Optimization for FM-Signal Modulation Index, Providing the DISS Information- Energy Efficiency

To provide the specified altitude for the DISS operation, the certain power budget of the radio system (signal power margin) is required, which can be "spent" for losses due to the signal propagation and reflection from the irradiated surface. At that the signal/noise (S/N) ratio at the receiver output shall not be less than the minimum acceptable S/N ratio (S/N ratio enough for the speed measurement with specified accuracy).

In broad comprehension, power budget of the radio system is determined not only by the radiated FM-signal power, the receiver sensitivity and the antenna pattern gain, but also by energy efficiency of a radiated signal. The energy efficiency criterion is the maximization s/n ratio at the receiver output. When using in the DISS as information, first (or third) signal harmonics is required, that this harmonic had maximum peak value. This is solved by selecting the optimal value of the FM-signal modulation index. This specific task is discussed in the article.

The optimum modulation index of FM-signal is determined and analysis of energy losses for the signal conversion in the receiver is performed. It is found that the optimal modulation index m = 1.84 leads to additional conversion losses up to -20 dB at certain ranges Dn= D1 (1+ 2n), where D1 = 234.4m at FM = 320 kHz. However, the usage of quasioptimal modulation index m = 1.5 gives the sufficient reduction in these losses, and there are virtually no signal conversion losses at altitude higher than 1500m.

Use of the meander signal as the modulating signal is analyzed that is applied in the DISS. It is found that the quasioptimal modulation index m = 1.5 for the sine modulation law is energy-effective (quasioptimal) also for the meander modulation law. Conducted experimental tests confirmed the basic results of theoretical studies. Based on obtained results the conclusions are made, the main one is as follows: to increased the information-energy efficiency of the FM-signal power in the DISS, it is reasonable to use the modulation index m = 1.5.