Design and implementation of an underlay control channel for NC-OFDM-based networks

This paper designs an underlay control channel for noncontiguous-OFDM-based cognitive networks. Noncontiguous OFDM (NC-OFDM) provides a fast and flexible manner of accessing disjoint parts of the spectrum and is ideally suited for dynamic spectrum access. While similar to OFDM, NC-OFDM explicitly restricts transmission to only certain subcarriers that are free of incumbent transmissions. In particular, this paper considers designing a control channel for a cognitive network consisting of multiple point-to-point (p2p) links that operate over a wide bandwidth that might encompass some primary transmissions. In such a scenario, control channel becomes vital not only to share basic transmission parameters but also to aid timing and frequency recovery of NC-OFDM transmission; a nontrivial problem in itself. The proposed design is a low-power underlay transmission that spans the entire bandwidth regardless of any incumbent transmissions and uses direct sequence spread spectrum (DSSS). The control channel operates in one of two modes. The first mode aids timing and frequency recovery through a two-step process, while the second mode is used for control data transmission. To enable multiple access, the p2p links use orthogonal pseudo-noise (PN) sequences. The proposed control channel is implemented on USRPs in the ORBIT testbed using GNU Radio. Experimental results suggest robust timing and frequency offset recovery even in the presence of concurrent primary transmissions and support for about 10 to 20kbps over a 1 MHz bandwidth at an uncoded symbol-error-rate of about 10-2 under typical operating conditions.