# On the Spectral Efficiency for Distributed Massive MIMO Systems
## 1 Introduction  
The prognosis for average user data consumption in 2026 is expected to be 35 GB/month compared with approximately 9 GB/month currently [1, 2], which implies a 25% compound annual growth rate. In 2026, the most consumed user applications will continue to be based on streaming video. It is anticipated that these applications will require up to 27 GB/month per user, compared with 6 GB/month today. Applications contributing to this dramatic increase in 2026 include: (i) 1080 p full HD (1920 \(\times\) 1080); (ii) full HD virtual reality (VR); and (iii) 4K UHD (3840 \(\times\) 2160). Beyond 2026, more demanding video streaming applications are expected to take user traffic to new levels, such as: (i) 8K UHD (7680 \(\times\) 4320); and (ii) volumetric data streaming [3]. Consequently, this prediction shows an increase of approximately 4 times in 5 years until 2026. If we linearly extrapolated this result to 2030, we obtain an increase factor of between 20 and 30 times the current traffic of the users. It is expected that the above usage cases and any future cases will require the same type of 5G key performance indicators (KPIs) including: (i) new target values (e.g., higher data rates, lower latency, higher reliability...); and (ii) new cut-off hybrids across the three types of user services for 5G, enhanced mobile broadband (eMBB), ultra-reliable low-latency communications (uRLLCs) and massive machine-type communications (mMTCs) [4]. Technological challenges and goals for the short (SEVO), medium (MEVO) and long (LEVO)-term evolution of 5G are shown in Figure 1.  
Within this framework and perspective, technological advances are mandatory to fulfill the user requirements. Thus, multiple-input multiple-output (MIMO) is fundamental for the development of mobile networks. In particular, massive MIMO technology expects to increase the transmission speed on the network by using multiple antennas at the transmitter (TX) and receiver (RX), and high frequencies, known as millimeter waves (mmWaves) [5]. However, the propagation channel for mmWaves is particularly exposed to attenuation and fading issues. Therefore, it will be necessary to connect the user equipment (UE) to several base stations (BSs) simultaneously. Systems with multiple BSs may be known as distributed MIMO (D-MIMO) systems or coordinated multipoint (CoMP) systems. CoMP term usually refers to systems where a suppression is sought through channel awareness being distributed by the backhaul network [6]. These systems will allow the multi-connectivity between BS and UE, which will require a deep study on the designing criteria on the network in order to provide an optimal network operation. Massive MIMO and coordinated interference schemes have been proposed as solutions to mitigate high interference and thus increase spectral efficiency [7]. However, the main issue of this technique is the increased system overload in the fronthaul [8] and backhaul [9] networks. A solution for cooperation in transmissions is to find a balance for the number of coordinated cells in order to decrease the overheads [10; 11]. These coordinated transmissions are typically based on three techniques: coordinated beamforming, coordinated transmissions and coordinated scheduling [12; 13; 14; 15]. Moreover, new strategies are emerging to improve the performance of the previous techniques, such as those based on games or deep learning [16; 17; 18]. Previous work by [PERSON] and [PERSON] [6] demonstrate that the full knowledge of the propagation channel provides an increase in the data rate for the CoMP systems. However, it also proves that both the sharing of full CSI and BSs coordination imply a drastic increase on the backhaul overhead, as stated previously in [9]. For that reason, the interference mitigation may not be worthy in practical cases. Nevertheless, it is preferable to use multi-BS systems to increase the data rate of the network, since they avoid overheads. For this, the joint spectral efficiency of the distributed MIMO systems is sought.