Project description Dynamic Spectrum Management and System Behavior in Cognitive Radio

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1 Project description Dynamic Spectrum Management and System Behavior in Cognitive Radio 1. Background During the last few decades, the severe shortage of radio spectrum has been the main motivation always used by researchers in the field of wireless communications. It has been believed that this shortage is mainly due to the physical scarcity of radio spectrum and to the rapid spread of diverse devices with wireless-interaction capability, such as mobile phones, laptop computers, home appliances, wireless tags, etc. Traditional and common approaches to solve this problem have been to increase the number of bits that can be transmitted per unit time and frequency, resulting in high capacity within a given frequency bandwidth. To this end, considerable research effort and fund have been spent to develop advanced wireless access technologies, and a lot of research is still ongoing all over the world. However, a recent report published by the federal communication commission (FCC) in US has shown a surprising finding, which highlights a different cause of the shortage of frequency resource: In many bands, spectrum access is a more significant problem than physical scarcity of spectrum, in large part due to legacy command-and-control regulation that limits the ability of potential spectrum users to obtain such access [1]. Thus, the large part of the licensed spectrum is not utilized most of the time and space, and the frequency spectrum is actually abundant. We have been trying to put more signals into congested frequency bands even if there are almost free frequency bands next to them. This paradoxical fact has resulted from the complicated and old regulations, which prevent us from utilizing more flexible and open access to these abundant bands (the regulation is old in a sense that it has been made to match the state-ofthe art technologies from the 1920s!). Apparently, in order to increase the efficiency of our natural spectrum resource utilization, more flexible spectrum management techniques and regulations are required. 2. Cognitive Radio: Another approach to increase spectrum efficiency One of the most important findings from the measurements reported in [1] is that a large portion of the radio spectrum is not in use for significant periods of time in certain areas. Thus, there are a lot of spectrum holes, which are defined as a set of frequency bands assigned (licensed) to a user (we call this user as a primary user), but, at a particular time and specific geographic location, not being utilized by that user [2]. On the other hand, the report also pointed out that most of the unlicensed spectra are heavily accessed by users and have high spectrum utilization thanks to the possibility of open access with relaxed regulations. These observations lead us to a key idea: the spectrum utilization can be drastically increased by allowing secondary users to access to the spectrum holes that are unutilized by the primary user at certain time and space. Cognitive radio has been proposed as a means to achieve such dynamics. A cognitive radio senses the spectral environment over a wide frequency band and exploits this information to opportunistically provide wireless links that can best meet the demand of the user, but also of its radio environments. The cognitive-radio devices have two important functionalities: spectrum sensing and adaptation. A secondary terminal first senses the spectrum environment in order to learn the frequency spectra unoccupied by primary users. Once such a spectrum hole is found, the secondary terminal adapts its transmission power, frequency band, modulation, etc., so that it minimizes the interference to the primary users. Even after starting the transmission, the secondary terminal should be able to detect or predict the appearance of a primary user so that it makes the spectrum available for the primary user. Basically, the primary users should not change their communication infrastructure due to these operations. Thus, these sensing (including the detection) and adaptation of the secondary users must be done independently of the primary users. Figure 1 shows an example of the spectrum utilization with ideal operation of cognitive radio. In the area 1 which is a region within communication range of primary users, the secondary users build communication links with frequency f 1 while the primary users are not active on the communication links. On the other hand, in the area 2 which is out of communication range of primary users, the secondary users can continuously utilize this frequency band without the interference from/to the primary users. Note that, in the conventional system without cognitive radio, the frequency band f 1 cannot be utilized by any user at any location. Thus, cognitive radio allows users to utilize a frequency band more densely in time and space, thereby leading to a drastic increase of the total spectrum efficiency.

