Received: 13 August 2018 Revised: 23 October 2018 Accepted: 19 December 2018 DOI: 10.1002/dac.3902 RESEARCH ARTICLE Spectral efficiency improvement with SISO and SIMO in M-QAM over millimeter-wave links Mohamed Al-Nahhal 1 Tawfik Ismail 2,3 Mohamed E. Nasr 1 1 Faculty of Engineering, Tanta University, Tanta, Egypt 2 National Institute of Laser Enhanced Science, Cairo University, Giza, Egypt 3 School of Engineering and Applied Science, Nile University, Giza, Egypt Correspondence Tawfik Ismail, National Institute of Laser Enhanced Science, Cairo University, Egypt. Email: tismail@niles.cu.edu.eg Summary This paper proposes a spectral efficiency improvement technique for millimeter wave (mmWave) links. The proposed technique provides an efficient utiliza- tion of the mmWave link capacity. This technique is applied in three cases the single-input single-output (SISO), single-input multiple-output (SIMO) with the maximal ratio combining and with the equal gain combining. The M-ary quadrature amplitude modulation scheme is used in our work. The power series expansion is used for deriving closed-form expressions for bit error rate (BER) performances in all studied cases. The BER closed-form expressions are con- firmed by the numerical solution of the integral equations. The simulation results show that a high spectral efficiency can be accomplished by the proposed technique. As well as the derived expressions closely match with the numeri- cal solution of integration expressions at different values of modulations order the Rician factor. For instance, the spectral efficiency gain achievement is 8 at signal-to-noise ratio (SNR) equals 34 dB in the case of SISO system whereas in the case of SIMO system, the same gain is achieved at SNR equals 24 dB. As well as the BER performance is enhanced from 1.188 × 10 -4 ,7.112 × 10 -4 , 4.164 × 10 -3 , and 3.286 × 10 -2 to 8.717 × 10 -16 ,1.119 × 10 -12 ,1.308 × 10 -9 , and 4.905 × 10 -6 for M = 4, 16, 64, and 256, respectively, at SNR equals 30 dB. KEYWORDS equal gain combining, maximum ratio combining, millimeter wave links, M-ary quadrature ampli- tude modulation 1 INTRODUCTION The explosive growth of data rates demanded has become a great challenge in future 5G and beyond networks. 1 In partic- ular, the transport networks that connecting radio access with core networks is critical importance. They are required to support high data rates, high spectral efficiency, and real-time services. Millimeter wave (mmWave) system is one of the well-known solutions that can be used to build effective transport networks due to its large bandwidth. Furthermore, a high spectral efficiency for the mmWave link can be achieved, by employing an adaptive transmission modulation (ATM) technique. 2-5 Many probability distributions are used to model the effects of fading on the received signal on mmWave links such as Nakagami-m, 6,7 Fluctuating Two-Rays, 8 Rayleigh, and Rician distributions. The common distribution is used to model the mmWave channel is the Rician distribution. 6, 9-11 This distribution is described by Rician factor K, which is the ratio between the mean power of the line-of-sight (LOS) path and the mean power of the remaining non-LOS paths. 12 The Int J Commun Syst. 2019;e3902. wileyonlinelibrary.com/journal/dac © 2019 John Wiley & Sons, Ltd. 1 of 11 https://doi.org/10.1002/dac.3902