超密集网络中子信道和功率分配研究

超密集网络中,严重的小区间干扰制约了终端用户的数据速率,针对该问题,提出一种基于簇优先级的资源分配方案。该方案分为3个步骤:首先,采用基于图论的染色算法为毫微微接入点(Femtocell Access Points,FAPs)分簇;然后,以簇内每个毫微微用户(Femtocell User Equipments,FUEs)的待发送数据量、排队等待时延以及受干扰强度等作为优先级,计算每个簇的优先级,高优先级的簇可最先获得信道增益好的子信道;最后,利用卡罗需-库恩-塔克(Karush-Kuhn-Tucker,KKT)条件和注水算法为FUEs分配功率。仿真实验表明该方案能够有效地减小Femtocell间的干扰,并能够极大地满足用户的需求,同时提升系统的吞吐量和频谱效率。     

An interference-aware virtual clustering paradigm for resource management in cognitive femtocell networks

Femtocells represent a promising alternative solution for high quality wireless access in indoor scenarios where conventional cellular system coverage can be poor. They are randomly deployed by the end user, so only post-deployment network planning is possible. Furthermore, this uncoordinated deployment creates severe interference to co-located femtocells, especially in dense deployments. This paper presents a new architecture using a generalised virtual cluster femtocell (GVCF) paradigm, which groups together FAP into logical clusters. It guarantees severely interfering and overlapping femtocells are assigned to different clusters. Since each cluster operates on different band of frequencies, the corresponding virtual cluster controller only has to manage its own FAPs, so the overall system complexity is low. The performance of the GVCF algorithm is analysed from both a resource availability and cluster number perspective. Simulation results conclusively corroborate the superior performance of the GVCF model in interference mitigation, particularly in high density FAP scenarios.     

基于拍卖的混合接入异构网络频谱共享

针对Overlay与Underlay模式的传统频谱共享系统网络效益较低的问题，提出了一种基于拍卖机制的频谱共享。当宏用户靠近飞蜂窝接入点时，飞蜂窝接入点与宏用户之间的链路性能比宏基站至宏用户的性能要好。此时宏基站希望将该用户卸载至飞蜂窝网络中，获得更好的通信服务质量的同时还能为宏基站节约能量。在保证飞蜂窝网络授权用户的QoS前提下，为激励网络内的混合接入，宏基站将自己的剩余频谱供飞蜂窝接入点使用，并对飞蜂窝接入点进行补偿。每个飞蜂窝接入点独立地决定自己的竞价，宏基站根据收集到的竞价决定最优分配策略。提出的基于拍卖机制的混合频谱共享能够使异构网络中的宏基站和飞蜂窝接入点的效用函数最大。并且补偿函数的引入能更好地激励飞蜂窝采用混合接入模式。     

有效的基于软频率复用长期演进femtocell网络的小区间干扰协调方案

家庭基站是一种小体积、低发射功率的基站,它为长期演进(LTE) femtocell双层网络提供更好室内覆盖的同时也增加了整个系统的容量。然而,femtocell和宏基站(MeNB)之间的干扰不容小觑。针对二者间的干扰问题,提出一种基于软频率复用(SFR)的有效的LTE femtocell网络的小区间干扰协调(ICIC)方案。该方案中,为避免共信道干扰,先对宏小区进行SFR频域资源分配,然后让femtocell用户使用宏基站未占用的频域资源;并且当femtocell位于宏小区中心区域时,不使用同一扇区边缘区域占用的频带。仿真结果表明,此方案减小了不同类型用户间的干扰,整个网络的吞吐量比没有使用ICIC技术的情况提高了14%,同时小区边缘用户的平均吞吐量至少提高了34%。     

基于家庭基站密度的自适应无线资源分配策略

针对飞蜂窝(又叫家庭基站)中干扰严重、资源利用率低等问题,提出一种基于家庭基站密度的自适应无线资源分配策略。通过频率分割抑制宏蜂窝与飞蜂窝间的干扰,依托资源复用和功率控制抑制飞蜂窝之间的干扰,并基于自组织网络技术实现家庭基站接入点(FAP)的自动配置。仿真和性能分析表明,策略在最大限度提高无线资源利用率的同时,基本实现了零干扰,并将系统总吞吐量提高了20%;尤其适用于家庭基站密集或无线资源紧张的应用场合。     

