LTE-A异构网络中的干扰协调机制研究
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摘要
随着移动互联网的快速发展,用户对数据业务的需求日益增强。在线游戏、视频通话、高速下载等应用对蜂窝网络的速率和延迟等性能要求越来越高。另一方面,有统计资料表明未来2/3的电话和90%以上的数据业务是在室内发生。然而由于信号的穿墙损耗,宏蜂窝网络很难保证室内用户的速率需求。因此,3GPP (3rd Generation Partnership Project)组织在LTE (Long Term Evolution) Release10版本中引入了低功率节点,用于拉近用户和基站间的距离,从而提升用户吞吐量。这种在宏蜂窝网络中叠加低功率节点的网络架构被称为异构网络(Heterogeneous Networks, HetNets)。
为了最大化频谱利用率,3GPP倾向于采用同频组网方式。宏基站之间、低功率节点之间的同层干扰和宏基站与低功率节点之间的跨层干扰成为了制约异构网络性能的关键因素。相比于传统宏蜂窝网络,低功率节点具有部署随机、封闭用户接入、覆盖增强等技术特点,使得异构网络的干扰情况尤其复杂。因此,LTE-A异构网络的小区间干扰协调技术对异构网络具有重要意义。本文以Macro-Femto网络和Macro-Pico网络为例,对异构网络的小区间干扰协调技术进行了系统性的研究。主要贡献和创新点如下：
1)研究Macro-Femto异构网络中的下行干扰问题,提出了一种基于干扰图的频域干扰协调方案。
在干扰图建模中,图顶点为位于家庭基站干扰区域内的宏小区用户或家庭基站,边代表干扰关系,权重代表干扰程度。权重根据区域平均信道准则确定。将分簇问题建模为图论中最大K割问题,并提出一个启发式算法——基于干扰图的混合分簇方案(HCIG)来近似求解。HCIG方案能同时减弱Macro-Femto网络中的三种下行干扰,提高网络吞吐量。
2)研究Macro-Femto异构网络中的Femto之间的干扰问题,提出了一种基于家庭基站协作的干扰协调方案。
在3GPP标准中,邻居家庭基站间存在X2接口,使得家庭基站间的协作变为可能。利用家庭基站间的辅助数据传输,本文提出一种名为虚切换的协作机制以消除邻居家庭基站间的干扰。虚切换的核心思想是将用户平面和数据平面进行分离。当虚切换发生时,控制信息由原始服务家庭基站承载,而数据信息由邻居干扰基站辅助传输。虚切换对用户和移动管理实体透明,只涉及家庭基站间的控制信令交互。并且信息交互是通过有线X2链路,因此该方案开销低、成功率高。
3)研究Macro-Pico异构网络中的宏基站对Pico用户的干扰问题,分析基于ABS (Almost Blank Subframe)的时域干扰协调方案中ABS比例的设置问题。
采用覆盖增强技术时,宏基站会对Pico扩展覆盖范围内的用户造成较强的下行干扰。通常采用基于ABS的时域干扰协调来消除此类干扰。本文利用泊松点过程网络模型分析了在采用覆盖增强技术下的Macro-Pico网络的性能,得到用户到服务基站的距离分布、用户的SIR分布、用户的平均吞吐量。在此基础上,分析了在给定覆盖增强偏置值时ABS比例的合理取值范围。该工作对实际网络中覆盖增强偏置值和ABS比例的设置具有一定的参考意义。
4)研究Macro-Pico异构网络中的覆盖增强机制对用户中断概率的影响,提出一种负载感知的泊松点过程网络模型。
注意到传统泊松点过程网络模型没有考虑基站的实际负载,而只是简单假设基站满负载,本文将基站负载和按需资源分配方案加入到传统泊松点过程模型中,得到更加接近实际情况的负载感知的干扰分析模型。利用该模型分析Macro-Pico网络中覆盖增强机制对用户中断概率的影响,分析得到在给定覆盖增强偏置时用户中断概率的表达式。
5)研究Macro-Pico异构网络中的宏基站和Pico基站间的干扰问题,提出一种负载自适应的干扰协调方案。
为了使得Pico能够更好地为热点用户提供通信资源,本文提出一种基于热点区域的有界覆盖增强方案来扩大Pico的覆盖范围。基于有界覆盖增强方案,进一步提出了负载自适应的动态覆盖控制算法来实现宏基站和Pico基站间的负载均衡,并提出混合时频域干扰协调算法来减轻宏基站对Pico扩展覆盖范围内的用户的干扰。负载自适应的动态覆盖控制算法及混合时频域干扰协调算法能够在用户QoS保证和网络容量之间实现较好的折衷。
With the rapid development of mobile Internet, mobile users put forward a higher demand for the data rates. Applications such as online games, video calls, high-speed downloads have led to the increasing demand for higher data rates and lower latency of the cellular networks. On the other hand, it is estimated that2/3of calls and over90%of data services occur indoors. However, due to the building penetration loss, more power from the cellular base station will be needed to provide indoor coverage. Therefore,3GPP (The3rd Generation Partnership Project) has introduced low power nodes in LTE (Long Term Evolution) Release10to make users get closer to base stations, thus improve the user throughput. The architecture of deploying low power nodes on the basis of traditional macrocell network is called Heterogeneous Networks (HetNets).
