CoSFR: coordinated soft frequency reuse for OFDMA-based multi-cell networks with non-uniform user distribution

Inter-cell interference coordination (ICIC) technology has been extensively studied to improve service quality of users near cell boundaries. Most ICIC schemes available make an assumption that users in the same cell are distributed uniformly. This is reasonable but incomplete. Mobility may result in users distributed non-uniformly, so this paper proposes a novel semi-static ICIC scheme, coordinated soft frequency reuse (CoSFR), for multi-cell networks with non-uniform user distribution. A finer cell partition structure with a tunable parameter r is proposed. With this structure, dedicated user classification rules and opportunistic scheduling strategies are designed for different cells. Assisted by a simple but efficient coordination scheme, a cell with overloaded cell-edge traffic, named aggressor cell, calls adjacent neighboring cells for help. Some reuse opportunities can be created after coordination and the aggressor cell can expand its cell-edge band to alleviate the overloaded state. No additional entity is required for being compatible with the flat network structure of LTE/LTE-Advanced. In addition, CoSFR can deal with the scenario that more than one cell suffering overloaded cell-edge traffic and it can also be applied to irregular cells with minimal modifications. Simulation results show that more than 97.4 % users are satisfied with their data rate. Average cell-edge user throughput is raised substantially and the outage probability declines significantly.     

Resource Allocation with Partitioning Criterion for Macro-Femto Overlay Cellular Networks with Fractional Frequency Reuse

The interference mitigation technique based on fractional frequency reuse (FFR) provides improved cell-edge performance with similar overall cell capacity as that of systems with the frequency reuse factor of one. Furthermore, frequency sub-band allocation by FFR has the benefit of allowing flexibility for the deployment of femto-cells through frequency partitioning. Determination of a proper frequency partitioning criterion between the cell-center and the cell-edge, and between the cells with femto-cells is an important issue. In addition, time resource partitioning introduces another degree of freedom to the design of time-frequency resource allocation. In this paper, we propose a novel time-frequency resource allocation mechanism using FFR for a macro-femto overlay cellular network. Feasible frequency sub-band and time resource is allocated to the cell-center and the cell-edge region in a cell by the proposed partitioning criterion and the time partitioning ratio. We provide a guideline for how to determine the partitioning criterion for the regions and how to design the amount of time resource. We derive the average capacity of macro-cells and femto-cells, and introduce a new harmonic mean metric to maximize the average capacity of the regions while achieving the fairness among users in a cell.     

Multi-beam cooperative frequency reuse for coordinated multi-point transmission

Coordinated multi-point (CoMP) joint transmission is considered in the 3rd generation partnership project (3GPP) long term evolution (LTE)-advanced as a key technique to mitigate inter-cell interference and improve the cell-edge performance. To effectively apply CoMP joint transmission, efficient frequency reuse schemes need to be designed to support resource management cooperation among coordinated cells. However, most of the existing frequency reuse schemes are not suitable for CoMP systems due to not considering multi-point joint transmission scenarios in their frequency reuse rules. In addition, the restrictions of frequency resources in those schemes result in a high blocking probability. To solve the above two problems, a multi-beam cooperative frequency reuse (MBCFR) scheme is proposed in this paper, which reuses all the available frequency resources in each sector and supports multi-beam joint transmission for cell-edge users. Besides, the blocking probability is proved to be efficiently reduced. Moreover, a frequency-segment-sequence based MBCFR scheme is introduced to further reduce the inter-cell interference. System level simulations demonstrate that the proposed scheme results in higher cell-edge average throughput and cell-average throughput with lower blocking probability.     

On the need for dynamic downlink intercell interference coordination for realistic Long Term Evolution deployments

Intercell interference is the main issue limiting the capacity of modern orthogonal frequency‐division multiple access based cellular networks. Recently, extensive research work has been carried out in this field, and intercell interference coordination techniques have been recognized as key enablers of current (and future) cellular technologies. In this article, (i) a comprehensive survey of the most representative contributions is provided together with (ii) a generic methodology to measure their actual merit. The performance of several interference avoidance strategies has been evaluated both from system and user point of view in the context of a Long Term Evolution (LTE)‐based network considering not only synthetic cellular scenarios but also realistic deployments. Our literature review indicates that there is a need for adaptive/operator‐customizable low‐complex intercell interference coordination (ICIC) schemes suitable for realistic LTE deployments. Results obtained by means of a comprehensive set of simulations corroborate and support this premise. In this article, it is shown that simultaneous gains in terms of spectral/energy efficiency and fairness can be achieved through dynamic mechanisms with respect to both classic hard reuse schemes and static ICIC techniques. Besides numerical results, a novel merit assessment methodology based on several weighted performance metrics is proposed. Our findings show that dynamic schemes outperform static techniques by around 20–35% in realistic deployments. Copyright © 2012 John Wiley & Sons, Ltd.     

