Physical Communication 39 (2020) 100991 Contents lists available at ScienceDirect Physical Communication journal homepage: www.elsevier.com/locate/phycom Full length article Performance analysis of relay-aided millimeter-wave communications with optimal and suboptimal combining at destination Hadi Hashemi , Javad Haghighat * , Mohsen Eslami Department of Electrical Engineering, Shiraz University of Technology, Shiraz, Iran article info Article history: Received 10 June 2019 Received in revised form 31 October 2019 Accepted 25 December 2019 Available online 7 January 2020 Keywords: Millimeter-wave communications Relay networks Fluctuating Two-Ray channel Maximal-Ratio Combining Selection Combining Outage probability abstract We analyze the performance of a relay network in millimeter-wave (mmWave) band, in two cases when the optimal, Maximal-Ratio Combining (MRC), or a suboptimal, Selection Combining (SC), receiver is employed at destination. The recently proposed Fluctuating Two-Ray (FTR) channel model is applied to model communication in mmWave band. Unlike existing works on the topic which restrict their analysis to channels with integer-valued Nakagami-m fading figures, we extend the analysis to more practical cases, where the fading figure may take arbitrary real values greater than 0.5. We also derive equations which enables us to relate the FTR channel parameters to the underlying network parameters, namely distances between the nodes, and path loss exponents. We analyze the system in both cases where a line-of-sight link between the transmitting and receiving nodes is either present or absent. Distribution of the Signal-to-Noise Ratio (SNR) at combiner’s output, as well as system outage probability and spectral efficiency are analytically derived. Throughout extensive simulations, we study and compare the performance of MRC and SC receivers for different system setups. We also consider the effect of applying antenna arrays with different antenna patterns, as well as the effect of interference from other nodes, on the system performance. The results suggest that despite its simple and low-complexity structure, in most cases, SC receiver demonstrates an encouraging performance and shows small performance losses compared to the optimal MRC receiver. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Millimeter-wave (mmWave) communications presents itself as a promising solution to arising spectrum scarcity problem, thanks to the fact that it could exploit the wide range of mmWave spectrum for data transmission and reception [1]. Consequently, mmWave communications is expected to play a key role in de- velopment of the fifth generation (5G) of cellular networks [2–5]. Many developing 5G technologies, including Vehicle-to-Vehicle (V2V) [6], Device-to-Device (D2D) [7] and Machine-Type Com- munication (MTC) [8] are expected to exploit mmWave commu- nication schemes in order to overcome the scarcity of spectrum that is caused by the presence of considerably large number of si- multaneously active nodes in 5G networks. In addition, mmWave communications is starting to explore potential applications as large as massive Internet of Things (IoT) [9] and also is proposed to be integrated in many other sectors such as High-Speed Train (HST) communications [10]. Communication in mmWave band faces its own particular challenges, including very large penetration loss and significantly * Corresponding author. E-mail addresses: h.hashemi@sutech.ac.ir (H. Hashemi), haghighat@sutech.ac.ir (J. Haghighat), m.eslami@sutech.ac.ir (M. Eslami). high scattering effects, which restrict the application of mmWave communication to short range transmissions. These unique fea- tures of mmWave communication schemes also call for devel- opment of new channel models which are capable to accurately track the actual channel behavior in practice. To this end, many recent works including [11–16] focus on introducing proper chan- nel models for mmWave communications. These channel models are mostly extensions of models previously applied in RF band, but include more parameters to address changes required to fit the model for mmWave environments. One of the most rec- ognized models is the recently introduced Fluctuating Two-Ray (FTR) model [11]. The FTR model includes many important wire- less channels models, e.g. Rayleigh, Nakagami-m, and Rician, as its special cases. In [11] the authors compare the behavior of the FTR model with an available field data, and demonstrate that the FTR model closely tracks the field data. Severe fading effects experienced in mmWave communica- tion channels, limit the coverage and performance of mmWave communication networks. A promising solution to improve the coverage and performance, is to apply relays. Relay deployment assists the source in communicating its message to the desti- nation more reliably, and also extends the coverage range in cases where the destination is located in remote places out of the direct communication range of the source. Thanks to their https://doi.org/10.1016/j.phycom.2019.100991 1874-4907/© 2020 Elsevier B.V. All rights reserved.