Cognitive Radio: Smart Use of Radio Spectrum

Transcription

1 Cognitive Radio: Smart Use of Radio Spectrum Miguel López-Benítez Department of Electrical Engineering and Electronics University of Liverpool, United Kingdom Tokyo, Japan,

2 Contents Introduction Cognitive radio Cognitive functions Applications Conclusion 2

3 Contents Introduction Cognitive radio Cognitive functions Applications Conclusion 3

4 Introduction Spectrum management: Allocation: Frequency bands Services Assignment: Frequency bands Operators Fixed spectrum management: Spectrum bands are allocated/assigned statically Exclusive use licence In use since early days of radio communications Easy avoidance of interference Usable spectrum has already been allocated 4

5 The UK Frequency Allocation Table 5

6 Is spectrum exhausted? Spectrum usage is concentrated on certain portions A significant amount of spectrum remains unused Inefficient spectrum usage! Source: M. López-Benítez et al., Spectral occupation measurements and blind standard recognition sensor for cognitive radio networks, Proc. 4th Int l. Conf. Cognitive Radio Oriented Wireless Networks and Comms. (CrownCom 2009), Hannover, Germany, June 22-24,

7 Fixed vs. Dynamic spectrum access Fixed spectrum management: Easy avoidance of interference Inefficient usage of spectrum! Inability to roll out new radio technologies and services! Solution? Dynamic Spectrum Access: Source: M. López-Benítez, Spectrum usage models for the analysis, design and simulation of cognitive radio networks, PhD Thesis, Department of Signal Theory and Communications, Technical University of Catalonia, Barcelona, Spain, July

8 Opportunistic spectrum access Analogy: Spectrum = Motorway Band = Lane Interference = Collision Concepts: Primary user (w/ licence) High priority Police car, ambulance car, etc. Secondary user (w/o licence) Low priority Regular car 8

9 Opportunistic spectrum access 9

10 Opportunistic spectrum access Time domain OSA: Spectrum hole or white space P S P S P S 10

11 Opportunistic spectrum access Time domain vs. Space domain OSA: Time domain OSA Space domain OSA 11

12 Opportunistic spectrum access Advantages of DSA/OSA: Higher spectrum efficiency Several systems can coexist in the same spectrum: Roll-out of new radio technologies, services, networks Reduced cost of spectrum (i.e., inexpensive services for the end-user) Drawbacks: Concept is simple in theory, but challenging in practice. Need for a more sophisticated, smart technology: Cognitive radio 12

13 Contents Introduction Cognitive radio Cognitive functions Applications Conclusion 13

14 Cognitive radio J. Mitola: Radio etiquette is the set of RF bands, air interfaces, protocols, and spatial and temporal patterns that moderate the use of radio spectrum. Cognitive radio extends the software radio with radio-domain model-based reasoning about such etiquettes. S. Haykin: Cognitive radio is an intelligent wireless communication system that is aware of its surrounding environment (i.e. its outside world), and uses the methodology of understanding-by-building to learn from the environment and adapt its internal states to statistical variations in the incoming RF stimuli by making corresponding changes in certain operating parameters (e.g. transmit power, carrier-frequency and modulation strategy) in real-time, with two primary objectives in mind: highly reliable communications whenever and wherever needed and efficient utilization of the radio spectrum. F. K. Jondral: A CR is an SDR that additionally senses its environment, tracks changes, and reacts upon its findings. A CR is an autonomous unit in a communications environment that frequently exchanges information with the networks it is able to access as well as with other CRs. 14

15 Cognitive radio 15

16 Cognitive radio Main features: Cognitive capability: Senses RF environment and learns about it. Activity of licensed users Temporal and spatial variations of environment Identifies portions of unused spectrum Selects the best spectrum and transmission parameters Reconfigurability: Adapts operating parameters accordingly. Frequency of operation Modulation Transmission power Communication protocol 16

17 Cognitive radio as OSA enabler Cognitive radio is the key enabling technology for DSA / OSA! 17

18 Contents Introduction Cognitive radio Cognitive functions Applications Conclusion 18

19 Cognitive radio functions Functions of a CR system: Spectrum awareness: Identify free portions of the spectrum and detect the presence of licensed users when a user operates in a licensed band. Spectrum selection: Select the best available spectrum. Spectrum sharing: Coordinate access to this channel with other users. Spectrum mobility: Vacate the channel when a licensed user is detected and smoothly move the transmission to another channel with minimal disruption. Source: M. López-Benítez, Cognitive radio, Chapter 13 in Heterogeneous cellular networks: Theory, simulation and deployment, CUP

20 Spectrum awareness Spectrum awareness: Identify free portions of the spectrum and detect the presence of licensed users when a user operates in a licensed band. Methods: Beacon signals Geolocation databases Spectrum sensing 20

21 Spectrum awareness (beacon signals) Beacon signals: Primary transmitters broadcast beacon signals: Spectrum usage, power, coverage, etc. Secondary users tune and decode the signal. 21

