Csaba Vulkan

Impact of Intra-LTE Handover with Forwarding on the User Connections

In order to ensure long-term competitiveness of 3G technology, a new study item has been started by 3GPP to define the long term evolution (LTE) of 3G. The scope of the LTE concept is to improve the system performance in terms of data rate, throughput, latency, coverage and cost. LTE introduces a pure packet based architecture, with distributed mobility management, where active mode handover decisions are done by E-UTRAN Node-Bs (eNB). During handover, downlink user data is forwarded between the involved eNBs. This paper provides an overview of the LTE and the intra-MME/SAE Gateway handover procedure. The impact of the forwarding on the user connections is evaluated with NS2 based simulations.

Network side lightweight and scalable YouTube QoE estimation

YouTube is a prominent Over The Top (OTT) application increasingly used on mobile devices for watching online videos. Due to the mobile environment, cellular technology specific reasons and the delivery mechanism adopted by YouTube, providing seamless customer experience is not straightforward. Being a popular application whose availability is important from customer satisfaction point of view, operators must deploy mechanisms capable of network wide YouTube QoE measurement and degradation detection. Known solutions aim to provide a comprehensive set of QoE influence factors such as the exact stallings for each video session. However, in order to generate network wide insight to a statistically relevant set of all YouTube downloads, such mechanisms may not represent an optimal balance between the provided level of details and their cost or complexity. Instead, this paper proposes a lightweight, real time, network side QoE estimation method suitable for network wide deployment. The proposed solution is able to detect YouTube videos with QoE degradations through novel metrics that were validated with real network measurements. The paper also presents the statistical analysis of a large set of videos showing how the method can evaluate the YouTube QoE in a network.

Enhanced Mobility Load Balancing Optimisation in LTE

Self-Organising Network (SON) use cases are seen as key enablers of efficient Long Term Evolution (LTE) system operation. Mobility Load Balancing Optimisation (LB) is a prominent SON use case defined by 3GPP with the scope of balancing the load in a given coverage area by reconfiguring the handover thresholds. When overload is detected at a given cell, the LB reduces the handover thresholds of those user equipments that can be redirected to the underutilised neighbouring cells. This mechanism improves the user plane performance in the overloaded cell. Despite the potential benefits, the available LB mechanisms have two major deficiencies, both of them causing user experience degradation: on the one hand, they tend to overestimate the load on the air interface, resulting in unnecessary handovers; on the other hand, they are either transport agnostic or have limited capability to consider the transport load during their decision process. This paper provides an overview of the LB related problems and proposes an improved solution including accurate evaluation of the radio load and a transport aware mechanism for end-to-end optimisation. The performance of the solution was evaluated by simulations.

Congestion Control in Evolved HSPA Systems

Evolved HSPA is a major evolutionary step of the WCDMA based 3GPP technology that provides high end user data rate and increased cell throughput. It can be deployed over the legacy 3G architecture, but an alternative packet based, flat architectural solution is introduced too, that enables the later transition to LTE. Smooth transition requires that the new, flat systems are first deployed as complementary solutions of the existing infrastructure. This will lead to heterogeneous radio systems that will be served by a shared packet based transport network. This co-existence raises fairness issues as transport congestion is handled differently by legacy and flat systems in addition to the fact that due to Radio Layer 2 protocol aspects, the Iub traffic is not TCP friendly. This paper discusses the architecture of the heterogeneous systems, the fairness problem in case of congestion on shared transport links and proposes a congestion control mechanism that is able to enforce fair resource usage. The solution is evaluated with simulations.

Throughput analysis of the proportional fair scheduler in HSDPA

High speed downlink packet access (HSDPA) provides increased user data rate by introducing additional functionalities at the Node B, such as fast packet scheduling based on the instantaneous radio link quality. One of the algorithms that is able to provide this capability is the proportional fair (P-FR) scheduler. In this paper we describe an approximate analysis method for the mean throughput of the proportional fair scheduler with two traffic classes. We extend the existing results that are based on the assumption of continuous rate distributions to the more realistic discrete distributions. The analysis with discrete distributions entails two problems that do not appear in the continuous case, namely that the priority of several users can be equal with a non-zero probability and that the sample path of throughputs does not always converge to a mean throughput. The proposed approximate method is evaluated with NS2 simulations.

