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In this letter, we construct new encoder-independent upper bounds on the free distance for the rate (n-1)/n convolutional encoders that are formed by using rate 1/2 encoders and combined with 2/sup n/-level constant-envelope full-response CPFSK signals of single-h, by generalizing the bounding technique employed for the existing bounds. The new bounds coincide with the existing bounds for h/spl les/0.25, but are generally better for h>0.25. We also present actual encoder combinations achieving the constructed new bounds with short constraint lengths.
Date of Publication: Dec. 2001