Macrocell User Equipments Research Articles

Hierarchical-Game-Based Uplink Power Control in Femtocell Networks

Femtocells are viewed as a promising option for mobile operators to improve coverage and provide high-data-rate services in a cost-effective manner. However, cross-tier interference mitigation is considered to be one of the major challenges. Distributed-game-based power control approaches are effective for interference management in wireless networks. In this paper, we focus on the optimal power allocation for uplink transmission in two-tier femtocell networks and take into account the different service requirements and design objectives of macrocell user equipment (MUE) and femtocell user equipment (FUE) devices. The framework of hierarchical game with a multiple-leader-multiple-follower model is adopted to investigate the uplink power allocation problem. By using hierarchical game, on one hand, the utilities of both MUE and FUE devices are maximized; on the other hand, the uplink protection of MUE devices is highlighted. To obtain the game equilibrium (GE) distributively, we develop the iterative power update rules for both MUE and FUE devices. Moreover, given channel uncertainty, a robust hierarchical game is formulated and solved distributively. For implementations, the running process of the proposed hierarchical games is analyzed in detail. Finally, numerical results show the convergence of the hierarchical games without and with channel uncertainty. Moreover, it is also demonstrated that, in either case, the GE is unique and that the game is effective.

Distributed Resource Allocation for Femtocell Networks: Regret Learning with Proportional Self-belief

Femtocell networks promise improvement in network quality and performance for dense wireless networks, but will suffer from inter-cell interference if resource management is not properly employed. This paper presents distributed joint resource allocation (sub-channel and power) to address co- and cross-tier interference issues in two-tier heterogeneous femtocell networks. Due to uncoordinated nature of femtocell base stations (HeNB) deployment, the interactions among self-interested HeNBs are formulated using game-theoretical tools. Then, we designed individual utility function for every HeNB in order to enforce cooperative behaviour among HeNBs as well as to avoid cross-tier interference towards macrocell user equipments within HeNB coverage. Based on the designed utility function, we propose a fully distributed adaptive learning algorithm with a proportional self-belief concept that can lead to correlated equilibrium with fast and decisive convergence. Finally, performance analysis on the proposed algorithm done in simulated environment showed positive results indicating improvements in terms of co- and cross-tier interference mitigation as compared to generic regret-based learning scheme and utility functions.

Research on user pairing algorithm for LTE femtocell uplink

In this article, a new simulated annealing based bit error rate pairing scheduling (SA-BPS) algorithm for LTE femtocell uplink virtual multiple input multiple output (V-MIMO) systems is proposed, which uses Round Robin (RR) criterion to decide the first user, and simulated annealing (SA) algorithm with objective function of bit error rate (BER) is suggested to decide the pairing users. The SA-BPS is considered not only to be used for inner layer user pairing within femtocell user equipments (HUEs) to increase spectrum efficiency, but also to be used for cross layer user pairing between HUEs and macrocell user equipments (MUEs) to cancel the cross layer interference from MUEs to HUEs. As a growth control coefficient is used in SA-BPS to control the growth of average BER of users in the pairing group, rapid user BER deterioration can be prevented by adopting a proper coefficient. Simulation results show that, the SA-BPS outperforms RPS, DPS, and the conventional SA algorithm in terms of BER performance, whether used for inner layer user pairing or cross layer user pairing. Also, the mode switching signal to interference ratio (SIR) threshold between inner and cross layer user pairing are obtained by simulation in three cross layer interference power distribution scenarios.

Dynamic Cell Range Expansion-based interference coordination scheme in next generation wireless networks

Deploying Picocell Base Station (PBS) throughout a Macrocell is a promising solution for capacity improvement in the next generation wireless networks. However, the strong received power from Macrocell Base Station (MBS) makes the areas of Picocell narrow and limits the gain of cell splitting. In this paper, we firstly propose a Dynamic Cell Range Expansion (DCRE) strategy. By expanding the coverage of the cell, we aim to balance the network load between MBS and PBS. Then, we present a cooperative Resource block and Power Allocation Scheme (coRPAS) based on DCRE. The objective of coRPAS is to decrease interference caused by MBS and Macrocell User Equipments, by which we can expand regions of Picocell User Equipments. Simulation results demonstrate the superiority of our method through comparing with other existing methods.

