EXTENDING THE REACH OF COGNITIVE RADIO AN OVERVIEW OF THE PAPER BY PRESTON F.MARSHALL Proceedings of the IEEE , Vol. 9

1. EXTENDING THE REACH OF COGNITIVE RADIOAN OVERVIEW OF THE PAPER BY PRESTON F.MARSHALLProceedings of the IEEE , Vol. 97, No. 4, April 2009

2. OBJECTIVES OF THE PAPER TO UNDERSTAND • THE RESEARCH PROGESS IN COGNITIVE RADIO (CR) TECHNOLOGIES • THE NEED FOR MULTIPLE COGNITIVE REASONING TECHNOLOGIES • A MODEL OF COGNITIVE RADIO • THE ARGUMENT FOR COGNITIVE RADIO DEPLOYMENT ? • WHAT TO LOOK FORWARD TO IN THE FUTURE ?

3. Recent Progress • The transition of Cognitive Radio Research has driven down two fundamental paths • Path 1 being Research on Dynamic Spectrum Access ( DSA ) • Path 2 being Research on Learning based Cognitive radio algorithms. Lets look briefly at each of these Paths.

4. Recent ProgressPath 1 – DSA • Deployment of prototype DSA systems has been done by a number of Commercial and Governmental Organizations. • One of the Main applications of the DSA systems has been towards Television white space which has evolved into the IEEE 802.22 Wireless Regional Area Networks [1] • Proposed approaches for white space utilization vary from centralized to fully distributed and fused sensing. • Multiple Resolutions of spectrum sensing with both fast(1ms) and slow sensing cycles ( 25 ms) have been proposed and Implemented [3].

5. Recent ProgressPath 1 – DSA • The sensing can also be done in the Analog domain • One such approach relies on analog Spectral analysis and temporal correlation with only the correlation difference requiring to be digitized [3]. • This lowers the energy consumption as less processing is required and hence cost reduction is also achieved. • Much of DSA has been focused on white space. However, Research on effects of Adjacent channel Interference are actively gaining momentum.

6. Recent ProgressPath 2 – Learning Algorithms for CR • Prototyping of Learning based CR algorithms has already been done. • The first task in developing these algorithms is effective representation of Knowledge. • Yarkin and Arslan [10] described a radio knowledge representation language that provides a mechanism to organize multiple sources of environmental and location awareness into an integrated representation. • Researchers have extended this knowledge representation to describe the state of knowledge of the radio components of the transceiver [11]

7. Recent Progress • A number of Cognitive radio experimental platforms( in terms of the hardware and software) have been built that can be used for testing new schemes of CR Research • The Kansas University Agile Radio is one such platform [13] • The Defense Advanced Research Projects Agency (DARPA) has initiated the Wireless Network after Next (WNaN) Program[14] • The objective of the program is to demonstrate that Cognitive Radio can at least provide the same performance of the Conventional radio BUT with reduced performance components through the use of adaptation to mitigate the effects of the environment. Thus, Reducing the cost • The WNaN covers a range of 900MHz – 6GHz. Has 4 independent transceivers. Has high quality front-end filters to enable efficient utilization of DSA. Uses Commercial quality RF circuits for Transceiver functionality

8. The Need for Multiple Reasoning Technologies • The Definition of Cognition in CR is not unique and is defined differently by Researchers. Thereby CR technologies are built around these Definitions of CR • For Example, Mitola[15] defines CR as system with an Intellectual Process using the Concepts of Artificial Intelligence. Thus CR algorithms were built around this perception of Cognition. • On the other hand, there is broader sense of the word Cognition, at least technologically. • For example, The DARPA Next Generation Communications (XG) program reported success in DSA using Fixed algorithms and policies [17] • Thus, as long as there a sense of cognition ( like being aware of its surrounding and taking decision ) , it doesn’t matter how things are done. What matters is What is to done ?