2 System X with licensed frequency f 1 (Primary user) Communication Area of System X System Y (Secondary user) frequency f 1 frequency time Area 1 f 1 time Primary user Secondary user Area 2 Busy period for primary users Busy period for secondary users Figure 1 Frequency utilization of primary and secondary users in Cognitive Radio Environment 3. Application Scenarios and Potential One of the scenarios considered as an application of cognitive radio technology is the spectrum pooling in which some specific and limited licensed spectrum (military, government, public interest, etc) is rented for public access [3][4]. The other scenario is the renting of radio spectrum from one mobile operator to the others according to the profile of the spectrum use [5]. There is also one scenario under standardization process in IEEE working group, which was set up on October The working group is currently developing a standard for a cognitive radio-based air interface for license-exempt devices (e.g. WiMAX devices) operating in a spectrum allocated to the TV broadcast service on a non-interference basis [6]. Furthermore, the ultra wide-band (UWB)- based system can also be considered as an instance of cognitive radio since it tries to overlay signal with licensed bands in an interference-free manner [7]. The application of cognitive radio technologies will have a great impact on wireless communication industry. First, the cognitive radio makes it easier for manufacturers to introduce and spread new technologies thanks to the increase of license-free operations (i.e. the secondary use of the licensed band). This is the same trend as the introduction of unlicensed bands which has induced a drastic increase of short-range devices such as wireless local area network (WLAN) and Bluetooth. This also helps small venture companies to start new wireless businesses. Second, system operators get more opportunities to utilize their owned spectrum by renting it to other users or system operators. This secondary market can reduce the burden for large operators to keep expensive licensed spectrum. Thanks to the above benefits brought to manufacturers and system operators, users can get new services with cheaper prices, but also with higher quality. Furthermore, cognitive radio technologies offer the international roaming capability to users since terminals can sense and understand the communication environment abroad, and adapt their transmission according to the spectrum environment in different countries. Finally, for the government, the benefit is not only limited to the efficient use of the natural resource of radio spectrum. The increase of the available radio resource can stimulate the invention of novel technologies, and can enhance the worldwide competence in this field. Especially, by leading the research and development with this new way of utilizing the spectrum, there is a great chance to propose a new worldwide standard. Thus, it is now a great chance for Denmark to take an initiative in this research area and to originate a Denmark-branded worldwide standard. There are several prerequisites for cognitive radio technologies to be successfully introduced into wireless industries. Most importantly, there must be considerable changes to the current licensing rules and regulations for the spectrum utilization. Discussions concerning this issue have already been started at FCC in the US [1][2], and the European countries and regulation bodies will inevitably follow such a trend in the immediate future. Next, cognitive radio requires innovative and unprecedented techniques (hardware/digital signal processing/spectrum management techniques) in order to sense and adapt to the spectrum. This observation has recently ignited an intensive research worldwide. However, being a green-field research, most of the available studies are still at a conceptual level, and the core technologies and system analysis for realizing cognitive radio are yet to be invented and developed. Thus, we have now a good opportunity to give a big impact

3 on the field of wireless communication research by showing concrete enabling technologies and system performance for cognitive radio. 4. Research Content This project is divided into two main tasks which tackle on challenging issues in the cognitive radio system. Task 1. Dynamic Spectrum Management The key information to prevent the interference from a secondary user to a primary user is the spectrum environment at the receiver of the primary user. As shown in Figure 2, even if a secondary transmitter (ST1) does not detect the signal of a primary transmitter (PT1), the transmission of ST1 can cause interference to the primary receiver (PR1). Many solutions to this so-called hidden terminal problem have been proposed for homogeneous system where a transmitter and a possibly interfered terminal can exchange information on the interference condition. However, in cognitive radio, the primary and secondary users cannot necessarily exchange information. In this case, a secondary terminal needs to estimate the actual spectrum environment in a separate place. Such a remote sensing in a cognitive radio system is a challenging open issue. The cooperation among many secondary terminals (e.g. secondary terminals surrounding ST1 in Figure 2) at different places can be exploited to achieve this remote sensing. Once the available spectrum is found, the secondary user must decide the transmission formats and their parameters. In cognitive radio, the primary user allows a certain level of interference from the secondary users, and this requires careful specification of the regulation, that is, the interference level and interference patterns that the secondary user is allowed to cause to the primary users. However, if we first specify the allowable interference and then develop the spectrum access algorithms, we may end up in a situation that is similar to the present regulation, where the rigid rules are severely restricting innovation in the spectrum access techniques. Therefore, in our approach, the spectrum usage rules and the spectrum usage algorithms will be addressed jointly. Only in that manner can a good trade-off be achieved between system performance and etiquette of spectrum utilization. The above problem will be also considered for the case with different primary users requiring diverse limits on interference levels. In addition, there can be internal gradation among the secondary users, so that a secondary user from a low-priority class should give precedence to the one from a higher class. The existence and specification of such priority classes are also necessary issues to be investigated. PT1 Primary user PR1? SR1 Secondary user ST1 Figure 2 Hidden terminal scenario in cognitive radio Task Goal The goal of this task is to develop dynamic spectrum management techniques and propose regulations for cognitive radio system. The advanced techniques for spectrum sensing, signal/power adaptation, resource allocation, and interference management must be developed together with the regulations and protocols to utilize the radio spectrum. The proposed techniques and regulations must offer a good trade-off between system capacity and etiquette of spectrum utilization. The developed rules and protocols will be proposed to European and worldwide regulation bodies and standardization organizations such as IEEE and ETSI.

