Full length article: LTE-Advanced and the evolution to Beyond 4G (B4G) systems

To encourage data offloading in heterogeneous networks, range expansion techniques have been introduced in LTE-Advanced system. By adding a positive bias in cell selection process, more users can select picocells as their serving cells even the picocell is not of the strongest signal. Also, to protect offloaded users from severe macrocell interference, a specific subframe called almost blank subframe (ABS) is introduced. Since macrocell is kept silent during ABSs, the offloaded users can be allocated without strong macrocell interference within ABSs. It is noted that the offloaded user's average data rate highly depends on the amount of ABSs rather than the bias value because the scheduling opportunities of offloaded users are limited within ABSs. And from the user's point of view, the cell which is expected to give high data rate should be selected as a serving cell. In this paper, we propose a cell selection scheme based on expected user data rate which is a function of the ABS ratio. Simulation results show that the performance of our proposed cell selection scheme can improve average user rate and increase the number of offloaded users as the ABS ratio increases.

Cell selection for range expansion with almost blank subframe in heterogeneous networks

Enhanced Inter-cell Interference Co-ordination (eICIC) is a time-domain multiplexing technique for improving the performance of legacy co-channel Heterogeneous Networks (HetNets). eICIC offers several benefits, including a more equitable distribution of user traffic across the macro and embedded pico cells (i.e., load balancing), in turn leading to better pico cell utilizations and better edge user throughputs. eICIC uses two mechanisms to achieve load balancing: (i) a cell selection bias to increase the pico coverage area, enabling the pico to attract more users, and (ii) macro muting ("Almost Blank Sub-frame", ABS) to improve SINRs of range extended pico users. Network load in the cellular system varies continuously as a result of user mobility and traffic dynamics (the varying amount of data pending for a user - arising from packet arrivals and eventual departures). Previous work did not consider dynamically changing network conditions in HetNets. Hence, they studied the benefits of eICIC over legacy HetNets arising from static optimization of eICIC parameters. Our approach is dynamic, and further improves performance by keeping the HetNet continuously optimized for improved user experience, by adapting the ABS in response to dynamic variations in network load. We present several analytical formulations, which allow for simple optimizations and an intuitive understanding of the desired system response to load variation. Exhaustive system simulations illustrate the performance gains along different dimensions, such as spectral efficiency, fairness, mean file transfer times, number of file transmissions, number of file drops, and mean queue lengths. Furthermore, we study the impact of different mobility scenarios, traffic mixes, and adaptation rate. We also provide a brief discussion of practical considerations in implementing Dynamic eICIC.

Dynamic eICIC — A Proactive Strategy for Improving Spectral Efficiencies of Heterogeneous LTE Cellular Networks by Leveraging User Mobility and Traffic Dynamics

Heterogeneous networks (HetNets) are expected to be one of the major performance enhancement enablers of LTE-Advanced. Due to significantly challenging interference scenarios, enhanced inter-cell interference coordination (eICIC) based on almost blank subframes (ABS) is the key technology enabler that makes HetNets a reality. The key factor determining the gain of ABS-based eICIC is the configured number of ABS which depends closely on the number of UEs requesting protection from ABS. In this paper, we study the optimal amount of ABS for synchronous ABS configuration by formulating this optimization as a network-wide utility maximization problem. We firstly propose a distributed method to determine victim UEs protected by ABS for any given amount of ABS via dynamic programming, and then find the optimal amount of ABS by evaluating the overall system utility. A remarkable feature of the proposed solution is that only limited information exchange via backhaul is required and the optimal amount of ABS could be individually derived at each eNB. Extensive simulations demonstrate that our solution can not only improve the overall system throughput significantly but also provide better network-wide fairness.

Optimized time-domain resource partitioning for enhanced inter-cell interference coordination in heterogeneous networks

For satisfying rapidly increasing data rates at hotspots and enhancing coverage in buildings, small cells, such as femtocells, picocells, and microcells, are deployed in LTE-A. Femtocells are typically installed at hotspots and overlay with the macrocell to improve energy efficiency and data rates. In macro-femto HetNets, the handover issue is more important than that in macrocell networks. On one hand, more frequent handovers are triggered because the coverage range of a femtocell is small, and multiple femtocells are overlaid. On the other hand, some schemes, such as load balancing, aimed at improving network performance, will also cause frequent handover in macro-femto HetNets. Therefore, handover has a significant impact on the performance of macro-femto HetNets. In this article, we first survey the state-of-the-art handover techniques that are aimed at keeping ongoing connections uninterrupted or ensuring the quality of service of mobile users. Then we introduce load-balance-related handover schemes. Moreover, an energy-efficient handover scheme is presented. At last, we point out interesting research issues on handover schemes in macro-femto HetNets.