In order to maximize the use of spectrum resources,3GPP tends to share the same frequency band between the macrocell and small cells. The co-layer interference among macrocells and that among small cells, as well as the cross-layer interference between macrocells and small cells have become an important factor in restricting the performance of HetNets. Compared to the traditional macro base stations, low power nodes are usually deployed randomly with the closed access mode and range expansion technology, which makes the interference problem of HetNets more complicated. Therefore, the Inter-Cell Interference Coordination (ICIC) technology is one of the most important technologies in LTE-A HetNets. In this dissertation, taking the typical HetNets scenarios Macro-Femto and Macro-Pico networks as an example, we study the ICIC technologies in LTE-A HetNets. The main contributions and innovations are listed as follows:
1) This dissertation studies the downlink interference problems of the Macro-Femto network, and proposes an interference graph based frequency-domain ICIC scheme.
In the construction of the interference graph, the vertexes are all femto base stations and the users which are in the interference regions of femtocells, the edge represents the interference relationship between two vertexes, and the weight represents the degree of the interference. The weight of edges is characterized by the Regional Average Channel State (RACS) metric. Then we model the optimal clustering problem as the MAX K-CUT problem in the graph theory, and propose a heuristic algorithm named Hybrid Clustering based on Interference Graph (HCIG). The HCIG algorithm could reduce all the three types of interference in Macro-Femto networks and improve the network throughput.
2) This dissertation studies the co-layer interference problem among femtocells, and proposes an ICIC scheme based on the cooperation between femtocells.
In the3GPP LTE standards, there is an X2interface between neighbor femtocells, which makes the cooperation between femtocells become possible. By enabling a femtocell to relay the data of its neighbor femtocell, we propose a virtual handover scheme to cancel the interference between femtocells. The main idea of virtual handover is to separate the user plane and control plane. When the virtual handover occurs, the control messages are transferred by the original serving femtocell, and the data messages are transferred by the aided neighbor femtocell. The virtual handover is transparent to the user and the Mobility Management Entity (MME), and it only needs the control message exchange between femtocells. Furthermore, the message exchange is through the wired X2link, which makes the virtual handover scheme have a low overhead and a high probability of success.
3) This dissertation studies the interference from the macrocell to picocell users, discusses the ratio of Almost Blank Subframe (ABS) in the ABS-based time-domain ICIC scheme.
When Range Expansion (RE) is used in the Macro-Pico network, the users in picocell expanded regions will suffer high interference from the macrocell. Generally, ABS-based time-domain ICIC scheme is utilized to cancel this type of interference. We use the Poisson Point Process (PPP) model to analyze the performance of the Macro-Pico network with RE, and get the distribution of distance between users and base stations, SIR, average user throughput. Then we get the reasonable range of ABS ratio when the range expansion bias is given. The work is helpful to the settings of range expansion bias and ABS ratio in the practical system.
4) This dissertation studies the impact of range expansion on the user outage probability, proposes a load-aware PPP model.
Observed that the load of base stations is not taken into account in the traditional PPP model, which is simply assumed that all base stations are fully loaded, we take the effects of load and QoS into the consideration, and propose a more practical load-aware PPP model. Based on the proposed model, we analyze the impact of range expansion on the user outage probability, and get the outage probability expression when the range expansion bias is given.
5) This dissertation studies the interference problem of the Macro-Pico network, proposes a load adaptive ICIC scheme.
