Cell-Free Massive MIMO Based Communication System For Green Internet Of Things

With the coming of 5G,wireless communication has immersed into all fields of society,greatly changing the way of information interaction in human society.Among them,the Internet of Things(IoT)is one of the important driving forces for its development in the future.For future smart city,industrial IoT and other application scenarios,devices are widely distributed in small and medium-sized areas,with high requirements for bandwidth and transmission rate.Meanwhile,it is difficult to replace the power supply,and the cost of backhaul links is large.How to deploy a green wireless communication system with high coverage,low cost and guaranteed service quality is an urgent problem in such scenarios.On the one hand,the mature low power wide area technology in the current market provides high coverage,low energy consumption and low-cost solutions.However,due to its limited bandwidth,it is only suitable for low-speed large-scale IoT scenarios.On the other hand,the mainstream 5G enabled cellular IoT,can meet the requirements of device speed and service quality,but due to the inherited cellular network architecture,the cells densify and covering area continues to shrink,making inter-cell interference and frequent handovers more serious,which results in system expense,making it unfit for the above application scenarios.Hence,this paper studies the green IoT communication system performance based on cell-free massive MIMO(Multiple-Input Multiple-Output).Aiming at the problems of uneven distribution of devices and difficult power supply,a cell-free downlink transmission scheme based on SWIPT is studied.Aiming at the problem of poor load balancing and high backhaul link cost,an uplink transmission scheme based on access point selection is investigated.The specific research contents and innovations of this paper are summarized as follows:1)Two transmission schemes,time switching and power splitting,are studied in the downlink cell-free transmission system,which maximizes the minimum achievable rate of devices and ensures the harvested energy.Considering the access points with multiple antennas,the devices first send the uplink pilots,the access points complete the channel estimation locally,and sends the RF signal to devices through conjugate beamforming,and the devices harvest the energy and decode the information through the receivers.To reduce the near-far effect,the interference from other devices,while ensures the system coverage,and the energy collection,a joint optimization scheme of energy division and power control is proposed and solved by an iterative method based on dichotomy.Finally,simulations verify that the proposed scheme can ensure the system coverage,improve the achievable rate of devices,and meet the energy supply requirements.Through the simulation analysis,the spectrum efficiency and energy performance of the two working modes are compared as well.2)The uplink cell-free transmission scheme with access point selection mechanism is studied.Considering the presence of pilot pollution,the uplink transmission power of devices is small,and the effect of power control is limited compared with that in downlink,resulting in unbalanced load of the access point and poor system performance.Therefore,the access point selection mechanism with load balancing function is added in the signal detection.First,the paper deduces the closed form expression of the achievable rate of devices in uplink transmission.Aiming at maximizing the minimum achievable rate of devices,a greedy algorithm is used for pilot allocation.A joint optimization scheme of access point selection vector design and power control is proposed,and the optimal solution is obtained by solving the generalized eigenvector problem and geometric programming problem.Then,it is noted that all access points serving all devices increases unnecessary backhaul link expense.Hence,an access point selection scheme is proposed.Finally,through simulations,the necessity of the access point selection scheme is confirmed in terms of backhaul utilities.Meanwhile,the optimal transmission scheme can effectively improve the system performance compared with other schemes.The simulation results also show that the proposed algorithm can quickly converge.