多径干扰对再入遥测系统性能的影响分析

Analysis the effect of multipath interference on the performance of the reentry telemetry system

多径干扰在频域表现为在PCM/FM的频谱中产生凹陷。我们通过用一直射路径和一延时小于一个比特间隔的镜面反射路径来模拟再入遥测多径信道,并结合一延时解调器,得到了系统的性能方程式,它是多径信道参数r、θ和τ的函数。然后就有和无预检波滤波器分别分析了多径干扰对系统性能的影响。通过分析得出,系统的比特错误概率都强烈依赖于凹口的位置,而在无预检波滤波器时,系统的平均比特错误概率与凹口的宽度(时延τ)无关,主要由凹口的深度(反射系数r)确定,此时由于多径干扰引起系统性能损失的上界为1/(1-r)2,它高出实际系统性能的损失值不到1dB。在有预检波滤波器的情况下,系统的平均比特错误概率与凹口的宽度、凹口的深度和滤波器的带宽都有关。当τ/Tb=0.15时系统性能最差,而滤波器的带宽B越窄,码间干扰越严重,在τ/Tb=0.15且BTb=1时,得到了系统性能损失的上界为:1/(1-r)2r,它超过系统性能的实际损失同样小于1dB。

In the frequency domain, the multipath interference manifests itself as a null in the PCM/FM spectrum. Using a line-of-sight path and a specular reflection path whose delay related to the line-of-sight signal is less than a bit time to model the re-entry telemetry multipath channel, and with a delay detector, we get the system＇s performance equation which is the function of the multipath channel parameters r, θ and τ, Then we respectively analyse the effect of multipath interference on the performance of system without or with predetective filter. According to the analysis, we can conclude that both the two cases the bit error probability of the system is strongly dependent on the null location. But the average bit error probability of the system without predetective filter is not dependent on the width of the null （the delay T ） but mainly determined by the depth （reflective coefficient r）. Here, the up-bounded of system performance loss caused by multipath interference is about 1/（1-r）^2, which overstates the actual loss of performance less than ldB. With the predetective filter, the average bit error probability of the system depends all on the null＇s width, null＇s depth and predetective filter＇s band-width. When τ/Tb=0.15, the system＇s performance is the worst, and the smaller the filter＇s band-width B, the more serious the ISI. When τ/Tb=0.15 and BTb=1, we have gotten the up-bounded of system performance loss： 1/（l-r）^2r, which also overstates the actual loss of performance less than 1dB.