Semi‐distributed dynamic inter‐cell interference coordination scheme for interference avoidance in heterogeneous networks

Inter‐cell interference (ICI) is a major problem in heterogeneous networks, such as two‐tier femtocell (FC) networks, because it leads to poor cell‐edge throughput and system capacity. Dynamic ICI coordination (ICIC) schemes, which do not require prior frequency planning, must be employed for interference avoidance in such networks. In contrast to existing dynamic ICIC schemes that focus on homogeneous network scenarios, we propose a novel semi‐distributed dynamic ICIC scheme to mitigate interference in heterogeneous network scenarios. With the goal of maximizing the utility of individual users, two separate algorithms, namely the FC base station (FBS)‐level algorithm and FC management system (FMS)‐level algorithm, are employed to restrict resource usage by dominant interference‐creating cells. The distributed functionality of the FBS‐level algorithm and low computational complexity of the FMS‐level algorithm are the main advantages of the proposed scheme. Simulation results demonstrate improvement in cell‐edge performance with no impact on system capacity or user fairness, which confirms the effectiveness of the proposed scheme compared to static and semi‐static ICIC schemes.     

中继蜂窝网络中小区间和小区内的资源分割

文章提出了一种应用于固定中继蜂窝网络中基于复用分割的小区间频谱分配框架,通过引入有效复用因子这一指标来优化小区内基站和中继之间的资源分割,并比较了不同的链路带宽分配方案对系统性能的影响。理论分析和仿真结果显示,该方案可以在保证小区边缘用户的吞吐量的同时提高小区容量。     

A Simulation Framework for Evaluating Interference Mitigation Techniques in Heterogeneous Cellular Environments

Femtocells present an attractive solution for the improvement of a mobile network’s services providing better data rates and coverage. Since their deployment results to a heterogeneous network where two layers must utilize the available spectrum, issues of interference arise. A method to address this challenge, is investigating the locations of the newly installed FBS, and enforcing a power controlled transmission of all FBSs that achieves optimal and fair overall performance. Another option that becomes available in inter-cell interference cancellation (ICIC) macrocell environments, is utilizing the available spectrum to complete or partly avoid co-channel operation. In this work, we provide a simulation framework that allows the creation of custom, high configurable, user defined topologies of femtocells with power control and frequency allocation capabilities. It allows the investigation of the margin of improvement in interference when these methods are applied and may work as a decision tool for planning and evaluating heterogeneous networks. To showcase the framework’s capabilities, we evaluate and study the behaviour of custom deployed femtocells/macrocells networks and examine the cross-tier interference issues. Facilitated by the framework, we enforce and evaluate each interference mitigation technique for different femtocells’ deployment densities. Finally, we compare the results of each method in terms of total throughput, spectral efficiency and cell-edge users’ performance.     

OFDMA系统静态软频率复用技术研究

在OFDMA系统中,小区间同频干扰是主要干扰。在B3G/4G系统中,为了减少小区间干扰,静态软频率复用技术已得到了广泛的应用。介绍了2种静态软频率复用方案:经典软频率复用和基于功率重用的软频率复用。在此基础上,详细研究了使用改进型比例公平调度算法的功率重用软频率复用系统。通过仿真,分析并证实了静态软频率复用技术可对边缘用户吞吐量带来极大提升,同时给出并验证了系统中改进型比例公平调度器的最佳配置参数。     

Downlink Radio Resource Management Through CoMP and Carrier Aggregation for LTE-Advanced Network

