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

Abstract

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.

A Base-Band Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM signals

“…Finally, by using (8) and remembering the fact that N/M = N vPHY , we show that the time-domain representation of the kth narrow sub-band, B k (i), is recovered by the k-th branch of the channelizer (see Figure 5)…”

“…Therefore, based on the opportunistic use of vacant spectrum, one of the use cases of the proposed wireless hypervisor is instantiating it as a Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) PHY, where one or several vPHYs could, based on spectrum availability/occupancy, have their data mapped into subcarriers that do not interfere/overlap with primary users in a Dynamic Spectrum Access (DSA) scheme, and thereby enabling efficient use of the spectrum [8]. For this use case, an NC-OFDM PHY node would employ the proposed hypervisor architecture as a wide-band OFDM modulator, where the subcarrier mapping module depicted in Figure 4 maps the modulated symbols only into subcarriers Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 15 November 2019 doi:10.20944/preprints201911.0180.v1…”

“…At the receiver side, the signal is demodulated and decoded as a wide-band NC-OFDM signal where some subcarriers had their values set to zero. The PHY receiver for this signal is depicted in the bottom right corner of Figure 4, and is nothing but a generic OFDM demodulator/decoder where the subcarrier demapper module needs to know beforehand which subcarriers are active during a given transmission interval [8]. Figure 6 depicts a possible snapshot of the spectrum usage in the TV white-space when the proposed architecture is used as a dynamic packet-based PHY layer with allocated bandwidth being dynamically changed according to vacant spectrum.…”

“…In general, due to the presence of primary and secondary users in shared licensed bands and of competing (i.e., opportunistic) users in unlicensed bands, the available spectrum for users in a cognitive radio environment is fragmented and its use is intermittent, i.e., the available spectrum is split into non-contiguous chunks and is not used all the time. This intermittent and fragmented spectrum availability calls for a flexible and agile transmission scheme of the desired signals [8]. The concurrent transmission of several narrow orthogonal frequency-division multiplexing (OFDM)-based signals allows for the selective use of the available chunks of spectrum, which enhances the spectrum utilization, consequently improving the area throughput.…”

Achieving maximum bit rate in a cognitive radio network with physical layer network coding

“…Because of high flexibility and capability of spectrum shaping, OFDM can be a perfect choice for the systems working under spectrum pooling provisions . Previous studies investigated OFDM‐based CRNs where the network also benefits from network coding. Orthogonal frequency division multiplexing with PLNC also renders a powerful combination to cope with nonflat fading channels as well as symbol offset problems .…”