As the number of devices using wireless spectrum has increased, availability of usable spectrum for the licensed devices is a concern. Cognitive radio (CR) plays an important role in addressing this problem with dynamic spectrum access. In the past few years there has been a large amount of research on addressing different aspects of cognitive radios. Orthogonal frequency division multiplexing (OFDM) has been suggested as one of the candidates for dynamic spectrum access in CRs due to its flexible and efficient use of the spectrum. Non-contiguous OFDM (NC-OFDM) is a method of transmission where some of the subcarriers in OFDM are nulled and only the remaining subcarriers are used for transmission. We are working on a data and control channel design for a communication system using NC-OFDM. We are not only doing analysis on MATLAB using our model but also testing with USRPs using GNURadio. You can find the code for my projects on https://github.com/r4tn3sh
Non-contiguous OFDM (NC-OFDM), is well suited to enable spectrum usage in such dynamic environments. NC-OFDM is similar to OFDM, except that the transmission is restricted to certain sub-carriers. Non-contiguous sub-carrier access may arise either due to the presence of Primary users, or due to the frequency selective nature of the channel, or to ensure fair allocation of spectrum. Enabling the use of NC-OFDM for dynamic spectrum access requires two key challenges to be addressed first. First, NC-OFDM requires an agreement between transmitter receiver about the set of subcarriers being used in that link (i.e. frequency synchronization), andsecond, timing and frequency offset estimation must be established to ensure successful data detection.
Although there are several techniques for detecting the start of a OFDM frame (timing synchronization) and to estimate frequency offset, they are not directly applicable to NC-OFDM due to concurrent nature of transmissions. We therefore propose a low data rate CDMA-like underlay control channel to address the above two issues. Such a control channel is envisioned to be low power and to span all available bandwidth. Multiple point-to-point links access this control channel by using different spreading codes. A two stage timing and frequency offset estimation technique is proposed using this channel. Once synchronization is established, channel is used to transmit control information such as sharing the active sub-carrier indices. Forsuch a control channel we study the accuracy of timing synchronization, data rates achieved in control channel and effect on the performance of NC-OFDM-based data transmission.