Analytical evaluation of downlink interference mitigation in multi‐macrocell/femtocell networks with frequency & cell partitioning

In this paper, we propose an interference mitigation method to suppress the downlink interference in multi‐macrocell/femtocell networks, and analytically evaluate the interference mitigation and average rate performances. Specifically, the proposed interference mitigation method consists of three steps: frequency partitioning, cell partitioning, and sub‐band allocation. In the frequency partitioning step, the whole downlink frequency band is divided into nine non‐overlapping sub‐bands. In the cell partitioning step, each macrocell is divided into four macrocell regions and three femtocell regions for macrocells' and femtocells' communications, respectively. In the sub‐band allocation step, each macrocell or femtocell region is allocated a sub‐band to guarantee that any two neighboring macrocell/femtocell regions use different sub‐bands. Conducted simulation results show that the proposed method is effective in mitigating the downlink interference and improving the average downlink per‐channel rate in multi‐macrocell/femtocell networks. In summary, the major contribution of the proposed interference mitigation method is that the downlink interference can be mitigated without cooperation between macrocells and femtocells, while the full frequency utilization of the macrocell is achieved. Copyright © 2016 John Wiley & Sons, Ltd.     

An efficient interference mitigation approach via quasi‐access in two‐tier macro‐femto heterogeneous networks

Two‐tier heterogeneous networks, where the current cellular networks, that is, macrocells, are overlapped with a large number of randomly distributed femtocells, can potentially bring significant benefits to spectral utilization and system capacity. In a two‐tier network, the cross‐tier interference needs to be handled properly. Unlike the downlink interference, the uplink (UL) interference at femtocell caused by macrocell user equipment (MUE) has not been addressed sufficiently. When an MUE is located near the coverage of femtocell, its transmit power may cause UL interference to the femtocell receiver, especially for the closed subscriber group femtocells that share the entire frequency spectrum with macrocell. We propose a novel quasi‐access strategy, which allows the interfering MUE to connect with the interfered femtocell access point (FAP) while only via UL. It can significantly alleviate the UL interference at the FAP as well as its neighbors, in the meantime, benefit the macro‐tier. Copyright © 2013 John Wiley & Sons, Ltd.     

Pricing and power control for energy‐efficient radio resource management in cognitive femtocell networks

Cognitive femtocell has been considered as a promising technique that can improve the capacity and the utilization of spectrum efficiency in wireless networks because of the short transmission distance and low transmit power. In this paper, we study the win–win solution of energy‐efficient radio resource management in cognitive femtocell networks, where the macrocell tries to maximize its revenue by adjusting spectrum utilization price while the femtocells try to maximize their revenues by dynamically adjusting the transmit power. When the spectrum utilization price is given by macrocell, we formulate the power control problem of standalone femtocells as an optimization problem and introduce a low‐complexity iteration algorithm based on gradient‐assisted binary search algorithm to solve it. Besides, non‐cooperative game is used to formulate the power control problem between collocated femtocells in a collocated femtocell set, and then low complexity and widely used gradient‐based iteration algorithm is applied to obtain the Nash‐equilibrium solution. Specially, asymptotic analysis is applied to find the approximate spectrum utilization price in macrocell, which can greatly reduce the computational complexity of the proposed energy‐efficient radio resource management scheme. Finally, extensive simulation results are presented to verify our theoretical analysis and demonstrate the performance of the proposed scheme. Copyright © 2013 John Wiley & Sons, Ltd.     

Outage probability in Poisson‐cluster‐based LTE two‐tier femtocell networks

This paper analyzes two‐tier orthogonal frequency‐division multiplexing (OFDM)‐based cellular structure, when the traditional macrocell structure is extended with femtocells. The benefit of using femtocells is the capacity and coverage extension capability. To fulfill strict quality of service requirements in next‐generation mobile networks such as Long Term Evolution (LTE) or LTE‐Advanced, capacity and coverage enhancing becomes rather important. On the other hand, adding small cells such as femtocells next to macrocell modifies the interference pattern of the current region. Therefore, the number of small cells in a given area should be limited. In this paper, we provide an analytic framework to calculate the outage probability for a macrocell user in OFDM‐based femtocell networks when the deployed femto base stations are composing an independent Poisson cluster process such as Thomas cluster process. Cluster‐based femtocell modeling offers accurate network planning for mobile operators. In this cluster‐based realization, we give an interference characterization and consider the outage probability for a randomly deployed user when communication channel is infected with Rayleigh fading. Copyright © 2014 John Wiley & Sons, Ltd.     

