Coverage Performance Analysis of FeICIC Low Power Subframes

Although the Almost Blank Subframes (ABSF) proposed in heterogeneous cellular networks can enhance the performance of the Cell Range Expansion (CRE) User Equipments (UEs), it significantly degrades macro-cell total throughput. To address this problem, the Low Power Subframes (LPSF) are encouraged to be applied in macro-cell center region by the Further-enhanced Inter-cell Interference Coordination (FeICIC). However, the residual power of the LPSF which interferes the CRE UEs, and the proportion of the LPSF affect the downlink throughput together. To achieve a better rate coverage probability, appropriate LPSF power and proportion are required. In this paper, the analytical results of the overall Signal to Interference and Noise Ratio (SINR) coverage probability and the rate coverage probability are derived under the stochastic geometric framework. The optimal region bias ranges for maximizing the rate coverage probability are also analysed. The results show that the ABSF still outperform the LPSF in terms of rate with the optimal range expansion bias, but lead to a heavier burden on the back-haul of the pico-cell. However, with a static range expansion bias, the LPSF provide better rate coverage than the ABSF. Also, in a low range expansion scenario, the reduced power of the LPSF has negligible effect on the rate coverage with the optimal resource partitioning

Capacity Analysis of LTE-Advanced HetNets with Reduced Power Subframes and Range Expansion

The time domain inter-cell interference coordination techniques specified in LTE Rel. 10 standard improves the throughput of picocell-edge users by protecting them from macrocell interference. On the other hand, it also degrades the aggregate capacity in macrocell because the macro base station (MBS) does not transmit data during certain subframes known as almost blank subframes. The MBS data transmission using reduced power subframes was standardized in LTE Rel. 11, which can improve the capacity in macrocell while not causing high interference to the nearby picocells. In order to get maximum benefit from the reduced power subframes, setting the key system parameters, such as the amount of power reduction, carries critical importance. Using stochastic geometry, this paper lays down a theoretical foundation for the performance evaluation of heterogeneous networks with reduced power subframes and range expansion bias. The analytic expressions for average capacity and 5th percentile throughput are derived as a function of transmit powers, node densities, and interference coordination parameters in a heterogeneous network scenario, and are validated through Monte Carlo simulations. Joint optimization of range expansion bias, power reduction factor, scheduling thresholds, and duty cycle of reduced power subframes are performed to study the trade-offs between aggregate capacity of a cell and fairness among the users. To validate our analysis, we also compare the stochastic geometry based theoretical results with the real MBS deployment (in the city of London) and the hexagonal-grid model. Our analysis shows that with optimum parameter settings, the LTE Rel. 11 with reduced power subframes can provide substantially better performance than the LTE Rel. 10 with almost blank subframes, in terms of both aggregate capacity and fairness.Comment: Submitted to EURASIP Journal on Wireless Communications and Networking (JWCN

Towards UAV Assisted 5G Public Safety Network

Ensuring ubiquitous mission-critical public safety communications (PSC) to all the first responders in the public safety network is crucial at an emergency site. The first responders heavily rely on mission-critical PSC to save lives, property, and national infrastructure during a natural or human-made emergency. The recent advancements in LTE/LTE-Advanced/5G mobile technologies supported by unmanned aerial vehicles (UAV) have great potential to revolutionize PSC. However, limited spectrum allocation for LTE-based PSC demands improved channel capacity and spectral efficiency. An additional challenge in designing an LTE-based PSC network is achieving at least 95% coverage of the geographical area and human population with broadband rates. The coverage requirement and efficient spectrum use in the PSC network can be realized through the dense deployment of small cells (both terrestrial and aerial). However, there are several challenges with the dense deployment of small cells in an air-ground heterogeneous network (AG-HetNet). The main challenges which are addressed in this research work are integrating UAVs as both aerial user and aerial base-stations, mitigating inter-cell interference, capacity and coverage enhancements, and optimizing deployment locations of aerial base-stations. First, LTE signals were investigated using NS-3 simulation and software-defined radio experiment to gain knowledge on the quality of service experienced by the user equipment (UE). Using this understanding, a two-tier LTE-Advanced AG-HetNet with macro base-stations and unmanned aerial base-stations (UABS) is designed, while considering time-domain inter-cell interference coordination techniques. We maximize the capacity of this AG-HetNet in case of a damaged PSC infrastructure by jointly optimizing the inter-cell interference parameters and UABS locations using a meta-heuristic genetic algorithm (GA) and the brute-force technique. Finally, considering the latest specifications in 3GPP, a more realistic three-tier LTE-Advanced AG-HetNet is proposed with macro base-stations, pico base-stations, and ground UEs as terrestrial nodes and UABS and aerial UEs as aerial nodes. Using meta-heuristic techniques such as GA and elitist harmony search algorithm based on the GA, the critical network elements such as energy efficiency, inter-cell interference parameters, and UABS locations are all jointly optimized to maximize the capacity and coverage of the AG-HetNet

