Spectrum Agile Radio

The most primitive way of obtaining a spectrum agile radio is to vary the carrier frequency of the SU whenever it needs to switch from one frequency to another. The spectrum manager tells the SU transmitter and receiver which frequency to use.
Along with the carrier frequency, the bandwidth of the SU also keeps changing as per user needs. So the radio should have circuits (primarily filters) whose bandwidth is controllable. In other words, tunable filters which are tough to design.

Another scenario that may occur in cognitive radios is the lack of contiguous unused spectrum. It is possible that there is enough bands of frequencies unused but they are scattered over the whole scan range, and the bandwidth requirement of the user is larger than the biggest spectrum hole. In this scenario, the above mentioned spectrum agile radio (which can change its carrier frequency and bandwidth) fails. The figure below depicts this case.

To use the scattered (small) spectrum holes, OFDM was proposed as the baseband modulation technique. In OFDM, the total baseband bandwidth is divided into sub-carriers (or sub-channels) and data is put onto those sub-carriers. FFT processor is used for doing OFDM modulation and demodulation.
To put data just in the spectrum holes, only those sub-carriers that lie in the hole carry SU data and the rest of them are filled with zeros. Since the transmitter and the receiver know which sub-carriers are being used, SU communication is possible as shown below.

Allocation of spectrum holes to multiple SUs is shown below

The power spectrum of the output of different secondary users look something like this.