Long Term Evolution-Advanced (LTE-A) offers several new technologies to improve the performance of the user. However, poor received signal and interference from adjacent cells in the cell-edge area can reduce the efficiency of using individual technology. Therefore, the cell-edge users have lower throughput compared to the other users in the cell and LTE-A standard. An efficient downlink radio resource management scheme is proposed in this paper by combining the coordinated multipoint transmission and reception technique along with carrier aggregation technique to achieve higher throughput for the cell-edge user and better overall performance. The proposed method jointly transmits multiple component carriers to the cell-edge user from different cells to increase the bandwidth, strengthen the received signal, and reduce the interference while it satisfies several constraints. Modified largest weighted delay first packet scheduling algorithm is deployed for resource allocation, which takes into account the delay parameters, the probability of packet loss, and data rates of the user. The obtained system-level simulation results show that the proposed method significantly enhances the throughput performances, spectral efficiency, and fairness index, compared with the existing conventional methods.

     

多小区OFDMA系统中分数频率复用方法的优化分析

在以OFDMA为基本多址方式的第四代移动通信系统中,频率复用作为提高系统性能的有效方法受到了广泛的关注,特别是分数频率复用和软频率复用。然而,如何进一步优化基于这些频率复用方法的OFDMA系统还有待研究。本文分析了多小区OFDMA系统负载和频率集合分配比例对小区总吞吐量和小区边缘用户数据速率的影响。同时,给出了在保证小区边缘用户服务质量的条件下,使小区总吞吐量最优的小区中心频率分配比例,并用仿真进行验证。仿真结果还表明了,随着分配比例的增大,小区总吞吐量先增大后减小。     

Frequency Reuse Techniques with a Distributed Antenna System

The performance of cellular networks is strongly limited by inter-cell interference. In order to reduce this interference, several techniques have been proposed, e.g., the frequency reuse techniques and distributed antenna system (DAS). This paper investigates the combinations of hard frequency reuse (HFR) and soft frequency reuse (SFR) techniques with DAS in a unique cell architecture, which are called DAS–HFR and DAS–SFR, respectively. This paper analytically quantifies the performance of the downlink multi-cell for DAS–HFR and DAS–SFR in terms of the average spectral efficiency. This also shows, the most appropriate frequency reuse technique depends not only on the average achievable data rate inside the cell, but also on the guaranteed achievable data rate (the minimum achievable data rate which is necessary to be obtained regardless of geographic location). The results show that DAS–SFR improves the achievable data rate of cell edges in a multi-cell environment as compared to a DAS–HFR when frequency reuse factor 1 is utilized. The results also show that DAS–SFR significantly increases the system capacity as compared to the DAS–HFR when frequency reuse factor 3 is utilized.     

大规模MIMO系统中基于图着色的半动态导频分配

针对大规模MIMO系统中存在的导频污染问题,提出了一种基于图着色的半动态导频分配方案。首先,推导出各个小区可以复用导频的用户与其服务基站的最大距离门限。然后,根据该门限值将系统内各小区的用户分为小区中心用户和小区边缘用户;允许各个小区门限值之内的用户（小区中心用户）复用相同的导频;最后,基于图着色理论协同分配各个小区的小区边缘用户的导频,达到降低导频干扰的目的。仿真结果显示,该方案减少了用户间的导频干扰,有效地提升了整个系统的上行可达和速率。     

TD—LTE系统频率复用仿真分析

主要根据小区间干扰协调的原理,分别从上行和下行的角度探讨ICIC的三种实现方式和各自的应用效果,分析LTE系统所采用的技术和OFDM的技术特点,说明了频率复用对于LTE系统的重要性。从干扰协调的角度对干扰与容量损失的关系、干扰余量的取定等做了定量的分析和计算,并给出了干扰协调设计要点的总结。给出了针对于不同频率复用系数下SINR的计算方法。实验表明,该分析具有一定的现实意义。     

Selective interference alignment and neutralization in coordinated multipoint using multiuser MIMO

