【作者】 刘鹏；

【导师】 李建东；

【作者基本信息】 西安电子科技大学 ， 通信与信息系统， 2015， 博士

【摘要】 为满足爆炸式增长的移动业务需求,蜂窝网络可以通过缩小小区面积以提高链路传输质量并减少每个小区的业务量。覆盖面积较小的小区(如毫微小区、家庭基站小区)和提供广域覆盖的传统宏蜂窝网络形成了异构网络。对于网络运营商而言,家庭基站利用现有的回程网络与宏基站通信,从而降低了运营成本。而对于用户而言,家庭基站更加易于安装。基于上述优点,本博士论文着重讨论由家庭基站和宏基站构成的异构网络。在第三代合作伙伴项目(3GPP)中,异构网络中的干扰问题被分成两类：跨层干扰(宏小区和家庭基站小区之间的干扰)问题和同层干扰(家庭基站小区之间的干扰)问题。典型的干扰抑制方案是给相互干扰的小区分配正交的信道,然而正交信道分配方法会导致频谱利用率下降。为了提高频谱利用率,需要更精细化的解决方案。而对于跨层干扰问题,家庭基站接入基于IP的有线回程网络,该网络回程时延波动较大,使得利用实时信息交换来抑制干扰的方法难以实现。另外,家庭基站的部署没有特定的规划并且其服务用户数较少,不同用户的干扰情况不具备统计相似性,这使得应用在宏小区间的干扰协调方法无法直接解决同层干扰问题。考虑到传统的干扰抑制方案在解决跨层干扰问题和同层干扰问题上的局限,本博士论文提出了一系列干扰管理策略。首先,针对跨层干扰场景,设计了二维的资源分配方法以适应信道和业务的动态变化。其次,提出了基于凸优化的算法来进一步提高现有同层干扰管理策略的性能。另外,针对同层干扰场景,结合准入控制和资源分配,设计了基于图论的解决方案。最后,探究了带有串行组解码的上行同层干扰管理方案。本文在国家自然科学基金重点项目“无线网络的干扰管理与容量研究”(61231008),“高等学校创新引智计划基金资助项目”(B08038),“国家重点实验室专项基金”(ISN1002005, ISN090305),长江学者和创新团队发展计划(IRT0852)的共同资助下,主要的贡献和结果如下：1、在异构网络的跨层干扰场景下,提出了二维的资源分配方案以适应时频信道变化。该方案通过在优化模型中引入了组稀疏项来避免跨层干扰,通过对资源分组从而使其对业务量大的突发性业务具有鲁棒性。该方案具有合作开销小、分布式等特点,适用于长期发展计划(LTE)已有协议。仿真研究了合作周期和分组大小对不同性能测度的影响,结果表明较之单独进行频域分配或单独进行时域分配的传统方案而言,本文所提方案具有明显的性能增益。2、在多用户正交频分复用(OFDM)网络中,研究了分布式同层干扰管理解决方案中的关键优化问题,提出了基于凸优化的信道和功率联合分配方案。由于该资源优化模型中同时存在离散变量和连续变量,不易求解,我们将该模型重新整理成为只有功率变量的连续优化问题,并对其进一步松弛得到易于求解的凸问题。我们采用内点法求解这个松弛后的凸问题,并且给出了相应的算法复杂度分析。仿真结果表明所设计的方案具有高的能效和低的计算复杂度。3、在同层干扰场景中,提出了基于图论的高效算法,以最大化具有资源需求的用户数和频谱空间效用。一方面,在干扰受限的家庭基站网络中,同一时刻可能无法保障所有用户的资源需求。另一方面,家庭基站网络的部署应尽可能重复使用资源。针对上述两个方面,我们联合准入控制和资源分配并建立了多目标优化模型,以尽可能的接入更多的用户和复用资源。典型的求解方法是迭代求解一系列松弛了的问题从而逐步逼近最优解。由于上述求解方法具有极高的计算复杂度,我们提出了基于弦图的启发式算法。仿真结果表明在网络密度较大的场景中,该启发式算法能以较低的计算复杂度接入较多的用户和带来较高的资源复用率。4、在上行同层干扰场景中,研究了带有串行组解码的干扰管理技术。在密集部署的家庭基站网络中采用高级接收机技术来减缓上行同层干扰。利用串行组解码器可以提高资源复用率,用户可以机会地使用相邻家庭基站小区预先分配的频谱资源而不影响相邻小区用户的正常通信。为了最好地解码有用信号,串行组解码器首先会解码一些干扰信号。本文将带有串行组解码器的多小区资源分配问题建模成为一个信道分配、速率分配和解码组分配的联合问题。在满足保障比特率业务(GBR)的速率需求约束下,最大化加权可变比特率业务(VBR)的和速率。本文证明了该分配问题是NP-hard问题,并提出了贪婪算法对其求解。求解思想是让GBR用户尽可能地使用相邻小区占用的信道,同时给VBR用户释放更多的“无干扰”信道以便其进行高速率数据传输。为了快速而粗略地评估上述方案带来的性能增益,我们提出了半解析的评估框架来估算其带来的潜在的吞吐量增益。同时,通过蒙特卡罗仿真,我们研究了GBR和VBR业务比例、用户数以及速率需求的配置变化对所设计方案的性能影响,结果表明该方案和传统的正交资源分配方案相比带来10%-100%的吞吐量增益。更多还原