Analytical TCP throughput model for high-speed downlink packet access

This study deals with the throughput analysis of data services over high-speed downlink packet access (HSDPA) systems. The achievable throughput is calculated with an approximate analytical method based on the Padhye model that has two input parameters: the packet loss probability and the transmission control protocol (TCP) round trip time. The proposed solution is to calculate these parameters with an equivalent queuing network model of the HSDPA system that takes into account the possible congestion points in the system and the protocol layers that have dominant impact on the delay and packet drop. The modelling considerations and the analysis method are described in detail. Finally, the model is validated with a performance study of the HSDPA system that is executed with detailed NS2- based simulations too. The proposed method is found to be reasonably accurate requiring less computational effort than the simulations.

Insight based dynamic QoE management in LTE

Long Term Evolution (LTE) subscribers expect seamless Quality of Experience (QoE) during their ubiquitous Internet access. Due to the high degree of application and content diversity, the amount of system resources that enable good QoE is application session specific and varies on a wide scale. Accordingly, good QoE requires dynamic and adaptive application session level resource management. The standard LTE Quality of Service (QoS) architecture was designed to provide bearer level differentiation thus it has neither the insight to the application sessions nor the granularity to provide proper QoE for each application session. Existing proprietary enhancements operating via context agnostic and static rule based bearer prioritization or intra-bearer scheduling are not able to allocate resources according to the actual QoE requirements of the application sessions. This paper describes a solution that maximizes the level of QoE and uses the system resources efficiently by dynamically adapting the QoS configuration to the estimated demand of individual application sessions. Simulations show that the solution is superior to the existing static methods that fail to provide proper QoE.

Efficient LTE PDCP buffer management

Long Term Evolution (LTE) evolved Node Bs (eNB) have dedicated buffers for each Radio Access Bearer (RAB) implemented at the Packet Data Convergence Protocol (PDCP) layer to receive and prepare user data for transmission over the air interface. These buffers multiplex the connections established by the active applications of a single user. Modern applications use the Transmission Control Protocol (TCP) that has a greedy mechanism to transfer data by trying to use all the available resources. If not managed, TCP sources may send large bursts that may eventually overload the PDCP buffer causing tail drops and quality degradation. In order to prevent these incidents, buffer management mechanisms are required. As this paper shows, neither the standard 3GPP Discard Timer nor the conventional Active Queue Management (AQM) techniques are able to properly manage the PDCP buffer. As a solution, two alternative mechanisms are proposed, which are more suited to the PDCP specific environment. Their superior performance is shown with detailed simulations.

Two-way TCP performance issues and solutions in asymmetric LTE radio access

Due to the joint evolution of mobile applications and cloud services, mobile users routinely act as both content consumers and producers. Nowadays there is an increasing trend in both the downlink (DL) and uplink (UL) data traffic, which are predominantly Transmission Control Protocol (TCP) based. Simultaneous DL and UL TCP transmission over an asymmetric link causes significant throughput degradation in the direction with higher capacity whenever TCP data and acknowledgment (ACK) segments are not treated separately. The radio access links of the Long Term Evolution (LTE) systems are asymmetric with higher capacity in the DL than in UL, making LTE prone to the asymmetry problem. In this context, the degradation is caused by multiplexing data and ACKs in the same User Equipment (UE) buffer before UL transmission. This paper proposes and evaluates three novel alternatives that solve the problem at the network side, i.e., transparently to the end devices. Simulation results indicate that the solutions can efficiently detect and resolve the asymmetry problem and they are able to reach the theoretical optimum within 1-5%. Excellent performance is provided even in the presence of complex web traffic.

Integrated Mobility Load Balancing and Traffic Steering mechanism in LTE

Mobility Load Balancing (MLB) is a Self-Organising Networks (SON) use case with the scope of detecting and resolving radio overload. In case of overload, the MLB triggers the handover of cell edge users towards less loaded neighbour cells to better align the traffic demand with the capacity available on the air interface. This, however, also increases the load on the transport links of these cells; therefore, MLB should consider the transport status in order not to cause transport congestion while resolving air interface overload. This paper proposes a general MLB framework that, unlike existing mechanisms, has efficient means to properly consider the transport load and congestion in addition to the radio load. The solution is not limited to any particular transport network topology and it also acts as a Traffic Steering (TS) mechanism as it resolves transport overload by redirecting users to neighbour cells with spare transport resources. The performance of the proposed framework was evaluated with simulations. Results indicate that the solution improves the overall system performance by balancing both radio and transport load.