# Signal Space Cognitive Cooperation

## TL;DR

This paper introduces a novel signal space cognitive cooperation scheme that enhances data rates in underlay spectrum sharing networks by transmitting two signals in two time slots and optimizing relay selection and power allocation.

## Contribution

It proposes a new transmission scheme using signal space diversity in cognitive cooperative networks, with analytical performance evaluation and relay selection strategies.

## Key findings

- Improved data rates with two signals transmitted in two time slots.
- Closed-form error probability expressions derived for the proposed scheme.
- Diversity gain achieved through asymptotic analysis.

## Abstract

In this work, a new transmission scheme, signal space cognitive cooperation, is introduced by applying the idea of signal space diversity in an underlay spectrum sharing decode-and-forward multi-relay cooperative network. In the proposed structure, the secondary source signal is rotated by a certain angle and then the source and the secondary best relay transmit the in-phase and the quadrature components of the rotated signal. As a consequence, two source signals, rather than one, are transmitted in two-time slots which improves data rates considerably, compared to the conventional cognitive cooperative schemes. In this work, proactive relaying mode is used in which the best relay is selected based on the max-min selection criterion before executing the transmission. Considering both statistical and the instantaneous channel state information of the feedback channel between the primary receiver and the secondary network, two power allocation methods are adopted at the source. For both methods, closed-form expressions of error probability are derived. Moreover, asymptotic analysis is performed and diversity gain is obtained to provide further insights about the system performance. Finally, analytical expressions are verified by Monte-Carlo simulations.