Enhancing cell-edge performance: a downlink dynamic interference avoidance scheme with inter-cell coordination

LTE uses single carrier-frequency division multiple access (SC-FDMA) scheme for the uplink transmission which limits the flexibility of the allocation process as the sub carriers assigned to a single user have to be contiguous. We propose an LTE uplink interference-aware search-tree based scheduling scheme using the high interference indicator (HII) and the overload indicator (OI) messages that are exchanged over the X2-interface. HII indicates the potential uplink inter-cell interference (ICI) to other cells while OI indicates the detection of low, medium or high uplink ICI from other cells. The exchanged HII and OI messages are used to scale down the proportional-fair (PF) scheduling metric before being processed by the search-tree scheduler to make interfered physical resource blocks (PRBs) less favorable to cell-edge (CE) UEs. Unlike previous work which handles these problems separately, the proposed integrated RRM framework jointly takes into account 1) interference information exchanged between neighbor eNBs, 2) constraints on max power per UE and equal power on different PRBs, and 3) constraints on the contiguity in PRBs allocation due to SC-FDMA mechanism. It is also fully integrated with the LTE uplink open and closed-loop fractional power control for proper power allocation. The proposed interference-aware scheduler reduces the effect of the generated ICI mainly on CE users and also manages to provide fair resource sharing between CE and cell-center (CC) users. Simulation results show that the proposed framework improves CE users' throughput by around 5% compared to soft frequency reuse (SFR). It also reduces the probability of not being served to 1% -- compared to 10% in SFR.

Effective inter-cell interference mitigation has been extensively studied because of its outstanding cell-edge signal quality improvement capability. Conventional static inter-cell interference coordination strategies, including fractional frequency reuse and soft frequency reuse, have received much attention owing to their effectiveness in mitigating interference and low complexity in implementation. However, they are less effective when dealing with dense uneven traffic distributions and dynamic traffic demands and thus incur low spectrum utilization in some cells and spectrum shortage in others. This paper proposes a universal frequency reuse scheme in a two-layer Long Term Evolution-Advanced heterogeneous network to ensure good throughput for all user equipment (UE), especially UEs at cell edge. The proposed scheme allows each cell to use all the spectrum resources, limited by an orderly regulation of all sub-bands. This scheme minimizes the potential occurrence probability of inter-cell co-sub-band interference through an intra-cell sub-band resource management. Furthermore, a graph-theoretic based sub-band allocation algorithm is developed to optimize UE throughput performance, especially for the cell-edge low signal to interference noise ratio UEs. A comprehensive performance comparison among different frequency reuse schemes is conducted by considering performance metrics, including cell-edge throughput, average throughput, and signal to interference noise ratio cumulative distribution function. Simulation result shows that the universal frequency reuse scheme outperforms other two schemes significantly. Copyright © 2016 John Wiley & Sons, Ltd.

Indoor coverage and capacity are the major limitations of the current cellular systems. In such perspective, femtocells deployment has attracted considerable interest. However, the random and co-channel deployment of femtocells poses severe interference management challenges, especially at the cell edges. Recently, Fractional Frequency Reuse (FFR) has emerged an attractive technique for co-tier and cross-tier interference in multi-tier cellular deployment. However, most of the previous work analyzes the performance of FFR for perfect cell geometry models. Applying regular resource of the standard FFR scheme to the realistic cellular networks with high irregularity in the cell geometry and channel conditions leads to sub-optimal performance. In this paper we investigate the performance of FFR for two-tier cellular network based on irregular cell geometry. To capture the realistic network scenario the position of Macro Base Stations (MBSs) and Femto Base Station (FBSs) are modeled with Hard Core Point Process (HCPP) and Poisson Point Process (PPP) respectively. Furthermore, Sectored-FFR scheme with optimal sub-band allocation is proposed in this paper and the capacity of the system is evaluated. Simulation results show that the proposed Sectored-FFR scheme outperforms the conventional FFR schemes in terms of throughput and capacity.

Interference mitigation has been identified as a key challenge for emerging cellular technologies based on Orthogonal Frequency Division Multiple Access, such as Long Term Evolution. In this context, static intercell interference coordination including Fractional Frequency Reuse (FFR) have been adopted by mobile operators as a good alternative to improve the quality of service at cell edges. Nevertheless, recent results made evident the need for additional research efforts as default FFR configurations only offer tradeoffs in which spectral efficiency is severely penalized. Moreover, the performance of such baseline designs has been showed to be poor in realistic cellular deployments featuring irregular cell patterns. This paper solves this problematic by introducing a novel multiobjective optimization framework based on evolutionary algorithms that jointly takes into account system capacity, cell edge performance, and energy consumption. With respect to important reference schemes, the proposed algorithm succeeds in finding FFR configurations achieving gains between 10 and 40 % in terms of system capacity while simultaneously improving cell edge performance up to 70 %.

