NonCoherent Detection for Cooperative OFDMBased System Over TimeVarying Fading Channels Sara Al Maeeni, Sami Muhaidat, and Ibrahim Abualhaol Electrical and Computer Engineering Department, Khalifa University Email:{sara.almaeeni,sami.muhaidat,ibrahim.aboalhaol}@kustar.ac.ae Abstract—In this paper, we investigate a noncoherent detec tion recursive scheme for a multirelay cooperative orthogonal frequency division multiplexing (COFDM) wireless system. We consider a distributed spacetime block code (STBC) system over timevarying fading channels, in which sourcerelay, relay destination, and sourcedestination links experience different Doppler spreads. The cooperative diversity scheme is based on the amplifyandforward (AaF) protocol and the decoding algorithm is derived based on the generalized maximum likelihood sequence estimation (GMLSE). We show that our proposed decoder allows recursive implementation, that does not require any channel state information (CSI). Simulation results demonstrate that the recursive decoder achieves sufficient results even under quasi static fading assumption. The bit error rate (BER) performance of the proposed scheme over timevarying channel, exhibits near quasistatic channel performance at low to medium signalto noise ratio (SNR). Using the assistance of 5 relays and under particular assumptions, the gain advantage is almost 5 dB. I. I NTRODUCTION Cooperative communication has been shown to provide an efficient way to combat wireless fading [1]. The key feature of cooperative transmission is to create multiple virtual antennas during the transmission. This enhances the reliability of wireless network and reduces the power consumption signif icantly. The cooperative Relaybased system consists of three components; source, relay, and destination. In a cooperative scenario, when the source communicates with the destination, the source node is collaborating with partner "relay" to create a virtual array of antennas, although each one has only one antenna. The creation of array reduces the effect of fading channel by enhancing the diversity at the destination. Most of the recent literatures on cooperative diversity as sume the full knowledge of channel state information (CSI). In fading channels when the coherence time is large enough, the channel estimation can be carried out through the use of pilot symbols [2]. However, sufficient resources should be allocated for pilot symbols to get accurate CSI. Other proposed solutions such as differential scheme have been introduced to overcome the problem of phase carrier recovery in fast fading channels [3], [4]. Although these schemes considered as a good practical solutions, they can be justified only for narrow band cooperative scenarios and not for highspeed broadband applications. Most of the existing works on noncoherent detection using AaF relaying for cooperative systems consider over at fading channels [4], [5]. One of the main challenges in highspeed broadband applications, is the depressive nature of frequency selective fading channels. This problem causes a form of distortion between consecutive transmitted symbols known as intersymbol interference (ISI). Highspeed broadband digital communication systems should be designed to handle such severe ISI. An efficient approach to mitigate ISI is orthogonal frequency division multiplexing (OFDM). The main feature of OFDM systems is their ability to convert the operating wide band channel into a number of parallel frequencyat narrowband subcarriers, resulting in a lowcomplexity solu tion. OFDM has been already adopted by major manufacturers and by standardization bodies for wide range of applications. Examples are digital audio/video broadcasting, and wireless local area networks [6], [7]. Another noncoherent detection scheme is proposed in [8] which does not assume any spacetime coding that exploit the advantages of orthogonal structure of the spacetime coding. Moreover, the computational cost of the method which is based on convex optimization and is relatively high. In this paper, we consider a noncoherent detection scheme for multirelay cooperative orthogonal frequency division mul tiplexing (COFDM) wireless networks over timevarying fad ing channels. The cooperative scheme is based on amplify andforward (AaF) protocol and the receiver’s detection is based on generalized maximumlikelihood sequence estima tion (GMLSE). In [9], it has been shown that the GMLSE rule can be reduced to a recursive expression which allows the im plementation of Viterbitype algorithm. It has been shown that the system performs near the optimum over at fading channel. However, this work considers the same detection algorithm for broadband cooperative scenario. Due to the distributed structure of the system as well as the underlying channel model assumption, the practical implementation of a noncoherent receiver is infeasible. Therefore, a special emphasis is imposed on distributed spacetime block code (STBC) system to exploit its inherent orthogonal structure and maintain low decoding complexity. In [10], three cooperative protocols that vary the degree of broadcasting and receive collision are proposed. It has been proved that Protocol III achieves a full spatial 978-1-4799-2452-3/13/$31.00 ©2013 IEEE 197