Effect of imperfect channel state information and co-channel interferences on two-hop fixed gain amplify-and-forward relay networks with beamforming Min Lin 1,2 , Heng Wei 3, * ,† , Kang An 3 , Jian Ouyang 4 and Yongming Huang 5 1 PLA University of Science and Technology, Nanjing, China 2 National Mobile Communications Research Laboratory, Southeast University, Nanjing, China 3 College of Communication Engineering, PLA University of Science and Technology, Nanjing, China 4 Institute of Signal Processing and Transmission, Nanjing University of Posts and Telecommunications, Nanjing, China 5 School of Information Science and Engineering, Southeast University, Nanjing, China SUMMARY This letter investigates the joint effects of imperfect channel state information and co-channel interferences on a two-hop fixed gain amplify-and-forward (AF) relay network with beamforming. Specifically, the analytical expressions of the outage probability and the average symbol error rate for the AF relaying are derived. More- over, the asymptotic analysis at high signal-to-noise ratio is also presented to reveal the diversity order and array gain of the considered AF relay system. Finally, computer simulations are given to confirm the validity of the analytical results. Copyright © 2014 John Wiley & Sons, Ltd. Received 22 August 2013; Revised 25 March 2014; Accepted 13 May 2014 KEY WORDS: amplify-and-forward relaying; beamforming; imperfect CSI; co-channel interference 1. INTRODUCTION Recently, multi-antenna relay technology has attracted many researchers as a low-cost strategy to increase the capacity, reliability, and coverage of wireless communications without requiring extra power or bandwidth (see [1–5] and the citations therein for example). When channel state informa- tion (CSI) is available, a practical transmission scheme for multi-antenna relay networks is beamforming (BF), which applies maximum ratio transmission (MRT) at the transmitter and max- imal ratio combining (MRC) at the receiver to maximize the output signal-to-noise ratio (SNR). Of the various relay schemes, the amplify-and-forward (AF) is of particular interest because of its low complexity. Although the performance of a two-hop AF relay network with BF has been well stud- ied in [1, 2], the authors have limited their analysis to the perfect CSI, which is a too restrictive assumption in practice [6–9]. This observation promotes the investigation of feedback delay on var- iable gain AF relaying in [7, 8] and that of imperfect channel estimation on fixed gain AF relaying in [9], respectively. However, the main drawback of the aforementioned works is that they have not taken the co-channel interference (CCI) into account. In addition to the imperfect CSI, CCI often severely degrades the performance of wireless systems. Under this situation, the impact of CCI on AF relaying has first been studied in [10] and [11] for single antenna scenario and later extended to the cases of multi-antenna BF over Rayleigh fading channels in [12–15]. Furthermore, the authors of [16] and [17] have studied a more general case, by considering *Correspondence to: Heng Wei, College of Communication Engineering, PLA University of Science and Technology, Nanjing, China. † E-mail: weiheng63@163.com Copyright © 2014 John Wiley & Sons, Ltd. INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS Int. J. Commun. Syst. 2014 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/dac.2822