Cellular communication systems especially LTE mandates the use of a simple and effective downlink power control of signals for controlling randomly dispersed user's throughput, interference, connectivity link. Users at the cell edge experience low throughout due to packet retransmission, and packet drop scenario. Due to the above problems, the operator loses revenue from the cell edge users due to poor usage of the spectrum. In this paper we propose a novel downlink optimal power allocation scheme (DOPAS) which maintains a constant cell edge spectral efficiency of a LTE cell for a given user density distribution and traffic type. DOPAS can be used to enhance the cell edge throughput without compromising the throughput of the users present closer to the eNodeB. DOPAS uses the convex optimization method in allocating right power control for both cell edge and near cell users and thus guaranteeing cell edge spectral efficiency. The cell edge throughput using the proposed algorithm is compared against the homogenous power allocation scheme where the cell power is uniformly distributed across the resource blocks of the scheduled users. Results showed that using DOPAS, there is a significant rise in cell edge throughput as compared to the modified homogeneous power allocation scheme. The improvement varied from 20-80% depending on the number of users present in near cell and cell edge region resulting in maintaining an operator defined cell edge spectral efficiency.

Interference management has been recognized by the industry as a key enabler for 4G systems. Emerging technologies include multicarrier systems such as LTE and WiMAX for which effective management of intercell interference is of utmost importance in order to improve the Quality of Service (QoS) at cell edges. Static Intercell Interference Coordination (ICIC) techniques such as Soft Frequency Reuse (SFR) are aimed at alleviating this problem; however the usage of baseline SFR designs (schemes without optimization) only offers tradeoffs between cell edge performance and spectral efficiency and performance is indeed far from optimal as results herein confirm. Thus, this paper presents a novel multiobjective algorithm in order to address this problem and achieve effective optimization of SFR implementations. Results show that the proposed algorithm succeeds in finding good-quality SFR configurations enhancing simultaneously network capacity and cell edge performance while reducing energy consumption with respect to baseline designs and previous proposals.

A novel frequency planning algorithm for mitigating unfairness in wireless LANs

The project to define Long Term Evolution (LTE) of UMTS cellular technology started in year 2004 is well established to the world. The subsequent evolve of LTE-Advanced however targets for enhancement in average spectrum efficiency and cell-edge user throughput. The limiting factor of the system performance at cell-edge is inter-cell interference (ICI). The challenge that is discussed in this paper focuses on ensuring the overall satisfaction mainly cell-edge system performance through the analysis on Fractional Frequency Reuse (FFR) and hybrid Macro system which involves small cell technology — Femtocell technology. This paper discussed the three frequency allocation schemes. The schemes include without the application of Femtocell, with randomly application of Femtocell and with preplanned Femtocell scheme. The FFR mechanism divides the cell into two regions (inner and outer). The inner radius and inner region resource is determined base on overall average throughput and overall user satisfaction.

Recently, Studies show that to date the demand for mobile high data rate occurs mainly in buildings. Moreover based on these studies sixty percentages of voice calls and ninety percentages of data traffic originate indoors. So indoor coverage which is associated with high quality services and high data rate that do not use extra radio resources is even more critical. femtocell technology is presented to cope with these challenges. One of the most important technical challenges faced by the femtocells is interference management. To cope with it many solutions have been presented by different researches. Fractional frequency reuse (FFR) is a solution in order to mitigate the inter-cell interference especially for the cell edge users. Furthermore it improves the total capacity of the network. In this paper we combine the LTE macrocells with the architecture of FFR and cognitive radio OFDMA based femtocells. And we choose Underlay approaches for secondary spectrum allocation. In our proposed scheme femto-macro and macro-macro interference are considered. Further for power control and capacity improvement, water-filling algorithm is taken into consideration. The simulation results show that the proposed scheme enhances the overall network throughput.

