In the presented work, the performance analysis of an underlay cognitive relay network with a hybrid-decode/amplifyand-forward relaying protocol is undertaken. Since the system is an underlay system, the secondary network will communicate simultaneously with the primary network. Hence interference temperature will act as a constraint to keep the secondary communication from interfering with the primary transmission. To ensure this, secondary transmit powers need to be kept below a pre-defined threshold level. Severe multi-path fading and shadowing or a long distance between the secondary source and the destination make a direct link between the two infeasible. These nodes can only be linked by a relay between the two. All the radio links in the system are characterized by the extended generalized-K fading distribution. The closed form expressions for the outage probability and ergodic channel capacity of the system are derived. The system outage performance and capacity with hybrid-decode/amplify-and-forward relay is compared to the performance with amplify-and-forward and decode-and-forward relays. Optimum relay position for best outage performance is also investigated. Further, the results of the performance analysis of the system are also illustrated by the numerical plots.

Introduction

Due to the increased data applications and services worldwide, global data traffic and volume has increased exponentially, giving rise to spectrum scarcity. This scarcity is more due to the under utilization of the available spectrum than due to actual lack of it. As a remedy to this problem, the cognitive radio networks came into existence in late twentieth century [1],[2]. These networks provided a means to alleviate the problem of spectrum scarcity. The said networks constitute multiple user networks that enable unlicensed (secondary) users to access a channel belonging to a licensed (primary) user. The access may be through one of the three laid down methods viz. overlay, underlay or interweave [3]. The channel has to be free before the secondary user (SU) can access it in overlay system, [4]. The SU has to vacate the channel if the primary user (PU) needs access to the channel as the PU will always have a higher priority. Underlay systems can give both PU and SU access to the channel simultaneously as in [5], provided the SU does not interfere with the PU signal while communicating. For this the powers transmitted by the secondary nodes need to be kept below the pre-defined threshold values. The secondary user in the interweave systems, employs some spectrum awareness methods to explore and then opportunistically utilize the unused spectrum, also called the spectrum holes as explained in [6]. In this work, the ultra wideband antenna configuration is used for spectrum sensing and the reconfigurable antenna is used for communication for secondary transmission on the radio channel.