Handover schemes in heterogeneous LTE networks: challenges and opportunities

Pico cell has been brought to cellular networks as a new technology to deliver cost-effective coverage and capacity to hard-to-reach places, and also increase capacity by cell splitting in hotspots with very dense phone usage. However, according to existing strongest receiving signal association method, pico cells with the extremely low transmitting power are significantly limited in the presence of macro coverage. Thus, the new introduced pico frequency resource will not be fully exploited in this unbalanced user distributions among macro and pico cells. In this paper, by jointly considering the long term channel condition and load balance in this macro-pico coexisted heterogeneous network, we formulate the cell selection problem into a network-wide proportional fairness optimization. Both optimal solution (by dynamic programming) and cost efficient algorithm (greedy heuristics) are proposed. In 3GPP LTE-A heterogeneous network scenario, the system-level performance evaluation shows that, compared with the conventional strongest signal cell selection scheme, the proposed optimized fairness cell selection methods could achieve significant performance gains especially for the cell edge users.

Optimized Fairness Cell Selection for 3GPP LTE-A Macro-Pico HetNets

In 3GPP LTE-Advanced Release-12, the feature of dual connectivity (DC) has been introduced to enhance small cell networks from the aspects of higher layer architecture and physical layer procedures, which can improve mobility robustness and per-user throughput as well as avoid frequent hard handover among small cells. To guarantee the UE maximum transmit power is not exceeded, the conventional power scaling and simple semi-static power splitting between two network points were considered for DC. However, both schemes would degrade the DC performance in terms of uplink coverage and user throughput due to the uncoordinated scheduling at two network points. To address this issue, an enhanced power control scheme is proposed which could conduct uncoordinated adjustment of uplink power and data rate for DC. The simulation results are provided to demonstrate the performance advantages of the proposed scheme.

An Enhanced Power Control Scheme for Dual Connectivity

Heterogeneous deployment is a promising solution to enhance both system capacity and coverage, where the planned macro cell is overlaid with pico cells. However, higher transmit power of macro eNB (MeNB) results in small coverage for pico cell, which causes insufficient offloading from macro cell to pico cell and inefficient cell splitting gain exploration. Cell range expansion (CRE) is one of alternative cell associations to enhance offloading, but would bring severe interference on the uplink reception of macro cell. Hence, the use of enhanced inter-cell interference coordination (eICIC) techniques is required to realize the promised performance of macro user equipments (MUEs). In this paper, an adaptive uplink power control scheme is proposed for heterogeneous network (HetNet) with CRE, wherein the uplink power control parameters of pico cells could be dynamically optimized based on admissible interference level of macro cell. In addition, the proposed scheme manages to improve the performance of pico UEs (PUEs) on condition that the Quality of Service (QoS) of MUEs can be well guaranteed. Numerical simulation results verify the validity of the proposed scheme.

Uplink power control and interference coordination for heterogeneous network

In view of the new feature after introducing the relay nodes (RNs) into the conventional cellular networks, a novel soft frequency reuse (SFR)-based inter-cell interference coordination (ICIC) scheme is proposed in the relay enhanced cellular networks. On the basis of original ICIC scheme for conventional cellular network, RNs in the proposed SFR scheme reuses part of neighboring cell-edge bands to improve both the cell average throughput and ell-edge UEs' throughput. Furthermore, for the sake of taking cell throughput and user fairness into consideration, a weighted proportional fairness (WPF) scheduling algorithm is also developed for in-band relay in this paper. Simulation results show that the proposed scheme can perform better on the SINR of cell-edge UEs and cell throughput in comparison with the previous schemes, in addition, the user fairness has also been guaranteed with efficient resource allocation approach.

Jianguo Liu

Papers

Optimized time-domain resource partitioning for enhanced inter-cell interference coordination in heterogeneous networks

Optimized Fairness Cell Selection for 3GPP LTE-A Macro-Pico HetNets

An Enhanced Power Control Scheme for Dual Connectivity

Uplink power control and interference coordination for heterogeneous network

Inter-cell interference coordination based on soft frequency reuse for relay enhanced cellular network