In wireless communication, the concept of coordinated multipoint (CoMP) transmission is more attractive, and it transpired the notion based on interference management techniques. Interference alignment (IA) and interference neutralization (IN) methods can substantially align and neutralize the interference signals. The existing work in the CoMP transmission using multiuser multi-input and multi-output (MU-MIMO) had long-held problems such as specific limit of intercell (ICI) and cochannel interference (CCI) cancelation that contain low performance in cell-edge users. The proposed framework of the transmission signal in selective interference alignment and neutralization (SIAN) CoMP MU-MIMO system transmits multiple data streams in multipath by using downlink coordination between base stations and receiver side. This work contributes the individual perspectives to implement the IA and IN to align and cancel the interfered signals at the receiver side. Once the perspectives mentioned above are executed, zero-forcing (ZF) and rechanneling filter are applied on the receiver side to eliminate residual ICI. In addition, spectral efficiency significantly improves the achievable data rate and enhances the performance of cell-edge users and reduces the CCI at the receiver side. Furthermore, the antenna configuration signals are decoded to get the exact version of interference-free signals with null path loss of signal transmission. The effectiveness of the proposed framework analyzes and verifies the numerical evaluation of the achievable degree of freedom. Finally, the simulation demonstrates the comparison of the proposed CoMP scheme with MIMO ZF and non-CoMP schemes, which significantly improve the performance of spectral efficiency for cell-edge users.     

Improving the capacity in wireless networks through integratedchannel base station and power assignment

The limited availability of radio frequency spectrum will require future wireless systems to use more efficient and sophisticated resource allocation methods to increase network capacity. In this work, we propose a joint resource allocation algorithm (JRAA) that makes the channel base station and power assignment in a wireless network with an arbitrary number of base stations and mobiles attempting to minimize the number of channels needed to provide each user in the system with an acceptable radio connection. We compare the performance of the JRAA for both the forward (downstream) and reverse (upstream) directions, in terms of the achievable traffic capacity, with some bounds on the performance of the maximum packing (MP), clique packing (CP), and reuse partitioning (RP) techniques, which are usually used as benchmarks on the capacity that can be achieved by any traffic-adaptive dynamic channel assignment strategy, where the quality is guaranteed by the reuse distance. Those performance results verify the improvement that can be achieved by the integration of the channel base station and power assignment. Finally, several versions of the two-way channel assignment problem are studied and evaluated     

Load-adaptive frequency reuse scheme for inter-cell interference coordination in relay networks

Cellular relay networks adopting orthogonal frequency division multiple(OFDM)technology has been widely accepted for next generation wireless communication due to its advantage in enlarging coverage scale as well as improving data rate.In order to improve the performance of user equipments(UEs)near the cell edge,especially to avoid the interference from inter-cell and intra cell,an enhanced soft frequency reuse scheme is adopted in this paper to assure inter-cell interference coordination(ICIC).Compared with traditional frequency allocation work,the proposed scheme is interference-aware and load-adaptive,which dynamically assigns available frequency among UES under certain schedule method in variable traffic load condition and mitigates interference using information provided by interference indicator.It can improve signal-to-interference plus noise ratio(SINR)of the UE in each sub channel thus enable the system achieve better throughput and blocking probability performance.Simulation results prove that the proposed scheme may achieve desirable performance on throughput,blocking probability and spectral utilization in the sector under different traffic load compared with other schemes.     

Biorthogonal Frequency Division Multiple Access Cellular System with Angle Division Reuse Scheme

In conventional OFDMA cellular systems, mobile stations (MSs) suffer from large ICI in fully loaded cellular environments with full cell frequency reuse, especially at the cell-edge. The fundamental cause is that the signals from serving Base Station (BS) and interference BSs, are modulated by same exponential bases, at same subcarrier. In this paper, a generalized low-complexity fractional Fourier transform (FrFT) based biorthogonal frequency division multiple access (B-OFDMA) cellular system with multiple angle division reuse scheme (MADR) scheme for inter-cell interference (ICI) cancellation is proposed. FrFT angle is regarded as a kind of time-frequency combination resource (TFCR), and it can be optimally allocated to each BS of the cellular system, based on simplified minimal base correlation coefficient (MBCC) criteria, which confirms the inner-cell mutual orthogonality between modulating bases at different subcarriers, and inter-cell mutual approximate orthogonality between modulating bases at same subcarriers. Therefore, at the receiver, ICI can be dramatically suppressed by MMSE equalization and correlative detection in respective optimal FrFT domain. Extensive system simulations are conducted for various practical scenarios to demonstrate the superior performance of the proposed FrFT MADR scheme in bit error rate (BER) and system throughput, especially for cell-edge MSs, compared with conventional OFDMA cellular with different ICI cancellation schemes and scheduling schemes.     

