening window adjustment approach for an MS in the sleep mode with real-time traffic is present. Instead of window-based operation specified in the IEEE Std. 802.16e-2005, a cycle-based mechanism is considered in the proposed approach. With the consideration of tolerable delay constraint, the approach dynamically adjusts the ratio of listening window to sleep window for each sleep cycle based on the number of both buffered and retransmitted packets. It is worthwhile to mention that the concept of flexible listening window adjustment approach has been proposed by the author and is adopted in the IEEE 802.16m standard draft. For an MS in the sleep mode with non-real-time downlink traffic, a statistical sleep window control approach is provided based on the concepts of IEEE 802.16m sleep mode operation. The proposed approach constructs a discrete-time Markov-modulated Poisson process for representing the states of non-real-time traffic. Furthermore, a partially observable Markov decision process is exploited to conjecture the present traffic state. Based on the estimated traffic state and the considerations of tolerable delay and/or queue size, two suboptimal policies for maximizing energy conservation are proposed within the approach.