Design Considerations of OFDM Waveform for Millimeter-Wave Integrated Communication and Long-Range Sensing in High Mobility Channels

Cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) is a promising waveform for integrated sensing and communication (ISAC). However, to avoid inter-symbol interference (ISI), the cyclic prefix (CP) duration must exceed the channel delay spread, which is determined by the channel length. Although beamforming in millimeter-wave (mmWave) systems can significantly reduce the delay spread for communication, radar sensing imposes distinct constraints. In particular, the maximum target detection range dictates the required CP duration, potentially requiring up to 50% of the symbol duration in long-range scenarios. This extended CP increases energy consumption and reduces spectral efficiency, as a substantial portion of transmission is allocated to CP rather than data symbols. Furthermore, Doppler shifts in high-mobility channels induce inter-carrier interference (ICI), further degrading radar performance. To address these challenges, which commonly arise in long-range and high-mobility applications such as UAVs and vehicular scenarios, we propose a novel resource element allocation scheme that employs a block-type pilot structure within the OFDM time-frequency grid. A dedicated low-PAPR pilot symbol, featuring nulls on the odd-indexed subcarriers and repeated periodically across the time-frequency grid, is utilized to support both communication and radar sensing functions. The proposed design facilitates effective long-range sensing and ICI mitigation without extending the CP length. Corresponding signal processing techniques are developed and analyzed to support the proposed approach.