Comparison of CSI and Fixed Gain Relay of Two-hop Wireless Link under Small-Scale Fading Sarwar Jahan Department of Electronics and Communication Engineering East West University Dhaka, Bangladesh E-mail: sjahan@ewubd.edu Md. Imdadul Islam Department of Computer Science and Engineering Jahangirnagar University Savar, Bangladesh Email: imdad@juniv.edu M. R. Amin Department of Electronics and Communication Engineering East West University Dhaka, Bangladesh E-mail: ramin@ewubd.edu Abstract— In this paper, we show the explicit analytical model of CSI (Channel State Information) and Fixed Gain Relay of 2-hop wireless link under dissimilar fading environment. The two schemes can be compared in context of common fading condition and common amplitude of signal. The CSI scheme is alike adaptive algorithm to provide optimum result at the expense of process time. Fixed Gain relay system provides a constant gain at relay without the knowledge of channel. Because of this there is a possibility of a receiver to experience unnecessary high amplitude signal or possibility of getting huge attenuation of signal. Actually it is very difficult to prove theoretically which scheme is better under which environment. In this paper we compare models based on outage probability and SER. The finding of the paper is: The CSI model works better in adverse channel condition i.e. at low SNR at receiving end but fixed gain relay is better under friendly condition i.e. at higher SNR under the same fading environment. Keywords- Rayleigh fading, outage probability, SER, cdf of combined r.v.s and threshold SNR. I. INTRODUCTION To increase the performance and the reliability of the wireless link, two hop mechanism can be used. It also gives coverage extension and improving robustness. Here a relay is placed between the transmitter and the receiver. In this two hop wireless links, the link between transmitter to relay is called 1st hop and the relay to the receiver is called 2nd hop. These two hops are affected by different types of fading called asymmetric fading environment. Relays can be the user nodes (mobile stations) or fixed terminals (infrastructure relays). According to their processing functionality, Relays can be divided in to three categories [1]. They are: Amplify-and-Forward (AF), Decode-and-Forward (DF) and Code-and-Forward (CF). Amplify-and-Forward relay is the simplest form of the relays. As the name suggests, it amplifies the received signal and retransmits [2]. AF can be further categorized (according to the gain) as variable gain (VG) and fixed gain (FG) relaying. VG uses instantaneous Channel State Information (CSI) of the received path, to choose the amplification gain [3], while FG uses long term statistics of the inward channel when choosing the amplification gain [4]. Therefore, the instantaneous CSI is not required for FG. This is also known as ‘semi-blind’ relay. Fixed gain relay does not to measure the channel from the source and it has less system complexity and overhead. CSI is defined as known channel properties of a wireless communication link. This information describes how a signal propagates from the transmitter to the receiver. It also represents the unify effect of scattering, fading and power deterioration with distance. Several works have been done in this two hop wireless link. In [5], the authors investigate the end-to-end performance of a dual-hop fixed gain relaying system with semi-blind (fixed gain) relay under asymmetric fading environments. They have analysed the results under different scenarios, such as varying average SNR, fading parameters per hop, the choice of the semi-blind gain and the location of relaying node. The authors in [6], have analysed the performance of a two hop CSI based amplify-and-forward relay with considering the co-channel interference at the relay. The performance analysis of two-hop decode-and-forward (DF) relaying protocol has been done in [7]. They have considered mixed faded channels by evaluating outage probability and average bit-error probability. Cheng et. al. investigated end-to-end performance with fixed gain relays over independent non-identical composite Nakagami- lognormal fading channels using mixture gamma distribution [8]. They have shown the numerical and simulation results with considering outage probability, the average symbol error rate and the nth moments of end-to-end signal to noise ratio (SNR). Some other works in these two hop wireless link have been found in [9-13]. In our previous work [14], we compared the performance of a two-hop wireless link under a different combination of space diversity in the first and second hop of the amplify-and- forward (AF) case incorporating the use of transmit antenna selection (TAS) on both link with weak direct link, distributed Alamouti scheme in two-hop link and single relay antenna International Journal of Computer Science and Information Security (IJCSIS), Vol. 15, No. 6, June 2017 6 https://sites.google.com/site/ijcsis/ ISSN 1947-5500