We propose and experimentally investigate a new technique, based on optical heterodyne generation, which converts two optical binary amplitude-shift keyed (2-ASK) into a single microwave quaternary ASK (4-ASK) one. Results show that 4-ASK signals are properly generated even when the 2-ASK signals are propagated through 40 km optical links from a geographically distributed optical network under real conditions of temperature, pressure, humidity, and wind. © 2012 Wiley Periodicals, Inc.