Distributed Resource Allocation in Two‐Hierarchy Networks

In this paper, a new distributed resource allocation algorithm is proposed to alleviate the cross‐tier interference for orthogonal frequency division multiplexing access macrocell and femtocell overlay. Specifically, the resource allocation problem is modeled as a non‐cooperative game. Based on game theory, we propose an iterative algorithm between subchannel and power allocation called distributed resource allocation which requires no coordination among the two‐hierarchy networks. Finally, a macrocell link quality protection process is proposed to guarantee the macrocell UE's quality of service to avoid severe cross‐tier interference from femtocells. Simulation results show that the proposed algorithm can achieve remarkable performance gains as compared to the pure waterfilling algorithm.     

Dense femtocell networks power self‐optimization: an exact potential game approach

Femtocell is regarded as a promising technology to enhance indoor coverage and improve network capacity. However, highly dense and self‐organized femtocells in urban environment will result in serious inter‐femtocell interference. To solve this problem, this paper proposes a distributed power self‐optimization scheme for the downlink operation of dense femtocell networks. First, a novel convex pricing mechanism is presented to price the transmit power of femtocells and construct the utility function of femtocells. Then, a noncooperative game framework for power self‐optimization of femtocells in dense femtocell networks is established on the basis of the exact potential game theory, which is demonstrated to converge to a pure and unique Nash equilibrium. Finally, combined with firefly algorithm, an effective power self‐optimization algorithm with guaranteed convergence is proposed to achieve the Nash equilibrium of the proposed game. With practical LTE parameters and a 3GPP dual‐strip femtocell model, simulation results show that the proposed game with convex pricing mechanism increases the femtocell network throughput by 7% and reduces the average transmit power of femtocells by 50% in dense femtocell networks, with respect to the compared schemes. Copyright © 2014 John Wiley & Sons, Ltd.     

ESPS scheme for two‐tier interference suppression in a heterogeneous network with cooperative femtocells

The coexistence of a macrocell and a number of femtocells often leads to a two‐tier heterogeneous network, where the co‐tier interference (CotIN) and cross‐tier interference (CrotIN) both degrade users' quality of service. In order to suppress these interferences, we propose a precoding scheme in a heterogeneous network with cooperative femtocells, called CotIN elimination and CrotIN suppression with precoding criterion selection (ESPS) scheme. In this scheme, we first eliminate the CotINs of each user by applying the QR decomposition to channel matrix. Then the CrotINs of macrocell users and femtocell users are suppressed via the macrocell base station (MBS) and femtocell access points (FAPs) with precoding criterion selection, respectively. By using the ESPS scheme, spatial resources can be efficiently exploited. In addition, our ESPS scheme requires little information exchange between MBS and FAPs without iteration and thus significantly reduces the implementation complexity. Furthermore, the robustness is increased through introducing the information of channel uncertainty into the ESPS when channel estimation or quantization error exists. The performance analysis for the ESPS demonstrates that the ESPS is practical in the heterogeneous networks. Finally, simulation results show that the ESPS can decrease users' bit error rates and increase their transmission rates. Copyright © 2015 John Wiley & Sons, Ltd.     

Performance analysis of an advanced heterogeneous mobile network architecture with multiple small cell layers

