cognitive radio

1. Cognitive Radio (CR)
1
Large amount of white space in time and frequency
• Unused portions of licensed spectrum is called spectrum holes or White
Spaces.
• Dynamic spectrum sensing helps with identifying and assessing quality of
unused channels

2. Cognitive Radio (CR)
2

3. Cognitive Radio: Basic Idea
3
Cognitive Radio (CR) technique has been proposed by the Federal
Communication Committee (FCC) as a solution to such spectrum shortage
problem.
Cognitive radios enhance the control process by adding:
 Intelligent control of radio;
 Ability to sense the environment;
 Processes for learning about environmental parameters;
 Awareness of its environment: signals & channels;
 Awareness of capabilities of the radio;
 Ability to negotiate waveforms with other radios.

4. Definitions
4
FCC:
“A radio that can change its transmitter parameters based on
interaction with the environment in which it operates.”
IEEE USA
“A radio frequency transmitter/receiver that is designed to
intelligently detect whether a particular segment of the radio
spectrum is currently in use, and to jump into (and out of, as
necessary) the temporarily-unused spectrum very rapidly,
without interfering with the transmissions of other authorized
users.”
Why So Many
Definitions?

5. 5
Cognitive Radio Applications

6. Cognitive Radio Wireless Networks
• Two types of cognitive users
1) Primary users (PUs) or licensed users
2) Secondary users (SUs) or unlicensed users
• Two types of cognitive radio networks
1) Synchronous
2) Asynchronous
• Three types of spectrum sharing modes
1) Overlay
2) Underlay
3) Interweave

7. PUs & SUs
• Primary User: A user who has higher priority or legacy
rights on the usage of a specific part of the spectrum.
• Secondary User: A user who has a lower priority and
therefore exploits the spectrum in such a way that it does not
cause interference to primary users.
• Spectrum Sensing: The task of obtaining awareness about
the spectrum usage and existence of primary users in a
geographical area.

8. PUs & SUs
• SUs can sense occupancy
of licensed channels and
use licensed channels
when they are not
occupied by PUs based on
spectrum sensing results.
• PUs and SUs form
primary wireless network
and secondary wireless
network (cognitive radio
network), respectively.

9. 1) Synchronous Cognitive Radio Networks
• Time axis is divided into slots
• SUs are synchronized with PUs
Same knowledge on the time-slot boundary
• PUs spectrum activity is consistent during a given time slot
An example of Synchronous CRN with interweave spectrum sharing mode

10. 2) Asynchronous Cognitive Radio Networks
• SUs are NOT synchronized with the PUs
• Different licensed channels for different types of PUs can
also be asynchronous

11. Cognitive Radio Spectrum Sharing
Modes
• Overlay
Cognitive radios overhear and enhance PU’s` transmissions
• Underlay
Cognitive radios constrained to cause minimal interference
to PUs
• Interweave
Cognitive radios find and exploit spectral holes to avoid
interfering with PUs
Knowledge
and
Complexity

12. Spectrum Sharing Modes (1)
Type 1: Overlay
• SUs may use a part of their energy to assist communications of PUs
through cooperative communication techniques and the rest of
energy to transmit their own signals.
• Interference from SUs' signals can be compensated with gain for
PUs' signal quality through cooperation of SUs.
• It requires SUs to know PUs' packets before PUs begin their
transmissions
PU
S
SU
R1
PU
D
SU
R2

13. Spectrum Sharing Modes (2)
Type 2: Underlay
• SUs transmit signals in such a low-power level that interference
caused by SUs is below noise floor of the spectrum.
• In the view point of PUs, transmissions by SUs are nothing but noise
with low-level power.
• SUs and PUs can co-exist in same spectrum
• If PUs’ topology and transmission power do not change
• No need to sense the channel
• Do not cause packet collision for PUs for both synchronous and
asynchronous cognitive networks

14. Spectrum Sharing Modes (3)
Type 3: Interweave
• Listen first and talk later
• Exclusive utilization
• SUs frequently sense channel
• SUs do not need to worry about interference temperature constraint of
PUs
• Relax power constraints imposed onto SUs as compared with
underlay spectrum sharing mode.
• No interference to PUs in synchronous CRNs
• Inevitable interference to PUs in asynchronous CRNs due to half-
duplex nature of wireless radios