Dynamic MIMO Precoding for Femtocell Interference Mitigation

This paper studies interference mitigation in heterogeneous cellular networks consisting of traditional macrocells and newly envisioned femtocells. The mutual interference between macrocells and femtocells arises as a result of decentralized femtocell deployment and backhaul delay. To mitigate downlink interference between the femtocell clients, known as Home User Equipments (HUEs), and macrocell clients, known as Macrocell User Equipments (MUEs), we present methods of dynamic distributed beamforming that are fully compatible with MIMO precoding mechanisms in existing LTE standard releases. We develop three MIMO beamforming schemes for interference mitigation that take into account the Quality of Service (QoS) requirement of both femtocell and macrocell clients. These new heterogeneous MIMO precoding strategies improve flexibility in resource provisioning and signaling requirement while responding to different QoS needs. We also present MUE mean throughput analysis by applying order statistics to our proposed methods. Moreover, we provide an approximate closed form for the mean throughput in terms of basic transmitter, channel, and receiver parameters. Furthermore, we extend our proposed interference control precoding schemes to spatial multiplexing for MIMO transmissions. Finally, we extend our solution to tackle the more general case involving multiple MUEs, multiple HUEs, and multiple femtocells.

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

SummaryTwo‐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.

Resource Allocation in Two-Tier Heterogeneous Networks through Enhanced Shadow Chasing

This work studies femtocell resource allocation in shared spectrum heterogeneous networks. We propose an enhanced shadow chasing method for femtocell resource allocation to achieve interference mitigation in heterogeneous networks serving both Macrocell User Equipments (MUEs) and Home User Equipments (HUEs). Shadow chasing home eNB (HeNB) uses Downlink Control Information (DCI) together with over-heard macrocell user ACK/NAK feedbacks and CQI reports to assign its own downlink resources to mitigate downlink interference to MUEs. Since the HeNB receives outdated DCI due to backhaul delay, we derive a likelihood metric for each resource unit being either empty or assigned to a (low-interference) outdoor MUE based on a finite-state Markov chain model for each resource unit. By dynamically separating MUE and HUE assignments, the enhanced shadow chasing can better control the downlink interference to MUEs for QoS assurance. It effectively reduces the probability of resource collision and MUE interference compared to schemes not considering backhaul delay effect or user feedbacks.

Backhaul-Aware Interference Management in the Uplink of Wireless Small Cell Networks

The design of distributed mechanisms for interference management is one of the key challenges in emerging wireless small cell networks whose backhaul is capacity limited and heterogeneous (wired, wireless and a mix thereof). In this paper, a novel, backhaul-aware approach to interference management in wireless small cell networks is proposed. The proposed approach enables macrocell user equipments (MUEs) to optimize their uplink performance, by exploiting the presence of neighboring small cell base stations. The problem is formulated as a noncooperative game among the MUEs that seek to optimize their delay-rate tradeoff, given the conditions of both the radio access network and the -- possibly heterogeneous -- backhaul. To solve this game, a novel, distributed learning algorithm is proposed using which the MUEs autonomously choose their optimal uplink transmission strategies, given a limited amount of available information. The convergence of the proposed algorithm is shown and its properties are studied. Simulation results show that, under various types of backhauls, the proposed approach yields significant performance gains, in terms of both average throughput and delay for the MUEs, when compared to existing benchmark algorithms.

