<< Cognitive Radio for military communications has been an intensive topic of research in recent years. Its main applications range from the utilisation of unused frequencies to interoperability among large communication systems in all layers >>

Cognitive Radio (CR) systems obtain information about their environment and adjust their operation accordingly to provide required services to end users. Regarding spectrum use, future wireless systems equipped with CR capabilities could dynamically access new frequency bands, and at the same time protect higher-priority users on the same bands from harmful interference[3]. For future mobile communication systems, CR techniques present a promising opportunity for costefficient access to spectrum bands to meet growing user demands. The emergence of CR techniques, especially in the terrestrial domain, has recently played a significant role in wireless research.

The focus in CR research has remained strongly on terrestrial civilian networks, although activities such as Software and Cognitive Radio for European Defense (SCORED) addressed the same issue from the point of view of military communications. In addition to terrestrial applications such as utilising the unused frequencies, known as white spaces in between high-power TV-transmitters, CR has been proposed to other applications such as LTE to allow more efficient spectrum utilisation and frequency sharing between terrestrial and satellite systems. Research has been carried out in this field and it has been found that the key issue is to either avoid or manage the interference between systems sharing the spectrum, which also applies to tactical communications. In fact, CRs, due to their inherent environment sensing and transmission adaptation capabilities, are perfect communication platforms to construct tactical communication systems.

Cognitive Radio
Future wireless communications will demand radio technologies providing significantly higher capacity, bit rates and flexibility than existing systems. In addition, wireless access should cover the entire population, including rural and distant areas.

CR has been an active topic of research for some years now. CR technologies have been proposed to improve spectrum occupancy by exploiting the unused parts of the spectrum without interfering with primary users who have either higher priority or legacy rights[4], [3].

These radios are aware of their environment and the available resources. They learn from the environment and adapt to variations in the environment in real-time. In many cases, awareness of the environment equals awareness of the radio spectrum obtained through its own active measurements or from external sources such as public databases[1].

Spectrum awareness is not the only thing that can guide cognitive operation[2]. Time, space, and energy are other possible radio resources to be aware of and which guide operation. Relaying and various Multiple-Input and Multiple-Output (MIMO) methods can be used opportunistically to exploit spatial opportunities[2]. As an example, sensing combined with beamforming could give more accurate information about spectrum use in the vicinity.

In addition to being aware of surrounding environment and current situation, CRs also need mechanisms to utilise the information. Cognition can be applied in multiple layers starting from physical layer adaptation, including antennas, radio resource management on link layer controlling the spectrum deployment in time and frequency domains, and network functionality comprising routing, including selection of the used radio system. Figure 1 shows a general cognitive cycle for characterising the operations of Cognitive Radio System (CRS). According to the definition, CRS has capabilities to obtain knowledge, adjust according to the knowledge and learn from the results. The definition is broad, and detailed techniques for creating the CRS functionalities have not yet been defined.

Figure 1: The cognitive cycle

Interview

CR Improves Spectrum Utilisation Ari Hulkkonen Senior Manager, Elektrobit CR aims to introduce secondary usage of the spectrum resources without interfering with the primary usage of the licensed users. How does CR ensure this? The secondary usage of the spectrum is one important application for CR. CR technologies can also be applied in many other ways to improve efficiency, throughput and coverage of the primary network. An example of another quite different CR scenario is extending the primary network coverage by providing satellite spots to support a terrestrial cellular network. However, the secondary usage is an important application especially from tactical communications applications point of view as the principles used in the secondary spectrum utilisation can also be applied to tactical communications, typically not to cause interference but to avoid it. In the secondary spectrum utilisation scheme, the CR devices must know if the frequency they intend to use is occupied or is it free. Typically, the knowledge is based on frequency allocation information in databases provided by either authorities or companies managing the data in their servers. Then, power limits have been given and the CR devices are not authorised to exceed those limits. Very low power devices may also deploy spectrum sensing or combined approach deploying both the database method and sensing. The problem with sensing is that a single receiver will never be able to detect the primary user with 100 per cent reliability. Thus, collaborative sensing where all the nodes of the network may provide information of the spectrum has been introduced. When the database information is not available, collaborative sensing is the only way to reliably detect the primary network and it is therefore one of the key methods applicable to tactical communications applications. Wireless communications in future will demand radio technologies providing significantly higher capacity, bit rates and flexibility than existing systems. In what way does CR fit the bill? In order to provide new Internettype multimedia services for the mobile users, much more bandwidth is needed. In the physical radio layer, MIMO techniques and the deployment of multicarrier air interface have improved the spectrum efficiency and data rates that can be provided to the users. Examples of systems utilising these technologies are LTE, WiMAX, WiFi, etc. However, this is not enough as the number of mobile users is increasing exponentially. The fact is that more bandwidth must be allocated to the mobile systems in response to the increasing need. However, the only problem is that every single frequency applicable to mobile communications has already been allocated. To improve the overall system capacity, in cellular systems, the solution so far has been to introduce smaller and smaller cells so that the spectrum can be re-used more efficiently. However, this increases the number of access points required, which increases the system cost and its maintenance cost. It also creates more overhead in the networks due to the control traffic required. Another way to improve the spectrum efficiency is to use advanced radio resource management (RRM) systems that allow the lowest possible frequency re-use patterns to be deployed. CR is foremost among technologies that targets to improve the spectrum utilisation, thus improving the overall spectrum efficiency of the communications system and enhances the instantaneous system throughput and overall capacity. CR technologies, for example, allow the unused frequencies to be deployed, not to mention the advanced interference management and avoidance, which also improves the system capacity. You have been a part of numerous research projects being carried out by defence organisations to study the feasibility of CR in tactical environment. Can you tell us about them? Currently, one of the leading activities which I would like to emphasise is the Finnish Defence Forces (FDF) Technology Programme 2013, in which one of the key technology areas addressed is a highly mobile ad hoc network based on CR. The Programme 2013 comprises three main activities: Protection, Command System and Situational Awareness. Each activity has been divided into two or more projects, which focusses on specific technology topics. The Command System activity consists of two projects: Wireless Air Interface (WAI) and Intelligent Networks (IN). The WAI project studies the air interface with a focus on methods that improve the LPD and LPI performance of the wireless systems. This comprises waveform development, antennas and as a special topic, the deployment of HF radio. The IN project then combines all this into a single wireless cognitive network and adds the security issues. The FDF Technology Programme started in 2013 and will be completed in 2016.

