隨著網際網路發展，藉網路得知生活上所需資訊已成為生活中的需求；其中，結合行動通訊技術與其他技術提供更多元網路服務是行動通訊技術所探討的議題。目前 NB-IoT 通訊系統中，基站 (eNB) 僅對已完成 Radio Resource Control (RRC) 連線之使用者裝置 (UE) 進行分配上下行資源。當閒置狀態 UE 欲藉 eNB 上網時，須執行 Random Access Procedure (RAP) 建立 RRC 連線。此研究的動機來自觀察眾多裝置進行 RAP 時，一旦發生碰撞所造成的成本將非常顯著。換言之，當 UE(s) 於指定系統通道資源上傳送特殊且唯一之前導訊號 (preamble) 供 eNB辨識 UE 登入請求後分配上行資源給 UE 傳送 RRC 連線請求控制訊息，若多個 UE(s) 選擇相同的子載波 (subcarrier) 進行 preamble 傳送將導致 UE(s) 之間產生碰撞。由於eNB 無法分辨是否有碰撞發生，這些 UE(s) 將同時於所配置之上行資源傳送個別 RRC 連線請求控制訊息，使得 eNB 無法解讀並透過 HARQ (Hybrid Automatic Repeat Request) 機制要求重送，後續重傳依然失敗直到HARQ 重傳次數上限；之後，所有碰撞之裝置將重新執行 RAP。不難理解，碰撞機率與 UE 數量呈正比；碰撞除了導致上線延遲外，亦造成無謂傳輸與耗電。5G 通訊系統已納入 massive MTC 應用場景，如何降低大量設備進行 RAP 發生碰撞機率將是重要且具專利價值之技術。本研究主要針對窄頻物聯網 (Narrow Band Internet of Thing，NB-IoT) 進行探討，嘗試導入 Wifi 中 Random Backoff 演算法於 RAP 機制中，預期透過此設計降低 UE 間相互干擾。特別強調的是，此設計除了欲降低整體碰撞機率目標外，同時考慮所提演算法與規範完全相容，確保此研究的產出具實用與專利價值。 ;With the popularity of smart mobile devices, surfing the Internet anytime, anywhere has become a kind of necessity for modern life. How to combine the mobile communication technologies with other technologies to support versatile services in mobile networks is one of major issues.In NB-IoT network, the base station (eNB) only schedules the channel resource for the devices (UE) which has established the rardio resource control (RRC) connections. For the UE staying in IDLE mode, it has to perform the random access procedure (RAP) in order to establish RRC connection with the eNB. The motivation of this research is from observing the behavior and performance of RAP in machine type communications. When the number of UE(s) performing RAP excesses a certain threshold, collions will occur and result in remarkable resource wastage and delay. In other words, a UE randomly selects one subcarrier and transmit the preamble on the specified channel. The preamble is a specified sequence of a certain length and the system provides only one preamble for all devices to perform RAP. The eNB searches the preamble from subcarriers and then allocate uplink resource(s) to UE(s) based on the number of detected preambles . The UE then transmits RRC connection setup request message to the eNB on the granted uplink resource. If more than one UE selects the same initial subcarrier to transmit the preamble to eNB, their signals will overleap with each other and the eNB can not tell how many UE(s) transmit the preamble and therefore their following messages will collide together. Then, the eNB utilizes the Hybrid Automatic Repeat Request (HARQ) feedback to notify those UE(s) to retransmit message again. The collisions can not be resolved and retransmissions will continue until reaching the maximal HARQ retries. After then, all involved UE(s) restart RAP again. Obviously, the collision probability is linerly proportional to the number of UE(s) and more collisions will prolong the access delay, power and bandwidth wastage. Due to 5G network has included the massive MTC (mMTC) usage case, how to efficiently reduce the collision probability in mMTC scenario could be an important and patenable technology.This project focuses on the Narrow Band Internet of Thing (NB-IoT) and try to integrate the RAP with the radndom backoff solution which has been adopted in WiFi networks in order to minimize the interference among UEs. We also emphasize that the proposed scheme will not only reduce collision probability in RAP but also consider the full compatability with current specifications. The major advantages of compatability are twofolds: 1) getting rid of standardization process and 2) the interesting of industry.
