Research On Green Energy Efficiency Of Massive MIMO

Energy efficiency will be more important in the future mobile communication technology.Green communication becomes the direction in the development of the next generation of mobile communication technology.The massive MIMO technology is supposed to be one of the most promising technologies in the future 5G systems from the perspective of improving both the spectrum and energy efficiency of the system.The spectrum efficiency of massive MIMO systems will be increasing proportionally to logarithmic growth of the number of antennas with the increase of the space freedom of the wireless channel.Under the condition of limited power,Optimum rate can be obtained with equal power of each antenna in the limit of total power.Although the multi antenna array can make the influence of the small scale fading and noise tend to vanish,the pilot contamination problem caused by the reuse of the pilot in the adjacent cell becomes one of the most serious problems in massive MIMO systems.Energy efficiency is one of the indicators to measure the ‘green' of the massive MIMO technology,which is a measure of the ability to transmit the information bits per joule energy.But researchers have only focused on the impact of the transmission power for a long time.Three issues as follows become more and more important.(1)energy consumption of the circuit in the link;(2)in the downlink,interference of the hardware impairment to the performance of the system such as the phase noise due to the non-ideal RF module and the phase quadrature;(3)in the uplink,the influence of quantization precision of the receiver A/D converter to the loss of information and energy consumption.All the above factors have an effect on the energy efficiency of the system.In this paper the influence of the three factors is analyzed in the massive MIMO system.The main research contents are as follows:1)In the downlink,a large number of radio frequency modules is used to send signals,which is interfered by the hardware impairment.In this paper,a large scale MIMO system model and circuit trains model are established considering the circuit energy consumption.The relationship between the hardware impairment and the system spectrum and energy efficiency is deduced,theoretically,and the influence of the hardware impairment and the pilot pollution is compared.Simulation results show that the high spectrum efficiency of the large scale MIMO technology is at the expense of energy consumption,which is also reduced by hardware impairment.However,in the multi-cell scenario,pilot pollution plays an important role in the system performance degradation,thus pilot design is the key to improve the energy efficiency of massive MIMO system.2)In the uplink,a large number of A/D converters are used for signal quantization,the precision of A/D converter which is directly related to the energy consumption of the system and the loss of information bits.In this paper,the model of quantized MIMO system is established,and the relationship between the number of quantization bits and the spectrum efficiency and energy efficiency is derived.To improve the energy efficiency,the particle swarm optimization algorithm is used to optimize the number of quantization bits and the number of antennas,and a new schedule for the selection of antenna is proposed.Simulation results show that the improved quantization precision cannot always improve the spectrum efficiency,and there exists an optimal quantization bit number in low accuracy(maybe1~6 bits),which can make the system energy efficiency maximum.A/D converter with low energy consumption,high precision and multi antenna selected,otherwise,low precision and a small amount of antenna selected,can effectively improve the system energy efficiency.