Fractional Frequency Reuse (FFR) is considered to provide enhancement in total throughput and an important reduction of the outage probability in two-tier macrocell-femtocell networks. However the allocation in FFR can create high cross-tier interferences to users located in the boundaries of the various zones. We propose a FFR scheme to alleviate the downlink cross-tier interference for users in these particular zones, joint with a QoS- aware analytical model to derive the blocking probabilities for different cell zones. The optimal parameters for cell channel partitioning in the proposed FFR-scheme are integrated to enhance the proposed system. Numerical results demonstrate the performance enhancement of the proposed joint FFR allocation and admission control scheme. Compared to current FFR and admission control strategies, our scheme permits to increase overall traffic by up to 40 % at cell center and 30 % at cell edge, for any given blocking probability.

In next generation broadband wireless networks, the orthogonal frequency division multiple access (OFDMA) technique provides a flexible physical interface for mobile users (MS) to share the radio resources. Since OFDMA allows MSs in different cells to reuse the same frequency subchannels to increase network spectrum utilization, MSs at the edge of a cell suffer from severe inter-cell interference from co-channel MSs in the neighboring cells. In WiMAX and LTE networks, fractional frequency reuse (FFR) is employed to mitigate the interference for edge users, but may incur a cost in terms of a lower overall spectrum utilization. In this paper, we propose a novel frequency planning scheme to improve the signal-to-interference (SIR) ratio of edge MSs in a multi-cell OFDMA system. A cell is partitioned in a way that minimizes the interference to the edge areas while sustaining a full frequency reuse factor (FRF) for each cell. Simulation results show that our scheme substantially outperforms FFR methods in terms of SIR under the condition of full frequency reuse and high traffic load.

The severe interference in full-duplex (FD) cellular network largely degrade the system performance, especially for cell edge users. In this paper, hybrid full/half duplex cellular network which is composed of a mixture of FD and half duplex (HD) base-stations (BSs) is focused. In order to relieve the received inter-cell interference, frequency reuse (FR) is applied. The adopted FR schemes include traditional full FR and two kinds of fractional FR (FFR) named Strict FFR (StFFR) and Soft FFR (SoFFR). Stochastic geometry is adopted to evaluate the system performance. Tractable results of the ergodic rate and coverage probability for different FR schemes are obtained. Through the numerical results, we can observe that full FR and StFFR schemes can improve the user coverage probability at the expense of system spectrum efficiency (SE). SoFFR can improve both the coverage probability and the system SE. The numerical results also show that if the residual Self-interference is not very strong, operating all the BSs in FD mode can achieve the best performance, otherwise, it is better to operate all the BSs in HD mode.

Distributed antenna system (DAS)-aided unity frequency reuse (UFR) and fractional frequency reuse (FFR) transmission scenarios are investigated in this paper, employing the classic multiobjective of nondominated sorting genetic algorithm II (NSGA-II) for maximizing cell throughput and the coverage. More specifically, coordinated multipoint (CoMP) cooperation is invoked among the distributed antennas (DAs) and the base station (BS) in support of the mobile stations (MSs) roaming at the cell edge, while considering a range of practical impairments. We demonstrate that the received signal-to-interference ratio (SIR) of non-CoMP transmissions follows the lognormal distribution by taking into account both fast fading and large-scale shadowing and path-loss effects. Our simulation results demonstrate that DAS-aided cooperation is capable of achieving a fivefold increased throughput over that of the traditional arrangement. Explicitly, an average throughput per channel of 6.61 bits/symbol may be achieved.

Joint virtual edge-clustering and spectrum allocation scheme for uplink interference mitigation in C-RAN

The cell edge users suffer from low signal strength and interference from neighboring cells. Additionally, in a vehicular scenario, vehicle penetration loss (VPL) exacerbates and reduces signal strength badly at the cell edge, which results in poor coverage and low quality of service (QoS). Outage probability and throughput are important parameters that cell edge users need to analyze. Recent research shows that non-orthogonal multiple access (NOMA) and amplify and forward relaying based moving relay (MR) are promising techniques for handling these issues. The performance of decode forward relaying based MR has not been analyzed yet. In this paper, we propose a cooperative NOMA system with full duplex decode forward (FD-DF) relaying based MR on the roof of a vehicle for a downlink network to analyze the performance of cell edge vehicular users. We derive the outage probability capacity of vehicular cell edge and non vehicular users. We analyze vehicular users’ outages and throughput performances in terms of transmit signal-to-noise ratio (SNR), power allocation coefficient, and residual interference levels. Simulation results show that FD-DFR based MR performs better than systems with half duplex decode forward (HD-DF) relaying based MR and without MR.