Research On Key Technologies Of UAV-aided Cooperative Communications

Maintaining a stable signal coverage is a prerequisite for ensuring communication reliability,and an increase in the barrier or transmission distance of an object significantly affects the coverage area of the signal.Cooperative communication is an effective means of alleviating the above problems.However,most traditional cooperative communications based on fixed relays cannot meet the flexibility of future mobile communication scenarios.To address the above problems,in this thesis,we use the unmanned aerial vehicle(UAV)as the carrier of relay communication to enable the flexible application of mobile communication scenarios.At present,the existing UAV-assisted cooperative communication technology has made breakthroughs in channel modeling and resource management,but system capacity,communication interruption and energy consumption are still the technical bottlenecks of UAV-assisted cooperative communication.In order to achieve efficient,stable and low-power UAV-assisted cooperative communication,we will select the trajectory planning,duplex(half-duplex(HD)/full-duplex(FD))mode and multi-hop UAV relay for UAV.The key technologies of the three aspects of the transmission strategy were studied and the following results were achieved.(1)We studied the single UAV relay trajectory planning for unknown receiver locations.Firstly,according to the position of the known receiving end and the road distribution,the kinematics law is simulated to predict the position of the unknown receiving end.Then based on the location prediction,a new UAV relay trajectory planning algorithm is proposed based on the air-ground channel.After that,according to the principle of power optimization,the method of adaptive control of transmission power is further adopted to adapt to the trajectory planning of the UAV relay.Finally,we achieved the highest possible savings of 21.1%in transmit power.(2)The duplex mode selection strategy for single UAV relay system is studied.Due to the existence of Self-Interference(SI),the system capacity of FD mode may be lower than the HD mode under certain conditions.This section studies the duplex mode switching conditions under the two transmission protocols:Amplify-and-Forward(AF)and Decode-and-Forward(DF).First,we compare the system capacity difference between the HD and FD modes of the UAV relay in different spatial locations,and derive the Residual Self-Interference(RSI)threshold for duplex mode switching.Then,the adaptive transmit power of the UAV with the largest system capacity is obtained,followed by giving out the RSI threshold under power adaptive control.The analysis results show that using the proposed RSI threshold under power adaptive control for duplex mode switching selection can increase system capacity and save 13.7%of transmit power.(3)The hybrid duplex mode transmission strategy for multi-hop UAV relay is studied.Firstly,based on the relay transmission of multi-hop duplex UAV,we obtain the multi-hop transmission mode of different UAV relays and different duplex transmission combinations as well as their outage probability.Then,we aim to minimize the probability of outage and adaptively control the transmit power.Finally,we analyze the outage probability and transmission power consumption of the UAV relay position change under different UAV relay numbers and different duplex transmission combinations.The analysis results show that the hybrid duplex mode transmission strategy using power adaptive control can reduce the probability of interruption by nearly 50%.