# A Survey on Massive MIMO Systems in Presence of Channel and Hardware Impairments
## 1 Introduction  
Increasing number of users and strong demand for high data rate have forced researchers to increase the capacity and spectral efficiency of next generation wireless communications. The required data rate for 5 th generation (5G) cellular communication is predicted to be 10 Gbps [1]. Different technologies for increasing the capacity of 5G, such as small cell communications, millimeter wave communications, and massive multiple input multiple output (MIMO) technology, have been suggested [2]. Small cell communication increases spectral efficieny by increasing the ability of frequency reuse, since it utilizes small cells such as femto cells and pico cells [3]. Millimeter wave communication, on the other hand, provides a large frequency band from 30 GHz to 300 GHz to increase the capacity of 5G [4]. In massive MIMO technology, a large number of antennas is used at the BS which increases the spatial resolution and consequently increases the spectral and power efficiency of the communications systems. Each of these technologies can separately increase the capacity of wireless communication systems considerably, and using them together can increase the capacity of the system even more [2]. However, these technologies face some challenges which have to be solved to reach the efficient implementation stage. In this paper our focus is on massive MIMO systems. MIMO systems have attained considerable attention in the past two decades. Since they increase the capacity and reliability of the communication systems, they have been employed in many wireless standards [5]. Early studies on MIMO systems were on point-to-point MIMO where two systems with multiple antennas communicate with eachother [6]. Later, attention was drawn to multi-user MIMO (MU-MIMO), where a BS with multiple antennas communicates with several single-or multiple-antenna mobile stations (MS). MU-MIMO has some advantages when compared with point-to-point MIMO. For example, expensive digital processors and power amplifiers are at the BS side and the MSs can have single antennas and be inexpensive [5; 7]. However, MU-MIMO with nearly the same number of BS antennas and number of MS and frequency division duplex (FDD) mode transmission is not a scalable technology [7]. In other words, this technology is not responsive to the increasing number of users and the high data rate demand. Therefore, massive MIMO systems have been suggested. Massive MIMO was first suggested in 2010 in [8]. These systems are like MU-MIMO except that there is large number of antennas at the BS (100 antennas or more). In fact, in massive MIMO systems the number of BS antennas is much larger than the number of users. It is shown that in massive MIMO systems, as the number of BS antennas tends to infinity, the effect of small scale fading vanishes, the number of users in a cell becomes independent of the cell size, and the spectral efficiency will be independent of the bandwidth [5]. In fact massive MIMO relies on the law of large numbers to average out noise and small scale fading [7]. Regarding these advantages, massive MIMO has also been applied to wireless sensor networks (WSN), where the decision fusion center is equipped with large number of antennas [9; 10; 11; 12; 13]. The results show that massive MIMO can improve the performance of WSNs, and the power saving in these networks replicates the power savings obtained in massive MIMO mobile communications. References [14; 15] provide an overview on 5G technologies and signal processing techniques, while [16] gives a tutorial on modulation formats of 5G. There are also some tutorial papers specifically on massive MIMO systems. Reference [17] gives survey on channel measurement and modeling in massive MIMO systems. It also discusses some typical application scenarios along with key techniques in the physical and network layers. In [18; 19], surveys on massive MIMO challenges from a detection and linear precoding perspective have been provided, respectively.