Cell ID extension in femtocell environments

In this paper, we solve a cell identification problem in a femtocell environment. Wireless communication standards specify physical cell identifiers (PCIs) for user entities (UEs) to discriminate data from different cells and for networks to identify cells. However, in the densely deployed area, the number of PCIs for femtocells may not be sufficient. The scarcity of PCIs results in identification ambiguity due to duplicated PCIs in a certain area, which may result in handover failure. Using time synchronization between femtocells and macro-cells, we propose a scheme to efficiently identify cells in a femtocell environment. The proposed scheme defines the virtual cell identifier (vID) of a femtocell as the system frame number offset of the femtocell with respect to a macro-cell and differentiates femtocells with combinations of vIDs and PCIs. Our scheme can increase the number of cell identifiers up to 1024 times using even coarse local time synchronization without changing the existing physical layer specification. The improvement in cell identification resolution reduces redundant message transactions, resulting in network performance enhancement. Since our scheme demands only local time synchronization between adjacent cells, it can be applied to asynchronous as well as synchronous wireless systems. We verify the proposed scheme via simulations in various environments.     

超密集网络中基于干扰协调的资源分配算法

超密集网络中,严重的小区间干扰制约了终端用户的数据速率,针对该问题,该文提出一种基于干扰协调的资源分配方案。该方案分为两个模块：第一模块基于毫微微接入点(Femtocell Access Points, FAPs)间的干扰程度,将干扰强的FAPs分到同一簇内,同簇内的FAPs共享频带资源,通过FAPs间的协作使不同簇之间实现频谱的复用;第二模块基于最大功率和最低速率的公平性准则进行最优功率分配,动态分配资源。仿真结果表明,该算法在超密集网络场景下能够有效控制FAPs间的干扰,最大化系统吞吐量。     

基于竞争窗口递减因子自适应调整退避算法

包括IEEE802.11、802.15.4标准在内的许多无线网络协议都采用二进制指数退避机制管理数据的重发。在动态分布式的网络环境中,二进制指数退避算法固定的竞争窗口递减方式难以适应动态变化的网络规模。针对这一问题,提出了一种改进的回退机制,该机制通过引入竞争窗口递减因子,自适应地调整无线节点的等待时间,以实现网络吞吐量的最大化。同时,在算法实现上提出一种启发式算法以跟踪网络中竞争节点数量的改变。在IEEE 802.11DCF协议中以相同的物理层参数进行仿真,结果表明改进算法提高了网络吞吐量,降低了分组平均接入时延。     

Location-based spectrum allocation and partitioning scheme for cross-tier interference mitigation in macro-femtocell networks

In this paper, we address the cross-tier interference problem in a two-tier network which consists of overlaid femtocells within a macrocell range. In the considered network, macro and femto base stations are assumed with the capabilities of power control and beamforming, differently from user terminals. Under such environments, we devise a decentralized solution where each femtocell determines whether to use shared spectrum or partitioned spectrum, considering users’ location information. Introducing two simple distance thresholds, we can optimally partition the spectrum and allocate them without detailed channel information. Simulation results demonstrate that our decentralized scheme achieves comparable performance to the centralized one with low overhead and feedback delay.     

A Stackelberg game-based spectrum allocation scheme in macro/femtocell hierarchical networks

Femtocells widely deployed in a macrocell form hierarchical cell networks, which can improve indoor coverage and network capacity, and have been regarded as one of the most promising approaches. However owing to the absence of coordination between the macro and femtocells, and among femtocells, decentralized spectrum allocation between macro and femtocell users become technically challenging. In this paper, a dynamic spectrum allocation scheme based on Stackelberg game is proposed, in which macrocell base stations as leaders and femtocell base stations as followers are players, and the same spectrum is the resource that players will choose assigning to users for minimizing the affected interference among each other. The Stackelberg equilibrium is defined and proved to be existed, which is also the optimal spectrum allocation manner. Simulations were conducted to study the impact of femtocells on the macrocells regarding throughput, outage probability and spectrum efficiency. And the comparison results show that the proposed scheme might be a solution for efficiently allocating the spectrum in hierarchical cell networks, as the improvement in terms of throughput, outage probability and spectrum efficiency had been achieved.     

