3.COGNITIVE RADIOA cognitive radio is a transceiver which automatically detects available channels in wireless spectrum and accordingly changes its transmission or reception parameters so more wireless communications may run concurrently in a given spectrum band at a place. This process is also known as dynamic spectrum management.

The principle of cognitive radio is temporal, spatial and geographical re-use of licensed spectrum.

4. UNDERLAY/OVERLAY Overlay communication exploits that part of spectrum which is currently not being used by primary user.

Underlay communication is done in that part of spectrum in which primary user is already present, under some power threshold to reduce the interference to primary users. 5.OVERLAY COGNITIVE RADIOCognitive radios use flexible spectrum access techniques for identifying under-utilized spectrum and to avoid harmful interference to other radios using the same spectrum.

Such an opportunistic spectrum access to under-utilized spectrum, when the frequency is assigned to licensed primary services, is referred as overlay spectrum sharing.

Overlay sharing requires new protocols and algorithms for spectrum sharing.

6.UNDERLAY COGNITIVE RADIOOpen access to most of the radio spectrum, even if the spectrum is licensed for a dedicated technology, is permitted by radio regulation authorities only for radio systems with minimal transmission powers in an underlay cognitive radio.

Spread spectrum, Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) or Ultra-Wide Band (UWB) techniques are used to spread the emitted signal over a large band of spectrum so that the undesired signal power seen by the incumbent licensed radio devices is below a designated threshold. The transmission power is strictly limited in underlay spectrum sharing to reduce the possibility for a potential interference.

8.UNDERLAY TRANSMISSIONIn Underlay scenario, secondary transmission is made in the presence of primary user under power constraint so that it will not interfere with primary transmission.

9. CONCEPT OF NC-OFDMNon-contiguous OFDM Carriers are deactivated according to the spectrum condition.Deactivation of subcarriers protect primary user interference. Parameters needed for deactivation of subcarriers is determined by the spectrum sensing.