合作式傳輸網路容量分析

Abstract

近年來，為了改進無線通訊的效能(例如：吞吐量)，各種不同的合作式傳輸技術皆被紛紛提出。當我們在考慮合作式傳輸技術的效能改進量時，大部份目前的研究都是著重在單對傳輸的效能(連結容量)。然而，使用合作式傳輸所引進的第三個裝置(中繼者)會增加所謂的「安靜區域」。安靜區域是一正在傳輸的傳輸對所預約的空間區域，其他位在安靜區域的裝置此時都不能夠傳輸。因此，在無線網路中同時能夠傳輸的傳輸對數量會減少。換句話說，雖然合作式傳輸能提供單對傳輸對較好的吞吐量，但是整體網路的吞吐量卻有可能沒有改善。 在本論文中，我們以網路容量(network capacity)取代連結容量來探討合作式傳輸。我們比較無線網路中直接和合作式傳輸的空間可重複利用率(spatial reuse)。為了達到公平的比較，我們從媒介接取層的角度來考慮功率設定，所以，我們將直接傳輸和合作式傳輸所使用的平均功率消耗定為相同。我們建立一個數學模型來計算空間可重複利用率和推導網路容量。我們的數值分析結果顯示，可使用合作式傳輸的區域至少減少了40%。此外，和存在連結容量的區域相比，如果一個系統想透過合作式傳輸獲得更多網路容量的效能改善，有網路容量的區域會變小更多，最多甚至會減少到100%，也就是說使用合作式傳輸所宣稱得到效能改善的區域，如果考慮空間可重複利用率的影響此區域會變得更小或甚至找不到。

Lately, various cooperative transmission schemes are introduced to wireless communication systems to improve the performance (e.g., throughput). When considering the performance gain using cooperative transmission, most current researches focus on the performance of a single transmission pair (i.e., link capacity). However, the introduction of a third ``relay' node enlarges the so-called quiet zone where no other nodes can transmit during an active cooperative transmission. As a result, the number of simultaneous transmission pairs in a network adopting cooperative transmission is reduced. In other words, although the cooperative transmission can provide better throughput for a single transmission pair, the overall network throughput may not be improved. In this thesis, we study the impact of cooperative transmission on the network capacity, instead of the link capacity. In particular, we compare the spatial reuse of direct and cooperative transmission schemes in a wireless network. To make fair comparison, we consider a power setting from the aspect of the MAC layer so that the average power consumption in direct and cooperative transmissions are kept the same. We develop a mathematical model to calculate the spatial reuse and derive the network capacity. Our numerical results show that the region where we can use cooperative transmission decreases at least 40\%. Besides, when comparing with the region where exists link capacity, if a system wants to get more performance gain by cooperative transmission, the region where exists network capacity decreases much more (even to 100\%). In other words, the region with performance gain when using cooperative transmission will become smaller or even no longer exist if we consider the influence of spatial reuse.