In order to make picocells provide more communication resources to the hotspot users, we propose a hotspot-based bounded range expansion scheme to expand the coverage of picocells. Based on the bounded range expansion scheme, we propose a Load-adaptive Dynamic Range Control (LDRC) algorithm to achieve the load balancing between the macrocell and picocell, and give a hybrid approach of time and frequency domain ICIC to reduce the interference from macrocell to users in picocell expanded regions. The LDRC and proposed ICIC scheme could achieve a good trade-off between QoS guarantee and system capacity.
为了最大化频谱利用率,3GPP倾向于采用同频组网方式。宏基站之间、低功率节点之间的同层干扰和宏基站与低功率节点之间的跨层干扰成为了制约异构网络性能的关键因素。相比于传统宏蜂窝网络,低功率节点具有部署随机、封闭用户接入、覆盖增强等技术特点,使得异构网络的干扰情况尤其复杂。因此,LTE-A异构网络的小区间干扰协调技术对异构网络具有重要意义。本文以Macro-Femto网络和Macro-Pico网络为例,对异构网络的小区间干扰协调技术进行了系统性的研究。主要贡献和创新点如下：
1)研究Macro-Femto异构网络中的下行干扰问题,提出了一种基于干扰图的频域干扰协调方案。
在干扰图建模中,图顶点为位于家庭基站干扰区域内的宏小区用户或家庭基站,边代表干扰关系,权重代表干扰程度。权重根据区域平均信道准则确定。将分簇问题建模为图论中最大K割问题,并提出一个启发式算法——基于干扰图的混合分簇方案(HCIG)来近似求解。HCIG方案能同时减弱Macro-Femto网络中的三种下行干扰,提高网络吞吐量。
2)研究Macro-Femto异构网络中的Femto之间的干扰问题,提出了一种基于家庭基站协作的干扰协调方案。
在3GPP标准中,邻居家庭基站间存在X2接口,使得家庭基站间的协作变为可能。利用家庭基站间的辅助数据传输,本文提出一种名为虚切换的协作机制以消除邻居家庭基站间的干扰。虚切换的核心思想是将用户平面和数据平面进行分离。当虚切换发生时,控制信息由原始服务家庭基站承载,而数据信息由邻居干扰基站辅助传输。虚切换对用户和移动管理实体透明,只涉及家庭基站间的控制信令交互。并且信息交互是通过有线X2链路,因此该方案开销低、成功率高。
3)研究Macro-Pico异构网络中的宏基站对Pico用户的干扰问题,分析基于ABS (Almost Blank Subframe)的时域干扰协调方案中ABS比例的设置问题。
采用覆盖增强技术时,宏基站会对Pico扩展覆盖范围内的用户造成较强的下行干扰。通常采用基于ABS的时域干扰协调来消除此类干扰。本文利用泊松点过程网络模型分析了在采用覆盖增强技术下的Macro-Pico网络的性能,得到用户到服务基站的距离分布、用户的SIR分布、用户的平均吞吐量。在此基础上,分析了在给定覆盖增强偏置值时ABS比例的合理取值范围。该工作对实际网络中覆盖增强偏置值和ABS比例的设置具有一定的参考意义。
4)研究Macro-Pico异构网络中的覆盖增强机制对用户中断概率的影响,提出一种负载感知的泊松点过程网络模型。
注意到传统泊松点过程网络模型没有考虑基站的实际负载,而只是简单假设基站满负载,本文将基站负载和按需资源分配方案加入到传统泊松点过程模型中,得到更加接近实际情况的负载感知的干扰分析模型。利用该模型分析Macro-Pico网络中覆盖增强机制对用户中断概率的影响,分析得到在给定覆盖增强偏置时用户中断概率的表达式。
5)研究Macro-Pico异构网络中的宏基站和Pico基站间的干扰问题,提出一种负载自适应的干扰协调方案。
为了使得Pico能够更好地为热点用户提供通信资源,本文提出一种基于热点区域的有界覆盖增强方案来扩大Pico的覆盖范围。基于有界覆盖增强方案,进一步提出了负载自适应的动态覆盖控制算法来实现宏基站和Pico基站间的负载均衡,并提出混合时频域干扰协调算法来减轻宏基站对Pico扩展覆盖范围内的用户的干扰。负载自适应的动态覆盖控制算法及混合时频域干扰协调算法能够在用户QoS保证和网络容量之间实现较好的折衷。
With the rapid development of mobile Internet, mobile users put forward a higher demand for the data rates. Applications such as online games, video calls, high-speed downloads have led to the increasing demand for higher data rates and lower latency of the cellular networks. On the other hand, it is estimated that2/3of calls and over90%of data services occur indoors. However, due to the building penetration loss, more power from the cellular base station will be needed to provide indoor coverage. Therefore,3GPP (The3rd Generation Partnership Project) has introduced low power nodes in LTE (Long Term Evolution) Release10to make users get closer to base stations, thus improve the user throughput. The architecture of deploying low power nodes on the basis of traditional macrocell network is called Heterogeneous Networks (HetNets).