Interference Coordination in Heterogeneous Networks: Stochastic Geometry Based Modelling and Performance Analysis

Recently data traffic has experienced explosive increase with the proliferation of wireless devices and the popularity of media-based free services. The academic and industry of mobile communications have predicted an estimated $1000$x increase in traffic volume for the forthcoming 5G networks. This traffic explosion stimulates the deployment of heterogeneous networks (HetNets) with small cells (SCs) underlying in the traditional macrocells, which has been considered as a promising technique to contribute to the $1000$x traffic capacity gain. Initially, licensed spectrum bands are expected to be used in SCs, thus the SC deployment introduces the cross-tier interference between SCs and macrocells, which degrades the downlink signal to interference plus noise ratio (SINR) of user equipments (UEs) severely, especially for the edge UEs in a ultra-densely deployed scenario. To alleviate this cross-tier interference between SCs and macrocells, unlicensed spectrum bands are advocated to be used in SCs. Specifically, with the aid of carrier aggregation, the $5$ gigahertz (GHz) unlicensed band has become an option for SCs in the Long Term Evolution (LTE)-Unlicensed (LTE-U) scheme, but the $5$ Ghz unlicensed band has already been used by WiFi networks. Thus downlink cross-tier interference also occurs between LTE-U and WiFi networks. Accordingly, downlink cross-tier interference is inevitable no matter licensed or unlicensed spectrum band (i.e., 5 GHz) is used in SCs, and interference coordination schemes, such as further enhanced inter-cell interference coordination (FeICIC) for macrocells and SCs, and Licensed Assisted Access (LAA) for WiFi networks and LTE-U networks, have been proposed to mitigate these cross-tier interferences. In this dissertation, we mainly focus on the modelling and performance analysis of HetNets with the aforementioned two interference coordination schemes (i.e., FeICIC and LTE-LAA) under the stochastic geometry framework. Firstly, as the configuration of reduced power subframe (RPS)-related parameters was not well investigated in a two-tier HetNet adopting RPSs and cell range expansion (CRE), we derive the analytical expressions of the downlink coverage probability and rate coverage probability in such a HetNet. The optimal settings for the area of macrocell center regions, the area of SC range expansion regions, and the transmit power of RPSs for maximizing the rate coverage probability are analysed. As compared with the rate coverage probability in the two-tier HetNet with almost blank subframes (ABSs), which is proposed in the previous version of FeICIC, i.e., the enhanced inter-cell interference coordination (eICIC), the results show that ABSs outperform RPSs in terms of the rate coverage probability in the two-tier HetNet with the optimal range expansion bias, but lead to a heavier burden on the SC backhaul. However, with static typical range expansion biases, RPSs provide better rate coverage probability than ABSs in the two-tier HetNet. Secondly, the conventional FeICIC scheme ignores the potential of RPSs being adopted in both tiers of a two-tier HetNet without CRE, which is envisioned to improve the SINR level of edge UEs in both tiers. Accordingly, we study the downlink coverage probability and rate coverage probability of a two-tier HetNet applying with our proposed scheme. The results reveal that adopting RPSs in both tiers not only improves the coverage probabilities of edge UEs, but also increases the rate coverage probability of the whole two-tier HetNet. Thirdly, in both previous works, strict subframe alignment (SA) was assumed throughout the whole network, which is difficult to maintain between neighbouring cells in reality. Consequently, we propose a novel subframe misalignment (SM) model for a two-tier HetNet adopting RPSs with SM offsets restricted within a subframe duration, and analyse the coverage probability under the effects of RPSs and SM. The numerical results indicate that the strict SA requirement can be relaxed by up to $20\%$ of the subframe duration with a loss of below $5\%$ in terms of the downlink coverage probability. Lastly, since stochastic-geometry-based analysis of the coexisting LTE-LAA and WiFi networks, which adopt the carrier-sense multiple access with collision avoidance (CSMA/CA) as the medium access control (MAC) scheme and share multiple unlicensed channels (UCs), was missing, we analyse the downlink throughput and spectral efficiency (SE) of the coexisting LTE-LAA and WiFi networks versus the network density and the number of UCs based on the Matern hard core process. The throughput and SE are obtained as functions of the downlink successful transmission probability (STP), of which analytical expressions are derived for both LTE-LAA and WiFi UEs. The results show that the throughput and SE of the whole coexisting LTE-LAA and WiFi networks can be improved significantly with an increasing number of accessible UCs. Based on the numerical results, insights into the trade-off between the throughput and SE against the number of accessible UCs are provided. All the derived results have been validated by Monte Carlo simulation in Matlab, and the conclusions observed from the results can provide guidelines for the future deployments of the FeICIC and LTE-LAA interference coordination schemes in HetNets

