LTE-A 異質網路下協同排程與功率控制之演算法設計

近年來由於用戶的增加，無線網路的需求節節上升，然而傳統的大型基地台受制於隨距離遞減的訊號強度，無法提供範圍內所有用戶足夠的服務品質；於是包含多種傳輸節點的異質性網路逐漸受到重視，該類型網路透過增加傳輸節點的密度，能更有效率的在不同空間共享相同的頻帶資源。儘管在過去數年中，同質性網路的資源規劃問題諸如：合作式排程、干擾消除等受到很大的關注，異質性網路在相對應的各領域仍未被充分討論。在本篇論文中，我們考慮包含數個低功率節點(Remote Radio Head)及一個大型基地台的異質性網路下合作式排程的相關問題，且加入了多點合作式通訊(Corrdinated-Multi Point, CoMP)的可能，使得網路擁有更高的規畫自由度。為了進一步分析這個問題，我們首先利用數學模型，將這包含多傳輸點的網路排程問題抽象化，並引用比例公平(Proportional Fair)作為最佳化目標。然而，由於通盤化的運算過於複雜，無論在設計或實作上皆有困難，因此我們藉由合理的推導與簡化，將其轉化為一合理規模的混和整數非線性規劃問題。針對該問題特性，我們提出了一個兩相迭代式演算法，將問題拆解成資源分配及功率調整兩組子問題，並透過網路特性的應用，分別對低功率節點和大型基地台做出不同的處理。簡單來說，資源分配子問題考量的是眾多節點在各通道的合作情形，有別於傳統貪婪演算法(Greedy Algorithm)之資源分配方式，大型基地台會考慮多個功率級別，避免落入局部最佳解；而功率調整子問題則是對通道間的資源做出更適合的調配，由於低功率節點對其他節點的影響較低，我們採用較為效率的演算法求解，以期在運算效能上有所提升。模擬結果說明所提之演算法能有效的處理目標問題，並在各項指標上優於現有的其他方法，尤其比較傳統同質性網路下設計之資源規劃時更為明顯，在使用一半的運算時間下，在使用者的平均速率上仍提供15\%的增幅。而加入多點合作式通訊更使得排程多樣性顯著增加，並因而提升整體網路的輸出效率。

關鍵字

異質網路；協同排程；多點協調技術；功率控制；比例公平

並列摘要

In recent years, the requirement of cellular network is highly increasing due to fast growing mobile users.However, conventional macro cell with large coverage can not provide advanced channel efficiency because of physical limitation. Thus, heterogeneous networks (HetNets) which increasing density of transmitting node attracts extensive attention. In this thesis, we investigate the problem of coordinated resource allocation for HetNets with low-power RRHs. The scenario we consider is the configuration when RRHs share the same cell ID with the macro BS such that coordinated multi-point (CoMP) operations among transmission points can be performed in a finer granularity without undesirably triggering frequent handover operations. While transmission scheduling in homogeneous networks (HomoNets) has been well developed in the literature, this is not the case for CoMP-enabled HetNets. We first formulate an optimization problem for resource allocation from multiple transmission points to a set of users demanding fair service. Since the problem incurs high complexity, we transform the formulation into an iterative problem for coordinated scheduling under CoMP. To solve the mixed-integer non-linear problem, we propose a two-phase iterative algorithm for sub-channel resource allocation and cross-channel power allocation. Briefly, in the sub-channel sub-problem each sub-channel is handled independently of others for resource allocation while in the cross-channel sub-problem the coupling among sub-channels is considered for proper allocation of the power budget to individual sub-channels. Simulation results show that the proposed algorithm can effectively solve the target problem with noticeable performance gain compared to related approaches. We also observe that conventional strategy of UE selection in HomoNets is not suitable to scheduling in HetNets. CoMP-enabled HetNets have more flexibility in scheduling for improving the cumulative throughput.

並列關鍵字

Heterogeneous network ； Coordinated scheduling ； Coordination multi-point ； Power control ； Proportional fair

參考文獻

[1] J. Lee, Y. Kim, H. Lee, B. L. Ng, D. Mazzarese, J. Liu, W. Xiao, and Y. Zhou, “Coordinated multipoint transmission and reception in LTE-advanced systems,” IEEE Communications Magazine, vol. 50, no. 11, pp. 44–50, 2012.

[2] J. woo Cho, J. Mo, and S. Chong, “Joint network-wide opportunistic scheduling and power control in multi-cell networks,” IEEE Transactions on Wireless Communications, vol. 8, no. 3, pp. 1520–1531, 2009.

[3] L. Li, C. Xu, and M. Tao, “Resource allocation in open access OFDMA femtocell networks,” IEEE Wireless Communications Letters, vol. 1, no. 6, pp. 625–628, 2012.

[7] L. Li, M. Pal, and Y. Yang, “Proportional fairness in multi-rate wireless lans,” in Proceedings of IEEE INFOCOM, 2008, pp. 1004–1012.

[8] K. Son, S. Chong, and G. Veciana, “Dynamic association for load balancing and interference avoidance in multi-cell networks,” IEEE Transactions on Wireless Communications, vol. 8, no. 7, pp. 3566–3576, 2009.

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LTE-A 異質網路下協同排程與功率控制之演算法設計

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