Lecture 9. Back to Basics, Forward to 6G: Reinventing OFDM for a Unified Air Interface

As the wireless industry transitions from 5G-Advanced toward the 6G horizon, the search for the “perfect” waveform has come full circle. Despite years of exploration into alternative candidates, Orthogonal Frequency Division Multiplexing (OFDM) and its DFT-spread variant (DFT-s-OFDM) remain the most resilient and scalable foundations for future systems. In this sense, 6G is not abandoning the past—but returning to it with a deeper understanding of its limitations. However, the 6G air interface cannot be a mere incremental evolution. It must resolve fundamental tensions between power efficiency, spectral containment, hardware constraints, multi-connectivity, and extreme flexibility. The fragmented notion of waveform flexibility introduced in 5G has fallen short; 6G instead calls for a unified and elastic waveform architecture capable of seamlessly operating across terrestrial and non-terrestrial networks, while adapting to vastly different device capabilities and service requirements. In particular, the integration of terrestrial networks (TN) and non-terrestrial networks (NTN) introduces fundamentally different channel dynamics—ranging from severe Doppler shifts and long propagation delays to heterogeneous link budgets—which directly challenge conventional waveform assumptions and demand a more robust, timing- and frequency-resilient OFDM framework. This keynote presents a multi-dimensional rethinking of OFDM and DFT-s-OFDM within a unified 6G framework, where peak-to-average power ratio (PAPR) and out-of-band (OOB) emissions are no longer treated as secondary impairments, but as central design dimensions. It further explores how next-generation OFDM can natively support integrated sensing and communication, linking waveform design with ambiguity function characteristics for high-resolution environmental awareness. Beyond classical perspectives, the talk highlights emerging approaches in channel representation, estimation, and prediction, enabling robust operation under extreme mobility and dense multi-connectivity. It also introduces the concept of elastic waveform generation, where a single OFDM-based framework can scale from high-throughput broadband links to ultra-low-power signals for wake-up radios and ambient IoT. The keynote ultimately argues that OFDM is not obsolete—but unfinished. Its evolution into a flexible, sensing-aware, and energy-conscious waveform architecture will form the backbone of 6G, enabling a unified, hardware-aware, and intelligent air interface for the next generation of wireless systems.