Inter-cell interference mitigation using adaptive reduced power subframes in heterogeneous networks

With the remarkable impact and fast growth of the mobile networks, the mobile base stations have been increased too, especially in the high population areas. These base stations will be overloaded by users, for that reason the small cells (like pico cells) were introduced. However, the inter-cell interference will be high in this type of Heterogeneous networks. There are many solutions to mitigate this interference like the inter-cell interference coordination (ICIC), and then the further enhanced ICIC (Fe-ICIC) where the almost blank subframes are used to give priority to the (victim users). But it could be a waste of bandwidth due to the unused subframes. For that reason, in this paper, we proposed an adaptive reduced power subframe that reduces its power ratio according to the user’s signal-to-interference-plus-noise ratio (SINR) in order to get a better throughput and to mitigate the intercell interference. When the user is far from the cell, the case will be considered as an edge user and will get a higher priority to be served first. The results show that the throughput of all users in the macro cells and pico cell will be improved when applying the proposed scheme in term of throughput for the users and the cells

A Study about Heterogeneous Network Issues Management based on Enhanced Inter-cell Interference Coordination and Machine Learning Algorithms

Under the circumstance of fast growing demands for mobile data, Heterogeneous Networks (HetNets) has been considered as one of the key technologies to solve 1000 times mobile data challenge in the coming decade. Although the unique multi-tier topology of HetNets has achieved high spectrum efficiency and enhanced Quality of Service (QoS), it also brings a series of critical issues. In this thesis, we present an investigation on understanding the cause of HetNets challenges and provide a research on state of arts techniques to solve three major issues: interference, offloading and handover. The first issue addressed in the thesis is the cross-tier interference of HetNets. We introduce Almost Blank Subframes (ABS) to free small cell UEs from cross-tier interference, which is the key technique of enhanced Inter-Cell Interference Coordination (eICIC). Nash Bargain Solution (NBS) is applied to optimize ABS ratio and UE partition. Furthermore, we propose a power based multi-layer NBS Algorithm to obtain optimal parameters of Further enhanced Inter-cell Interference Coordination (FeICIC), which significantly improve macrocell efficiency compared to eICIC. This algorithm not only introduces dynamic power ratio but also defined opportunity cost for each layer instead of conventional zero-cost partial fairness. Simulation results show the performance of proposed algorithm may achieve up to 31.4% user throughput gain compared to eICIC and fixed power ratio FeICIC. This thesis’ second focusing issue is offloading problem of HetNets. This includes (1) UE offloading from macro cell and (2) small cell backhaul offloading. For first aspect, we have discussed the capability of machine learning algorithms tackling this challenge and propose the User-Based K-means Algorithm (UBKCA). The proposed algorithm establishes a closed loop Self-Organization system on our HetNets scenario to maintain desired offloading factor of 50%, with cell edge user factor 17.5% and CRE bias of 8dB. For second part, we further apply machine learning clustering method to establish cache system, which may achieve up to 70.27% hit-ratio and reduce request latency by 60.21% for Youtube scenario. K-Nearest Neighbouring (KNN) is then applied to predict new users’ content preference and prove our cache system’s suitability. Besides that, we have also proposed a system to predict users’ content preference even if the collected data is not complete. The third part focuses on offloading phase within HetNets. This part detailed discusses CRE’s positive effect on mitigating ping-pong handover during UE offloading, and CRE’s negative effect on increasing cross-tier interference. And then a modified Markov Chain Process is established to map the handover phases for UE to offload from macro cell to small cell and vice versa. The transition probability of MCP has considered both effects of CRE so that the optimal CRE value for HetNets can be achieved, and result for our scenario is 7dB. The combination of CRE and Handover Margin is also discussed