The integration of small cell technologies into the current mobile network operators is a necessity for providing capacity and coverage improvement in the future mobile networks (5G). This integration paves the way for heterogeneous networking. In this paper, a novel heterogeneous architecture for the efficient integration of small cell technology into the current mobile networks is developed, namely advanced heterogeneous mobile network (AHMN). AHMN architecture consists of a stack of multiple cell layers wherein the upper layer is the macrocell layer while under this layer, a number of lower small cell layers are formed. Focusing on femtocells and metrocells, as the most typical paradigms of small cells, a femtocell layer which serves the indoor traffic activity of femtocell users is considered, while the metrocell serves the outdoor traffic activities as well as the overflow traffic from femtocells. The overall heterogeneous network (HetNet) is completed with the macrocell overlay layer, which serves only the macrocell users and the overflowed traffic from the underlay metrocell layer. In the proposed AHMN architecture, the metrocell layer is deployed as a complementary layer between the macrocell and femtocell layers and facilitates the handover traffic interaction between the edge layers. Meanwhile, the mobility management in this architecture is critical and hence, the interaction between successive network layers, due to the handover (HO) traffic, is analyzed. Furthermore, for each network layer, a guard channel scheme is proposed in order to minimize the HO dropping rate of the mobile users. We show both analytically and by simulation the capability of AHMN in offloading traffic and reducing the blocking/dropping probability compared with the traditional macrocellular network.     

UMTS Multi-Service Uplink Capacity and Interference Statistics of Femtocells

In this work, the multiservice uplink capacity of single and multiple femtocells is given. The COST231 multiwall and multifloor indoor propagation model has been used to calculate the indoor propagation loss. Results show that the uplink capacity of a deployed femtocell will reduce by 2 % if two extra femtocells are deployed in the same building higher and lower of it. Results also show that the uplink capacity is slightly affected if there are several femtocells deployed in the buildings around the one at which the femtocell under study is already exists. It is demonstrated that uplink capacity is interference limited if the femtocell is deployed to serve the users in three floors. Results show that the uplink capacity will be interference and noise limited if the femtocell is deployed to serve the users in five floors. Finally, it is found that the effect of the interference due to the uniformly distributed users within the macrocell around the femtocell is insignificant.     

Power Control Based on Maximum Power Adaptation in Two-Tier Femtocell Networks

In recent years, femtocells are receiving considerable attention in mobile communication as a cost-effective means of improving indoor coverage and capacity. A significant technical challenge in the deployment of a large number of femtocells is the management of interference from the underlay of femtocells onto the overlay of macrocell. In this paper, a reasonable and effective interference suppression scheme based on the adaptive adjustment of femtocell users’ maximum transmission power is proposed. The highlight of the scheme is the joint design of macrocell users’ uplink communication protection and femtocell users’ optimal power allocation. The scheme restricts the cross-tier interference at macrocell base station below a given threshold and ensures the optimization of femtocell users’ power allocation at each adjustment phase. Last, admission control is also considered, aiming to exploit the network resources more effectively. Simulation results show the superiority of the proposed scheme over the scheme based on the Signal-to-Interference-Plus-Noise Ratio adaptation. We also give some reference on utility function selection by setting different coefficients in the utility function, and show the effectiveness of admission control in both fixed and random network topologies.     

An efficient handoff algorithm based on received signal strength and wireless transmission loss in hierarchical cell networks

Compared with the macrocell systems, the femtocell systems allow users to obtain broadband service with high data rate by using lower costs of transmit power, operation and capital expenditure. Traditional handoff algorithms used in macrocells cannot well satisfy the mobility of users efficiently in hierarchical macro/femto cell networks. In this paper based on the received signal strength (RSS) and wireless transmission loss, a new handoff algorithm in hierarchical cell networks called RWTL-HO is proposed, which considers the discrepancy in transmit power between macrocell and femtocell base stations. The simulation results show that compared with the conventional algorithm, the proposed algorithm improves the utilization of femtocells by doubling the number of handoffs; and in comparison with the handoff algorithm based on combining the RSSs from both macro and femto cell base stations, reduces half the number of redundant handoffs.     

Outage Analysis of LTE-A Femtocell Networks with Nakagami-m Channels

The concept of extending traditional macrocell cellular structure with small cells (like femtocells) in next-generation mobile networks (e. g., Long Term Evolution Advanced) provides a great opportunity to improve coverage and enhance data rate. Femtocells are cost efficient, indoor base stations. These femtocells can operate in closed mode i. e. only restricted users connection are allowed. Therefore, if the number of deployed femtocells is significant, that can dramatically modify the interference pattern of a macrocell. Thus mobile service providers have to pay attention for the number of simultaneously operating femtocells and encroach, if necessary, to provide appropriate service level to every mobile user. In this paper we provide an analytic framework to characterize the upper bound of service outage probability for a potential macrocell user in a two-tier mobile system, when the radio channels are infected by Nakagami- \(m\) fading. In our proposal the femtocells are operating in closed mode and deployed into a designated macrocell, hence every femtocell increases the interference level. The spatial location femtocells is modelled with Poisson cluster process. Compared to traditional grid structure or completely spatial random Poisson point process femtocell deployment, cluster based layout may provides more life realistic deployment scenario. To evaluate the upper bound of service outage we use the tools of stochastic geometry.     