A Reverse Auction Framework for Hybrid Access in Femtocell Network

In the two-tier macro-femto heterogeneous network, hybrid access is regarded as the most ideal access control approach to mitigating macro-femto cross-tier interference and enhancing overall network performance. However, the implementation of hybrid access is hindered by a lack of incentive market mechanism to motivate private femtocell owners to offer access permissions to macro users. In this paper, we propose a reverse auction framework for access permission transaction between a macrocell operator and multiple femtocell owners to promote hybrid access. Our goal is to maximize social welfare while guaranteeing the truthfulness of the auction. Since the coverage of multiple femtocells may overlap, we partition each cell to adjust the granularity of access permission availability. We first propose a Vickery-Clarke-Grove (VCG)-based mechanism, which costs the least among all auction mechanisms that produce maximum social welfare. As the VCG mechanism is too time-consuming, we propose two alternative truthful mechanisms, namely, generalized second-price and suboptimal mechanism. We further extend the auction framework to the scenario where femtocell owners have heterogeneous valuations for access permissions in different locations.     

Interference coordination based on random fractional spectrum reuse in femtocells toward Internet of Things

Femtocell is featured with a low-power and low-cost cellular access in indoor environments, and thus offers an effective yet flexible way to implement information exchange over Internet of Things. In femtocell networks, the dense deployment of home eNodeBs causes severe inter-cell interferences and imposes heavier load on the scarce frequency spectrum. In this paper, we propose an inter-femtocell interference coordination scheme to enable random and fractional reuse of frequency resources in a 3D in-building scenario. Specifically, we consider the regular femtocell deployment, where all the femtocells are divided into two groups and two neighboring femtocells will be classified into different group. Each group is initially allocated with a half of frequency resources. To more sufficiently utilize the spectrum, either group of femtocell is allowed to transmit over the frequency assigned to the other group of femtocell in a random way at the cost of introducing some interferences, i.e., reuse based on a specified probability. This probability is determined by maximizing the geometric mean of users’ average throughput, such that the fairness among users is guaranteed simultaneously. Moreover, an equivalent scheme generated from full frequency reuse between two femtocells groups is also given. Here, either group of femtocell will avoid transmitting over fractional frequency randomly with a certain probability and the interference to the adjacent femtocell can be reduced. The simulation results show that the proposed schemes could obtain the larger system average rate and edge user performance compared with the baseline schemes. Moreover, the geometrical average throughput per user achieved by our method is highest, and a more fair resources allocation can be realized .     

Subchannel allocation for the OFDMA-based femtocell system

In femtocell networks, due to their small cell size, we can achieve higher spatial diversity from the channel reuse between multiple femtocells. In addition, if femtocells operate on OFDMA systems, each subchannel can be reused separately among femtocells, improving system efficiency more significantly. However, due to a large number of femtocells and their uncoordinated and irregular deployment, we need to treat intercell interferences very carefully in OFDMA-based femtocell networks, which makes developing efficient resource allocation schemes more difficult.     

Interworking wireless mesh networks: Problems,performance characterization,and perspectives

