Published in Microwaves & RF
Since the introduction of multi-carrier (MC) orthogonal frequency-division multiplexing (OFDM) in 1966 by Chang, Bell Laboratory, the MC modulation waveform has demonstrated its robust link performance with high modulation bandwidth efficiency in wireless communication applications. They include today’s Wi-Fi and Long-Term Evolution (LTE) technology.
Traditional single-carrier (SC) modulation-based wireless systems, such as quadrature amplitude modulation (QAM), shaped-offset quadrature phase-shift keying (SOQPSK), and the like, have shown a higher transmitter power efficiency when compared to OFDM-based systems while having a comparably good modulation bandwidth efficiency. However, it’s known that the link quality of an SC-based system such as SOQPSK-TG (Telemetry Group) deteriorates noticeably in a wireless channel with multipath, especially in time-varying multipath channels.
Over decades of research efforts, various channel-equalization algorithms for QAM and other SC modulations have been proposed and utilized. For example, frequency-domain adaptive equalization algorithms were proposed and experimented on.
Sparse adaptive-channel equalization algorithms were invented to combat wireless link degradation in terrestrial multipath environments,6 as was sparse equalization of SOQPSK-TG for aeronautical telemetry applications. Those equalization algorithms have mostly been in the experimental stage, not being widely implemented in wireless-communication applications for their limited capacity to deal with rapidly time-varying wireless channels.
For example, it may occur in an aeronautical telemetry communications link when a test article (TA, i.e., an aircraft serving as a test platform) is flying at low altitudes, or during take-off, landing, or taxiing. These typical time-varying multipath cases remain a major challenge for link integrity in aeronautical telemetry wireless communications.