Literature survey of cognitive radio networks and their research challenges.

1. Cognitive Radio Networks:
Technology Survey & Research
Challenges
Presented By:
Vatsala Sharma
PhD (ECE) I year
College of Technology And Engineering
MPUAT, Udaipur

2. Outline
• Introduction to Cognitive Radio Network
• Objectives of Cognitive Radio Networks
• CR Network Architecture
• Main Issues in Cognitive Radio Networks
• CRN Standards
• Spectrum sensing & analysis
• Dynamic spectrum allocation & sharing
• Cognitive Radio platforms
• Future Research Directions
• Conclusion

3. Introduction to Cognitive Radio Networks
•A cognitive radio is an intelligent radio that can be programmed and configured
dynamically.
• Radio that automatically detects available channels in wireless spectrum, accordingly
changes its transmission or reception parameters to allow more concurrent wireless
communications in a given spectrum band.
• FCC assigns spectrum to licensed holders,
known as primary users (PU).
• Users who have no spectrum licenses,
known as secondary users (SU).
• FCC has been considering more flexible and
comprehensive uses of the available spectrum,
through the use of cognitive radio technology. Figure 1: Spectrum usage

4. DYNAMIC SPECTRUM ACCESS
• CR Technology works on the principle of dynamic spectrum access, where
secondary users utilize spectrum holes.
• A spectrum hole is a band of frequencies assigned to a primary user, but, at
a particular time and specific geographic location, the band is not being
utilized by that user.

5. Software
Radio
• Dynamically support
multiple variable
systems, protocols
and interfaces
• Interface with
diverse systems
• Provide a wide
range of services
with variable QoS
Conventional
Radio
• Supports a fixed
number of systems
• Reconfigurability
decided at the
time of design
• May support
multiple services,
but chosen at the
time of design
Cognitive Radio
• Can create new
waveforms on its
own
• Can negotiate new
interfaces
• Adjusts operations
to meet the QoS
required by the
application for the
signal environment
• SDR+intelligence
How is a Cognitive Radio Different from Other Radios?

6. Objectives of Cognitive Radio Networks
• spectrum sensing - determine the spectrum holes
• spectrum decision - select the best spectrum opportunities to meet the user
communication requirements
• spectrum sharing - coordinate access to the selected channels with CR
neighbours
• spectrum mobility - switch to the selected opportunities
• maintain seamless communication during spectrum handovers
• avoid any harmful interference to primary users.

7. CR Network Architecture
In a CR network architecture, the components include both a secondary
network and a primary network.

8. CR architectural approaches
The architecture of CR networks can either be centralized or distributed.
• Centralized Approach - Spectrum allocation and access are controlled by a
central entity (e.g., a base station)
• Distributed Approach – Spectrum Allocation and access controlled by CR users
Centralized Approach Distributed Approach

9. Main Issues in Cognitive Radio Networks
1) Self-coexistence - One of the most important and specific issue of CR is to avoid
secondary users to harmfully interfere with primary users. Overlay and underlay are two
possible spectrum access techniques.
2) Accurate Sensing - Sensing aims to determine if a channel is idle or busy in terms of
primary user activity.
3) Signalling - CR scenarios require the exchange of control information between CR
devices for spectrum sensing and sharing. Most CR MAC protocols use a common
control channel (CCC), which facilitates signalling and also neighbour discovery in CR
adhoc networks.
4) Optimized spectrum decision - secondary users are expected to dynamically choose the
best available channels and transmission parameters.
5) Seamless spectrum handover - Seamless transition with minimum quality degradation is a
fundamental goal for any spectrum handover scheme.

