Janne Ali-Tolppa

On the limits of PCI auto configuration and reuse in 4G/5G ultra dense networks

Increased demand for higher user throughput has led to deployment of multi-layer networks commonly called heterogeneous networks (Hetnets). Therein, small cells are deployed alongside traditional macro cells, in many cases on the same spectrum. Such scenarios complicate the configuration of network parameters such as the Physical Cell Identity (PCI). A number of approaches have as such been proposed to automate the allocation of PCIs in such scenarios. These approaches seek to address the two conflicting objectives for PCI assignment in a hetnet scenario: 1) the need for optimal performance by avoiding conflicts, against 2) the requirement to separate the different layers and avoid any need to share knowledge among the layers. However, as the density of small cells increases evolving the Hetnets into what are called Ultra Dense Networks (UDN), these approaches reach their limits. In this paper, we study the performance of the current PCI allocation strategies in such UDN scenarios and evaluate their break down points. Our results show that these strategies do not adequately address PCI allocation for the UDN scenario. Specifically, we observe that PCI assignment in one layer requires knowledge of the assignments in the other layer, otherwise the consequence is a very high count of PCI confusions.

A minimum spanning tree-based approach for reducing verification collisions in self-organizing networks

The verification of Configuration Management (CM) changes has become an important step in the operation of a mobile Self-Organizing Network (SON). Typically, a verification mechanism operates in three phases. At first, it partitions the network into verification areas, then it triggers an anomaly detection algorithm for those areas, and finally generates CM undo requests for the abnormally performing ones. Those requests set the CM parameters to a previous stable state. However, verification areas may overlap and share anomalous cells which results in a verification collision. As a consequence, the verification mechanism is not able to simultaneously deploy the undo requests since there is an uncertainty which to execute and which to potentially omit. In such a case, it has to serialize the deployment process and resolve the collisions. This procedure, though, can be negatively impacted if unnecessary collisions are processed, since they might delay the execution of the queued CM undo requests. To overcome this issue, we propose an approach for changing the size of the verification areas with respect to the detected collisions. We achieve our goal by using a Minimum Spanning Tree (MST)-based clustering approach that is able to group similarly behaving cells together. Based on the group they have been assigned to, we remove cells from a verification area and prevent false positive collisions from being further processed. Furthermore, we evaluate the proposed solution in two different scenarios. First, we highlight its benefits by applying it on CM and Performance Management (PM) data collected from a real Long Term Evolution (LTE) network. Second, in a simulation study we show how it positively affects the network performance after eliminating the false positives.

Fluid capacity for energy saving management in multi-layer ultra-dense 4G/5G cellular networks

There is a major demand for reducing energy consumption in mobile networks and it is expected become even more vital in the future (5G) multi-layer Ultra Dense Networks (UDNs), in which the number and density of cells in the different layers will grow dramatically. In these networks, multiple geographically overlapping layers are deployed to increase the capacity and throughput, but also increasing the energy consumption. In this paper we present an end-to-end solution that manages energy saving mechanisms in order to scale the provided capacity to the traffic. Assuming a Heterogeneous Network (HetNet) deployment, the solution dynamically selects cells to activate and/or deactivate considering the prevailing network load and the expected spectral efficiency of those cells. Evaluation in a small HetNet scenario showed that the proposed solution is able to reduce the energy consumption by more than 30%.

A Steiner tree-based verification approach for handling topology changes in self-organizing networks

In todays Self-Organizing Networks (SONs) we differentiate between closed-loop functions, which have a predefined absolute goal, and such that form an action plan that achieves a high expected utility. Both function types perform changes to Configuration Management (CM) parameters, but only the second type may re-adapt the action plan in order to maximize the utility. A SON verification approach is one member of this particular function class. It is seen as a special type of anomaly detection that divides the network into sets of cells, triggers an anomaly detection algorithm for those sets, and finally generates CM undo actions for the abnormally performing cells. Unfortunately, one of the challenges verification strategies are facing are network topology changes. Typically, cells are switched on or off when energy saving features are enabled. However, enabling or disabling cells can negatively influence a verification mechanism which may create a suboptimal action plan or even blame certain CM changes that actually did not harm performance. In order to overcome this issue, we present an approach that is based on Steiner trees. In graph theory, a Steiner tree is a Minimum Spanning Tree (MST) whose costs can be reduced by adding additional vertexes to the graph. We use this tree to filter out anomalies caused by topology adjustments and such induced by other CM changes. In this paper, we also evaluate the proposed solution in several scenarios. First, in a simulation study we evaluate the functions that are used to build the Steiner tree. Second, we show how it positively affects the network performance when having concurrent CM and topology changes.

