Baseband

The baseband part of cognitive radio is generally Orthogonal Frequency Division Multiplexing (OFDM). The basic block diagram of OFDM is given below.

The input bit stream is read 'k' bits at a time and mapped into symbol constellation, i.e. a symbol stream of complex numbers. This stream is parallelized into M channels. These M channels are then mapped to form one OFDM symbol. This OFDM symbol has M data sub-carriers, P pilot sub-carriers and N-(M+P) zeros. The zeros are inserted at dc and at the edges. Zero at dc is to avoid corruption of data due to dc offsets and LO leakage. The pilot data is inserted to estimate the channel for equalization and frequency synchronization. The number and location of pilot carriers depends on the method used for channel estimation. The signal at the input of IFFT is the frequency domain representation of the signal that is sent over the channel. A typical signal is shown below.

The frequency domain signal is converted into time domain signal by the IFFT block. A cyclic prefix is added to the data stream which is then serialized.

At the receiver side the data is parallelized and the cyclic prefix is removed. The FFT block converts the signal into frequency domain. Equalization is performed in the frequency domain, which is nothing but division of each channel by the gain of the transfer function of the transmission medium for that particular frequency. This data is then serialized and symbol demodulation is performed to obtain the bit stream.

If we make a tunable narrowband RF front end then the baseband bandwidth F_B would be around 100 MHz. See the last post for details about the need for narrowband front end.

In the case of cognitive radio, each SU uses only some of the data carriers. The spectrum manager will provide the SU with the locations of these data carriers. All other locations (frequencies) will be filled with zeros. These will include frequencies where PU is present.

Issues
1. Adjacent Channel Interference: When a SU is putting data in a sub-carrier, the resulting spectrum is not confined in that frequency alone. There is small amount of power transmitted in all the adjacent sub-carriers.

The resultant baseband spectrum is actually a sinc function. This will cause interference to the PU (as well as other SU). The interference is significant only at the adjacent channels as the sinc function decays pretty fast. The tolerable interference to the adjacent channel has to be defined and that may vary from one PU band to another. So for each sub-carrier, the amount of power permissible has to be calculated.

Guard bands, windowing techniques, insertion of cancellation carriers etc. are possible solutions for reducing adjacent channel interference.

2. Pilot carrier positioning: In the case of normal OFDM system, pilot carriers are usually scattered uniformly among the sub-carriers and the channel estimation at data carriers is obtained by interpolation from the pilot carriers. In cognitive radio, since only few of the sub-carriers are used for data (rest are zeros), a method for finding the location of pilot carriers needs to be investigated.

One of the possible solutions is to distribute them among each cluster of spectrum holes, i.e. there should be at least some P% of sub-carriers should be pilot carriers in each cluster of holes being used.

Another solution is to use long training sequences for channel estimation. In this technique, all the sub-carriers (being used by an SU) are used to send NT number of training symbols which are used to estimate the channel in each of the sub-carriers. This technique is useful only in slow fading channels. In case of fast fading channels, prediction of channel estimation will have to be employed with the training symbols correcting the predicted estimate from time to time.

3. Interference from PU: The received baseband signal of the SU will contain data from the PUs and other SUs. An FFT of the signal is taken, which gives the frequency domain information of the signal. Only those sub-carriers where the transmitter has put the data is used as the output and the rest is ignored.
A question arises whether the PU will add noise to the SU. The other SUs will not be an issue as they will be orthogonal.

4. Synchronization: In normal OFDM systems, the transceiver and receiver need to be time and frequency synchronized to avoid errors in demodulation. In case of many SUs sending orthogonal signals in the same band, the carriers of all the SUs need to be synchonized.