弹性波透地通信分层大地信道的多径特性

Multipath characteristics of through-the-earth stratified medium channel of elastic wave signal communiaction

通过分析弹性波在平行地层中传播的界面反射及衰减特性,得到了分层均匀介质中弹性波信号传播的多径信道模型,导出了多径延迟与各层的厚度以及层内弹性波纵波的传播速度的关系,以及多径/直达信号比(RMD)的近似算式,同时也探讨了因层间反射导致的多径衰落情况。在神华集团上湾煤矿煤层及围岩钻孔资料基础上建立地层模型并进行了仿真,结果表明:在直达信号正常情况下,多径信号的幅度较直达信号低20 dB以上,多径干扰基本可以忽略,此时即使传输信号带宽大于信道相关带宽Bc,由多径导致的衰落仍然是近似平坦的;如果直达信号被遮蔽(比正常值低20 dB以上),信号呈明显频率选择性衰落,衰落深度约15 dB。仿真结果还说明使用直接序列扩频的透地通信,较高的码速率有利于抑制多径效应。

By analyzing the interface reflecting and power attenuation characteristics of elastic wave during propagating in parallel stratified earth medium,the multipath channel model of longitudinal elastic wave propagating in homogeneous stratified medium was achieved,and the relationship between multipath delay,medium layer thickness and longitudinal wave velocity was derived,as well as multipath/direct signal ratio approximate calculation.Meanwhile,the multipath fading caused by layer reflecting was studied.Based on the drill column data of adjacent rock in Shangwan Coal Mine of Shenhua Group,the channel model was established.Simulation results show：if direct signal is normal,the amplitude of multipath signal is lower than the direct signal＇s of 20 dB,so the interference of multipath signal caused by layer interface reflection can be neglected,and it proved that even the transmission bandwidth of signal is wider than channel correlation bandwidth Bc,the multipath fading is still a approximately flat fading.If the direct signal is shadowed（-20 dB to normal）,the stratified earth media channel is a frequency selective fading channel,and the fading depth is up to about 15 dB.Also,it illuminates that in spread spectrum communication,high chip rate is benefit for multipath interference rejection.