# Analysis and Evaluation of Performance Gains and Tradeoffs for Massive MIMO Systems
## 5 Simulation Results
Energy efficiency degrades due to increase circuit power consumed for each antenna and the impact of non-orthogonal interference from other cells i.e., (interference factor \(\beta\) is changing from 0.01 to 0.11), spectral efficiency degrades, as the degree of interference among cells is high. In spite the effect of circuit power consumed, array gain and spatial multiplexing gain will degrade due to interference of non-coherent channels among cells. Degree of channel estimation will limit array gain loss while interference from non-perpendicular channels of other cells will limit the degree of freedom that is introduced for each user, hence decreasing the spatial multiplexing gain. Massive MIMO strengthens these gains via increasing the number of antennas. Figure 10 explains how increasing the number of antennas will raise the achievable sum rate under perfect and imperfect CSI in massive MIMO multi cell system. As mentioned before growing the number of radio propagation channels will increase the degree of freedom which means growing the number of served terminals via coherence interval, i.e., \"high spatial multiplexing gain\". The gap between the curves is due to MRT precoding, which is limited by interference due to the reuse of non-orthogonal pilot signals in a multi-cell system. Figure 8: Outage probability vs. signal to noise ratio (dB) in multi-cell system with Y\({}_{th}\) = 10 dB, K = 10 and L = 7. In order to compare our results for massive MIMO with conventional MIMO, Figure 11 shows this comparison in terms of channel capacity for different values of signal to noise ratio (SNR). From Figure 11, channel capacity of conventional MIMO system increases linearly as the number of antennas grows up due to the following: (1) Exploiting the spatial multiplexing concept which it is so potent technique for increasing link capacity at high SNR levels; (2) Relying on maximum number of spatial data streams which it is bounded by the minimum number of antennas at the transmitter (base station) or receiver (single user). In the other side, the link capacity of massive MIMO system outgrows the link capacity of conventional MIMO system because of the following: (1) Exploiting Space Division Multiple Access (SDMA) technique that uses spatial multiplexing concept to enable higher data rate; (2) Higher comprehensive multiplexing gain is get according to the minimum number of antennas at users (multi-users are served simultaneously which induces for higher data rate) as mentioned before in Theorem 2. Figure 10: Achievable sum rate (bits/s/Hz) vs. the number of antennas (M) in multi-cell system with L = 7, K = 10, and transmit power = 10 dB. Figure 9: Energy Efficiency vs. Spectral Efficiency under the effect of pilot contamination in multi-cell system with L = 7, K = 10 and transmit power = 10 dB. Figure 12 shows that the massive MIMO technique outperforms conventional MIMO technique by enhancing system performance in term of outage probability through beamforming technique, which it is defined as a downlink multi-antenna technique weights the data before transmission, forming narrow beams and aiming the energy at the target user, then it improves the spatial diversity gain in massive MIMO via increasing the received SINR. As compared to conventional MIMO technique, massive MIMO technique will give the following important features of increasing the number of antennas in mobile communication systems: (1) Hiding the effects of thermal noise, fast fading and intra cell interference among users; (2) Rising the degree of freedom that were provided for serving terminals which means more robustness and link reliability (i.e., increased diversity gain); (3) Reducing uplink and downlink transmit powers through coherent combining and increasing antenna aperture, transmit power of each user terminal can be reduced inversely proportional to the number of antennas at the BS with no reduction in performance. Figure 11: Comparison between Conventional MIMO and Massive MIMO Systems in term of Capacity.