4 Time plan for Task 1 1 Post-doc with 1 professor and 2 assistant research professors Year Month Sub-task Description Scenario A system model and scenario are defined. As a starting point, only one type of Definition primary user will be considered. This scenario is used also for task Literature Survey Literature survey on different spectrum sensing technologies 5-7 Solutions Development Study solutions to achieve efficient remote sensing. A possible solution is the use of cooperative sensing. The solutions should have high accuracy. 8 Vacation 9-11 Evaluation The accuracy and overhead of the solution are evaluated. The evaluations will be made analytically and/or by simulations. 12 Documentation The scenario/solutions/results of the evaluations are summarized. Mile Stone 1: Design of efficient remote sensing techniques for cognitive radio with 1 type of primary user Documentation A publication based on the obtained results is prepared. 2-3 Literature Survey Literature survey on adaptive transmission strategies including power control and distributed protocols to control interference 4-6 Solutions Development Study solutions for adaptive transmission strategies and propose a regulation giving the best trade-off between system capacity and etiquette. 7 Vacation 8-10 Evaluation The solutions are evaluated from system performance and interference point of views. The evaluations will be made analytically and/or by simulations. 11 Documentation The solutions and results are summarized. Mile Stone 2: Transmission protocols and regulations for cognitive radio with 1 type of primary user Documentation A publication based on the obtained results is prepared Solutions Study solutions to handle different kinds of primary users. Adaptive resource Development allocations and regulations are also developed to handle multiple frequency bands. 3-4 Evaluation The solutions and regulations are evaluated. The evaluations will be made analytically and/or by simulations. 5-6 Solutions Development The adaptive transmission strategies are designed to handle multiple types of secondary users with different priorities. 7 Vacation 8-9 Evaluation The solutions and regulations are evaluated. The evaluations will be made analytically and/or by simulations. 10 Documentation The obtained solutions/results/regulations are written. Mile Stone 3: Transmission protocols and regulations for cognitive radio with different types of primary and secondary users Documentation A publication based on the obtained results is prepared. 12 Documentation A document to be submitted to regulatory organization is prepared. Mile Stone 4: Proposal of transmission strategies and regulations to regulatory organizations Task 2. Cognitive radio system dynamics The terminals in cognitive radio system adapt their local actions (transmission power, frequency, timing, sensing methods, modulation parameters, etc.) according to the surrounding environment so that the individual links as well as the overall system perform in an efficient and stable manner. In order to achieve the system stability, it is vital to understand how such local interactions are affecting the overall system behavior at a large scale. In the cognitive radio system, depending on the employed local actions, there can be positive or negative macro-level system behavior. The positive behavior results in the efficient and stable use of the radio spectrum while the negative behavior produces completely unused radio spectrum. Game theory [11][12] has been utilized to analyze such a system dynamics with many self-organizing entities that cooperate and compete with each other. Some of the works have analyzed the convergence and stability of cognitive radio system with dynamic decision making from the game theory [13]. However, they have only focused on a limited transmission technique (power control), and have not provided general rules for the local actions, which can lead a system toward a positive operating state. We can also exploit some tools commonly used to analyze the economical behaviors with free market, such as auctions and bargaining, since the cognitive radio introduces the free, but regulated market" approach of the spectrum usage, as compared with the strictly regulated approach in current wireless systems (which can be viewed as a completely regulated market without any auctions and bargaining). The cognitive radio network with these local interactions can be viewed as a complex adaptive system [9][10] which has gained considerable attention as a new science to analyze system behavior and dynamics. A complex system is expected to exhibit emergent phenomena which are macro-level system behaviors dependent on the local actions of individual entities. The