Self-Organization in Small Cell Networks: A Reinforcement Learning Approach

In this paper, a decentralized and self-organizing mechanism for small cell networks (such as micro-, femto- and picocells) is proposed. In particular, an application to the case in which small cell networks aim to mitigate the interference caused to the macrocell network, while maximizing their own spectral efficiencies, is presented. The proposed mechanism is based on new notions of reinforcement learning (RL) through which small cells jointly estimate their time-average performance and optimize their probability distributions with which they judiciously choose their transmit configurations. Here, a minimum signal to interference plus noise ratio (SINR) is guaranteed at the macrocell user equipment (UE), while the small cells maximize their individual performances. The proposed RL procedure is fully distributed as every small cell base station requires only an observation of its instantaneous performance which can be obtained from its UE. Furthermore, it is shown that the proposed mechanism always converges to an epsilon Nash equilibrium when all small cells share the same interest. In addition, this mechanism is shown to possess better convergence properties and incur less overhead than existing techniques such as best response dynamics, fictitious play or classical RL. Finally, numerical results are given to validate the theoretical findings, highlighting the inherent tradeoffs facing small cells, namely exploration/exploitation, myopic/foresighted behavior and complete/incomplete information.

On the performance of shared access control strategy for femtocells

Femtocells can be employed in cellular systems to enhance the indoor coverage, especially in the areas with high capacity growing demands and high traffic rates. In this paper, we propose an efficient resource utilization protocol, named as shared access protocol (SAP), to enable the unauthorized macrocell user equipment to communicate with partially closed-access femtocell base station to improve and enhance the system performance.The system model considers a femtocell that is equipped with a total of N separated antennas or channels to multiplex independent traffic. Then, a set of N1 channels is used for closed access only by the authorized users, and the remaining set of channel resources can be used for open access by either authorized or unauthorized users upon their demands and spatial locations. For this system model, we obtain the signal-to-interference ratio characteristics, such as the distribution and the moment generating function, in closed forms for two fading models of indoor and outdoor environments. The signal-to-interference ratio statistics are then used to derive some important performance measures of the proposed SAP in closed form, such as the average bit error rate, outage probability, and average channel capacity for the two fading models under consideration. Numerical results for the obtained expressions are provided and supported by Monte Carlo simulations to validate the analytical development and study the effectiveness of the proposed SAP under different conditions. Copyright © 2013 John Wiley & Sons, Ltd.

차세대 이동통신 시스템에서 동기신호를 이용한 펨토셀 탐색 기법

본 논문에서는 co-channel deployment 방식을 사용하는 closed access 펨토셀 주변에서 마크로셀 단말의 동기소실 문제를 해결하고 펨토셀의 전력 누수 때문에 발생하는 셀 탐색을 줄이기 위하여 CS(Common Signal)을 사용한 펨토셀 탐색 기법을 제안한다. CS는 특정 마크로셀 내의 펨토셀들이 같은 시간에 공통으로 전송하는 신호로, 펨토셀이 CS를 송신할 경우 마크로셀 내에서 마크로셀 기지국의 CINR(Carrier to Interference Noise and Ratio) 값이 크게 유지되어 closed access 펨토셀 주변에서 마크로셀 단말이 동기를 유지할 수 있다. 또한, CS는 단말이 CS를 사용하여 측정한 CSCINR(Common Signal Carrier to Interference Noise and Ratio) 값을 이용하여 펨토셀 존재 여부를 판단할 수 있도록 설계되므로, 마크로셀 단말은 마크로셀 기지국에 동기를 유지하면서 펨토셀 존재여부를 판단할 수 있다. 그리고 제안된 방식에서는 옥외에서 이동 중인 단말이 자신의 이동성을 이용하여 펨토셀 탐색 여부를 판단함으로써 단말의 펨토셀 탐색 빈도를 줄인다. 따라서 펨토셀 탐색에 대한 부담을 줄이고, 이에 대한 부가적 이득으로 핸드오버 시도의 빈도 감소를 기대할 수 있다. In this paper, we propose a femtocell searching technique which can prevent a macrocell UE(user equipment) from losing synchronism to its serving macrocell near closed access femtocells in co-channel deployment due to the leakage of femtocell signals by using a CS(Common Signal). The CS, commonly transmitted by femtocells in a macrocell at the same time, enables the macrocell UEs to be kept synchronized with their serving macrocells since the CINR(Carrier to Interference and Noise Ratio) of base stations in macrocell can be kept high even near closed access femtocells. Also, the CS is designed in such a way that a macrocell UE can recognize the existence of femtocell by using the metric CSCINR(Common Signal Carrier to Interference and Ratio) measured with CS. In addition, the proposed femtocell searching technique can reduce the frequency of femtocell searching trial by using the metric on mobility of a macrocell UE defined in this paper, and the reduction of the frequency of handover trial can be also expected as a byproduct.

