Research On Threshold Selection Algorithm For Adaptive Modulation Of Air-Ground Broadband Communication

The air-ground broadband communication channel is characterized by fast time-varying,which causes the system’s quality of service(QoS)to be unguaranteed,and the spectral efficiency of air-ground communication cannot be improved.The adaptive modulation technology can adjust the modulation mode to adapt to the channel quality according to the channel state information to ensure system QoS and improve spectral efficiency,which is an effective method to solve this problem.The key to realize the air-ground broadband adaptive modulation is to determine the switching threshold interval between different modulation modes.However,due to the variety of air-ground broadband transmission services,the transmission characteristics and channel characteristics of different transmission services are different,which poses a huge challenge to the threshold determination of air-ground broadband adaptive modulation.Therefore,the adaptive modulation threshold determination under different transmission service characteristics becomes a key issue for air-ground broadband adaptive modulation.The paper combines the project “Development and Testing of Air-ground Broadband Data Link Communication System Prototype” to divide the transmission links of transmitting air-ground broadband ATS service,AOC service,AAC service and APC service into ATS/AOC links with high QoS requirements and AAC/APC link with high throughput requirements.The corresponding adaptive modulation threshold selection algorithm is designed for ATS/AOC link and AAC/APC link respectively,which can guarantee QoS and effectively improve the spectral efficiency.The main work and results are as follows:(1)ATS/AOC transmission link is characterized by fast time-varying,the traditional land-based communication adaptive modulation threshold selection algorithm cannot accurately determine the threshold,guarantee system QoS,and improve the system spectral efficiency.To solve this issue,nondata-aided error vector magnitude(NDA-EVM)was considered as the metric of channel quality estimation of air-ground links,an instantaneous SER threshold selection algorithm based on NDA-EVM was proposed.According to the error mechanism of MQAM symbols,the relationship between NDA-EVM and SER over fading channels was derived,and the adaptive modulation threshold determination and update mechanism of ATS/AOC link was designed accordingly.Based on the one-to-one correspondence between modulation mode and channel state,the Markov channel model was established and the packet loss performance of the threshold selection algorithm was analyzed.The simulation results show that the proposed algorithm has lower algorithm complexity.Compared with the traditional SNR-based adaptive modulation threshold selection algorithm,the proposed algorithm has lower packet loss rate and higher spectral efficiency.When the packet arrival rate is 2packets/ms,the spectral efficiency of this algorithm is increased by 0.59bit/s/Hz compared with the traditional algorithm;when the arrival rate is 4 packets/ms,it is 0.78bit/s/Hz higher than the traditional algorithm.(2)AAC/APC transmission link is characterized by large amount of transmitted data.The instantaneous SER-constrained adaptive modulation threshold selection algorithm strictly limits the system QoS,which limits the throughput improvement and cannot support the large-capacity transmission of AAC/APC services.To solve the issue,average SER adaptive modulation threshold selection algorithm based on NDA-EVM was proposed.Based on the average SER performance and throughput performance of MQAM adaptive modulation system,the threshold optimization model which maximizes system throughput was established.The threshold selection problem was transformed into threshold optimization problem,and the adaptive modulation threshold selection algorithm based on average SER constraint was designed.The simulation results and analysis show that the average SER constraint algorithm based on NDA-EVM has higher spectral efficiency under the premise of guaranteeing QoS.Compared with the instantaneous SER threshold selection algorithm,the proposed algorithm improves the spectral efficiency by 0.64bit/s/Hz in LOS scenario,while 0.72bit/s/Hz in NLOS scenario.