Wireless broadband networks based on the IEEE 802.11 technology are being increasingly deployed as mesh networks to provide users with extended coverage for wireless Internet access. These wireless mesh networks, however, may be deployed by different authorities without any coordination a priori, and hence it is possible that they overlap partially or even entirely in service area, resulting in contention of radio resources among them. In this paper, we investigate the artifacts that result from the uncoordinated deployment of wireless mesh networks. We use a network optimization approach to model the problem as resource sharing among nodes belonging to one or different networks. Based on the proposed LP formulation, we then conduct simulations to characterize the performance of overlaying wireless mesh networks, with the goal to provide perspectives for addressing the problems. We find that in a system with multiple overlaying wireless mesh networks, if no form of inter-domain coordination is present, individual mesh networks could suffer from capacity degradation due to increased network contention. One solution toward addressing the performance degradation is to “interwork” these wireless mesh networks by allowing inter-domain traffic relay through provisioning of “bridge” nodes. However, if such bridge nodes are chosen arbitrarily, the problems of throughput sub-optimality and unfairness may arise. We profile the impact of bridge node selection and show the importance in controlling network unfairness for wireless mesh network interworking. We conclude that mesh network interworking is a promising direction to address the artifacts due to uncoordinated deployment of wireless mesh networks if it is supplemented with appropriate mechanisms.     

Automatic facial expression recognition using facial animation parameters and multistream HMMs

The performance of an automatic facial expression recognition system can be significantly improved by modeling the reliability of different streams of facial expression information utilizing multistream hidden Markov models (HMMs). In this paper, we present an automatic multistream HMM facial expression recognition system and analyze its performance. The proposed system utilizes facial animation parameters (FAPs), supported by the MPEG-4 standard, as features for facial expression classification. Specifically, the FAPs describing the movement of the outer-lip contours and eyebrows are used as observations. Experiments are first performed employing single-stream HMMs under several different scenarios, utilizing outer-lip and eyebrow FAPs individually and jointly. A multistream HMM approach is proposed for introducing facial expression and FAP group dependent stream reliability weights. The stream weights are determined based on the facial expression recognition results obtained when FAP streams are utilized individually. The proposed multistream HMM facial expression system, which utilizes stream reliability weights, achieves relative reduction of the facial expression recognition error of 44% compared to the single-stream HMM system.     

Exact and heuristic approaches based on noninterfering transmissions for joint gateway selection,time slot allocation,routing and power control for wireless mesh networks

Wireless mesh networks (WMNs) provide cost-effective alternatives for extending wireless communication over larger geographical areas. In this paper, given a WMN with its nodes and possible wireless links, we consider the problem of gateway node selection for connecting the network to the Internet along with operational problems such as routing, wireless transmission capacity allocation, and transmission power control for efficient use of wired and wireless resources. Under the assumption that each node of the WMN has a fixed traffic rate, our goal is to allocate capacities to the nodes in proportion to their traffic rates so as to maximize the minimum capacity-to-demand ratio, referred to as the service level. We adopt a time division multiple access (TDMA) scheme, in which a time frame on the same frequency channel is divided into several time slots and each node can transmit in one or more time slots. We propose two mixed integer linear programming formulations. The first formulation, which is based on individual transmissions in each time slot, is a straightforward extension of a previous formulation developed by the authors for a related problem under a different set of assumptions. The alternative formulation, on the other hand, is based on sets of noninterfering wireless transmissions. In contrast with the first formulation, the size of the alternative formulation is independent of the number of time slots in a frame. We identify simple necessary and sufficient conditions for simultaneous transmissions on different links of the network in the same time slot without any significant interference. Our characterization, as a byproduct, prescribes a power level for each of the transmitting nodes. Motivated by this characterization, we propose a simple scheme to enumerate all sets of noninterfering transmissions, which is used as an input for the alternative formulation. We also introduce a set of valid inequalities for both formulations. For large instances, we propose a three-stage heuristic approach. In the first stage, we solve a partial relaxation of our alternative optimization model and determine the gateway locations. This stage also provides an upper bound on the optimal service level. In the second stage, a routing tree is constructed for each gateway node computed in the first stage. Finally, in the third stage, the alternative optimization model is solved by fixing the resulting gateway locations and the routing trees from the previous two stages. For even larger networks, we propose a heuristic approach for solving the partial relaxation in the first stage using a neighborhood search on gateway locations. Our computational results demonstrate the promising performance of our exact and heuristic approaches and the valid inequalities     

