Delay-Doppler Modulation and Software Defined Radio Implementation

Modern wireless communication networks, including future 6G systems, are expected to support use cases in high-mobility environments. These scenarios often involve Non-Terrestrial Networks (NTNs), unmanned aerial vehicles (UAVs), vehicular communications, and high-speed railway systems. The Orthogonal Frequency-Division Multiplexing (OFDM) scheme is widely employed today thanks to its great properties in static multipath channels. The strength of OFDM lies in the use of the cyclic prefix (CP), which enables a low-complexity single-tap equalization at the receiver. While OFDM is expected to remain a baseline waveform in 6G systems, it faces significant challenges in rapidly time-varying environments. In such scenarios, the strong Doppler effect disrupts subcarrier orthogonality, leading to inter-carrier interference (ICI). Thus, OFDM performance degrades, and the great advantage of having a low complexity receiver is lost. To overcome OFDM's limitations in high-mobility scenarios, one approach is to employ it with additional precoding. Building on this idea, Orthogonal Time Frequency Space (OTFS) is a novel modulation scheme that has gained growing interest as a promising alternative to OFDM in strongly time-variant channels. In this thesis, channel estimation algorithms for OTFS systems are investigated. The presented embedded pilot approach inserts a single pilot and guard symbols within the delay-Doppler grid. Numerical results compare channel estimation performance accounting for both integer and fractional Doppler shifts. The outcomes show that the impact of fractional Doppler can be effectively mitigated with relatively simple additional signal processing, resulting in only minor performance differences. The ultimate objective of this thesis is the integration of the complete OTFS transceiver chain on a Software Defined Radio (SDR) platform. In particular, a loop-back configuration is implemented and preliminary results confirm the correctness of the system.