Welcome to Masayoshi Tanaka  Website  

Research activities of wireless & satellite communications in the following laboratories.
-  College of Industrial Technology, Nihon University, 
     Department of Electrical and Electronic
     Engineering.​
-  NTT Electrical Communications Laboratories.
  

We developed a precise analytical model of the multi-carrier system and a simplified method that can estimate the AM-to-AM and AM-to-PM constituents of spurious output separately.

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We are studying experimental and analytical studies of spurious emissions that a multi-carrier transmitter for satellite communications produces in the spurious domain that was newly defined in ITU WRC-03 as shown in Fig. 1.

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The radio frequency spectrum is a common resource and it is necessary to keep it as unpolluted as possible, making the best use of the most modern and cost-effective techniques. Between 1996 and 2003, ITU-R Study Group 1 has developed and adopted a number of new or revised recommendations dealing with unwanted emissions. We have to design or operate the future system in accordance with the recommendations.

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 It is particularly complex to understand the mechanism of spurious emissions for application to multi-channel or multi-carrier transmitters/transponders, where several carriers may be transmitted simultaneously from a final output amplifier. Figure 2 shows the factors generating spurious emissions.

 

In our study, we put special interest on the spurious emissions generated from the output power amplifier (PA) nonlinearity, because PA is a dominant component. The PA nonlinear characteristics determine the overall system linearity. Intermodulation products generated by this nonlinearity cause spurious emissions in the spurious domain.

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Improving the power efficiency of PA is a major concern because power resources are limited for satellite onboard equipment and efficient use of energy is also of great concern for terrestrial transmitters. In general, however, PAs are highly efficient near the nonlinear saturation region. Improving efficiency, therefore, conflicts with suppressing spurious emissions. Thus, it is particularly desirable to find the preferable condition to have a high-power amplifier (HPA) system effective in achieving high efficiency while satisfying the recommended spurious emission level.

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However, it may be difficult to experimentally confirm the condition for all of the probable multi-carrier operational modes as shown in  Fig. 3. To address the issue, we carried out theoretical as well as experimental studies of the characteristics of multi-carrier transmitters and discuss preferable system design methods and evaluation methods that are compatible with the recommendations.

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We performed a lot of measurement for the multi-carrier system composed of up to 256 QPSK modulated or unmodulated carriers for several output back-off conditions of a TWT amplifier. In addition, we developed a precise analytical model of the multi-carrier system, which includes nonlinear characteristics (AM-AM and AM-PM conversion) and multiple squared roll-off filtered QPSK multi-carrier modulation units.

 

The spurious domain emissions are strongly dependent on the nonlinear characteristics greater than the fifth order. We then approximated the characteristics by 9th order polynomials and determined their coefficients by the least mean square method to fit the HPA used for measurements.

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- Simplified spurious level evaluation method (Fig. 5)

A simplified mrthod that can estimate the AM-to-AM and AM-to-PM constituents of spurious output separately was devised. Based on this model, a new algorithm for estimating the spurious emission level by simple multiplications and additions using the known filter responses and one unmodulated CW measurement was proposed.

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The proposed evaluation method was accurate enough to determine whether a system is compatible with ITU recommendations without using simulation software or needing to conduct complex multi-carrier tests.