Orthogonal Chirp-Frequency-Division Multiple Access: System, Channel Estimation, and Pilot Optimization

Abstract

This paper introduces the orthogonal chirp-frequency-division multiple access (OCFDMA), a technique based on orthogonal chirp-frequency-division multiplexing that partitions the frequency band into independent chirp-based subchannels, referred to as voices. By leveraging the block-circulant property of the discrete modular chirp transform, OCFDMA assigns each user one or more voices to achieve orthogonal, interference-free, multiple-access transmissions, thereby enabling per-user processing and reducing transceiver complexity. User-oriented geometries are developed to allow flexible trade-offs between bit-error rate (BER) and data rate by adjusting the number and size of voices. Per-voice linear models are derived, and both minimum-variance unbiased and minimum mean-squared-error channel estimators are presented, along with optimal pilot designs. The proposed technique is compared with state-of-the-art chirp-based multiple-access techniques, such as orthogonal chirp-division multiple access (OCDMA), as well as orthogonal frequency-division multiple access (OFDMA)-related techniques widely adopted in current wireless standards. Numerical experiments show that OCFDMA achieves superior per-user BER, strong robustness to cyclic-prefix-length violation and carrier-frequency offset, competitive data rates, and a peak-to-average power ratio comparable to OFDMA for both downlink and uplink connections.