Raised-cosine filtering for modems
AssigneeHycom Incorporated
InventorsDavid M. Motley, John F. Stockman
Life of this patent
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Abstract
Filter quality is substantially improved (or complexity substantially reduced) in a digital PSK modem requiring an overall filter characteristic of the percent raised cosine type by using, in both the transmitter and the receiver, a filter having the same percent raised cosine response but at a higher cutoff frequency which satisfies the formula ##EQU1## where f.sub.f is the cutoff frequency (6 db) of the new filter, f.sub.o is one-half the baud rate of the modem, and .alpha. is the percent raised cosine expressed in decimal form.
Claims1
A digital modem including low-pass intersymbol interference suppression filter means required to have an overall response characteristic of the form
- H(f)=.tau.
- for 0.ltoreq.f.ltoreq.1/2.tau.(1-.alpha.), and ##EQU7## where H(f)=filter response as a function of frequency ##EQU8## and .alpha.=percentage of RC filter characteristic expressed in decimals
- the improvement comprising using, in both the transmitter and the receiver of said modem, separate low-pass filter means having a response characteristic of the form
- H(f)=.tau.
- for 0.ltoreq.f.ltoreq.1/2.tau.(1-.alpha.), and ##EQU9## at a cutoff frequency which is related to the cutoff frequency of said overall response characteristic by substantially the formula ##EQU10## where f.sub.f =6 db cutoff frequency of said separate low-pass filter means, and
- fo=6 db cutoff frequency of said overall response characteristic.
Dependencies are inferred from the claim text.