22 Spectrum awareness (beacon signals) Pros and cons of beacon signals: Perfect information Requires agreement primary-secondary Changes in legacy systems (technical & economical problems) 22

23 Spectrum awareness (databases) Databases: Regional DB contains relevant info Spectrum usage, TX location, frequency, power, coverage, etc. Geolocation needed (GPS) 23

24 Spectrum awareness (databases) Pros and cons of databases: Perfect / accurate information Relies on external system (technical, admin & legal problems) Need for geolocation in DSA/CR terminals (cost, location accuracy, etc.) Database updating rate: not suitable for dynamic bands 24

25 Spectrum awareness (spectrum sensing) Spectrum sensing: Sample primary signal and determine ON/OFF state by means of signal processing methods: Matched filter detection Energy detection Feature detection (cyclo-stationarity, pilots) Covariance based detection 25

26 Spectrum awareness (spectrum sensing) Primary transmitter Primary receiver CR user 2 No interaction between CR users and primary Tx/Rx Licensed band 2 CR users must rely on locally sensed signals to infer primary user activity CR user 1 Licensed band 1 Channels found occupied by CR user (licensed bands 1 and 2) are now avoided during communication between CRs 26

27 Spectrum awareness (spectrum sensing) Pros and cons of spectrum sensing: Does not rely on an external system No changes to primary (legacy) system (simple, inexpensive) Suitable for dynamic spectrum bands Inaccurate information (spectrum sensing errors) 27

28 Spectrum awareness (spectrum sensing) Problems of spectrum sensing: Hidden node problem (receiver uncertainty) Interference CR transmitter range Primary transmitter range Primary user Primary Base-station CR user Cannot detect the transmitter 28

29 Spectrum awareness (spectrum sensing) Problems of spectrum sensing: Hidden node problem (shadowing/fading) Interference CR user CR transmitter range Primary transmitter range Primary transmitter Cooperation Primary user 29

30 Spectrum awareness (spectrum sensing) Solution to problems of spectrum sensing: Detects the primary user correctly Primary transmitter BUSY CR user 3 Shadowing Cannot detect the primary user due to the obstacles Multi-path fading Weak signals are received due to the multi-path fading may not detect the primary user CR user 2 IDLE Primary user CR user 1 IDLE By exchanging their sensing information, CR users can detect the primary user under fading and shadowing environments 30

31 Spectrum awareness (spectrum sensing) Non-cooperative sensing: CR users detect primary signal independently through local observations. Cooperative sensing: CR users share their local observations to collectively detect primary signal. 31

32 Spectrum awareness 32

33 Spectrum selection Spectrum selection: Select the best available spectrum. Classification: Spectrum analysis Spectrum decision 33

34 Spectrum selection Spectrum analysis: Characterise spectrum holes based on certain metrics. RF metrics: Frequency Bandwidth Interference Emission limits Activity metrics Duty cycle Other statistics Spectrum decision: Select the most convenient spectrum Primary user Primary user Transmission range 8 4 CR user CR user Received power No PU: SU can transmit with max power PU: SU must reduce power 34

35 Spectrum sharing Spectrum sharing: Coordinate access to this channel with other users. P S1 S S2 P S3 S P S1 S S3 Classification methods: Architecture: centralised vs. distributed. Behaviour: cooperative vs. non-cooperative. Scope: intra-network vs. inter-network. 35

36 Spectrum mobility Spectrum mobility: Vacate the channel when a licensed user is detected and smoothly move the transmission to another channel with minimal disruption. 36

37 Spectrum mobility Occupied by primary users Idle spectrum band CR user A CR user B CR user A Spectrum handover CR user B 37

38 Cognitive radio functions Functions of a CR system: 38

39 Contents Introduction Cognitive radio Cognitive functions Applications Conclusion 39

40 Applications: Rural broadband TV transmitter IEEE : Broadband access in rural areas. Using TV channels (6-7-8 MHz). Coverage: 33km (100 km max). CR transmitter 40

41 Applications: Rural broadband 41

42 Applications: Public safety 42

43 Applications: Broadband mobile comms Mobile communication systems face a serious problem: Capacity has doubled every 5 years Traffic level has doubled every year! 43

44 Applications: Broadband mobile comms 44

45 Contents Introduction Cognitive radio Cognitive functions Applications Conclusion 45

46 Conclusion Spectrum problem: Spectrum use is not efficient nowadays Spectrum bands are allocated but not fully exploited Spectrum demand increases constantly Solution: dynamic / opportunistic use of spectrum Increases spectrum efficiency. Enables roll-out of new services at lower costs. Key enabling technology: Cognitive radio Senses the radio environment and learns about it Reconfigures and adapts dynamically to the operating conditions Important technical problems and challenges to be overcome Important applications in future wireless communication systems 46

47 The End 47

48 Further reading M. López-Benítez, Cognitive radio, Chapter 13 in Heterogeneous cellular networks: Theory, simulation and deployment, Cambridge University Press,

49 for your attention!