QoS-Aware and Energy-Efficient Resource Management in OFDMA Femtocells

We consider the joint resource allocation and admission control problem for Orthogonal Frequency-Division Multiple Access (OFDMA)-based femtocell networks. We assume that Macrocell User Equipments (MUEs) can establish connections with Femtocell Base Stations (FBSs) to mitigate the excessive cross-tier interference and achieve better throughput. A cross-layer design model is considered where multiband opportunistic scheduling at the Medium Access Control (MAC) layer and admission control at the network layer working at different time-scales are assumed. We assume that both MUEs and Femtocell User Equipments (FUEs) have minimum average rate constraints, which depend on their geographical locations and their application requirements. In addition, blocking probability constraints are imposed on each FUE so that the connections from MUEs only result in controllable performance degradation for FUEs. We present an optimal design for the admission control problem by using the theory of Semi-Markov Decision Process (SMDP). Moreover, we devise a novel distributed femtocell power adaptation algorithm, which converges to the Nash equilibrium of a corresponding power adaptation game. This power adaptation algorithm reduces energy consumption for femtocells while still maintaining individual cell throughput by adapting the FBS power to the traffic load in the network. Finally, numerical results are presented to demonstrate the desirable operation of the optimal admission control solution, the significant performance gain of the proposed hybrid access strategy with respect to the closed access counterpart, and the great power saving gain achieved by the proposed power adaptation algorithm.

Femtocell Sleep Mode Activation based Interference Mitigation in Two-tier Networks

Abstract The deployment of femtocells in conventional macrocells is expected to solve the indoor coverage problems in a cost effective manner. However the key requirement for efficient co-channel macro/femto cell deployment is the suppression of interference. This paper proposes a sleep mode activation procedure for femtocell base stations based on the interference mitigation for macrocell user equipments (MUEs) results of the system level simulation with realistic LTE-A system parameters have shown that the proposed procedure improves the signal quality for the MUEs and reduces the number of blocked MUEs up to 60% compared to no sleep mode scenario. Moreover, the algorithm presents power consumption saving opportunities of approximately 55% in femtocell networks with respect to no sleep mode activation scenario.

Distributed Pareto-Optimal Power Control for Utility Maximization in Femtocell Networks

This paper proposes two Pareto-optimal power control algorithms for a two-tier network, where newly-deployed femtocell user equipments (FUEs) operate in the licensed spectrum owned by an existing macrocell. Different from homogeneous network settings, the inevitable requirement of robustly protecting the quality-of-service (QoS) of all prioritized macrocell user equipments (MUEs) here lays a major obstacle that hinders the successful application of any available solutions. Directly targeting at this central issue, the first algorithm jointly maximizes the total utility of both user classes. Specifically, we adopt the log-barrier penalty method to effectively enforce the minimum signal-to-interference-plus-noise ratios (SINRs) imposed by the macrocell, paving the way for the adaptation of load-spillage solution framework. On the other hand, the second algorithm is applied to the scenario where only the sum utility of all FUEs needs to be maximized. At optimality, we show that the MUEs' prescribed SINR constraints are met with equality in this case. With the search space for Pareto-optimal SINRs substantially reduced, the second algorithm features scalability, low computational complexity, short converging time, and stable performance. We prove that the two developed algorithms converge to their respective global optima, and more importantly, they can be implemented in a distributive manner at individual links. Effective mechanisms are also available to flexibly designate the access priority to MUEs and FUEs, as well as to fairly share radio resources among users. Numerical results confirm the merits of the devised approaches.