9. The Need for Multiple Reasoning Technologies • Also, in a more Practical setting, the problem with learning based approaches is the assumption of feedback to the system. • This Feedback may not be possible in a complex network of the CR and may exist external to the system itself. • There are also practical considerations. Too much reliance of technology brings its own problems with it. • Lack of proper functioning, too much malware that exists in the internet today, would force participants to not trust each other and hence not co-operate

10. The Need for Multiple Reasoning Technologies • Regulators tightly control spectrum, due to the extreme social and economic consequences of interference to key infrastructure and services. • It is unlikely that they would accept the principle that, although a node’s behavior will initially be very poor, it will eventually learn to “play nice” in the spectrum (after causing any number of interference events while learning)

11. The Need for Multiple Reasoning Technologies • Thus, there is a strong argument that when improper behavior has negative impacts, the use of declarative techniques is advantageous. • These techniques are deterministic, verifiable and testable. • Thus, a Need arises for Partitioning of the CR system into two Parts • A Policy or Regulatory body • A Performance Optimization body

12. A Proposed Inclusive Model Of Cognitive Radio • Partitioning the Intelligence model of CR into endogenous ( within the system ) and exogenous ( external ) components • A DSA system partitioned into System Strategy Reasoner (SSR) and Policy Conformance Reasoner( PCR)

13. Inclusive Model Of Cognitive Radio • The PCR takes care that the device conforms to a certain kind of good behavior. For example, • doesn’t cause interference, • doesn’t violate any spectrum regulation policy • Is there a certain QOS achieved • The SSR deals with the performance optimization, based on the sensing it does. For example, • spectrum sensing, • choosing the optimal modulation scheme, • error-control codes etc • Thus we can see that, there seems to be a need for a mixture of a Learning model and a deterministic model. For example, The decisions that the PCR takes must be acceptable to other users of the network. Thus, there must be a common understanding between the players and this facilitates a certain aspect of determinism in the working of the PCR • Thus, a mixture of Learning based and deterministic algorithms is the right way forward.

14. Inclusive Model Of Cognitive Radio • The Cognition doesn’t have to have a spectrum or a link focus alone.. • We could include Cognition at multiple decision making levels of Wireless Communications. Such as, • What is the Optimal network routing ? • What is the Optimal network topology ? • How is Content management done ? • How is user interaction done ?

15. Inclusive Model Of Cognitive Radio • Thus ,introducing cognition to different levels brings along with it, its own constraints and performance optimization techniques. Thus, the challenge now is to find an integrated solution that would provide a best fit.

16. Inclusive Model Of Cognitive Radio • Thus, we generalize the intelligence model of the CR can be generalized as shown below • Each Environment would have its Policies. We could eliminate this one-to-one relationship between PCR and the Environments if the Policy language was general enough

17. Inclusive Model Of Cognitive Radio • For an example, lets look at the Spectral Environment and see what should be the PCR ( exogenous ) and SSR ( endogenous ) considerations

18. Inclusive Model Of Cognitive Radio Similarly for the Network Environment, some considerations of the PCR would be Validation of packet traffic of the devices to ensure no violations of network operating rules were made by the device A network firewall is one possible choice for the construction of a network PCR For a device Environment, the PCR would require that the CR algorithms not violate the operating limits of the device ( like duty cycle, peak power, amplifier bias etc ) Internet content filters are examples of PCR for the user environment A lot of the focus of PCRs has been for the Spectral Environment [19]

19. Inclusive Model Of Cognitive Radio • By looking at the considerations of the various environments, it looks like that the PCRs functionality are more Regulatory in nature while the SSRs are more associated with the actual communication system design • Thus, the PCRs at best cannot improve the performance. Its only goal being to Regulate certain actions. • The SSRs optimize the performance of the CR system • Now that we have discussed a model for the CR. What is the Justification for having the CR system itself ? • IS IT WORTH IT IF THE SYSTEM BECOMES COMPLEX ? • WOULDN’T THAT IMPLY INCREASE IN THE COST ?

