在日趨增加的頻寬需求下，傳統的細胞網路架構已經逐漸不敷使用。為了解決此問題，增設小基地台被認為一種可能的解決方案，增設小基地台雖可分擔大基地台的流量，但若採用傳統以接收訊號強度為判斷基準的基地台選擇機制，除了分擔流量的效果相當有限，大多數的大基地台仍會出現過載的情況。因此，目前廣受學界討論的做法是將原判定和大基地台連線卻因為大基地台的過載而連線失敗的使用者，卸載給小基地台。但若不考慮卸載目標基地台的負載情況，卸載的目標基地台仍可能為過載，進而造成因為過多無意義的卸載而導致的連線失敗。為解決此問題，我們提出了一種考量基地台負載情況的卸載機制，該機制可以根據各基地台的負載情況將使用者卸載，並成功地避免掉過多無意義的卸載而導致的連線失敗。根據模擬的結果，所提出的卸載機制能達到更高卸載成功的機率。

The development of small cells which shifts the traditional macro-based homogeneous network to the heterogeneous networks (HetNets) is regarded as one of the major breakthroughs to improve the network capacity. With the traditional Max-SINR association scheme, a user will choose the base station (BS) which offers the strongest received signal strength as its serving BS. Consequently, severe congestions may happen at the high-power macro base stations (MBSs) because most users tend to associate with the more powerful MBS rather than the low-power pico base stations (PBSs). To release the congestion problem, users can be shifted from the overloaded BSs to other BSs, known as user offloading. However, without taking the BS load into consideration, users may be offloaded to a neighboring BS that is overloaded, leading to failure cell association that is undesirable from both users' and operators' perspectives. In this work, we propose a load-based offloading scheme to address the aforementioned issue. We first formulate the optimal load-based offloading problem and shows that it is NP-hard. To facilitate practical implementation, a heuristic algorithm is proposed that is featured by the use of an offset parameter to mitigate offloading users to over-loaded BSs. From simulation results, the proposed heuristic algorithm is shown to increase the association rate and the average user rate, as compared to the traditional offloading scheme without considering the BS load.

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