Beyond Legacy OFDM: A Mobility-Adaptive Multi-Gear Framework for 6G

Abstract

While Third Generation Partnership Project (3GPP) has confirmed orthogonal frequency division multiplexing (OFDM) as the baseline waveform for sixth-generation (6G), its performance is severely compromised in the high-mobility scenarios envisioned for 6G. Building upon the GEARBOX-PHY vision, we present gear-switching OFDM (GS-OFDM): a unified framework in which the base station (BS) adaptively selects among three gears, ranging from legacy OFDM to delay-Doppler domain processing based on the channel mobility conditions experienced by the user equipments (UEs). We illustrate the benefit of adaptive gear switching for communication throughput and, finally, we conclude with an outlook on research challenges and opportunities.

Figures (3)

• Figure 1: The framework: the adaptively selects among three operating gears based on the mobility conditions and Doppler spread feedback from the . Gear 1 is used for low mobility (quasi-static), Gear 2 for medium mobility (time-varying), and Gear 3 for high/extreme mobility (fast-varying) scenarios.
• Figure 2: Throughput comparison across the three gears as a function of the velocity of the scatterers. Simulation parameters consider a system operating at $5.9$ GHz with a of $15$ kHz. The channel follows an exponential with a rich-scattering Rayleigh fading model. For Gear 2 (), $d_t$ denotes the pilot spacing along the time axis, where a lower $d_t$ (higher pilot density) provides increased robustness against Doppler spread at the cost of spectral efficiency. The shaded regions indicate the suggested operational range for each gear: Gear 1 for low mobility (green), Gear 2 for medium mobility (yellow), and Gear 3 for high-mobility/high-reliability scenarios (red).
• Figure 3: characterization of the gears. The curves show the probability of the instantaneous exceeding a given threshold $\gamma$ for Gear 1/2 (/) and Gear 3 at levels of $20$ dB and $25$ dB.