Hybrid Satellite-Terrestrial Cooperative Communication with Mobile Terrestrial Nodes Neeraj Varshney and Aditya K. Jagannatham Department of Electrical Engineering Indian Institute of Technology Kanpur Kanpur, India 208016 Email:
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Abstract—This work investigates the performance of hybrid satellite-terrestrial systems over time-selective fading links arising due to the node mobility with multiple relay based selective decode-and-forward cooperation. The aerial satellite-to-relay and satellite-to-destination links are non-identical time-selective shadowed Rician fading in nature, whose parameters depend on the elevation angle of the satellite, whereas the terrestrial relay-destination links are assumed to be non-identical timeselective generalized Nakagami faded. Closed form expressions are derived for the per-frame average symbol error rate (SER) and asymptotic SER floor considering the transmission of M ary PSK modulated symbols. It is observed that the timevarying nature of the links significantly degrades the system performance. Further, the impact of the satellite elevation angles at the terrestrial nodes is explicitly demonstrated through simulations, along with the effect of preamble versus midamble for channel estimation. The error rate of the system is seen to reduce significantly with increasing satellite elevation angle at the relay when the satellite-destination link experiences frequent heavy shadowing (FHS) and the relay-destination links are relatively strong. However, for other scenarios when the relay-destination links are relatively weak and satellite-relay links experience FHS, significant performance improvement can be seen by increasing the satellite elevation angle at the destination UE.
I. I NTRODUCTION Hybrid satellite-terrestrial systems have gained significant research interest due to their ability to provide satellite coverage inside buildings and other shadowed areas where line-of-sight communication is not possible because of the masking effect [1], [2]. This enables higher data rates, enhanced reliability and improved bandwidth utilization for broadcasting and navigation applications with lower costs. The masking aberration also affects outdoor communication scenarios and its effect becomes more pronounced at lower satellite elevation angles. Further, mobility of the destination user-equipment (UE) and other cooperative UEs serving as relay nodes induces Doppler, which results in time-selective fading [3] and the ensuing degradation of the end-to-end system performance [4]. Therefore, studying the effect of node mobility and the resulting time selective fading links on hybrid satellite-terrestrial communication systems together with the satellite elevation angles which impact the degree of shadowing in satellite-terrestrial links is critically important. A brief summary of related works in existing literature is presented next. The work in [5] analyzes the symbol error rate (SER) and outage performance of a hybrid satellite-terrestrial cooperative system over non-identical fading channels. However, the
work therein considers the amplify-and-forward (AF) protocol which introduces noise amplification at the relay nodes [6]. The work in [7] analyzes the SER of a hybrid/integrated satellite-terrestrial selective decode-and-forward (DF) cooperative network and presents results in terms of finite sums of Lauricella hypergeometric functions. Both these works however do not consider the impact of elevation angles as well as time-selectivity on the end-to-end performance. The ergodic capacity and average SER performance of a hybrid satellite-terrestrial cooperative system with fixed gain AF relaying has been analyzed in [8] wherein the aerial and terrestial links are assumed to experience shadowed Rician and Rayleigh fading respectively. A similar setup has been considered and analyzed in the presence of cochannel interference in [9], [10]. Further, several works such as [1], [2], [11], [12] have analyzed dual-hop hybrid satelliteterrestrial cooperative systems considering either AF or fixed DF relaying protocols at the relay node. However, none of the works in the existing literature consider time selectivity and its impact on the end-to-end performance, especially employing the robust selective DF cooperative protocol. This work investigates the performance of hybrid satelliteterrestrial systems over time-selective fading links with multiple relays and selective DF cooperation. Moreover, the satellite-terrestrial links in the land mobile satellite (LMS) system are assumed to be standard non-identical time-selective shadowed-Rician fading in nature, whose parameters depend on the satellite elevation angle. The relay-destination terrestrial links are assumed to be nonidentical time-selective generalized Nakagami faded. It is observed that the time-varying nature of the links leads to a discernible degradation of the end-to-end performance of the system. Further, the impact of the satellite elevation angles is explicitly demonstrated in this work. It can also be noted that the results for quasi-static channels in scenarios with stationary terrestrial nodes can be directly derived by setting the correlation parameter to unity. II. H YBRID S ATELLITE -T ERRESTRIAL S YSTEM M ODEL WITH M OBILE N ODES Consider a hybrid satellite-terrestrial communication (HSTC) system with selective DF based cooperation in which K terrestrial relay nodes cooperate with a satellite and selectively re-transmit M -ary PSK modulated satellite data to the desired destination UE over orthogonal channels. This work considers each relay node as well as the destination
UE to be mobile with arbitrary speeds which results in time-selective satellite-relay, satellite-destination and relaydestination links. Moreover, in contrast to previous works [1], [2], [13], the presence of a direct satellite-destination link with an elevation angle θSD is also assumed. The elevation angle for the link between satellite and relay r is denoted by θSRr , where r = 1, 2, · · · , K. In the two-phase system considered, the received signals ySD [k] and ySRr [k] at the destination and relay r respectively, corresponding to the transmission of symbol x[k] by the satellite in the k th signaling period, 1 ≤ k ≤ Nb , are given as, p (1) ySD [k] = P0 hSD [k]x[k] + wSD [k], p ySRr [k] = P0 hSRr [k]x[k] + wSRr [k], (2)
where 1 ≤ r ≤ K, P0 denotes the power transmitted by the source and hSD [k], hSRr [k] are the time-varying channel coefficients for the satellite-destination and satellite-relay r link respectively. Further, a set of relays participate in the second phase and forward the data symbol to the destination only if the decoding SNR exceeds a predefined threshold [6], [7], [14]. The received signal yRr D [k] at the destination corresponding to transmission by relay r is given by p (3) yRr D [k] = Pr hRr D [k]x[k] + wRr D [k],
where Pr denotes the power transmitted by relay r and hRr D [k] is the channel coefficient of the time-varying relay r-destination link. The quantities wSD [k], wRr D [k] and wSRr [k] above are the additive white noise samples at the destination, relay r respectively and can be modeled as zero mean circularly symmetric complex Gaussian random variables with variances η0 . Because of the time-selective nature of the links, it is often difficult to obtain the instantaneous CSI corresponding to each signaling period k. Hence, it is assumed that the channel coefficients hSRr [L] and hSD [L], hRr D [L] at the rth relay and destination UEs are perfectly estimated [15], [16] once at the Lth signaling instant of each frame and are subsequently employed to detect each symbol x[k], 1 ≤ k ≤ Nb , in the corresponding frame of length Nb . This is a valid assumption in typical wireless systems since the receiver tracking loop cannot estimate the channel gains in each signaling period k due to the significantly higher pilot overhead involved [3]. Also note that the model is general and can accommodate both a preamble or midamble for channel estimation by appropriate choice of L. Further, we model the time-selective nature of each link using the first order auto-regressive (AR1) process [17] as, ( p ρi hi [k + 1]+ 1−ρ2i ei [k], if 1≤k
