SIGNAL TO NOISE RATIOS FOR MULTIPATH COMBINERS. PART I. SUMMARY OF ANALYSIS.
RAYTHEON CO WALTHAM MASS RESEARCH DIV
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The paper analyzes the performance of an adaptive combiner for a multipath channel for a time-varying path structure. If the multipath structure is unknown but constant in time, the adaptive receiver can always attain the maximum possible processing gain although the gain stability requirement on the feedback loop may be very difficult to meet for very weak signals. If the multipath structure is a random process, the adaptive receiver can only function well if the weakest input signal to noise ratio is much larger than the fractional decorrelation from baud to baud p 1 - k. If this condition is satisfied, nearly all the possible processing gain will be achieved. The adaptive performance of an envelope type combiner is qualitatively the same as for a coherent type combiner, and differs quantitatively only by coefficients of order unity in practical cases. For a constant multipath structure, the loop gain in the reference channel should be as near unity as possible, and certainly 1 minus the loop gain must be much less than the input signal to noise ratio. For a time-varying multipath structure, there is an optimum value of 1 minus the loop gain equal to the geometric mean of the input signal to noise ratio and the baud to baud decorrelation for the coherent combiner. The processing gain will degrade by 3db if 1 minus the loop gain either decreases to the baud to baud decorrelation or increases to the input signal to noise ratio. The envelope combiner depends on the same variables in the same way apart from numerical coefficients of order unity. Author
- Radio Communications