軟性線路轉移之效能評估

Abstract

在行動通訊網路中，手機透過無線電介面和基地台聯繫，以達到和外 界通訊的目地。在一個細胞(一個基地台發出的信號的涵蓋區域)中，當有 使用者要求一個通話時，假如這個基地台還有閒置的頻道，則這個通話便 可以連結起來。否則，這個通話便不能被接通。當一個正在通話的手機從 一個細胞移動到另一個細胞時，在原來的基地台所使用的頻道便要釋放， 並向新的基地台要求一條頻道。這個過程稱為線路轉移。TDMA或FDMA系 統(例如: AMPS, GSM, DECT, PHS 等等)必須利用硬性線路轉移的方法。 在硬性區域線路中，舊的無線頻道必須在新的無線頻道建立前釋放。所以 ，手機在一個時間永遠只能和一個基地台做聯繫。而以CDMA技術為基礎的 行動系統則利用線路區域轉移的方法同時透過不同的基地台，使用多個無 線連結和外界通訊。在這個論文中，我們研究軟性線路轉移的方法並且和 硬性線路轉移比較它們之間的效能。研究的結果告訴我們，雖然在軟性線 路轉移中，手機可能會使用額外的連結，但是這個方法能夠在線路轉移的 過程中，使得連結的轉換有更大的成功機率。所以說，只要小心地安排細 胞之間相互覆蓋的區域，軟性線路轉移將會比硬性線路轉移提供更好的效 能。 In a mobile communication network, a handset communicates with the outside world through the radio contact to a base station(BS). When a call arrives at a cell(i.e., the coverage area of a BS). the destination(or the origination) handset is connected if a channel is available. Otherwise, the call is blocked.When a communicating handset moves from one cell to another, the channel in theold BS is released, and a channel is required in the new BS. This process is called handoff. For mobile systems with the radio interface based on TDMA or FDMA technologies(e.g.,AMPS, GSM, DECT, PHS, and so on), hard handoff is employed. In hard handoff, the old radio link is broken before the new radio link is established, and a handset always communicates with one BS at any time.On the other hand, mobile systems based on CDMA technology utilize soft handoff where a handset may communicate with the outside world using multiple radio links through different BSs at the same time. This thesis studies the soft handoff mechanism and compares its performance with hard handoff. The study indicates that although a handset may potentially consume extra radio links in soft handoff, the mechanism provides better opportunity to transfer the link successfully in handoff procedure. Thus, by carefully planning the overlay areas of cells, soft handoff can outperform hard handoff.