Lucas Chavarria Gimenez

From LTE to 5G for Connected Mobility

Long Term Evolution, the fourth generation of mobile communication technology, has been commercially deployed for about five years. Even though it is continuously updated through new releases, and with LTE Advanced Pro Release 13 being the latest one, the development of the fifth generation has been initiated. In this article, we measure how current LTE network implementations perform in comparison with the initial LTE requirements. The target is to identify certain key performance indicators that have suboptimal implementations and therefore lend themselves to careful consideration when designing and standardizing next generation wireless technology. Specifically, we analyze user and control plane latency, handover execution time, and coverage, which are critical parameters for connected mobility use cases such as road vehicle safety and efficiency. We study the latency, handover execution time, and coverage of four operational LTE networks based on 19,000 km of drive tests covering a mixture of rural, suburban, and urban environments. The measurements have been collected using commercial radio network scanners and measurement smartphones. Even though LTE has low air interface delays, the measurements reveal that core network delays compromise the overall round-trip time design requirement. LTEs breakbefore- make handover implementation causes a data interruption at each handover of 40 ms at the median level. While this is in compliance with the LTE requirements, and lower values are certainly possible, it is also clear that break-before-make will not be sufficient for connected mobility use cases such as road vehicle safety. Furthermore, the measurements reveal that LTE can provide coverage for 99 percent of the outdoor and road users, but the LTE-M or NarrowBand-IoT upgrades, as of LTE Release 13, are required in combination with other measures to allow for additional penetration losses, such as those experienced in underground parking lots. Based on the observed discrepancies between measured and standardized LTE performance, in terms of latency, handover execution time, and coverage, we conclude the article with a discussion of techniques that need careful consideration for connected mobility in fifth generation mobile communication technology.

Comparison and Extension of Existing 3D Propagation Models with Real-World Effects Based on Ray-Tracing

The next generation of cellular network deployment is heterogeneous and temporally changing in order to follow the coverage and capacity needs. Active Antenna Systems allows fast deployment changes by cell shaping and tilt adaptation which have to be controlled in self-organized manner. However, such kind of automated and flexible network operations require a Self Organizing Network (SON) algorithm that works based on network performance parameters being partly derived from the radio measurements. Thus, appropriate radio propagation models are not only needed for network planning tools but also for simulative lab tests of the developed SON algorithm controlling the flexible deployment changes enabled by Active Antenna Systems. In this paper, an extension of the existing 3D propagation model is proposed in order to incorporate the propagation condition variation effects, not considered so far, by changing antenna beam orientation like antenna tilting or when users are distributed in the third dimension (height) in multi-floor scenarios. Ray tracing based generated propagation maps that show the realistic propagation effect are used as 3D real world reference for investigation and model approval.

Protocol coding for reliable wireless bits under jamming: Concept and experimental validation

Practical digital communication systems are designed to operate well in typical conditions, where the bit/packet error rates are below a certain level. In packet-based systems, if Bob has a data to send to Alice then Bob first transmits a preamble/packet header followed by data. A necessary condition to receive the data correctly is to receive the packet header and therefore headers use a more robust encoding. Under typical conditions, the probability of error for the packet header is close to 0. However, when the received signal at Alice is very weak or she experiences excessive interference, the error rate for the data part of the packets goes to 1, while the error rate for the header part increases significantly. This causes the wireless connection to break and produces a rate waterfall effect in digital systems: either the wireless data rate is above a minimal rate R, or it is equal to 0. In other words, practical digital systems do not degrade the data rate gracefully towards 0 as the transmission conditions worsen. We have recently introduced the concept protocol coding to refer to strategies for sending information by using the degrees of freedom available when one needs to decide the actions taken by a particular communication protocol. Under typical operating conditions, protocol coding delivers relatively low data rate, but the transmitted bits are very reliable. In this paper we show that by applying protocol coding, the rate waterfall effect can be avoided in practically any wireless digital communication system. An example application of such a concept is the case in which a short critical message needs to be delivered to a severely interfered receiver. We describe an experimental setup which proves the viability of this concept in a WiFi system exposed to a commercial jammer of the ISM band.

Mobility Performance in Slow- and High-Speed LTE Real Scenarios

Mobility performance and the impact of handover data interruption times in real scenarios are studied by means of field measurements in an operational LTE network. Both slow- and high-speed scenarios are analyzed by collecting results from two different areas: Aalborg downtown and the highway which encircles the same city. Measurements reveal that the terminal is configured by the network with different handover parametrization depending on the serving cell, which indicates the use of mobility robustness optimization. Although the network is dominated by three-sector sites, no intra-site handovers are observed in the city center as cells on the same site often cover different non-crossing street canyons. Moreover, no handover failures are experienced in the measurements which confirms robust LTE mobility performance. The average interruption time, which is at least equal to the handover execution time, lays within a 24-29ms interval. Nevertheless, examples of delays larger than 100ms are occasionally observed. The studied scenarios are replicated in a system level simulator to investigate whether simulations are capable of reproducing similar mobility performance.

A Simple Statistical Signal Loss Model for Deep Underground Garage

In this paper we address the channel modeling aspects for a deep-indoor scenario with extreme coverage conditions in terms of signal losses, namely underground garage areas. We provide an in- depth analysis with regard to the path loss (gain) and large-scale signal shadow fading, and propose a simple propagation model which can be used to predict cellular signal levels in similar deep- indoor scenarios. The measurement results indicate that the signal at 800 MHz band penetrates external concrete walls to reach the lower levels, while for 2000 MHz wall openings are required for the signal to propagate. It is also evident from the study that the shadow fading at different levels of an underground garage are highly correlated. The proposed frequency-independent floor attenuation factor (FAF) is shown to be in range of 5.2 dB per meter deep. Therefore, the attenuation rate in the z dimension is much higher than the in-building attenuation in x and y dimension, which is often assumed at 0.6 dB/m.