20. A New Structure Of TheArgument For Cognitive Radio • CRs are viewed as more complex in comparison with conventional Radios • This would imply Cost insensitive applications. • An alternative view is that CR provides the opportunity to overcome the shortfalls of hardware performance and thus makes less capable hardware more reliable and effective. • The DARPAs WNaN program goal is to demonstrate that with reduced component performance, CR can achieve significant performance improvement

21. A New Structure Of TheArgument For Cognitive Radio • Thus, our perspective of Cognitive Radio Performance can be understood as shown below

22. A New Structure Of TheArgument For Cognitive Radio • An example of this would be Mitigation of Front end overload effects caused by high levels of energy in the receiver pre-selector bands. • A challenge for Spectrum management is to avoid radios operating simultaneously on the same frequency. An even difficult challenge is to avoid a strong signal near the assigned channel. Typical scenarios would be Military and Public safety applications • Non-conventional radio would require very high linearity in its Front-end to avoid the overloading effect • CR offer the opportunity to trade with the linearity such that at least one of its Front-end pre-selector bands are not overloaded.

23. THE NETWORK AND UPPER LAYER’SBEHAVIORS IN COGNITIVE RADIO • Much of the focus of CR has been on the Physical layer adaptation. • Network layer plays a significant role in the performance especially when utilizing the flexibility of the DSA and the CR model • DSA eliminates the specific assignment of individual links to frequencies and bandwidth. This provides the opportunity to re-organize ( Topology, routing ) the network.

24. THE NETWORK AND UPPER LAYER’SBEHAVIORS IN COGNITIVE RADIO • The tradition problem of the network layer was “How to route traffic, given a particular Topology” • In a Cognitive setting, question to be answered is “What is the right Topology to route the traffic “

25. THE NETWORK AND UPPER LAYER’SBEHAVIORS IN COGNITIVE RADIO • Lot of Research has been going on in the Networking community on its role in CR. • The DARPA’s WNaN program has incorporated 11 theses [14] in its program. Many of these relate to the network layers operations but solely the result of the flexibility of the DSA at the physical layer

26. What New Research is Required ? • CR is the answer to the current problems and challenges facing Wireless Communications today. • This will have major economical and social consequences which in turn would lead to new problems and challenges. • The Goal here is to foresee what kind of challenges we might have to face

27. What New Research is Required ? • Density and Scaling • Expressions of CR algorithms and Reasoning • Stability • CR Environmental Abstractions • Decision Theory in Cognitive Radio • Relationship of Content Flow and Cognitive Radio • The Practice of Interference Tolerant Operation

28. What New Research is Required ?Density and Scaling • Network Density will play a key role in Building CR systems • Prior work in Self-forming networks focused on discovering and exploiting ALL POSSIBLE CONNECTIVITY. However, the situation reverses as the network density increases. • The goal then is to keep things simple. Change the Topology to reduce the complexity and interactivity of the network.

29. What New Research is Required ?Density and Scaling • Conventional Mobile-Ad hoc network • Scalability of the network is highly limited because of mutual interference[25] • Another drawback with this network is that at any point only one node can transmit thereby limiting its scalability

30. What New Research is Required ?Density and Scaling • Now consider an CR network. Shown is the WNaN network • In this architecture each node connects to different set of nodes through multiple transceivers, frequencies. • Clearly one can see that the scalability of the network is much improved • The choice of DSA, The kind of topology, routing, interference environment all are key issues in determining the Density and Scalability of a network and is key Research area to be worked upon

31. What New Research is Required ? Expressions of CR algorithms and Reasoning • Cognitive Radio research has spurred different new techniques answering questions on channel selection, waveform selection, power control etc. • A potential outcome of this would be an island of technology capabilities, each targeting specific problems but not readily integrated with other solutions • One Problem as already mentioned is how does one go about Implementing the PCRs. • The DARPA XG program has reported development of two Policy Reasoners for spectrum policy conformance reasoning [19], [20]. However a more general Policy language needs to be developed • Reasoning is just one part of CR. Incorporating Learning into the endogenous component of the CR decision making offers rich dividends