Centralized dynamic frequency allocation for cell-edge demand satisfaction in fractional frequency reuse networks

Fractional frequency reuse (FFR) has emerged as a well-suited remedy for inter-cell interference reduction in the next-generation networks by allocating frequency reuse factor (FRF) of unity for the cell-center (CC) and higher FRF for the cell-edge (CE) users. However, this strict FFR comes at a cost of equal partitioning of frequency resources to the CE which most likely has varying demands in current networks. In order to mitigate this, we propose a centralized dynamic resource allocation scheme which allocates demand-dependent resources to CE users. The proposed scheme therefore outperforms the fixed allocation scheme of strict FFR for both CC and CE users. Complexity analysis provides a fair means of analyzing the suitability of proposed algorithm. We have also compared the proposed methodology with a reference dynamic fractional frequency reuse (DFFR) scheme. Results show maximum performance gain of up to 30% for 3 reference cells employing Rayleigh fading—through normalized area spectral efficiency (ASE) analysis for both fixed allocation and DFFR. Spectral efficiency analysis also indicates per-cell performance gain for both CC and CE users. Further, detailed three-dimensional ASE plots give insights into the affects to other cells. Due to dynamic nature of traffic loads, the proposed scheme is a candidate solution for satisfying the demands of individual cells.     

QoS Guaranteed Resource Allocation Scheme for Cognitive Femtocells in LTE Heterogeneous Networks with Universal Frequency Reuse

In this paper, we propose a novel resource allocation scheme for co-channel interference avoidance in LTE heterogeneous networks with universal spectrum reuse where both macro users (MUs) and cognitive femto base stations (FBSs) within the same macrocell coverage can dynamically reuse whole spectrum. Specifically, resource blocks (RBs) are shared between cognitive FBSs in underlay mode while the resource sharing among FBSs and MUs is in overlay mode. The macrocell is divided into inner and outer regions with the inner region further divided into three sectors. The proposed scheme addresses co-channel interference (CCI) by employing fractional frequency reuse (FFR) for RB allocation in the outer region of the macrocell and increase the distance of users that reuse the same RB within the macrocell. Part of RBs are allocated to the outer region of the macrocell with a FFR factor of 1/3, while the remaining RBs are dynamically allocated to each sector in the inner region of macrocell based on MUs demand to efficiently utilize the available spectrum. A basic macro base station (MBS) assistance is required by the FBS in selection of suitable RB to avoid interference with MU in each sector. With the proposed solution, both macro and femto users can dynamically access the whole spectrum while having minimum bandwidth guarantee even under fully congested scenarios. Moreover, the proposed scheme practically eliminates the cross-tier interference and the CCI problem in heterogeneous network reduces to inter-femtocell interference. The throughput and outage performances of the proposed scheme are validated through extensive simulations under LTE network parameters. Simulation results show that the proposed scheme achieves a performance gain of more than 1.5 dB in terms of SINRs of both macro user and femto user compared to traditional cognitive and non-cognitive schemes without bandwidth guarantee for femtocells.     

Multiple Connectivity and Spectrum Access Utilisation in Heterogeneous Small Cell Networks

In the context of heterogeneous and small cell networks, users will have the possibility to connect to multiple radio access (RA) carriers that will be available by a dense deployment of RA infrastructure consisting of high-power and low-power access nodes. Determining which RAs a user should be associated with and select from, for its downlink transmissions, depends on the long-term and short-term data rates that these RAs may offer to the user. In this study the multi-RA association and utilisation is decomposed into a multi-RA to user association problem that assigns multiple RAs to users, and a multi-RA selection problem that determines which of the assigned RAs should be used at any time for the user transmissions. As a solution to the first problem, we propose a distributed dual-based spectrum access scheme (DSA) that considers multi-connectivity, whilst, the second problem is solved by means of a heuristic multi-RA selection scheme that utilise different multi-radio transmit diversity (MRTD) schemes while taking into account different inter-cell interference coordination (ICIC) schemes. Our two-step approach is evaluated by means of simulations which demonstrate cell-edge user throughput performance improvements that exceed 100 % when the multi-connectivity DSA is employed. Further significant user rate and energy efficiency improvements up to 69 and 38 % respectively can be achieved when MRTD is combined with ICIC.