International Telemetering Conference Proceedings / October 23-26, 2000 / Town & Country Hotel and Conference Center, San Diego, California / The interoperability of Feher-patented Quadrature Phase Shift Keying (FQPSK) systems with future and legacy data links is discussed in this paper. In particular, the benefits of “forward” interoperability with 3G (3rd Generation) wireless systems such as WCDMA (Wideband Code Division Multiple Access) up to 40 Mchips/sec are highlighted. Other proposals of forward interoperability with future data links include an enhanced, ultra-bandwidth efficient FQPSK and 16-state FQAM (Feher’s Quadrature Amplitude Modulation) architectures. In addition, since FQPSK based systems have been proven in “dual use” systems and extensively tested and evaluated by the Department of Defense (DoD) and NASA, the analysis of “backward” interoperability with legacy data links such as GSM is also included in this paper.