Mobility Sensitivity Analysis for LTE-Advanced HetNet Deployments with Dual Connectivity

A mobility performance sensitivity analysis is presented for Dual Connectivity cases where the users can be served simultaneously by a macro and a small cell. The performance is assessed for the 3GPP generic simulation scenario and for two site-specific cases, with the aim of comparing the results coming from these different scenarios. The site-specific scenarios are based on detailed three-dimensional topography map data and advanced ray-tracing techniques. It is generally found that the first order statistics and overall conclusions are in good alignment for the considered environments. However, there are also a number of differences in the obtained performance results for the different cases, e.g. in terms of small cell time-of-stay statistics. The explicit modeling of streets and building topology, propagation in street canyons, etc., is found to have an important impact on the mobility performance. Especially the time-of-stay statistics and the location of handover failures differ between generic 3GPP case and the site-specific modeling scenarios. So in conclusion, trends and observations from simulations based on the 3GPP scenario are valuable, but should be supplemented by results from site-specific scenarios, as additional performance-determining effects are more accurately represented for such cases.

Analysis of Data Interruption in an LTE Highway Scenario with Dual Connectivity

This study evaluates whether last versions of Long Term Evolution with dual connectivity are able to support the latency and reliability requirements for the upcoming vehicular use-cases and time-critical applications. Data interruption times during handovers and cell management operations are evaluated by means of system level simulations for a high-speed scenario. The scenario models a highway covered by a macro layer and an ultra dense network of small cells distributed on both sides of the road. Results reveal that for single connectivity, and due to the large amount of handovers, terminals are unable to exchange data with the network about 5% of the time. This time is considerably reduced if dual connectivity with split bearer architecture is adopted, with less than 1%of time in data interruption. However, when adopting secondary cell group architecture, the relative data interruption time increases up to 6.9%.

Validation of Mobility Simulations via Measurement Drive Tests in an Operational Network

Simulations play a key role in validating new concepts in cellular networks, since most of the features proposed and introduced into the standards are typically studied by means of simulations. In order to increase the trustworthiness of the simulation results, proper models and settings must be provided as inputs to the simulators. It is therefore crucial to perform a thorough validation of the models used for generating results. The objective of this paper is to compare measured and simulated mobility performance results with the purpose of understanding whether simulation models are close to reality. The presented study is based on drive tests measurements and explicit simulations of an operator network in the city of Aalborg (Denmark) - modelling a real 3D environment and using a commonly accepted dynamic system level simulation methodology. In short, the presented results show that the simulated handover rate, location of handovers, radio link failures, and signal/interference level statistics match well with measurements, giving confidence that the simulations produce realistic performance results.

Towards Zero Data Interruption Time with Enhanced Synchronous Handover

This paper presents enhancements for lowering the handover interruption time in future wireless networks. We propose a selective data forwarding for the handover preparation phase, and the integration of the make-before-break procedure with the synchronous random access-less handover. To evaluate our proposals, we analyze the handover timing for typical and variable values of the user equipment and e-NodeB processing times, and X2 interface latencies. Our results show that the processing delays, reconfiguration times, and the X2 latency should be simultaneously reduced to minimize the data interruption time. Selective data forwarding during the handover preparation reduces the data interruption time by 18 % compared to the basic random access less handover with typical network delays. Make-before-break is the most suitable handover type for future low-latency applications, as it achieves zero data interruption independent of the latency of handover steps.

Mobility Management for Cellular Networks: From LTE Towards 5G

The ongoing design and standardization of the fifth generation (5G) new radio (NR) will enable new use cases and applications, imposing more challenging requirements in terms of mobility performance. As an example, 5G mobile networks should support seamless mobility with zero data interruption at each handover, even at high speeds. A prerequisite for research towards new 5G mobility solutions is to first understand what current procedures can achieve. The initial work of this thesis is therefore focused on the analysis of field-measurements of an operational Long Term Evolution (LTE) network in both slowand high-speed scenarios, observing rate of radio-link failures, handover failures, data interruption times, etc. It is found that the macro-cellular mobility performance is good with a low rate of failures. However, the measurements also reveal that the handover data interruption time can sometimes be hundreds of milliseconds and, therefore, presenting the first challenge to be addressed in order to fulfill the demanding 5G requirements. The field measurements are furthermore used to calibrate and validate system-level simulation models presented in the remainder of the thesis for benchmarking more sophisticated mobility solutions that are not yet implemented in the field. Secondly, studies of the mobility performance and the data interruption time for the more evolved LTE-Advanced (LTE-A) versions with dual connectivity are addressed. These studies are conducted for a variety of environments, including generic scenarios with hexagonal network topologies, non-uniform site-specific scenarios, pedestrian mobility and high-speeds up to 130 km/h. The impact of using different network architectures for implementing dual connectivity are assessed as well. Simulations results of a site-specific high-speed scenario shows that when adopting dual connectivity with secondary cell group (SCG) architecture, the overall data interruption time increases by 42 % compared to single-node connectivity. While, if dual connectivity is realized with the split-bearer architecture, the interruption time is reduced by 83 %. Furthermore, novel candidate solutions such as synchronous handover without random access (a.k.a. RA-less handover) and innovations with make-