CR in Tactical Communications
Tactical communications networks are operated in a dynamically changing environment, where interference and sudden changes in the network configuration and radio parameters take place. However, with traditional Combat Net Radios (CNR), it is rather challenging to guarantee a specific performance level for users as the system parameters have to be fixed and agreed beforehand. Problems were first limited through wellperformed frequency planning, and later the introduction of wideband radios featuring automatic frequency allocation solved many issues with co-site interference.

Since the introduction of voice transmission, the required information bandwidth has increased drastically. Today, the communication systems transfer images, video and data in addition to voice and data messages between users, thus increasing the throughput and capacity requirements to a new level. The introduction of MIMO and multicarrier techniques has provided more throughput and system capacity but the spectral efficiency has become a problem as the systems require more bandwidth.

CR offers new possibilities to further enhance the performance of a modern tactical communication system by introducing methods and mechanisms to avoid interference and interception, improve systemwide spectral efficiency and allow more flexible resource utilisation. In addition to terrestrial wireless links, satellites, UAVs and wired connections are combined in a hybrid system (Figure 2).

In addition to industry-driven projects targeting common applications, research activities have been carried out to study the applicability of CR to military communications. As an example, Defense Advanced Research Projects Agency (DARPA) has launched several programmes related to CR in the United States. DARPA’s neXt Generation Program (XG) aims to develop theoretical solutions for dynamic control of the spectrum, technologies and subsystems that enable reallocation of the spectrum and prototypes to demonstrate applicability to legacy and future military radio systems.

In Europe, EU-funded projects such as ARAGORN and SENDORA, activities funded by the European Space Agency (ESA) such as the ACROSS[5], and national projects and programmes such as TRIAL have developed CR solutions. In addition, the European Defence Agency (EDA) has launched its own projects to support the development of CR and its applicability to military communications. An example of such an activity is SCORED (Military Software-Defined Radio capabilities including applying Cognitive Radiobased Spectrum Management in the Security and Defence domains).

Figure 2: Cognitive Tactical Communication Network

Elektrobit has participated in numerous of these projects and is currently developing technology and solutions that allow deployment of CR functionality in practical applications. Following years of active work on MIMO and multicarrier technologies that provided a significant increase in the throughput of wireless links, CR is today one of the key technologies supporting the rapidly increasing requirements for system capacity. While spectrum resources are limited and bands have become more and more crowded, there is a need for methods that allow more efficient utilisation of radio resources.

EB Tactical Wireless IP Network (TAC WIN)
EB TAC WIN is a complete solution to building a tactical communications mobile ad hoc network for vehicle and stationary applications. With TAC WIN, battle groups can create high-data-rate wireless IP networks as backbones to support C2 data transmission during operations. The flexibility to use the EB solution in different frequency bands and network topologies provide cost effectiveness, ease of use and efficiency in various tactical communication scenarios. The EB TAC WIN is built with these basic components: the tactical router and the radio head unit.

TAC WIN can be deployed as an independent network or as part of a larger operative network supporting a great variety of applications and physical equipment connected to the same flexible and dynamic mobile ad hoc network with highspeed connections comparable to commercial internet services.

TAC WIN provides flexible routing functionality and interfaces to establish the connection between nodes and to other systems using either wireless or cable/ fibre communications. The wireless interface is provided by the router’s integrated SDR baseband section that allows various military or commercial waveforms to be run, depending on the customer’s requirements.

References

1. M. Höyhtyä, A. Hekkala, and A. Mämmelä, “Spectrum awareness: techniques and challenges for active spectrum sensing,” in Cognitive Wireless Networks, edited by F. Fitzek and M. Katz, pp. 353-372, Springer, 2007.
2. F. H. P Fitzek and M. Katz, editing, Cognitive Wireless Networks, Springer, 2007.
3. S. Haykin, “Cognitive radio: Brain-empowered wireless communications,” IEEE Journal on Selected Areas in Communications, vol. 25, pp. 201–220, February 2005.
4. J. Mitola III and G. Q. Maguire, Jr., “Cognitive radio: Making software radios more personal,” IEEE Personal Communications, vol. 6, pp. 13–18, Aug. 1999.
5. M. Höyhtyä, J. Kyröläinen, A. Hulkkonen, J. Ylitalo, and A. Roivainen, “Application of cognitive radio techniques to satellite communication,” in Proc. DySPAN, pp. 540¬–551, October 2012.

With inputs from Reima Kettunen, Juha Ylitalo and Marko Höyhtyä