毫米波通訊：蜂巢狀系統的特性與設計

Abstract

由於傳輸容量需求的急速增加，有大量頻寬的毫米波頻段近來成為下一世代無線通訊熱門的頻段。為了克服毫米波信號的嚴重路徑損耗(path loss)，必須在傳送鍵結的兩端都使用高增益的天線。和傳統使用全向性天線的蜂巢狀系統不同，毫米波的通訊系統需要準確的指向性。 在此篇論文中，我們考慮了在傳收兩端都裝載天線陣列的蜂巢狀系統並假設用戶端與基地台之天線波束方向皆週期的同步旋轉，在每一方向的停留時間都等長。 現有涵蓋容量(coverage capacity)往往以信號與雜訊加干擾比的涵蓋機率(coverage probability)來衡量。這種分析架構一則忽略了實際細胞搜尋的程序需要確認鏈路兩端之身分又假設用戶端只與路徑損耗最小的基地台連結。此外現有的分析同時考慮了路徑損耗、大/小尺度衰落(large and small scale fading)、鏈路的遮蔽(blocking)機率及基地台/用戶位置的隨機分佈以至於涵蓋機率之表示式非常複雜，無法清楚的看到相關參數之影響也難以從中預測在不同環境下的性能趨勢。 針對前者，我們的分析加入以下實務考量:即無論是在細胞搜尋或正規通聯階段，鏈路之建立需兩端皆能成功完成同步，而同步成功之機率只與信雜比或信號與雜訊加干擾比有關因此用戶未必只跟路徑損耗最小的基地台連結。就後者，我們忽略通道衰減效應並檢查在何種情境時干擾亦可不予考慮。若我們的主要目的 在求得細胞搜尋之成功機率高於某一定值(譬如95%)所需要的基地台佈建密度，電腦模擬證明了在許多狀況下這些簡化分析的確可準確的預測相關系統參數。這些簡化且能讓我們較容易的分析系統效能以及解釋環境或通道主要參數對涵蓋機率與搜尋成功率造成的影響，另外也能就不同通道環境與系統佈建密度對後項機率做預測。本論文也考慮了正確指向的機率及波束寬度、數量以及傳送功率間的權衡等系統工程之討論並建議適當的系統參數值選擇。

The large available bandwidth of millimeter-wave (mmWave) bands make it an attractive candidate band for next generation cellular communication systems. To overcome the associated severe path loss, directional antennas have to be employed at both sides of a link. Unlike conventional cellular systems in which UEs use omni antennas and the base stations (eNBs) use either omni or sector (low gain) antennas, an mmWave based system requires high pointing accuracy. In this thesis we consider the system in which both sides employ switch beam array antennas and the beam directions rotate synchronously and periodically. The signal-to-noise-plus-interference ratio (SINR) coverage probability performance has been analyzed for such a scenario before. The resulting analytic expressions are complicated and the relations among various system and channel parameters are far from clear. Through many simulation e?orts we analyze conditions in which channel fading and/or interference can be neglected. The simpli?cation allows us to analyze and interpret the the e?ects of other (major) system and channel parameters on the coverage probability and the optimal eNB density. It also enable one to predict performance in di?erent operating environments. As the SINR (SNR) coverage analysis assumes that an UE is always served by the eNB with smallest link (path) loss and no pointing error. We modify this assumption to analyze the cell search statistics in a more realistic condition where cell search is achieved only when both sides are able to synchronize and decode each other's ID, including those of the eNB, UE and beam position.