無線ATM 網路軟式移交之訊務轉載研究

Study on Traffic Shedding for Soft-Handoff in Wireless ATM Networks

在無線ATM（asynchronous transfer mode）行動通訊中，某個蜂巢式細胞內突然湧入許多使用者，將會造成此蜂巢式細胞的基地台訊務過載，形成所謂的熱點細胞（hot-spot cell），而鄰近蜂巢式細胞可能處於輕載狀態，造成系統訊務負載不平衡現象。由於頻寬資源是有限且共享的，因此為了使有限的頻寬資源更有效地使用，我們利用行動台越區移交的機制來減緩訊務負載不平衡的現象。本文提出一個適用於軟式移交（soft handoff）的完整性訊務轉載演算法（complete traffic shedding algorithm; CTSA），藉由調整移交門檻參數來有效地降低蜂巢式細胞基地台的負載。實際上我們是減少過載蜂巢式細胞的移交區域範圍，並不是調整蜂巢式細胞基地台傳輸功率。位於過載蜂巢式細胞邊緣附近的使用者能夠提早移交到鄰近輕負載的蜂巢式細胞，並延後鄰近蜂巢式細胞的使用者進入過載熱點細胞，來達到減輕負載的效果。最後，透過模擬的方式與其他已提出的方法互相比較。CTSA 演算法確實使輸出率(throughput)提高、軟式移交斷訊率（call dropping ratio）、新連線阻塞率（blocking ratio）及系統連線拒絕率（call refusing ratio）顯著降低。

In wireless ATM(asynchronous transfer mode)mobile communications,an abrupt increase of mobile users in a specific cell will cause traffic overloaded,creating so-called hot-spot cell. The neighboring cells may be inlight traffic-load condition; this situation causes an unbalanced traffic-loaddistribution in the system. However, the spectrum is a limited resource. Inorder to utilize the spectrum more efficiently, we make use of mobilehandoff strategies to alleviate unbalanced traffic-loads. This paper proposesa complete traffic shedding algorithm (CTSA) for soft handoff to efficientlyalleviate overloaded traffic in a cell by adjusting handoff parameters. The proposed algorithm virtually reduces the service coverage of heavilyloaded cells without adjusting the transmission power. The mobile userslocated in the outer area of the heavily loaded cell site are early handedover to the neighboring cell carrying a light load and mobile users inneighboring cells have handoff to heavily loaded hot-spot cell delayed, sothat the traffic-load of the heavily loaded cell is decreased. We comparedthe performance of other approaches with our scheme by simulation. TheCTSA algorithm obviously increases system throughput and decreases thesoft-handoff call dropping ratio, the new call blocking ratio, and the totalcall refusing ratio. The performance of CTSA is better than the othermethods examined.