用於大規模天線LTE/A系統之用戶定位波束成型法

Abstract

在傳統蜂巢式網路的系統中，細胞邊緣的使用者容易受到鄰近細胞訊號的干擾，因此，波束成型(Beamforming)技術被提出來增加細胞邊緣使用者的接收訊雜比。傳統的波束成型技術利用基地台掃描的方式來確認哪個方向的波束對個別使用者有較好的效果，但通常需要花費比較長的延遲時間及比較高的回傳資訊量。這些缺點在基地台端為大規模天線系統(massive antennas system)時，由於資訊量大增而顯得更加嚴重。本篇論文提出了一個以用戶端定位為基礎的波束成型提案來解決以上的問題。本提案主要的概念在於當基地台知道使用者的位置時，就可以利用此資訊對該方向進行波束成型傳輸。具體的實現方法如下: 我們首先將基地台服務的區域分成數個區塊，然後由使用者定位出自己座標之後回傳該區塊的索引值給基地台，再由基地台對使用者做波束成型。LTE/A系統規格裡有定義以位置參考訊號來定位的觀測時間差定位(OTDOA)法，我們因此提出一個兩階段的OTDOA定位來滿足定位所需的準確度。模擬結果顯示我們提出的方法比起沒有使用改良式波束成型技術的傳統傳輸，吞吐量增加了兩倍左右。 除此之外我們還使用接收訊號能量定位法(RSS)來進一步增加OTDOA定位準確度。

It is known that in cellular systems, the cell edge users are vulnerable to interference from neighboring cells. Beamforming has been proposed as an effective method to deal with the problem. Conventional beamforming employs a scanning scheme requiring long delay time and high overhead. These problems will be deteriorated when a massive antenna array is deployed in the basestation. This thesis proposes a location-based beamforming scheme to overcome the difficulties. The idea is that if a basestation (BS) knows a user’s location, it knows which direction to beamform. To implement, we first partition the coverage area of a BS into regions. Then, let the user estimate its location and report its region index to the BS. Finally, the BS conducts beamforming based on the reported information. LTE/A has defined reference signals allowing the application of a localization scheme referred to as observed- time-difference-of-arrival (OTDOA). Using OTDOA, we propose a two-stage localization method achieving satisfactory estimation accuracy. Numerical results show that the proposed method could double the throughput of the system without beamforming. To further enhance the performance, we also combine a received-signal-strength (RSS) method in the OTDOA.