現今使用者要求在低成本的環境下，提供更高的傳輸率、更廣的覆蓋率，而femtocell基地台是一個具有前瞻性的解決方案，適用於室內環境，例如辦公大樓、家用網路。本篇論文探討下行傳輸的能量效益最大化，如何在較低功率的耗損下，傳輸更多的資料。為提高femtocell網路的能量效益，依據功率分配、半雙工、流量分配等規劃，建立出的數學模型屬於混合整數非線性分數規劃(Mixed Integer Nonlinear Fractional Programming)，在Branch and Bound的框架下對原始模型作線性化後得出最佳解。
我們利用整數變數放鬆、對數項目的近似、RLT (Reformulation-Linearization Technique) 。以上步驟讓所有限制式為線性。接著使用CCT (Charnes Cooper Transform)使目標函式不再是分數型態，把模型化簡為混和整數線性規劃(Mixed Integer Linear Programming)，再經由CPLEX求解軟體，根據求解出來的結果，規劃出低耗損及高效率的傳輸方式。本篇論文的最後探討在femtocell系統下，增加使用者對於能量效益的影響。

Recently in the communication system, user request more capacity and more coverage with the low cost, and femtocell base station is a promising technical which provide higher data transmission rate and lower energy consumption, and it is suitable for indoor environment, such as enterprise building, home network. This paper aim to maximize the energy efficiency for downlink, also find how will the femtocell network performance be influenced if we increase more users in the indoor environment. We consider flow distribution, power control, and formulate the energy efficiency problem. The original model is a mixed integer nonlinear fractional which is NP-hard and hardly solvable. We should process some transforms on it than we can find the optimal solution based on the Branch and bound framework.
Within Branch and Bound framework, we employ a reformulation linearization technique (RLT) to relax the nonlinear terms. And we use Charnes-Cooper transform to simplify the fractional form of objective function, then the modified model is a mixed integer linear programming problem. Solved by CPLEX, we get the solution that has low cost and high transmission rate. Finally, we investigate the influences if we increase users in the femtocell system.

[1] A. Saeed, A. Akbari, M. Dianati, and M. A. Imran, "Energy efficiency analysis for LTE macro-femto HetNets," Wireless Conference (EW), Proceedings of the 2013 19th European, pp. 1-5, 2013..
[2] Z. Pan, J. Liu, and S. Shimamoto, "Neuron control-based power adjustment scheme for sleep two-tier cellular networks," 2014 IEEE Wireless Communications and Networking Conference (WCNC), pp. 3201-3206, 2014
[3] J. Kim, W. S. Jeon, and D. G. Jeong, "Effect of Base Station-Sleeping Ratio on Energy Efficiency in Densely Deployed Femtocell Networks," IEEE Communications Letters, vol. 19, pp. 641-644, 2015.
[4] M. Deruyck, D. De Vulder, W. Joseph, and L. Martens, "Modelling the power consumption in femtocell networks," Wireless Communications and Networking Conference Workshops (WCNCW), 2012 IEEE, pp. 30-35, 2012.
[5] Z. Pan and S. Shimamoto, "Cell sizing based energy optimization in joint macro-femto deployments via sleep activation," 2013 IEEE Wireless Communications and Networking Conference (WCNC), pp. 4765-4770, 2013.
[6] W. Vereecken, I. Haratcherev, M. Deruyck, W. Joseph, M. Pickavet, L. Martens, et al., "The effect of variable wake up time on the utilization of sleep modes in femtocell mobile access networks," Wireless On-demand Network Systems and Services (WONS), 2012 9th Annual Conference on, pp. 63-66, 2012.
[7] F.R. Yu, X. Zhang and V.C.M Leung, "Green Communications and Networking," New York:CRC Press, 2012.
[8] S. Y. Seidel and T. S. Rappaport, "914 MHz path loss prediction models for indoor wireless communications in multifloored buildings," IEEE transactions on Antennas and Propagation, vol. 40, pp. 207-217, 1992.
[9] Y. Bejerano and S.-J. Han, "Cell breathing techniques for load balancing in wireless LANs," IEEE Transactions on Mobile Computing, vol. 8, pp. 735-749, 2009.
[10] M. R. Garey and D. S. Johnson, “Computers and Intractability: A Guide to the Theory of NP-Completeness,” W. H. Freeman, pages 245-248, 1979.
[11] C. E. Gounaris, R. Misener, and C. A. Floudas, "Computational comparison of piecewise− linear relaxations for pooling problems," Industrial & Engineering Chemistry Research, vol. 48, pp. 5742-5766, 2009.
[12] J. E. Mitchell, "Branch-and-cut algorithms for combinatorial optimization problems," Handbook of applied optimization, pp. 65-77, 2002.
[13] Y. Nesterov, "Introductory Lectures on Convex Programming Volume I: Basic course," Lecture notes, 1998.
[14] Y. Pochet and L. A. Wolsey, Production planning by mixed integer programming: Springer Science & Business Media, 2006.