On the Practical Implementation of VFDM-based Opportunistic Systems: Issues and Challenges

Abstract

Vandermonde-subspace frequency division multiplexing (VFDM) is a physical layer technique for cognitive two-tiered networks, allowing for the coexistence of an orthogonal frequency division multiplexing (OFDM) legacy systemand a cognitive secondary system in a time division duplex mode. It consists of a linear null-space precoder used by thesecondary transmitter to effectively cancel the interference towards one or more primary receivers, while guaranteeing anon-negligible rate to a served secondary receiver. In this work, we propose an implementation of an experimental test-bedusing the new SDR4All platform developed at the Alcatel-Lucent Chair on Flexible Radio (SUPELEC) to take a step towardsa proof of concept of a VFDM-based system. We focus on the secondary link, where an opportunistic transmitter/receiverpair communicates over moderately frequency selective channels, characterized by very short root mean square (r.m.s.)delay spreads and non uniform power delay profiles (PDP). The obtained results show the practical feasibility of a VFDMtransmission over a secondary link. However, a significant bit error rate (BER) loss with respect to the previously shownachievable theoretical performance is evident. A thorough analysis of the structure of the VFDM precoder is carried out andthe impact of the channel characteristics on the performance of the opportunistic system is discussed. Numerical findingsdemonstrate that the potential BER drop can be addressed by designing a suitable flexible receiver able to deal with theeffect induced by non uniform PDP and short r.m.s. delay spread channels.