Peter Legg

A Simulation Study of LTE Intra-Frequency Handover Performance

Abstract-Intra-frequency handover is an important RRM procedure in LTE and the (self-) optimization of its performance, in terms of handover reliability and handover frequency, has been studied extensively by 3GPP. We present a simulation analysis of the handover performance in an LTE macro deployment, considering the impact and interdependency of key handover settings (offset, TTT, filter coefficient K) for UEs of different speeds. Different parameter combinations giving the same handover failure rate give approximately the same handover frequency; there is scope to reduce the latter by 10 to 20% by selecting the best combination. Failures, occurring mostly in the source cell, may be reduced through careful user plane design.

Optimised Iub flow control for UMTS HSDPA

In this paper, a strategy for exploiting the flow control signaling between the RNC and Node B for UMTS (Universal Mobile Telecommunications System) HSDPA (high speed downlink packet access) is presented. The flow control manages the transfer of data to the scheduler, that is located at the Node B. Simulation of a TDD mode cell demonstrates that the strategy achieves near minimum latency whilst at the same time limiting the data loss on inter-Node B handover. The loading of the flow control signaling on the Iub interface is acceptable.

Small Cell densification requirements in high capacity future cellular networks

In order to cope with the expected increase in traffic demand, the large-scale deployment of Small Cells (SC) becomes a cost-efficient way to realize the foreseen capacity explosion in the near future. In this paper we investigate, for a particular forecast in the 2020, what are the requirements in terms of capacity per area unit and how SC densification along with other design criteria can fulfill such demands. A wide set of illustrative numerical results are provided showing key design criteria such as frequency reuse, deployment planning, macro-cell power-down and transmitted power reduction, which, in conjunction with SC densification, have strong impact on the capacity delivered by the network.

A Centralised Approach to Power On-Off Optimisation for Heterogeneous Networks

The design of centralized algorithms and techniques allowing for an efficient utilisation of infrastructure in terms of energy consumption is one of the key challenges in heterogeneous networks (HetNets). In this study the energy efficiency in the HetNet scenario is formulated as an optimisation problem and an iterative improvement algorithmic approach to power on-off of network cells is devised and evaluated. The algorithm is based on the simulated annealing search approach and the obtained network configuration solutions are compared to a baseline configuration scenario where all cells are powered on. The optimization search is guided by an objective function which is defined on outage throughput and energy efficiency. Simulation results show that significant energy reductions gains can be achieved by switching off macro cells and with no loss of the cell edge user throughput. In some scenarios the algorithm generates solutions which considerably increase network throughput. The gains are more pronounced in configurations where pico cells are deployed at hot zones of user clusters.

A network controlled handover mechanism and its optimization in LTE heterogeneous networks

Mobility Robustness Optimization (MRO) is proposed in Long Term Evolution (LTE) networks to improve handover performance by 3GPP (3rd Generation Partnership Project). However, conventional MRO algorithms which tune cell (-pair) level handover parameters that control the generation of measurement reports have restricted gain because of the uneven interference distribution within the handover region in the spatial domain, and also the interference variation in time domain when the cell load changes. Mobility optimization is even more challenging in heterogeneous networks (HetNet) due to the more complex channel and load conditions there. In this paper, we propose a network controlled handover mechanism to solve these problems. The mechanism exploits measurement reports to identify the best target cell and channel quality measurements to realize a handover specific timing decision. System simulations show the new mechanism can be configured by a MRO algorithm to overcome many of the handover challenges in LTE HetNet.

Evaluation of LTE HetNet deployments with realistic traffic models

LTE Hetnet deployments using pico cells raise many questions regarding their deployment and operation. This study addresses some key questions using a system simulator and three different traffic models, FTP, HTTP and real-time video. Uplink and downlink are considered. Conclusions drawn from previous work are verified using these models. It is found that system throughput or user satisfaction is increased if macro UEs can be offloaded to the pico layer. This requires pico placement adjacent to user hot-spots and pico cell enlargement using cell range extension (CRE) or increased pico transmit powers. The pico count can be halved by a 6dB increase in pico size. CRE values of 6dB or greater benefit from interference reduction from almost blank subframes (ABS). Additionally, splitting the frequency band (re-use 2) reduces the cell area throughput except under high pico densities.

Mobility performance and suitability of macro cell power-off in LTE dense small cell HetNets

It is widely accepted that future capacity demands will require the deployment of dense small cell networks most surely co-existing with legacy macro cell deployments forming a so-called heterogeneous network (HetNet). Whereas network capacity can benefit from these dense small cell deployments, mobility management becomes increasingly complex to handle, especially in high mobility outdoor urban environments. This paper addresses the performance evaluation of Long Term Evolution (LTE) handover mechanisms in dense small cell HetNets. We assess the potential challenges and we discuss the suitability of Macro cell power-off in “sufficiently dense” small cell deployments. Extensive numerical results are presented through detailed system level simulations.

Packet data transmissions in a W-CDMA network-examples of uplink scheduling and performance

This paper addresses the problem of efficient scheduling of packet data users in a W-CDMA network. Two simple, centralised algorithms are described for the uplink, and their performance illustrated in the case of the FDD mode of UMTS, for a delay-tolerant service. Both algorithms are centralised schedulers that attempt to manage tightly the interference power in each cell. Simulation results illustrate the performance (delay, throughput) one might expect from such a system.

Soft Metric Assisted Mobility Robustness Optimization in LTE Networks

Mobility Robustness Optimization (MRO) is a use-case of Self-Organizing Networks (SON) for LTE that aims to optimize handover performance automatically. Conventional MRO algorithms tune handover parameters based on counts of handover failures and handover events. The algorithm requires many handovers per cell pair (e.g. 1000) to trigger a reliable adjustment. Thus it can result in a slow adaptation to changes that improve or deteriorate handover performance, such as network loading or UE speed. In this paper, we propose a new MRO algorithm taking into account soft metrics, such as handover command transmission time, derived both from failed and successful handovers. The proposed algorithm provides a faster reaction and lower failure rate than the conventional method, with no deterioration in the handover count.

Coverage and Capacity Optimization in E-UTRAN Based on Central Coordination and Distributed Gibbs Sampling

Coverage and capacity optimization is considered as one of the most important optimization items for cellular network. It is beneficial from energy consumption and interference deduction perspective if the downlink base station power can be regulated according to the UEs distribution and service requirements. A cellular base station downlink power regulation scheme is studied where the power regulation scheme makes use of UE (user equipment) measurements and an optimization method Annealed Gibbs Sampling. A central node (or function entity) is involved as for the purpose of performance monitoring and coordinating the optimization process. Simulation with a LTE network configuration shows that while the measurements and optimization computation are distributed across base stations, a network performance can be optimized with this scheme especially for the cell edge users. Further characteristics of this scheme are presented and discussed.