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Evolution of Wireless Communication Systems
The evolution of wireless communication systems is mainly driven by the introduction of new services and the availability of more advanced technologies. Over the last couple of decades, cellular networks have grown exponentially and the demand for new and improved services has become an important issue for operators. There is a need for new technologies to alleviate the capacity limitations of the network and to maintain the QoS demanded by the users. This has motivated the development of new standards like the Third Generation Partnership Project’s (3GPP) LTE standard [6] in order to provide higher data rates and an improved QoS in wireless networks. The 3GPP LTE standard has chosen Orthogonal Frequency Division Multiple Access (OFDMA) as the underlying modulation technology [7]. It is a spectrally efficient version of multi-carrier modulation, where the subcarriers are selected in such a way that they are all orthogonal to one another over the symbol duration. Physical layer DL transmissions are implemented using OFDMA while Uplink (UL) transmission uses Single Carrier Frequency Division Multiple Access (SC-FDMA). The main difference between both schemes is that in OFDMA data detection is performed in the frequency domain while in SC-FDMA it is done in the time domain averaging the noise over the entire bandwidth [52].
Despite being in the early days of roll-out, LTE has become the fastest developing system due to the fast speeds and high quality user experience that it offers. It has now been launched on all continents, by 156 operators in 67 countries. By 2013, there are 6.4 billion of mobile subscriptions globally [1]. Current commercial LTE deployments are based on 3GPP Release 8 and 9 [5].
Data traffic will continue to grow, along with mobile data subscriptions and an increase in the average data volume per subscription. In fact, overall mobile data traffic is expected to continue the trend of doubling each year [1]. This growth in traffic and services will bring new technical challenges to the operators, interference being one of the most performance-limiting. The demand for new mobile services and for higher peak bit rates and system capacity is tackled with the evolution of the technology to 4G. The 4G systems are based on 3GPP LTE and are progressing on a large scale, with 55 million users as of 2012 and from 1.6 to 2 billion users anticipated in 2018 [1].
Interference Scenarios in 4G Networks
During the past 20 years, there has been a massive growth in traffic volume, number of devices connected, and an increased demand for video data. Future cellular networks should be able to cope with this increased demand and handle all the traffic in an efficient way.
There are new technical challenges, and potential interference scenarios that vary with the type of deployments, requirements, high data rate and QoS levels. These new interference scenarios have been considered during the LTE Release 10 standardization:
• Macro-picocell interference (see figure 2.1).
• Macro-Home-eNodeB (HeNB) interference (see figure 2.2). Enhancements in the operation of the base stations, advanced terminal receivers and a new carrier type with reduced transmission of “always-on” signals are a requirement for high network energy efficiency [28]. With the transmission of control signaling, a non-negligible amount of energy is used by the power amplifier. Minimizing the transmission of “always-on” signals allows the base station to turn off circuitry if it has no data to transmit. By eliminating unnecessary transmissions, the interference is reduced, leading to improved data rates in the network. Another aspect to consider is that with the use of small-cells, sporadic traffic, and a reduction in the transmission of control signals, interference becomes time-varying. When designing new techniques, these new characteristics and requirements have to be taken into account.
Heterogeneous Networks and Interference
As the traffic demand grows and the RF environment changes, the network relies on cell splitting or additional carriers to overcome capacity and link budget limitations and maintain uniform user experience. Moreover, site acquisition for macro base stations with towers becomes more difficult in
dense urban areas. A more flexible deployment model is needed for operators to improve broadband user experience in a ubiquitous and cost-effective way. The concept of Heterogeneous Networks (HetNets) has been introduced in LTE to address the capacity and coverage challenges resulting from the enormous and continuous growth of data services. The traditional macro network is deployed to provide umbrella coverage and smaller nodes are added as an underlay network to alleviate coverage holes and traffic hot zones. These low-power nodes provide very high traffic capacity and user throughput locally, for ex. indoor and outdoor hotspot positions. The macro layer ensures service availability and QoS over the entire coverage area. HetNets include microcells, picocells, femtocells, and distributed antenna systems (remote radio heads), which are distinguished by their transmit powers, coverage areas, physical size, backhaul and propagation characteristics [33].
Table of contents :
Abstract
Contents
List of Figures
Acronyms
Notations
1 Introduction
1.1 Contributions and Thesis Outline
2 Background
2.1 Evolution of Wireless Communication Systems
2.2 Interference Scenarios in 4G Networks
2.2.1 Heterogeneous Networks and Interference
2.2.2 Femtocells
2.2.3 Machine-to-Machine Communications
2.3 Scheduling and Link Adaptation in LTE
2.3.1 Resource Allocation in LTE
2.3.2 Discontinuous Reception (DRX)
3 Performance Evaluation of Small-cell Deployments
3.1 Interference in Femtocell Deployments
3.1.1 System Model and Assumptions
3.1.2 Average Throughput Analysis of HARQ with Interference Cancellation
3.1.3 Performance of the HARQ Protocol in Femtocell Deployments with Interference
4 Mutual Information Analysis of Interference Networks
4.1 Key Challenging Applications
4.1.1 Heterogeneous Networks
4.1.2 M2M and Sparse Latency-Constrained Traffic
4.2 Related Work
4.3 Initial Analysis for Interference-free Networks
4.3.1 Signal Model and Assumptions
4.3.2 Modeling and Optimization of a Resource Scheduling Policy
4.3.3 Numerical Results
4.4 Interference Networks Analysis
4.4.1 Modeling and Assumptions
4.4.2 Simple Interference Analysis in Zero-outage
4.5 Practical Interference Networks Analysis
4.5.1 Rate Optimization (fixed across rounds)
4.5.2 Rate Optimization with an Outage Constraint
4.6 Methodology for Resource Allocation in Practical Interference Scenarios
4.6.1 Manhattan-topology for Small-cells
4.6.2 Macro/Small-cell Scenario
4.6.3 Physical layer Abstraction Models
5 Practical Scheduler Design for LTE Base Stations
5.1 Handling Interference in LTE Networks with HARQ and AMC
5.2 OpenAirInterface Implementation
5.2.1 Physical Layer and Resource Allocation
5.3 Application of the Scheduling Policies in LTE
5.4 Performance Analysis of the Scheduler
5.4.1 Results without interference
5.4.2 Results with one interferer
5.5 Scheduler under the Full LTE PHY/MAC Protocol Stack
5.5.1 Feasibility Evaluation
5.5.2 Interference Scenario Description
6 Conclusions and Areas for Further Research
6.1 Conclusions
6.2 Areas for further research
Appendix A Summary of the thesis in French
A.1 Abstract en français
A.2 Introduction
A.2.1 Contributions et cadre de cette thèse
A.3 Résumé du Chapitre
A.3.1 Evolution des systemes san fils
A.3.2 Interférence dans les reseaux 4G
A.3.3 Gestion et adaptation de liaison pour LTE
A.4 Résumé du Chapitre 3
A.4.1 Interférence dans les reseaux small cells
A.4.2 Modèle du system
A.5 Résumé du Chapitre 4
A.5.1 Applications clés
A.5.2 Analysis pour les réseaux avec interférence
A.6 Résumé du Chapitre 5
A.6.1 Implémentation sur OpenAirInterface
A.6.2 Application des techniques pour les modems LTE
A.7 Conclusion
Bibliography
