Introduction

In this project we aim to study the impact of multiple users in Cognitive Radios. Multi-user issues in Wireless have been studied with respect to diversity and transmit cooperation. 

As a first step we show that radio sensitivity required of an individual radio is not feasible with today's radios. In the presence of multipath and shadowing, a single user may receive a signal that is 40-50dB lower than nominal path loss. The presence of multiple Cognitive Radios may be the only way of avoiding the effects of severe fading. 

Fig. 1 show the the variation in sensitivity threshold when the number of users is increased. For a single user we need radio sensitivities of -115dBm for a 99.9% detection probability. Furthermore, this sensitivity relies on extremely rare events which are not well modeled by the Central Limit Theorem. Roughly, 10 users are needed to achieve half the threshold gains.

Fig 1: Variation of the Sensitivity threshold as the number of users is increased