呼叫准入约束下Macro-Femto小区选择GSPN模型研究

随着数据业务快速增长,仅靠宏蜂窝网络已无法满足流量需求;飞蜂窝网络可以有效分流宏蜂窝流量, 近年来 所构成的Macro-Femto架构得到了较广泛的部署。Macro-Femto网络部署的关键问题之一是小区选择,由于飞蜂窝网络频谱资源有限,进行小区选择时仍需考虑呼叫准入控制(Call Admission Control,CAC)。图形化的广义随机Petri网(Generalized Stochastic Petri Nets,GSPN)具有并行性、不确定性和异步特性以及强大的分析复杂系统的功能。针对Macro-Femto网络的呼叫准入约束机制,通过分析呼叫对信道的占用情形,在改进呼叫重审机制的基础上,建立了呼叫准入控制策略GSPN模型。利用建立的GSPN模型,分析了不同呼叫准入控制策略对新呼叫和切换呼叫阻塞率的影响,给出了部分重审机制的呼叫准入控制策略。在分析飞蜂窝的高速服务特性和宏蜂窝的高负载能力的基础上,结合部分重审呼叫准入策略和GSPN方法,提出了基于负载的飞蜂窝优先选择方案(Load-Based Femto-Priority Selection Scheme,LFP)。仿真结果表明,较典型的飞蜂窝优先选择方案,所提出的LFP方案对新呼叫的阻塞率和切换呼叫的掉线率分别降低了2.7%和4.6%。     

基于非合作博弈的femtocell 双层网络分布式功率控制

研究在频谱共享条件下家庭基站双层网络的分布式功率控制策略.将宏基站所能承受的干扰限度视为家庭基站的可分配资源,家庭基站以竞价形式对其"购买",从而构成宏基站与家庭基站以及家庭基站用户之间的博弈模型.分析了该博弈过程中纳什均衡解的存在性和惟一性,并给出了在非合作模式下指导家庭基站用户进行理性竞争的分布式功率调整算法.最后,通过仿真实验验证了所提出算法的有效性.     

IEEE 802.11ax密集WLAN的干扰协调策略

无线局域网(WLAN)中设备数量的指数级增长,导致激烈的信道争用及严重的同频干扰。当运行在同一频段上的两个WLAN在彼此感知范围内时,使用先监听后发送的分布式信道接入方式会引起资源浪费,降低网络吞吐率。提出一种密集WLAN场景下的干扰协调策略。根据IEEE 802.11ax中的基本服务集颜色对站点进行分组,提出协调交替目标唤醒时间策略(CAT),安排不同组内站点在不同的时间接收数据,从而避免信道争用,最小化干扰,同时通过优化下行数据包大小提高网络吞吐率。仿真结果表明,当两个WLAN距离很近时,CAT方案能够保证网络吞吐率。此外,在考虑误码率的情况下,吞吐率并非随着数据包的增大单调递增,而是存在一个阈值,超过该阈值后网络吞吐率即呈下降趋势。     

Energy efficiency of femtocell deployment in combined wireless/optical access networks

Optical/wireless convergence has become of particular interest recently because a combined radio wireless and optical wired network has the potential to provide both mobility and high bandwidth in an efficient way. Recent developments of new radio access technologies such as the Long Term Evolution (LTE) and introduction of femtocell base stations open new perspectives in providing broadband services and applications to everyone and everywhere, but the instantaneous quality of radio channel varies in time, space and frequency and radio communication is inherently energy inefficient and susceptible to reflections and interference. On the other hand, optical fiber-based networks do not provide mobility, but they are robust, energy efficient, and able to provide both an almost unlimited bandwidth and high availability.In this paper, we analyze the energy efficiency of combined wireless/optical access networks, in which LTE technology provides ubiquitous broadband Internet access, while optical fiber-based technologies serve as wireless backhaul and offer high-bandwidth wired Internet access to business and residential customers. In this contest, we pay a particular attention to femtocell deployment for increasing both access data rates and area coverage. The paper presents a novel model for evaluating the energy efficiency of combined optical/wireless networks that takes into account the main architectural and implementational aspects of both RF wireless and optical parts of the access network. Several hypothetical network deployment scenarios are defined and used to study effects of femtocell deployment and power saving techniques on network’s energy efficiency in urban, suburban and rural areas and for different traffic conditions.