Self-organising network management for heterogeneous LTE-advanced networks

This thesis was submitted for the award of Doctor of Philosophy and awarded by Brunel University LondonSince 2004, when the Long Term Evolution (LTE) was first proposed to be publicly available in the year 2009, a plethora of new characteristics, techniques and applications have been constantly enhancing it since its first release, over the past decade. As a result, the research aims for LTE-Advanced (LTE-A) have been released to create a ubiquitous and supportive network for mobile users. The incorporation of heterogeneous networks (HetNets) has been proposed as one of the main enhancements of LTE-A systems over the existing LTE releases, by proposing the deployment of small-cell applications, such as femtocells, to provide more coverage and quality of service (QoS) within the network, whilst also reducing capital expenditure. These principal advantages can be obtained at the cost of new challenges such as inter-cell interference, which occurs when different network applications share the same frequency channel in the network. In this thesis, the main challenges of HetNets in LTE-A platform have been addressed and novel solutions are proposed by using self-organising network (SON) management approaches, which allows the cooperative cellular systems to observe, decide and amend their ongoing operation based on network conditions. The novel SON algorithms are modelled and simulated in OPNET modeler simulation software for the three processes of resource allocation, mobility management and interference coordination in multi-tier macro-femto networks. Different channel allocation methods based on cooperative transmission, frequency reuse and dynamic spectrum access are investigated and a novel SON sub-channel allocation method is proposed based on hybrid fractional frequency reuse (HFFR) scheme to provide dynamic resource allocation between macrocells and femtocells, while avoiding co-tier and cross-tier interference. Mobility management is also addressed as another important issue in HetNets, especially in hand-ins from macrocell to femtocell base stations. The existing research considers a limited number of methods for handover optimisation, such as signal strength and call admission control (CAC) to avoid unnecessary handovers, while our novel SON handover management method implements a comprehensive algorithm that performs sensing process, as well as resource availability and user residence checks to initiate the handover process at the optimal time. In addition to this, the novel femto over macro priority (FoMP) check in this process also gives the femtocell target nodes priority over the congested macrocells in order to improve the QoS at both the network tiers. Inter-cell interference, as the key challenge of HetNets, is also investigated by research on the existing time-domain, frequency-domain and power control methods. A novel SON interference mitigation algorithm is proposed, which is based on enhanced inter-cell interference coordination (eICIC) with power control process. The 3-phase power control algorithm contains signal to interference plus noise ratio (SINR) measurements, channel quality indicator (CQI) mapping and transmission power amendments to avoid the occurrence of interference due to the effects of high transmission power. The results of this research confirm that if heterogeneous systems are backed-up with SON management strategies, not only can improve the network capacity and QoS, but also the new network challenges such as inter-cell interference can also be mitigated in new releases of LTE-A network

Improved interference management techniques for multi-cell multi-user MIMO systems

One major limiting factor for wireless communication systems is the limited available bandwidth for cellular networks. Current technologies like Long Term Evolution (LTE) and LTE-Advanced (LTE-A) have standardised a frequency reuse factor of 1 to enable more channel resources in each cell. Also multi-layer networks that consist of overlapping macro cells and small cells like pico cells, micro cells and femto cells have also been used to improve the capacity of the cellular network system. These multi-layer networks are known as heterogeneous networks or HetNets while the single layer, traditional cellular systems are referred to as homogeneous networks or HomoNets. Several interference management systems and techniques have been proposed in the past to deal with the effect of inter-cell interference (ICI) (i.e., the interference from a macro cell base station (BS) to a macro cell user in another macro cell) and inter-user interference (IUI) (i.e., the interference of another user's data signal to a given user within the same cell on the same time and frequency slot). Interference cancellation techniques such as beamforming, uses transmit pre-coders and receive beam-formers to limit the effect of interference. The interference alignment strategy ensures that the interference is aligned into a given subspace and leaves a residual subspace free for the desired signal. Coordinated scheduling/beam-forming and coordinated multi-point transmission (CoMP) have also been used to limit the interference within the cellular network. For HetNets, interference avoidance techniques based on radio resource management (RRM) have been used to limit the effect of interference within the network and improve the attainable system capacity. This thesis investigates the challenges of two main interference management techniques and proposes methods to alleviate these issues without impeding the expected performance already attained. The main techniques considered for HomoNets and HetNets are: CoMP transmission under the interference cancellation technique and resource block allocation (RBA) under the interference avoidance technique. The setbacks for the well known CoMP transmission strategy are high data overhead, energy consumption and other associated costs to the network provider. Further investigations were carried out and a joint selection of transmit antennas for the users was proposed with the main aim of preserving or exceeding the already achieved gains but obtaining a further reduction in the data overhead. Fully distributed RBA solutions are required, especially since future networks tend to become self-organising networks (SON). Another major consideration in choosing the resource blocks (RBs) for the users in each cell is the RBA metric. Since the capacity of the cell is dependent on the sum-rate of the users, it is important to consider the maximisation of the sum-rate or sum-SINR (i.e the sum signal to interference and noise ratio) when assigning RBs to users. The RBA technique aims to choose the RBs such that the interference within the cell is avoided. To achieve this, a RBA metric is required to obtain the qualification matrix before allocating RBs to the users. Many authors in the past have proposed several metrics for RBA but avoided any RBA metric that required a direct estimation of the interference power on each RB for each user's allocation. This is because the SINR or interference power on each RB for any user can only be obtained with pre-knowledge of already occupied RBs in neighbouring cells. In this thesis, two distributed RBA solutions based on a direct interference estimation was proposed to obtain the required qualification matrix for the RBA under the HomoNet and HetNet system models. The gains and advantages obtained are shown and analysed using the obtained simulation results. The issue of interference coupled with limited available channels remains a major limiting factor for HetNets. Therefore, it is very important to develop techniques that maximise the utilisation of available bandwidth for each cell while minimising possible interference from neighbouring cells. Finally, this thesis considers and investigates a possible joint solution using both interference avoidance and interference mitigation techniques. Hence two solutions are proposed: joint RBA plus beam-forming and joint RBA plus CoMP transmission, to further mitigate the high interference in HetNets. The simulation results have shown significantly improved system performance especially for a highly dense HetNet