32. What New Research is Required ?Stability • The logistics of acquiring, programming and operating wireless nodes limits the size and extent of networks that have been used for prototyping CR and ad-hoc wireless systems. • Thus analyzing individual nodes in the networks is not difficult and current simulation tools exist to aid in this process • But its dynamism starts increasing as the network starts growing and analysis of its behavior becomes increasingly difficult.

33. What New Research is Required ?Stability • Conventional radio assigns spectrum such that nodal operation would be orthogonal wrt other nodes. • In Cognitive radio spectrum sharing implies that decisions made on one node will ripple through other nodes and perhaps the entire network. Thus, raising the Issue of Stability of the Network • This ripple could have a Damped effect or Cascaded effect or an Oscillatory effect.

34. What New Research is Required ?Stability • Thus, from the point of Stability, it is necessary to look at a system perspective and be able to determine its characteristics ( Example, whether it has Transient, damped or Oscillatory response ) • This transition of analysis to a system perspective would require formal analytical tools ( for e.g. Theory of Control systems helps us to analyze stability of a System function, differential equations ) to determine the stability of network. This must become a fundamental driver to the design of the layers • Thus, today's concerns of Throughput and Latency would be on par with questions of Stability and Density

35. What New Research is Required ?Cognitive Radio Environmental Abstractions • The Environment of CR system is complex and vary geographically from place to place. • Techniques that work in one Environment don’t work well in others. • Most techniques rely on empirical data(spectrum data , network data) from a specific environment. Thus, a general consensus is required to capture the essentialities of environments into one general framework. This will enforce better techniques to be built that would work in most situations.

36. What New Research is Required ?Cognitive Radio Environmental Abstractions • With respect to Spectral characterization, no generalized approach exists. Generalization of Spectral occupancy though has received significant attention. • A framework for RF Spectral Measurements and inferences based on them has been developed by University of Kansas [28] • A number metrics exist to characterize Spectrum usage such as , Spectrum Utilization Index [29], percentage spectrum occupied and so on.

37. What New Research is Required ?Cognitive Radio Environmental Abstractions • All these studies show the opportunity that DSA, and in general CR, provides. • But, What is more important is how do these Indexes vary with respect to the given parameters of the system. This can seen below

38. What New Research is Required ?Decision Theory in Cognitive Radio • A lot of Optimized algorithms for DSA functionality are being developed. • The Optimization we talk about is with respect to giving autonomy to the individual nodes on making optimal decisions. • These decisions depend on Information that nodes senses and many a time would depend on other nodes for information gathering. • Even though the Information is not exhaustive, the time required for rendezvous, synchronization and measurements maybe significant . • Thus, we need to answer a Fundamental Question of Decision theory: • HOW MUCH IS A DECISION IMPROVED IF UNCERTAINTY IS REDUCED, TO THE COST OF OBTAINING IT ? • Thus, the first algorithm in Decision making should be • Whether to invest in acquiring channel state at all And not how to use Channel state to making Decisions

39. What New Research is Required ? Relationship of Content Flow and Cognitive Radio • CR research has been mainly focused on the Physical layer up till the network layer and always treated the application layer independent of it. How about Cognition at the upper layers as well ? • A great amount of dynamism is introduced if content location is not fixed. This allows the networking layers to flexibly choose routing and topology strategies.

40. What New Research is Required ?The Practice of Interference Tolerant Operation • Much of DSA research has been towards assuring non-interference operation • A more aggressive approach for CR would be for a stated amount of reliability in spite of a certain level of interference. This mechanism would hence allow interference-tolerant devices to work for a prescribed level of assurance. Hence reducing cost

41. What New Research is Required ?The Practice of Interference Tolerant Operation