Research On Multi-User Space-Time Modulation Schemes For Ultra-Reliable Low-Latency Communications

Ultra-Reliable and Low-Latency Communication(URLLC)is one of the fifth-generation mobile communication(5G)application scenarios,the physical layer design of which needs to be rethought to satisfy the stringent requirements of low-latency and high-reliability of the wireless communication link.Unfortunately,it is challenging for typical communication mechanisms to satisfy the two seemingly contradictory requirements,i.e.,high-reliability and low-latency,at the same time.Due to the considerable spatial freedom,significant antenna array gain,asymptotic orthogonal channel property,and high spatial diversity,massive multiple-input multiple-output(MIMO)techniques can maintain high reliability of communications under stringent low latency requirements.Combined with space-time coding schemes,the system's spatial diversity and spatial multiplexing can be further enhanced.Thus,massive MIMO techniques can improve the reliability of the system by compensating for the limited time diversity gain of short packet transmission,and are enabling techniques for URLLC.In this dissertation,we consider a multi-user massive MIMO enabling URLLC uplink transmission system.In such a system,a phase shift keying(PSK)modulated multi-user space-time modulation scheme is designed for short data packet transmission.With such a scheme,the pilot overhead of the considered system reduces,and the transmission efficiency also improves.Moreover,the reliability of the system is also improved by further exploiting collaboration gains between users.In detail,the main contributions can be summarized as follows:1.Given the short data packet transmission,the number of time slots is limited,which will result in a limited number of orthogonal pilot sequences for multiple users.As a result,the error performance of the uplink system will degrade.Against this background,a joint design framework for constellation design and user pairing with the unique factorable theory is proposed,which can reduce the orthogonal pilot overhead.In detail,in the same cell,each pair of users utilize the same pilot sequence,while the pilot sequences between pairs are mutually orthogonal.Besides,users in thesame pair exploit AAUDCP with phase-shift keying(PSK)constellations for space-time modulation.This scheme can reduce the pilot overhead while ensuring the unique identification of transmitted symbols.Second,to further improve the reliability of the system,the effect of the channel fading coefficient on the signal to interference plus noise ratio of the received signal is studied.Based on this,a low-complexity intelligent user pairing algorithm is proposed.Finally,the simulation results show that the proposed scheme can effectively improve the error performance of the system.2.To mitigate the interference between users,a power allocation scheme for the smart user pairing scheme is proposed with an emphasis on improving the system's energy efficiency and error performance.In detail,a space-time modulation constellation is firstly designed under the constraint of total power,and a detection scheme based on Euclidean distance is built.Then,the power optimization problem is established by maximizing the minimum Euclidean distance of the transmitted constellation points.By solving the optimization problem,the optimal parameters of power allocation are obtained.Simulation results verify that the proposed power distribution scheme can significantly improve the reliability of the system.