Secure OTFS Transmission via Joint Delay-Doppler Precoding and Time-Domain Noise Injection

This paper studied the secure transmission problem for a single-antenna orthogonal time frequency space (OTFS) system where a transmitter sends private information to legitimate receiver with the existence of a potential eavesdropper. Without multiple antennas, we propose a joint delay-Doppler (DD) domain and time domain secure transmission strategy, instead of the traditional ones based on multi-antenna precoding. The DD-domain precoding matrix and time-domain artificial noise (AN) vector are jointly optimized to maximize the system secrecy rate. The cases where eavesdropper's channel state information (CSI) is available and unavailable are both investigated. Moreover, a low-complexity projected gradient (PG)-based algorithm is developed. Numerical results show that the benefit of time-domain AN in terms of secrecy rate becomes more significant as the total available transmit power increases. In addition, the low-complexity PG algorithm can significantly reduce computational complexity with only negligible secrecy rate loss.