綠能蜂巢式網路下睡眠模態基地台之合作式策略

Abstract

在綠能蜂巢式網路中，如何選擇哪個基地台進入睡眠模態是一個熱門的議題。過去的研究顯示基於現今使用者的流量狀況與位置可以決定出很有效的開關基地台策略，搭配上擴大範圍的功能，可以使得開啟的基地台數變為劇減。但是為了關閉更多基地台而增加的傳輸能量，它的能量消耗也變得更為重要，與整個基地台中的運作能量一樣是最主要的消耗原因，而這卻在最近的研究中屢屢被忽略。 另外一方面，合作式技術在無線網路中扮演了重要的角色，理由是因為它能增加網路效率並能增加無線通道的傳輸品質，而它主要的功能是藉由兩個或多個基地台的同時傳輸，進而達到這些功能。也因此，藉由此吸引人的功能，我們可以用合作式技術去減少因為增加範圍而遽增的傳輸能量，但為了達到綠能的目的，如何同時決定哪一個基地台關閉，又哪些基地台與基地台之間要做合作變成一個很困難的問題，也因此這篇論文主要在探討設計一個有效率的演算法去同時解決綠能的目標並同時滿足使用者的需求。 在這篇論文當中，我們研究了在綠能蜂巢式網路下對睡眠模態基地台的合作式策略，我們藉由大量的文獻回顧去點出傳輸能量問題的重要性，並為了將睡眠模態基地台和合作式策略能夠同時運用上，我們設計了新的訊框並提出了合理假設去達成此目標，另外一方面，為了能夠有效的提出我們的演算法，我們將此問題用二位元整數最佳化 (Binary Integer Programming) 的方式去呈現，並藉由集合覆蓋的問題去證明屬於一個NP-hard的問題。然後藉由典型二位元整數最佳化的解決方式，我們使用了搜尋列舉法 (Search Enumeration) 還有拉式鬆弛法 (Lagrangian Relaxation) 去求出此問題的下界。藉由上面方法觀察到的一些特性，我們提出了兩種啟發式演算法，並說明為何它為線性時間演算法。 最後，從我們的模擬結果，我們可以發現我們的方法會比不考慮傳輸能量的綠能策略還要節能甚至也可以關閉更多的基地台，然後我們也同時發現基地台合作的數量會在不同的情況選擇不同的合作策略，有別於一般認定越多基地台合作就會有更好的效果，並用簡單的方式說明此現象的發生。跟沒有做任何綠能策略的狀況比起來，我們甚至可以省下三十到六十百分比的能量消耗，也可以證明了我們的方法是一個有效節能的合作式策略。

Switching off base stations (BS) is an effective and efficient solution to energy saving in green cellular networks. The previous works focus mainly on when to switch off BSs based only on the amount of current traffic demands of active users, and use the coverage extension functionality to cover users as more as possible. Based on this objective, not only the constant power but also the transmit power becomes the main energy consumption of BS. However, the previous researches do not consider transmit power into account when applying switching-off functionality. On the other hand, the cooperation technique is the key feature in next generation wireless networks, which most of researches use this technique to improve the throughput or enhance the spectrum efficiency via concurrent transmission by two or more BSs. As mentioned in the previous paragraph, the transmit power is ignored in energy saving paradigm. The cooperation technique is the remedy for solving this problem. However, jointly considering sleep-mode BSs and cooperation technique is the main challenge for the purpose of being green. Our goal is to design energy-efficient cooperation strategies to ensure our system is green and to satisfy users’ demands at the same time. In this thesis, we investigate the benefit of sleep-mode functionality of BSs and the cooperation technique in green cellular networks. To combine these two features, we introduce the new frame structure and its applicable scenario. Then, we formulate our problem into binary integer programming, and prove it is NP-hard. Based on our formulation, we solve the lower bound for this problem via Lagrangian relaxation with search enumeration. Furthermore, we propose two heuristic algorithms by the properties of energy saving and the limitation of bandwidth. In the simulation results, we can show that our algorithms are better than pure Power Control mechanism which lack of considering the transmit power not only in the number of switching-off BSs but also the total energy consumption. According to the results, we observe that the higher cooperation size doesn’t mean the better strategy for different situation. Compared to the original energy consumption when all BSs are opened, our algorithms can save 30 to 60% energy consumption, which shows our methods are indeed efficient energy-saving cooperation strategies.