Proactive caching with content‐based pricing for cooperative femtocells in two‐tier heterogeneous networks

With increase in the number of smart wireless devices, the demand for higher data rates also grows which puts immense pressure to the network. A vast majority of this demand comes from video files, and it is observed that only a few popular video files are requested more frequently during any specified time interval. Recent studies have shown that caching provides a better performance as it minimizes the network load by avoiding the fetching of same files multiple times from the server. In this paper, we propose to combine two ideas; proactive caching of files and content‐based pricing in macro‐femto heterogeneous networks. The femtocell access point (FAP) is allowed to manipulate its users' demand through content‐based pricing and serve the users' requests by proactively downloading suitable content into its cache memory which reduces the load of the femtocell. In addition, an incentive mechanism is also proposed which encourages the FAP to help macrocell users under its coverage zone by allowing access to its cached content and thereby reduces the macrocell load. The proposed content‐based pricing and proactive caching scheme for femtocells is modeled as a Stackelberg game among the macrocell base station and the FAP to jointly maximize both of their utilities. Performance analysis of the scheme is presented for a single femtocell scenario and compared with the conventional flat pricing‐based scheme via numerical examples. The results demonstrate significant reduction in network load using our proposed scheme.     

System design of cdma2000 femtocells - [femtocell wireless communications]

Femtocells extend the cellular network coverage and provide high speed data service inside homes and enterprises for mobiles supporting existing cellular radio communication techniques. They also provide additional system capacity by offloading macro network traffic. This article reviews the characteristics of cdma2000-based femtocell systems. It discusses design and deployment aspects such as carrier allocation, access control, efficient support for femtocell discovery by idle mobiles, and active call hand-in from macrocell to femtocell. The evolution of the cdma2000 standard for optimizing performance and enriching user experience with femtocells is also discussed.     

A preset threshold based cross-tier handover algorithm for uplink co-channel interference mitigation in two-tier femtocell networks

This paper considers the co-channel interference mitigation problem and proposes a preset threshold based cross-tier handover algorithm for uplink co-channel interference mitigation in two-tier femtocell networks. The proposed cross-tier handover algorithm introduces a preset threshold cross-tier handover policy, which takes into account both the time-to-stay (TTS) of a macrocell user equipment (MUE)/femtocell user equipment (FUE) in a femtocell/the macrocell, and the received signal to interference plus noise ratio (SINR) at a femtocell access point (FAP)/the macrocell base station (MBS) in making a cross-tier handover decision for an MUE/FUE. A cross-tier handover decision is made by comparing the TTS of an MUE/FUE in a femtocells/the macrocell and the SINR at a FAP/the MBS with a preset TTS threshold and different SINR thresholds. The objective of the preset threshold based cross-tier handover algorithm is to increase the received SINR at the MBS/FAPs and thus improve the network performance. The performance of the proposed cross-tier handover algorithm with the minimum power transmission and the optimal power transmission is analyzed, respectively. Numerical results show that the proposed preset threshold based cross-tier handover algorithm can significantly improve the network performance in terms of the outage probability, user sum rate, and network capacity.     

分层异构网络中基于斯坦克尔伯格博弈的资源分配算法

分层异构网络中家庭基站与宏基站之间往往存在干扰,如何分配资源以获得高谱率和高容量、保证用户性能一直是研究的重点。为了解决这个问题,本文提出了一种异构蜂窝网络中基于斯坦克尔伯格博弈的家庭基站与宏基站联合资源分配算法,算法首先基于图论的分簇算法对家庭基站和宏用户进行分簇和信道分配,以减少家庭基站之间的同层干扰和家庭基站层与宏蜂窝网络的跨层干扰;然后建立了联合家庭基站发射功率以及宏用户接入选择的斯坦克尔伯格博弈,推导出达到纳什均衡时的家庭基站发射功率的表达式,并据此为宏用户选择合适的接入策略。仿真结果表明,该算法能够有效地提高宏用户的信干噪比（SINR）,家庭用户的性能也得到改善。      

