To perform a standards and patent search, I checked numerous websites including www.nssn.org (ANSI’s search engine), www.patents.ibm.com (IBM’s patent search engine), www.uspto.org (US Patent Office), and www.itu.int (Internation Telecommunications Union). Below is a table of the keywords used at each search engine, the number of results, and a selection of links that correspond to the most closely related patents and standards:
 

Web Site	Keywords	Search Results	Document numbers
Standards
www.nssn.org/search.html	Modem	36 results, 5 used	T/CD 01-02E T/CD 01-03E T/CD 01-04E T/CD 01-05E T/CD 01-06E
	Quadrature modulation, QAM	none
www.itu.int	V.22, V.29, V.32	5 results	Rec. V.22, Rec. V.22bis, Rec. V.29, Rec. V.32, Rec. V.32bis
Patents
www.uspto.org	Quadrature AND modulation	98 results	5,838,727; 5,838,268; 5,838,208; 5,825,828; 5,781,076; 5,729,575;

References

Couch, Leon W. (1997). Digital and Analog Communication Systems. Upper Saddle River, NJ: Prentice-Hall, Inc.

Ukunev, Yuri. (1997). Phase and Phase-Difference Modulation in Digital Communications. Norwoord, Massachusetts: Artech House, Inc.