| Summary: | In the underlay spectrum sharing approach, cognitive radios have limited coverage area to keep interference protection for the licensed users. Multiple cooperative relays have shown benefit for allowing reliable communication with an increase in the radio coverage area. The performance of an underlay cognitive cooperative multiple relays network (CCMRN) is evaluated in case of imperfect channel state information (CSI). In this paper, CCMRN will be treated as a secondary network with multiple decode and forward relays, whereas the secondary user (SU) transmitter will select a single relay from the available set of relays. The relay selection process is based on obtaining maximum end-to-end effective capacity (C<sub>eff</sub>) as seen by the SU. By assuming cooperation between the licensed primary network and the unlicensed secondary network, the SU and the selective secondary relay can share the spectrum with occupying two orthogonal channels of two different primary users (PUs). According to the average interference power constraint of the PUs, the SU allowed maximum power transmission will be obtained. Network performance is assessed in terms of the SU maximum end-to-end C<sub>eff</sub> as well as outage probability (P<sub>out</sub>). Mathematical expressions for C<sub>eff</sub> and P<sub>out</sub> are derived for practical multipath Rayleigh faded channel. The analytical results illustrate the effect of the imperfect CSI on the performance metrics with applying various constraints such as available multiple relays group size, statistical delay quality of service, and average interference power.
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