Interference Avoidance through Pilot‐Based Spectrum Sensing Algorithm in Overlaid Femtocell Networks

Co‐channel interference between macro–femtocell networks is an unresolved problem, due to the frequency reuse phenomenon. To mitigate such interference, a secondary femtocell must acquire channel‐state knowledge about a co‐channel macrocell user and accordingly condition the maximum transmit power of femtocell user. This paper proposes a pilot‐based spectrum sensing (PSS) algorithm for overlaid femtocell networks to sense the presence of a macrocell user over a channel of interest. The PSS algorithm senses the pilot tones in the received signal through the power level and the correlation metric comparisons between the received signal and the local reference pilots. On ensuring the existence of a co‐channel macrocell user, the maximum transmit power of the corresponding femtocell user is optimized so as to avoid interference. Time and frequency offsets are carefully handled in our proposal. Simulation results show that the PSS algorithm outperforms existing sensing techniques, even at poor received signal quality. It requires less sensing time and provides better detection probability over existing techniques.     

Stackelberg game for backhaul resource allocation in the two-tier LTE femtocell networks

As one promising technology for indoor coverage and service offloading from the conventional cellular networks, femtocells have attracted considerable attention in recent years. However, most of previous work are focused on resource allocation during the access period, and the backhaul involved resource allocation is seriously ignored. The authors studied the backhaul resource allocation in the wireless backhaul based two-tier heterogeneous networks （HetNets）, in which cross-tier interference control during access period is jointly considered. Assuming that the macrocell base station （MBS） protects itself from interference by pricing the backhaul spectrum allocated to femtocells, a Stackelberg game is formulated to work on the joint utility maximization of the macrocell and femtocells subject to a maximum interference tolerance at the MBS. The closed-form expressions of the optimal strategies are obtained to characterize the Stackelberg equilibriums for the proposed games, and a backhaul spectrum payment selection algorithm with guaranteed convergence is proposed to implement the backhaul resource allocation for femtocell base stations （FBSs）. Simulations are presented to demonstrate the Stackelberg equilibrium （SE） is obtained by the proposed algorithm and the proposed scheme is effective in backhaul resource allocation and macrocell protection in the spectrum-sharing HetNets.     

Predictive Reservation for Handover Optimization in Two-Tier Heterogeneous Cellular Networks

The mobile network operators seek ways to increase their capacity and coverage in order to be able to deliver high quality services even under surging mobile broadband demand. The deployment of femtocells is the most efficient, economical and technically feasible approach to achieve this goal. However, a two-layered femtocell/macrocell heterogeneous networks where users can handover from a femtocell to the overlaid macrocell requires innovative traffic management techniques. While call handover from a femtocell to a macrocell can be easily implemented based on already known procedures, on the other hand, the handover towards a femtocell is more challenging. More specifically, due to the limited bandwidth available at each femtocell, it is not spectrally efficient to use a typical guard channel scheme in order to prioritize handover calls over new calls. Therefore, in this paper we introduce a dynamic pre-reservation scheme, which is based on the predictive channel reservation concept and adjusts dynamically the reserved resources. We show both analytically and by simulation that the proposed scheme can be efficiently applied to a two-layered femtocell/macrocell heterogeneous mobile network.     

双层分级蜂窝网最优频谱分配与定价

针对异频组网的双层分级蜂窝网,提出了一种基于纳什谈判解法的最优频谱分配与定价策略,该策略能激励家庭基站采用开放用户组模式,最大化频谱效益。通过Stackelberg博弈建模,分析了频谱定价与用户需求的关系。通过纳什谈判解法,获得了最佳的频谱分配与定价策略,按需地为宏基站与家庭基站分配了带宽资源,定量地分析了家庭基站所提高的频谱效益。仿真结果表明,该策略相比非合作博弈方法,可有效提高运营商以及家庭基站拥有者所能获得的频谱效益,部署家庭基站将提高蜂窝网络的总效益。