International Journal of Scientific & Engineering Research Volume 10, Issue 2, February-2019 823 ISSN 2229-5518 IJSER © 2019 http://www.ijser.org Analog- Digital Precoder for Downlink System on the Impact of Antenna Impairments Wasan Al- Masoody * and Naz Islam* Abstract- In this paper, the BER performance of the downlink multi-user multiple-input-multiple-output (MU- MISO) system for the perfect channel state information (CSI) is investigated. Rather than compensating or eliminating the effect of the mutual coupling among antenna elements, an optimization process is used by the aid of an analog-digital (AD) precoder. In this precoder, a conventional linear scheme is used in the digital domain, while a standard optimization technique is applied in the analog domain to manipulate the values of the load impedance and adjust the source of the antenna impairment which is the mutual coupling (MC) leading to the optimum values. The results are compared with the ideal case, where there is no mutual coupling, from one hand and with the conventional case, where there is mutual coupling. Although the results from the ideal case are much better than the results from the conventional one, the superior results can be achieved using the AD precoder. Index Terms— MIMO, BER, precoding, mutual coupling, analog digital precoding, perfect CSI, optimization. ——————————  —————————— 1 INTRODUCTION IN wireless communication systems, the applications of multiple-input multiple-output (MIMO) have resulted in performance gains over the conventional single-input single- output (SISO) systems. The precoding techniques associated with MIMO can transfer the computational complexity from the user side to the base station is more suitable for such studies and have been extensively studied [1]. For example, the capacity for achieving dirty paper coding (DPC) has been proposed to pre- subtract the interference before transmission [2]. However, due to the DPC’s impractical assumption and high computational complexity, this technique is difficult to implement [2]. On the other hand, suboptimal non-linear technique such as Tomlinson-Harshima precoding (THP) and vector perturbation (VP) have also been proposed [3]- [5]. However, the most common precoding approaches are the linear precoding which has been receiving increasing research attention due to its low computational complexity. Besides, the zero-forcing (ZF) precoding is a simple and effective technique with tolerable sub-optimal performance with a significant computational complexity reduction [6], [7]. C. B. Peel et al. in [8] developed a regularized form of ZF (RZF), by introducing a regularization factor, which improved performance, especially at low signal-to-noise ratios (SNRs). On the other hand, a correlation rotation linear scheme (also known as phase alignment) was proposed in [9], [10], where the interference has been exploited to further benefit the system performance. It was also shown that as the number of antennas increases, the computational complexity and cost for the non-linear approaches could be very high although they can provide rate benefits for the MIMO system [11-13]. Therefore, due to the complexity benefits of linear approaches over non- linear ones, the focus of this work will be on linear precoding schemes. Existing studies on the precoding scheme of MIMO systems usually assume an uncorrelated Rayleigh flat fading channel, which means there is no spatial correlation or mutual coupling impact among antenna elements. However, in practice when the antenna spacing is small, the spatial correlation and mutual coupling effects cannot be neglected [12], [13], [18- 21]. Therefore, many experimental studies have been conducted to investigate the correlation and mutual coupling. The effect of spatial correlation and mutual coupling is examined when a large number of antenna elements are fitted within a fixed physical space [18], [19]. Spatial correlation can be interpreted as a correlation between the received average signal gain and the spatial direction of the signal. Many experimental studies have been conducted on the spatial correlation effect [20], [22], [23]. The impact of the spatial correlation on the system performance of MIMO has also been investigated earlier [24-28]. The designs of the robust precoding scheme in spatially correlated channel were also studied [29-31]. The effect of mutual coupling induced by two real-world antennas has been analyzed. Matrix IJSER