Verification of Configuration Management Changes in Self-Organizing Networks

The verification of configuration management (CM) changes is an essential operation in a mobile self-organizing network (SON). Usually, a verification approach operates in three steps: it divides the network into verification areas, triggers an anomaly detection algorithm for those areas, and finally generates CM undo requests for the abnormally performing ones. Those requests set CM parameters to a previous stable state. However, the successful completion of this process can be quite challenging, since there are factors that might negatively impact its outcome. For instance, if a temporal degradation occurs during the optimization of a cell, a verification mechanism may wrongly assume that it is anomalous, and interrupt the optimization process. Furthermore, a verification strategy experiences difficulties when it faces verification collisions, i.e., conflicting undo requests that cannot be simultaneously deployed. At first, it has to determine whether the collision is a false positive one. Then, it has to resolve it by finding out which requests have the highest probability of restoring the network performance. In addition, it needs to consider the time that is given for rolling back CM changes. In this paper, we contribute to the area of SON verification by providing a solution that addresses those problems. Our approach makes use of constraint optimization techniques to resolve collisions as well as find the appropriate order for deploying undo requests. In addition, we use a minimum spanning tree-based clustering technique to eliminate false positive collisions. Further, we evaluate our solution in a simulation study in which we show its positive effect on the network performance.

An experimental system for verifying topology changes in mobile communication networks

Automatic Configuration Management (CM) parameter change assessment, the so-called Self-Organizing Network (SON) verification, is an important enabler for stable and high-quality modern mobile communication networks. However, it also presents a new set of challenges. While improving network stability and resolving unexpected conflicts caused by parallel configuration changes, SON verification can have trouble coping with very dynamic networks that exhibit frequent topology changes. Such changes can be, for example, due to energy saving features which try to maximize the energy efficiency of a mobile network by adapting the topology to the network traffic. The experimental system presented in this paper demonstrates a new paradigm for handling such changes. It extends the currently available SON verification principles by providing a solution for the assessment of the impact of topology changes and for correcting them, in case the changes lead to degraded performance. The system includes a wide variety of visualization capabilities, including the various network states, user behavior, and performance statistics.

Layer-independent PCI assignment method for Ultra-Dense multi-layer co-channel mobile Networks

Ultra-Dense Networks (UDNs) are Heterogeneous Networks (HetNets) that deploy a high density of small cells over-laying the traditional macro cells. If several Long Term Evolution (LTE) layers share the available spectrum, assigning the Physical Cell Identities (PCIs) becomes complicated due to the density and the diversity of the network. Since different layers can be managed by different Network Management (NM) and Self-Organizing Network (SON) solutions, it would often be desirable to be able to assign the PCIs in each layer independently. At the same time it must be ensured that the PCI conflicts, also between the layers, are minimized. When the small cell layer is managed independently, high cell density increases the probability that two small cells sharing the same PCI are neighbors to the same macro cell, thereby creating a conflict in inter-layer adjacencies, even when within the layers PCI conflicts are avoided. We propose a method that can minimize these inter-layer conflicts, while still allowing independent assignment between the layers. Starting with an initial intelligent guess for adequate PCI reuse distance, the solution then uses the Automatic Neighbor Relation (ANR) function to learn the full multi-layer network topology and optimize the PCI assignment. Comparing with state of the art strategies, our results show that the proposed approach maintains good performance without requiring the exchange of information across layers.

Optimistic concurrency control in self-organizing networks using automatic coordination and verification

In a mobile Self-Organizing Network (SON), the SON coordinator has been introduced to control the application of Configuration Management (CM) changes, in order to prevent conflicts between independent SON function instances running in parallel. However, there is always a trade-off between stability and efficiency. On one hand we need to avoid conflicts, on the other we want fast, parallelized execution of SON function instances. Additionally, the concept of SON verification has been developed to automatically detect and correct degradations that arise from unexpected side-effects of (parallel) CM changes made by SON functions or human operators. However, as the number of function instances increases in the future networks, the performance of the SON coordinator becomes critical, i.e. excessive serialization is no longer possible. In this paper, we show how both, SON coordination and verification, can work together and how the cooperation enables a more efficient SON without having to compromise on its stability. This can be achieved by extending the SON function execution coordination to SON verification and by dynamically adjusting the coordination policies between more relaxed and more strict concurrency control strategies based on the feedback from the verification.

Network Element Stability Aware Method for Verifying Configuration Changes in Mobile Communication Networks

Automatic Configuration Management CM parameter change assessment, the so-called Self-Organizing Network SON verification, is an important enabler for stable and high-quality modern SONs. However, it also presents a new set of challenges. While improving network stability and resolving unexpected conflicts caused by parallel configuration changes, the SON verification can also make the network optimization less dynamic. This flexibility can be desirable, especially when the network is in an unstable state. One such example is Network Element NE commissioning, after which it may be preferable for the SON functions to explore the configuration space more freely in order to find the optimal configuration. On the other hand, at some point in time, the NE has to converge to a stable configuration. To address these challenges, we introduce in this paper the concept of Network Element Virtual Temperature NEVT, which indicates the state of stability of a NE, and propose how it can be utilized to optimize the verification process. This approach is evaluated in a simulated environment and compared to other verification mechanisms. The results show that the proposed method allows the network to better react to changes without sacrificing on its stability.