我們的手機開機第一件事情就是與基地台達成時間同步。在下行鏈路的同步上，我們在一定的時間間隔採用廣播的方式，但上行鏈路的同步上就不能這麼做了，這麼做會大量消耗能源。上行鏈路的同步只發生在需要的時刻，本文後面將會提到哪些時刻需要做上行同步。然而，我們要如何得知用戶(UE)需要進行上行同步呢，我們將利用偵測前導碼(preamble)的方式來得知用戶(UE)要進行上行同步。;The first thing that starts with our mobile phone is to synchronize with the base station. In the downlink synchronization, we use the broadcast method at certain time intervals, but the uplink synchronization cannot do this. This will consume a lot of energy. Uplink synchronization occurs only when there is immediate necessity, and will be mentioned later in this article for uplink synchronization. However, how do we know that the user (UE) needs to perform uplink synchronization? We will use the method of detecting the preamble to know that the user (UE) is going to perform uplink synchronization.