Research On Adaptive Transmission Technology In Vehicular Network Environment

The existing V2X system supports the 140 km/h absolute speed and 280 km/h relative speed with 6 GHz carrier frequency.For 5G,200km/h absolute speed with mm-wave band(30GHz~70GHz)carrier frequency will be supported in eV2X system.However,the Doppler effect caused by relative displacement and fast time-varying channel caused by high-speed movement both seriously decrease the transmission capacity of the system.Therefore,under high mobility scenes,it is of great scientific significance to study transmission schemes that can effectively improve the performance of commnication systems.A novel channel quality indicator(CQI)prediction algorithm based on spatial filter is proposed to solve the problem that the predicted CQI does not match the actual channel condition due to the weak autocorrelation of fast time-varying channel in high-speed mobile scenes.Firstly,the adaptive modulation coding system in high-speed mobile scenes is modeled,and the received signal is projected into orthogonal subspaces by a spatial filter at the receiver.Then the Doppler compensation,channel estimation,channel prediction and signal-to-noise ratio(SNR)prediction are conducted in each subspace respectively.And the measured SNR vector is mapped to a scalar quantity through effective SNR mapping(ESM)algorithm,which is named as effective signal-to-noise ratio(ESNR).Finally,the reported CQI value is evaluated by looking up the CQI-ESNR table and reported to the transmit side.Numerical analysis shows that the autocorrelation performance is proportional to the order of spatial filter.Simulation results show that the proposed CQI prediction algorithm can both lower the computational complexity and improve the system throughput effectively.In order to suppress influence of channel fading,spatial diversity technology is widely used in wireless communications,especially spatial diversity and frequency diversity.Space-frequency coding technology can achieve both spatial diversity and frequency diversity by joint coding on transmit antenna and subcarrier,which can significantly improve the performance of OFDM system without increasing transmit power and bandwidth.According to the network deployment of high mobility scene,there are multiple relay nodes in the same logic unit,and these relay nodes are controlled by the same base station.However,in high mobility scene,different relay nodes in the same logic unit are far away from each other,and the time difference of arrival(TDOA)of receive signals will destroy the orthogonality of channel provided by traditional space-frequency coding.In order to solve this problem,a new orthogonal space-frequency coding scheme based on the TDOA is proposed.Firstly,the system is modeled according to the fast time-varying channel in high-speed mobile scenarios,and it is found that the TDOA causes phase rotation of the elements in the frequency domain equivalent channel matrix,which destroys the orthogonality of the channel.Then,according to the position information of the base station and the vehicle,the TDOA of receive signals is obtained.And the elements in the orthogonal space-frequency block coding matrix are pre-rotated by TDOA at the transmitter to pre-compensate the orthogonality of the channel.Performance analysis shows that the signal-to-noise ratio(SNR)of the traditional scheme weakens periodically with the increase of the TDOA,while the new scheme proposed in this paper suppresses the influence of TDOA perfectly.Finally,the simulation results show that the new orthogonal space-frequency coding scheme proposed in this paper is superior to the traditional orthogonal space-frequency coding scheme under any modulation conditions.Orthogonal space-frequency coding can not achieve full transmission rate when the number of transmit antennas is larger than two for the orthogonality of channels,while quasi-orthogonal space-frequency coding can achieve full transmission rate at a slight sacrifice of orthogonality.Based on the existing quasi-orthogonal space-frequency coding schemes,this paper proposes a new scheme to suppress the influence of TDOA.Firstly,the quasi-orthogonal space-frequency coding system with four antennas is modeled.It is found that the elements in the frequency-domain equivalent channel matrix is rotated in different degrees due to the difference of signal arrival time when conventional quasi-orthogonal space-frequency block coding is adopted at the transmitter.According to the estimated time-of-arrival(TOA)of the receive signals,the elements in the quasi-orthogonal space-frequency coding matrix are pre-rotated to varying degrees in our proposed scheme.Performance analysis shows that the diversity gain of the traditional quasi-orthogonal space-frequency coding scheme varies periodically with the increase of TDOA,while the new quasi-orthogonal scheme proposed in this paper can effectively suppress the influence of the signal arrival time difference to obtain the maximum diversity gain.Finally,the simulation results show that the BER performance of the proposed quasi-orthogonal space-frequency coding scheme is much better than that of the traditional scheme on the basis of full rate transmission.