10. 5) Cross layer design - to get better performance, the strict layer-based approach is often
violated in wireless networks, namely through cross-layer interactions.
6) Energy efficiency - The number of sensed channels must also be minimized through
appropriate prioritization mechanisms as sensing is one of the main sources of energy and
time consumption.
Cross layer design

11. The IEEE 802.22 standard is the first effort for achieving a CR international standard. It
defines CR techniques that are specifically targeted to enable unlicensed devices to
exploit television white spaces in the VHF and UHF bands (54862 MHz) in a non-
interfering basis for the deployment of Wireless Regional Area Networks (WRAN).
CRN Standards

12. SPECTRUM SENSING & ANALYSIS
First critical step towards dynamic spectrum management, having three aspects of
spectrum sensing.
a) Interference Temperature Model
The interference temperature approach used to enforce an interference limit
perceived by receivers. It is a measure of the RF power available at a receiving
antenna to be delivered to a receiver, reflecting the power generated by other
emitters and noise sources.
TI (fc,B) =
Where PI(fc,B) is the average interference power in Watts centered at fc
b) Spectrum Sensing
Enables the capability of a CR to measure, learn and be aware of the radio’s operating
environment, such as the spectrum availability and interference status. Spectrum
sensing techniques can be categorized as:
(i) Energy Detector
(ii) Feature Detector
(iii)Matched Filtering & Coherent Detection

13. Summary of main spectrum sensing techniques

14. c) Cooperative Sensing
• By taking advantage of the independent fading channels (i.e., spatial diversity) and
multiuser diversity, cooperative spectrum sensing is proposed to improve the reliability
of spectrum sensing, increase the detection probability to better protect a primary user,
and reduce false alarm to utilize the idle spectrum more efficiently.
• In centralized cooperative spectrum sensing, a central controller, e.g., a secondary base
station, collects local observations from multiple secondary users, decides the available
spectrum channels using some decision fusion rule, and informs the secondary users
which channels to access.
• In distributed cooperative spectrum sensing, secondary users exchange their local
detection results among themselves without requiring a backbone infrastructure with
reduced cost.

15. DYNAMIC SPECTRUM ALLOCATION & SHARING
Classification of spectrum allocation and sharing schemes

16. Considering the access technology of the secondary users, licensed spectrum sharing
can be further divided in two categories:
1) Spectrum underlay: In spectrum underlay secondary users are allowed to transmit
their data in the licensed spectrum band when primary users are also transmitting.
The interference temperature model is imposed on secondary users’ transmission
power so that the interference at a primary user’s receiver is within the interference
temperature limit and primary users can deliver their packet to the receiver
successfully.
2) Spectrum overlay: Spectrum overlay is also referred to as opportunistic spectrum
access. Unlike spectrum underlay, secondary users in spectrum overlay will only use
the licensed spectrum when primary users are not transmitting, so there is no
interference temperature limit imposed on secondary users’ transmission. Instead,
secondary users need to sense the licensed frequency band and detect the spectrum
white space, in order to avoid harmful interference to primary users.

17. COGNITIVE RADIO PLATFORMS
A. Wireless open-Access Research Platform (WARP)
B. SORA
C. COgnitive RAdio Learning (CORAL)
D. Berkeley Emulation Engine (BEE2)
E. Lyrtech’s small form factor (SFF) SDR

18. FUTURE RESEARCH DIRECTIONS
A. Seamless spectrum handovers
B. Proactive spectrum selection and interference avoidance
C. Interdependency between the propagation characteristics of radio signals and
the frequency band in usage
D. Alternatives to the common channel
E. Energy efficiency
F. Validation of CR protocols

19. CONCLUSIONS
• This presentation gives an overview of Cognitive Radio Network technology, which is
still in its infancy and aims to enable an efficient utilization of the radio spectrum, has
been briefly described under different perspectives, putting the emphasis on layer-2
issues, learning based on past experience, and implications on upper layer protocols.
• Provide the readers a global vision of CR concerning its principles, present state of
development, and possible future directions.
• It was shown that several challenging issues still need further investigation, making CR
an open research area, such as:
(1) seamless spectrum handovers;
(2) proactive spectrum selection;
(3) interference avoidance;
(4) energy efficiency;
(5) alternatives
•In addition, recently proposed dynamic spectrum management and sharing schemes are
reviewed, such as medium access control, spectrum handoff, power control, routing, and
cooperation enforcement

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