What is it

Cognitive Radio is an intelligent wireless communication system that scans the radio spectrum, detects unused spectrum and uses it for communication. When the licensed user (also called Primary User (PU)) of that spectrum comes up (or reclaims the spectrum), the cognitive radio (also known as Secondary User (SU)) moves to some other available frequency. So it primarily consists of a spectrum sensing unit, spectrum manager and spectrum agile handset. The spectrum sensing unit has to be fast and hence is power consuming. The spectrum management unit also ensures that the SUs dont have to switch their frequencies too rapidly. This is done by keeping the PU appearance probability in mind.

Need of Cognitive Radio
Though all the available spectrum is being allocated to users, RF measurement shows that about 30% of the spectrum is actually being used, on an average, across time and space axes. There is lot of congestion in the free ISM bands and there is a lot of fight over spectrum allocation to private companies. Usually the government owns the spectrum and licenses it to others for commercial use. Military and maritime navigation has been licensed with the majority of the RF spectrum, though they are not much in use at all times and at all places. So lot of licensed frequencies remain under-utilized.

The concept of Cognitive radio aims to tap this potentially available spectrum for commercial purposes. I should also state that this is just one aspect in which cognition is applied. Cognitive radios in generic sense as defined by Joseph Mitola is a wireless equipment which understands the radio environment and predicts the user needs and is capable of adapting itself to the requirements thereby providing efficient use of resources.

Requirements
There is only one requirement that the cognitive radio should and MUST follow. It should not interfere with the licensed users communication, i.e. the QoS of the licensed user should not be compromised. In other words, SU should be able to detect the PU with 0% error and SU should back-off within a prescribed time limit if the PU comes up.

Challenges

• Detection of PU with 0% error means detecting a weak PU signal embedded in noise, i.e. presence of PU should be identified even if the power of PU is comparable to noise.
• Strong interferers near a weak PU band makes the detection even more tougher.
• Back off within a prescribed time limit forces the spectrum sensing to be fast and dynamic. The sensing has to be done continuously which consumes lot of power.
• The settling times of the transmitter and receiver while they change the channel limits the SU communication QoS.
• The range over which the cognitive radios can scan and operate is theoretically infinite but a wide range puts severe design challenges of the hardware.
• Power spilled over from SU to the adjacent channel may cause interference to the PU occupying that frequency. This spillage should be within the limits prescribed by the PU. This essentially translates to the guard bands in the SU channels.