為了解決毫米波 (Millimeter Wave) 高衰減的特性，毫米波訊號利用波束成型 (Beamforming) 的技術來傳輸，將訊號朝一個特定方向集中發射，而非全向 (Omnidirectional) 發射。然而，高度方向性的波束對於使用者的地理位置是很敏感的。因此，在毫米波通訊的初始訪問 (Initial Access) 中，初始波束搜尋 (Initial Beam Search) 是必要的。而基於位置上下文信息 (Position Context Information) 的波束搜尋方法在使用者所提供的位置信息與其真實位置的誤差較大情況時，可能會遭遇預期外的延遲。於是，本論文將針對這種情況下做設計，透過限制上下文信息方法的嘗試次數，並混合盲搜尋方法，以避免方法在錯誤方向進行長時間的搜尋，降低其對毫米波通信系統所帶來的影響，來改善初始訪問階段的延遲。經由模擬的結果顯示，我們所提的方法比起基於位置上下文信息的 EDP (Enhanced Discovery Procedure) 方法在延遲上有平均20%左右的改善; 而在環境較差的情況下，更可達43%左右的改善。

In order to address the issues caused by the high-attenuation characteristics of the millimeter-wave, the millimeter-wave signals are transmitted using the beamforming technology. Therefore, signals are transmitted directionally rather than omnidirectionally. However, highly directional beams are very sensitive to the geographic location of a user. This makes the initial beam search is necessary for the initial access of the millimeter-wave communication. However, the beam search method based on the position context information may encounter an unexpected delay when the acquired position information of the user has a large error from the actual position. Specially focusing on this situation by limiting the number of attempts of the context information method along with the blind search method, time-consuming searches in the wrong direction can be avoided to mitigate the impact on the millimeter-wave communication system and to improve the initial access delay. Via simulations, we show that the proposed scheme has an average performance improvement of 20% or so as compared to the enhanced discovery procedure (EDP) scheme based on the position context information. It can even reach 43% or so of improvement when studying in a bad mode.

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