Challenges and Perspectives for the Future Broadband Wireless Communications for Railway
This chapter presents the challenges and perspectives for the future broadband wireless communications for railway. First of all, a section is dedicated to the next generation broadband technologies, such as Cognitive Radios and 5G technologies. Then, the current work groups in railway community are presented. Finally, the challenges and perspectives are detailed from different work programs such as in the context of the Joint Undertaking Shift2Rail.
Next Generation Broadband Technologies
The book aims to describe all the railway applications requiring broadband capacities. We focus on this part on the emerging promising technologies that can solve the problem of increasing needs in terms of throughputs and the problem of spectral congestion.
Spectral Aggregation and Cognitive Radio
Definitions and Standards
Among all the solutions for an Internet on board trains, the need of high capacity leads to solutions based on the aggregation of several available frequency bands. This technique is currently used in order to increase the capacity of the systems and allow broadband communications. The satellite and cellular solutions, presented in Chap. 2, are especially based on this aggregation technique. Another solution to optimize the use of the frequency spectrum is to rely on Cognitive Radios (CR).
Recently, the rapid growth in wireless communications has contributed to a huge demand on the deployment of new wireless services. The radio electromagnetic
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E. Masson and M. Berbineau, Broadband Wireless Communications for Railway Applications, Studies in Systems, Decision and Control 82,
spectrum is a limited physical quantity, and only a certain part of it is suitable for radio communications. The traditional way of governing this resource has been to administer licenses for portions of the spectrum, usually by a national agency such as US Federal Communications Commission (FCC). Almost all the usable portions of the spectrum are allocated for licensed users. Available electromagnetic spectrum for wireless transmission has become a highly valuable resource. However, recent researches published by the FCC  show that the traditional static frequency allocation policy is not efficient and results in poor spectrum utilization. In , a general survey of radio frequency bands (from 30 MHz to 3 GHz) is provided.
Figure 3.1 presents the average spectrum occupancy and highlights how low could be the spectrum occupancy in many bands. The dramatic increase in the demand for radio spectrum and the actual low spectral efficiency has spurred the development of a next generation wireless technology referred to as Cognitive Radio. Cognitive Radio concept, introduced by Mitola in 1999 , appears to be a tempting solution to the spectral congestion problem by using frequency bands not heavily occupied by licensed users. The International Telecommunication Union (ITU) validated the following definition: “Cognitive Radio system is a radio system employing technology that allows the system to obtain knowledge of its operational and geographical environment, established policies and its internal state; to dynamically and autonomously adjust its operational parameters and protocols according to its obtained knowledge in order to achieve predefined objectives; and learn from the results obtained” .
Fig. 3.1 Summary of spectrum band occupancy calculations from 
Mitola introduced the cognitive cycle and theorized some concepts which can be summarized as : broad sense adaptation to the environment, intelligence in the network and the terminal, independence of the terminal with respect to the network and the operator and independence of the user with respect to the technique.
A CR system can then adapt its behavior through three steps:
- • Information capturing: provided by sensors at all levels to obtain adequate information about radio interface, propagation, network, protocols, security and user requirements;
- • Decision making: decisions can be made based on training and/or knowledge bases. The stage of decision making can use information from sensors but also very broad concepts, such as technico-economic considerations via regulatory rules for spectrum use;
- • Adaptation: this part concerns the auto-reconflguration step which is provided by support technology, the Software Defined Radio technology.
Cognitive Radio systems need the development of Intelligent Mobile Terminal, Intelligent Infrastructure and Cognitive Manager that will allow the cooperation between infrastructure and mobile terminal.
Intelligent Mobile Terminal consists of essential stages, such as spectrum sensing, channel estimation, blind modulation recognition or source separation.
Spectrum sensing represents a primary and essential function of CR for dynamic spectrum access. The objective is to detect the white spaces and free frequency bands without causing interferences with Primary User (PU). The spectrum usage can be categorized into black, grey and white spaces. Spectrum sensing relies then on white spaces detection . In , a survey of spectrum sensing techniques and algorithms is performed. The algorithms can be classified into three main methods: coherence detection, non-coherent (blind) detection and feature detection. Channel estimation and equalization are also part of the main techniques to set up a Cognitive Radio system.
The Intelligent Infrastructure relies on an inter-layer architecture with potential alternatives for interacting with the heterogeneous access technologies. A basic system of hierarchical data enables learning by memorization in a local database and consolidation in a central knowledge base.
Finally, the decision engine (or “Cognitive Manager”) uses this information to control all components at all levels of the stack (software, operating system, hardware).
In , the subject of security issues in the special case of Cognitive Radios is addressed. It is stated that this subject has not yetbeen many studied compared to other issues for CR case. All communication systems need to be made secure to operate by techniques, such as authentication, authorization, encryption, accounting and nonrepudiation processes. Cognitive Radios cause unique security issues, such as the observation of a huge amount of information or the extensive use of collaboration as for the spectrum sensing techniques and the spectrum sharing.
The standardization issues have significant importance in the development of CR systems, since it encourages companies to invest in this domain. Several standards are already published or in a draft status. The most rising one is the IEEE 802.22 standard. IEEE 802.22 is a standard for Wireless Regional Area Network (WRAN) using white spaces in the television (TVWS) frequency spectrum .
The development of the IEEE 802.22 WRAN standard aims then to use Cognitive Radio techniques to allow sharing of geographically unused spectrum allocated to the television broadcast service, on a non-interfering basis. It is the first worldwide effort to define a standardized air interface based on CR techniques. The 802.22 working group deals with issues on physical layer and Medium Access Control (MAC) layer and also on spectrum sensing, geolocation and security issues . The novelty of the standard is that it treats at the same time coexistence mechanism between licensed TV band and unlicensed broadband networks, Cognitive Radio concept, super frame structure specially designed for incumbent protection and two layer security concept .
The IEEE P1900 standard committee was established as a result of the growing interest for dynamic spectrum access networks. The objective of the committee was to support the standards dealing with dynamic spectrum management and next generation radio developments. The Standards Coordinating committee 41 (SCC41) replace the P1900 since 2007. The seven working groups of the SCC41 are :
- • IEEE P1900.1: Definitions and Concepts for Dynamic Spectrum Access: Terminology Relating to Emerging Wireless Networks, System Functionality, and Spectrum Management;
- • IEEE P1900.2: Recommended Practice for the Analysis of In-Band and Adjacent Band Interference and Coexistence Between Radio Systems;
- • IEEE P1900.3: Recommended Practice for Conformance Evaluation of Software Defined Radio (SDR) Software Modules;
- • IEEE P1900.4: Architectural Building Blocks Enabling Network-Device Distributed Decision Making for Optimized Radio Resource Usage in Heterogeneous Wireless Access Networks;
- • IEEE P1900.5: Policy Language and Architectures for Managing Cognitive Radio for Dynamic Spectrum Access Applications;
- • IEEE P1900.6: Spectrum Sensing Interfaces and Data Structures for Dynamic Spectrum Access and other Advanced Radio Communication Systems;
- • IEEE P1900.7: White Space Radio Working Group.