5 cognitive radio system, as a complex system, has a potential to show such emergent phenomena in terms of interference, throughput, delay, etc. Task Goal The goal of this task is to understand the system dynamics of a cognitive radio and to derive the rules of local actions that can result in stable and efficient system operations. The major analytical tools will be game theory and existing methodologies for analyzing the economic behavior. We will observe emergent phenomena of the cognitive radio system and explore how the local interactions are affecting the emergent phenomena at a large scale. The obtained rules will be reflected on the design of detailed transmission strategies employed by each terminal. Time plan for Task 2 1 full-time Ph.D. student supervised by 1 professor and 2 assistant research professors Year Month Sub-task Description Literature Survey Get familiar with game theory and complex system. Literature survey on the research studying the dynamics of communication system 3 Documentation A document summarizing the possible problems is prepared. 4-5 Modeling Model the simple cognitive radio system defined in task 1 by using the tools of game theory and economics. 6-7 Evaluation Analysis of system behavior with many variations of local interactions and rules. Derive conditions of local actions that make the system work stably. 8 Vacation 9-10 Evaluation Continued 11 Documentation The evaluation results are written in a document. 12 Documentation A publication based on the obtained results is prepared. Mile stone 1: Analytical model of a simple cognitive radio system and its conditions for system stability Scenario A system model and scenario are redefined. Many kinds of primary users are Definition introduced. This scenario is also used for task Modeling Model the cognitive radio system with many kinds of primary users by using the tools of game theory and economics. 5-6 Evaluation Analysis of system behavior with multiple frequency bands. Derive resource allocation rules to make the system work stably. 7 Vacation 8-9 Evaluation Continued 10 Documentation The evaluation results are written in a document. 11 Documentation A publication based on the obtained results is prepared. Mile stone 2: Analytical model of a cognitive radio system with different types of primary users and its conditions Scenario Definition 2008 for the system stability A more complex system model and scenario are redefined with many kinds of secondary users with different priorities. This scenario is also used for task Modeling Model the cognitive radio system including many kinds of secondary users by using the tools of game theory and economics. 3-6 Evaluation Analysis of system behavior of cognitive radio system including multiple priority level of secondary users. Derive conditions to make the system work stably. 7 Vacation A publication based on the obtained results is prepared. 8 Documentation The evaluation results are written in a document. 9 Documentation A publication based on the obtained results is prepared. Mile stone 3: Analytical model of a complex cognitive radio system with multiple priorities of secondary users and its conditions for system stability Documentation Preparation of Ph.D. thesis Mile Stone 4: Finalized Ph.D. thesis 5. Research Environment The Research will be conducted at Wireless Networks Group (WING) in Center for Tele- InFrastructure (CTIF), Aalborg University. WING group consists of more than 20 research staffs with a broad range of expertise from wireless access to wireless networking technologies. The group has several years of experience in research topics related to this project including Ph.D projects, namely, coexistence mechanism in the unlicensed spectrum, radio resource allocation for wireless networks, digital signal processing techniques, etc. Especially, we have strong research experience on the protocol design under given regulation rules [14] but also on the proposal of regulations to improve system capacity in the unlicensed band [15][16]. These topics are closely related to this project, and our experience can be directly exploited to conduct the abovementioned research. Furthermore, WING group is running several collaboration research programs with many companies (Danish, European, and Asian operators/manufactures), which give us great opportunities to discuss practical and implementation aspects of cognitive radio.

