Telecommunications – Transmitter – Measuring – testing – or monitoring of transmitter
Reexamination Certificate
1998-11-04
2001-07-10
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter
Measuring, testing, or monitoring of transmitter
C455S102000, C455S103000, C455S114100, C455S012100, C330S12400D, C330S154000, C330S149000
Reexamination Certificate
active
06259899
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a system and method for determining the dispersion of intermodulation power.
BACKGROUND ART
Intermodulation is the production of spurious signals having frequencies corresponding to the sum and difference frequencies of the fundamentals and their harmonics produced by a nonlinear element, such as a diode or amplifier. For applications including, but not limited to, multi-user cellular satellite communications systems, the nonlinearity is typically created by High Power Amplifiers (HPAs) driven into saturation. However, these amplifiers operate more efficiently when they are driven near saturation. Thus, the system designer strives to optimize the performance of the HPAs by selecting an optimal operating point for all the HPAs based on the particular application.
For practical systems, the intermodulation (IM) power generated by the HPAs is dominated by third-order effects having sum and difference terms which are difficult to filter and therefore lead to nonlinear distortion. This distortion diverts a portion of the input power to various harmonics. As such, it is desirable to minimize the effect of IM power on the received signals. The signal to IM power ratio may be used to measure this effect. This ratio may be improved by either increasing the signal power or decreasing the IM power. However, an increase in signal power may actually result in more significant IM distortion depending upon the operating point of the corresponding amplifier(s). Reduction of IM power for a particular signal or beam may be accomplished by spatial dispersion of the IM power across multiple beams or cells. As such, it is useful to quantify the spatial dispersion of IM power which is the improvement in signal to IM power ratio from the HPAs on a satellite to the received signals on the ground.
Standard simulation techniques quantify IM power by generating time domain samples of the input waveform to each HPA, then using an HPA transfer characteristic to compute time domain samples of the output waveform from the HPA. This widely accepted simulation technique has a running time proportional to the product of the total number of time samples considered and the total number of HPAs in the simulation. For multi-user cellular systems, this technique requires long running times because the number of HPAs in such a system is typically large, and because such multi-user signals are typically wide-band, which requires a large number of samples in the simulation. For practical systems with several hundred users and over 100 HPAs, the standard simulation technique is prohibitively slow and thus not feasible.
SUMMARY OF THE INVENTION
Thus, it is an object of the present invention to provide a system and method for computing the dispersion of intermodulation (IM) power in a system that can be mathematically modeled as a matrix multiplication followed by an instantaneous nonlinearity and then followed by another matrix multiplication.
Another object of the present invention is to provide a system and method for analyzing intermodulation dispersion which is computationally efficient.
A further object of the present invention is to provide a system and method for computing dispersion of intermodulation power for analyzing a cellular system consisting of a large number of individual users, a large number of HPAs, and a complex multi-beam antenna.
A still further object of the present invention is to provide a system and method for quantifying the spatial dispersion of intermodulation power.
Another object of the present invention is to provide a system and method for evaluating intermodulation dispersion in selecting an optimal operating point for the HPAs in a satellite communication system.
In carrying out the above objects, and other objects, features, and advantages of the present invention, a system and method for analyzing dispersion of intermodulation power include performing a frequency domain analysis which tracks the frequency and amplitude for each user signal, considers only third-order intermodulation effects, and computes an improvement in signal to IM ratio between the HPAs and received signals without computing a signal to IM ratio at either the HPAs (source) or the received signal location (destination). In one embodiment of the present invention, the system and method include determining an input signal representation, determining signal power transmitted in each beam at each frequency and, determining signal power for each source of nonlinearity at each frequency. The system and method also include determining the signal power in each beam and each frequency, determining a representation for nonlinearities produced by each source of nonlinearity at a first frequency, calculating the far-field contribution at the first frequency due to call all sources of nonlinearity, and repeating the calculations for each frequency.
The present invention provides a number of advantages over prior art approaches. By using a frequency domain analysis and considering only third-order effects, the present invention has a computational efficiency far greater than standard simulation techniques. This approach does not require generating time domain samples of any signals in the system. As such, the present invention does not require specification of the operating point of the HPAs. Rather, the present invention computes the improvement in signal to IM ratio between the HPAs on the satellite and received signals on the ground without actually computing the signal to IM ratio at either location.
The present invention is computationally efficient enough to treat cases of a cellular system consisting of a large number of individual users, a large number of HPAs, and a complex multi-beam antenna. Having quantified the IM dispersion, the satellite system designer can use this information to select an optimal operating point for the HPAs in the system by evaluating performance for various configurations. This provides the opportunity for significant improvements of the efficiency and cost of the system.
The above objects and other objects, features, and advantages of the present invention will be readily appreciated by one of ordinary skill in the art from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.
REFERENCES:
patent: 4907004 (1990-03-01), Zacharatos et al.
patent: 5638024 (1997-06-01), Dent et al.
patent: 5966048 (1999-10-01), Thompson
patent: 5969572 (1999-10-01), Jeong et al.
patent: 6006111 (1999-12-01), Rowland
patent: 6061568 (2000-05-01), Dent
Berman Arnold L.
Mandell Michael I.
Gantt Alan T.
Gudmestad T.
Hughes Electrical Corporation
Hunter Daniel
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