Research On The Wireless Transmission Scheme Based On The Forwarding Enhancement

The rapid development of the information and communication technologies has changed human life and driven social progress.The vigorous development of mobile Internet and Internet of things provides high-speed and convenient user experience,which lead to a higher demands for wireless network capacity,reliability,and coverage.In order to achieve continuous wide-area coverage and meet the high-capacity communication requirements of hotspots,huge number of base station facilities and maintenance expenses are required in the traditional cellular network,which are contrary to the low-carbon and environmental protection in the future.As a technique to effectively enhance the system capacity and expand the coverage of the cellular,wireless forwarding can provide strong guarantees and supplements to the current cellular network.Especially when the cellular network cannot work normally in the natural disasters or emergencies,wireless forwarding equipments can be quickly established to recover the communications.Therefore,it is necessary to study the wireless forwarding technique.Considering the design of transmission scheme in wireless forwarding enhancement system,this dissertation studies the precoding,transmit diversity,equalization,and other technologies by using model establishment,algorithm design and performance evaluation and verification.The wireless forwarding enhanced transmission schemes are proposed for static,dynamic,different channel conditions and different forwarding equipments.Firstly,consider the imperfect channel information in the wireless forwarding system,the imperfect channels in the frequency division duplex and time division duplex systems are modeled respectively,and two robust transmission schemes are proposed.Then,the wireless forwarding technique without channel estimation is studied,and a low-cost transmission scheme is designed in the reconfigurable-intelligent-surface-assisted system.Since the first two researches assume the forwarding equipments are fixed,the transmission schemes based on orthogonal time frequency space modulation with the high-speed mobile forwarding equipments are studied to improve the reliability.The main contribution of this dissertation includes the following three aspects;1)For the problem of the non-robustness caused by imperfect channel state information in the relay system,two-way and multiway forwarding schemes in frequency division duplex and time division duplex systems are studied.First,the uncertainly channel of the forwarding channel is modeled based on different causes of channel uncertainty.In the frequency division duplex,a minimax problem is formulated considering both the CSI uncertainty and the relay power constraint,i.e.,to minimize the worst-case MSE.However,this minimax problem is non-convex and difficult to solve.Thus,we divide the original problem into two subproblems with respect to uplink and downlink channel uncertainty: the former one is solved by the Lagrangian method and the latter one can be manipulated into a Sylvester equation,which has a closed-form solution.In the time division duplex system,an overall iterative algorithm is proposed to minimize the mean square error.Simulations show that the proposed forwarding schemes with the two duplex systems has good robustness to the imperfect channel state information.2)For the problem of high channel estimation overhead in the reconfigurableintelligent-surface-assisted system,a low-cost transmission scheme is studied.First,an optimization problem without channel estimation is constructed by minimizing the mean square error.The original constrained problem is equivalently transformed into an unconstrained problem by the spherical coordinate representation.Then,the Bayesian optimization theory is adopted and regards the overall transmission process as a black box function.Based on the transmit and receive samples,the base station iteratively updates the precoding and the phase shift at the reconfigurable-intelligent-surface,which effectively reduces the training overhead compared to the channel estimation-based scheme.Since the proposed framework does not require an explicit expression of the objective function,the proposed framework can be flexibly extended to scenarios such as the wireless energy harvesting systems and the user fairness.3)For the problem of low reliability in high-speed mobile wireless forwardingenhancement system,the transmission schemes based on orthogonal time frequency space modulation are studied.First,consider the transceiver design of the orthogonal time frequency space modulation,a space-time-code-based transmit diversity scheme in the rectangular pulse shaping is proposed with orthogonal time frequency space modulation,which precludes the assumption of two consecutive frames remains unchanged in the ideal pulse shaping scheme.In order to reduce the overhead of the proposed transmit diversity scheme,an information mapping method in the delay-Doppler domain is proposed.Based on the cyclic shift technique,a transmit diversity scheme in the delay-Doppler domain is proposed,and its equivalence with the cyclic delay diversity in the time domain is proved.In view of the problem of low transmission spectral efficiency in high-speed mobile scenarios,nonlinear precoding schemes in the delayed-Doppler domain is proposed to achieve efficient parallel transmission of multiple data streams.Moreover,two linear receiver equalization schemes are proposed to realize low-complexity equalization.Finally,the proposed transmit diversity and precoding scheme based on orthogonal time-frequency space modulation is applied to the high-speed mobile wireless forwarding scenarios,which effectively improves the system reliability and spectrum efficiency.