6 Brief project description The currently employed regulation for the utilization of radio frequency spectrum has been preventing users from dynamically accessing temporally and spatially unused portion of the spectrum. This outdated regulation has been a major cause of the inefficient spectrum utilization, resulting in the artificial problem on the scarcity of the radio frequency. Cognitive radio, which is enabled with the radical change of the traditional regulation, has been proposed as a means to achieve more flexible and efficient utilization of the radio spectrum. A cognitive radio senses the spectral environment over a wide frequency band and exploits this information to opportunistically provide wireless links that best meet the demand of the user and its radio environments. The project aims at the development of enabling technologies and regulations for this innovative concept of cognitive radio, such as spectrum sensing and dynamic spectrum management. The project also analyzes dynamics and stability of the cognitive radio system by using game theory and economical tools. These analyses provide rules to stably and efficiently share our common natural resource, radio spectrum. The enough frequency resource obtained through the application of cognitive radio promotes the development of more advanced wireless technologies, which will be helpful for our health, security, education, etc.

7 Annotated references [1] Federal Communications Commission, Spectrum Policy Task Force, Rep. ET Docket no , Nov [2] S. Haykin, Cognitive Radio: Brain-Empowered Wireless Communications, IEEE Journal of Selected Areas in Communications, vol. 23, no. 2, Feb [3] J. Mitola III, Cognitive radio for flexible mobile Multimedia communications, in Proc. of 6 th International Workshop on Mobile Multimedia Communications (MoMuC), San Diego, CA, Nov. 1999, pp [4] T. A. Weiss and F. K. Jondral, Spectrum Pooling: An Innovative Strategy for the Enhancement of Spectrum Efficiency, IEEE Radio Communications Magazine, 42: S8-S14, March [5] B. Aazhang, J. Lilleberg, and G. Middleton, Spectrum sharing in a cellular system, in Proc. of IEEE Eighth International Symposium on Spread Spectrum Techniques and Applications (ISSSTA), Sydney, Australia, Aug. 2004, pp [6] IEEE working group [7] J. Lansford, Uwb coexistence and cognitive radio, in Proc. of International Workshop on Ultrawideband Systems and Technologies and Ultra Wideband Systems (UWBST & IWUWBS), Kyoto, Japan, pp , May [8] D. Cabric, S. M. Mishra, and R. W. Brodersen, Implementation issues in spectrum sensing for cognitive radios, in Proc. of Thirty-Eighth Asilomar Conference on Signals, Systems and Computers, Pacific Grove, CA, Nov. 2004, pp [9] J. Mitola. III and G. Q. Maguire. Jr., Cognitive radio making software radios more personal, IEEE Personal Communications Magazine, Vol. 6, no. 4, pp , Aug [10] G. G. Weisbunch, Complex System Dynamics, Reading, MA: Addison-Wesley, [11] D. Fudenberg and D. K. Levine, The Theory of Learning in Games. Cambridge, MA: MIT Press, [12] T. Basar and G. J. Olsder, Dynamic Noncooperative Game Theory, 2 nd ed. Philadelphia, PA:SIAM, [13] J. O. Neel, J. H. Reed, and R. P. Gilles, Convergence of cognitive radio networks, in Proc. of IEEE Wireless Communications and Networking Conference (WCNC), Atlanta, Georgia, Mar. 2004, pp [14] P. Popovski, H. Yomo, S. Aprili, and R. Prasad, "Frequency Rolling: A Cooperative Frequency Hopping for Mutually Interfering WPANs", in Proc. of the 5th ACM International Symposium on Mobile Ad Hoc Networking and Computing (MOBIHOC), pp , Tokyo, Japan, May, [15] H. Yomo, P. Popovski, and R. Prasad, "Adaptive Radio Resource Sharing for Collocated Wireless Personal Area Networks", in Proc. of the 6th international symposium on wireless personal multimedia communications (WPMC 2003), October, 2003, Yokosuka, Japan. [16] P. Popovski, H. Yomo, and R. Prasad, "Dynamic Adaptive Frequency Hopping for Mutually Interfering Wireless Personal Area Networks", accepted for IEEE Transactions on Mobile Computing