【Abstract】 Data explosion drives the evolution of the architecture of mobile networks. Shrinking cell size can meet the dramatically increasing demand for the mobile traffic due to the decreasing cell load and short transmission path. Together with macro cell which is used to provide wide coverage, they form heterogeneous networks (HetNets). Femtocell reduces the capital expenditure as there is no need dedicated wire line deployment between base stations (BSs), and at the same time it is more flexible to be deployed by users. Due to these merits, we focus on femtocells in this dissertation.HetNets bring new interference issues. Due to the two-tier architecture, the 3rd Generation Partnership Project (3GPP) technical report categorizes HetNet interference issues in two types:cross-tier interference (interference between macro cell and femtocells) and co-tier interference (interference among femtocells). A typical solution is partial channel deployment where orthogonal channels are allocated to femtocells and macro cell (to mitigate cross-tier interference) or interfering femtocells (to mitigate co-tier interference), which leads to the inefficient spectrum utility compared with co-channel deployment.To improve the spectrum utility, some fine-grained interference management schemes are necessary. However, for cross-tier interference issue, femtocell base station (FBS) directly plugs into an IP-based wire line backhaul network where delay cannot be predicted readily-making the real-time information exchange impossible. For the co-tier interference scenario, femtocells occupy small geographical regions in unplanned manners and serve small number of users, which makes the traditional statistic approach that treats the interference based on the law of large numbers less effective. As a matter of fact, we cannot directly apply the inter-cell interference coordination (ICIC) scheme popularly used in macro cells to femtocell networks.Concerning the limitations on the cross-tier scenario and co-tier scenario, this dissertation proposes a series of interference management schemes. First, a two-dimensional (2D) resource allocation scheme is designed to adapt channel variations and dynamic traffic in the cross-tier interference scenario. Second, a convex optimization based algorithm is proposed to further improve the performance of a popularly used co-tier interference management scheme. Third, a graph based solution is developed to join admission control and resource allocation in the co-tier interference scenario. At last, we investigate the uplink co-tier interference management with successive group decoding.The research works are supported by the National Natural Science Foundation of China (No.61231008); State Key Laboratory Foundation (ISN1002005, ISN090305); the 111 Project (No.B08038); Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0852). The main achievements and results of this dissertation are listed as follows:1.2D resource pattern optimization approach for interference avoidance in HetNets is proposed. By optimizing the resource pattern based on variable channel conditions, plus the introduction of group lasso term, our scheme exploits channel variations in both time and frequency domains and in addition avoids the interference. By grouping the resources in our optimization model, our scheme is flexible and robust against heavy burst traffic. It is also lightweight in terms of coordination overhead and completely distributed, making it suitable for the Long Term Evolution (LTE) protocol. Extensive simulations investigate the effect of coordination period and group number on different performance metrics and the effectiveness of our proposed method compared with sole frequency domain and time domain resource allocation approaches.2. Convex optimization based joint channel and power allocation scheme for multi-user orthogonal frequency division multiplexing (OFDM) networks is proposed. The investigated problem is the key problem in a distributed co-tier interference management solution, which is generally NP-hard. We reorganize the problem with only power variables and design a novel relaxation scheme to make it convex. The interior-point method is employed to solve the relaxation problem in the polynomial time, and the theoretical complexity is further presented. Simulation results show that our scheme can provide high energy efficiency compared with the existing methods and low computational complexity.3. Graph based efficient resource allocation scheme to maximize the number of users with Quality of Service (QoS) demands is proposed. Traffic class leads to higher demand for network designers on QoS provision. In the interference limited co-tier femtocell scenario, users’ diverse requirements may not be guaranteed at the same time. On the other side, femtocells are developed to reuse more resources. Concerning these two objectives, we join admission control and resource allocation and formulate a multi-objective optimization problem. The typical solution is solving the relaxation problem iteratively to approach the optimal solution but will be computationally prohibitive when the network scales up. We further develop a heuristic method based on the chordal graph, which shows to perform well in the case of large-scale networks.4. Uplink co-tier interference management in femtocell networks with successive group decoding is studied. We propose a new scheme to mitigate the uplink co-tier interference in dense femtocell networks, assuming that advanced receivers are employed by the FBSs. We exploit resource reuse by taking advantage of the successive group decoder (SGD) such that users can opportunistically access the entitled resources of nearby cells. The SGD decodes some interference signals in order to best decode the useful signal. Multi-cell uplink resource allocation with SGDs is formulated as a joint channel, rate and decoding group (CRG) allocation problem to maximize the weighted sum rates of the variable bit rate (VBR) users while meeting the rate requirements of the guaranteed bit rate (GBR) users. Due to the NP-hardness of the problem, we propose a greedy algorithm where the idea is to let GBR users opportunistically transmit on the channels of nearby cells and release more interference-free channels for high-rate transmission of VBR users. A semi-analytical framework is developed to provide a rough estimate of the potential throughput gain by the proposed technique. Simulation results show that the throughput gain over the conventional orthogonal resource allocation ranges from 10%-100% with different traffic proportion, number of users and rate requirement.更多还原