## 1 Introduction  
The use of satellite communication systems nowadays is widespread in many diverse applications, such as navigation, mobile communication, broadcasting and disaster relief. Therefore, their proper working in the above diverse practical scenarios is very important for both the users and service providers. One highly probable event in land mobile satellite communications is the difficulty in maintaining line-of-sight (LOS) communications [1; 2], because of the following propagation impairments: the blocking of signals caused by large obstacles (shadowing), severe attenuation (path loss) and the multipath channel exhibiting frequency selective fading may cause intersymbol interference [3; 4]. This situation is also commonly called masking when LOS is no longer available between satellite and terrestrial users, and it severely affects the indoor users or in the case of low satellite elevation angles. A number of works have studied the performance of hybrid satellite-terrestrial system (HSTS) [1; 2] in the presence of the masking effect, e.g., [5; 6; 7; 8; 9; 10]. The average symbol error rate (SER) of the fixed gain amplify-and-forward (AF) hybrid satellite terrestrial-system with generalized fading channels was derived in [6]. The analysis of cooperative relaying strategies combined with the delay diversity technique in a digital video broadcasting-satellite services to handhelds (DVB-SH)-compliant hybrid satellite/terrestrial network, for reducing the impairments caused by the masking, was given in [7]. A cooperative diversity scheme for mobile satellite multimedia broadcasting systems, utilizing space-time block codes and rate-compatible turbo codes to achieve diversity gains and additional coding gains, respectively, was analyzed in [8]. A hybrid satellite-terrestrial networking approach, where land mobile users have both satellite communication and cooperative-networking capabilities, to solve the masking problem was proposed in [9]. The use of different cooperative transmission schemes for the delivery of satellite services, in which mobile terminals forward the received signal from the satellite, was studied in [10]. The outage performance of an HSTS was analytically evaluated in [11]. The performance of a selective decode-and-forward (DF) protocol-based HSTS, employing the selection of the best relay terminal, was investigated in terms of outage probability by [12]. The symbol error probability of the hybrid/integrated satellite-terrestrial cooperative network described in [12] was evaluated in [13]. In [14], the authors considered the performance of AF-based relaying in a hybrid satellite-terrestrial free space optical (FSO) cooperative link, where a masked destination node was made to receive the relayed signals through an FSO link. The use of multiple antenna techniques in HSTS with masked ground node was considered in [15]. All of the above cited papers have contributed to our understanding of the performance analysis of HSTS; however, they have focused on the ideal case without co-channel interference (CCI). The assumption of no CCI is impractical nowadays due to the deployment of many wireless standards in the same spatial location and the increased practice of reusing the spectrum resource in traditional wireless networks [16; 17; 18]. The effect of CCI on the average SER of a particular HSTS was investigated in [19], where the DF protocol was used at the terrestrial relay, and the destination was corrupted by multiple Rayleigh faded interferers. The LOS path between satellite and destination node was assumed to be available in [19]. As is evident from the above literature survey, with the exception of [19], the effect of interference on the performance characteristics of dual-hop HSTS has not been investigated before in the open literature.