In order to maximize the use of spectrum resources,3GPP tends to share the same frequency band between the macrocell and small cells. The co-layer interference among macrocells and that among small cells, as well as the cross-layer interference between macrocells and small cells have become an important factor in restricting the performance of HetNets. Compared to the traditional macro base stations, low power nodes are usually deployed randomly with the closed access mode and range expansion technology, which makes the interference problem of HetNets more complicated. Therefore, the Inter-Cell Interference Coordination (ICIC) technology is one of the most important technologies in LTE-A HetNets. In this dissertation, taking the typical HetNets scenarios Macro-Femto and Macro-Pico networks as an example, we study the ICIC technologies in LTE-A HetNets. The main contributions and innovations are listed as follows:
1) This dissertation studies the downlink interference problems of the Macro-Femto network, and proposes an interference graph based frequency-domain ICIC scheme.
In the construction of the interference graph, the vertexes are all femto base stations and the users which are in the interference regions of femtocells, the edge represents the interference relationship between two vertexes, and the weight represents the degree of the interference. The weight of edges is characterized by the Regional Average Channel State (RACS) metric. Then we model the optimal clustering problem as the MAX K-CUT problem in the graph theory, and propose a heuristic algorithm named Hybrid Clustering based on Interference Graph (HCIG). The HCIG algorithm could reduce all the three types of interference in Macro-Femto networks and improve the network throughput.
2) This dissertation studies the co-layer interference problem among femtocells, and proposes an ICIC scheme based on the cooperation between femtocells.
In the3GPP LTE standards, there is an X2interface between neighbor femtocells, which makes the cooperation between femtocells become possible. By enabling a femtocell to relay the data of its neighbor femtocell, we propose a virtual handover scheme to cancel the interference between femtocells. The main idea of virtual handover is to separate the user plane and control plane. When the virtual handover occurs, the control messages are transferred by the original serving femtocell, and the data messages are transferred by the aided neighbor femtocell. The virtual handover is transparent to the user and the Mobility Management Entity (MME), and it only needs the control message exchange between femtocells. Furthermore, the message exchange is through the wired X2link, which makes the virtual handover scheme have a low overhead and a high probability of success.
3) This dissertation studies the interference from the macrocell to picocell users, discusses the ratio of Almost Blank Subframe (ABS) in the ABS-based time-domain ICIC scheme.
When Range Expansion (RE) is used in the Macro-Pico network, the users in picocell expanded regions will suffer high interference from the macrocell. Generally, ABS-based time-domain ICIC scheme is utilized to cancel this type of interference. We use the Poisson Point Process (PPP) model to analyze the performance of the Macro-Pico network with RE, and get the distribution of distance between users and base stations, SIR, average user throughput. Then we get the reasonable range of ABS ratio when the range expansion bias is given. The work is helpful to the settings of range expansion bias and ABS ratio in the practical system.
4) This dissertation studies the impact of range expansion on the user outage probability, proposes a load-aware PPP model.
Observed that the load of base stations is not taken into account in the traditional PPP model, which is simply assumed that all base stations are fully loaded, we take the effects of load and QoS into the consideration, and propose a more practical load-aware PPP model. Based on the proposed model, we analyze the impact of range expansion on the user outage probability, and get the outage probability expression when the range expansion bias is given.
5) This dissertation studies the interference problem of the Macro-Pico network, proposes a load adaptive ICIC scheme.
In order to make picocells provide more communication resources to the hotspot users, we propose a hotspot-based bounded range expansion scheme to expand the coverage of picocells. Based on the bounded range expansion scheme, we propose a Load-adaptive Dynamic Range Control (LDRC) algorithm to achieve the load balancing between the macrocell and picocell, and give a hybrid approach of time and frequency domain ICIC to reduce the interference from macrocell to users in picocell expanded regions. The LDRC and proposed ICIC scheme could achieve a good trade-off between QoS guarantee and system capacity.
引文
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