Design and Optimization of OTFS-Based Communication Systems

Sammanfattning

Orthogonal Frequency Division Multiplexing (OFDM) is a widely adopted modulation technique for modern 4G/5G wireless communication systems. While it exhibits robust performance in static or mildly time-varying environments, its susceptibility to inter-carrier interference (ICI) due to Doppler spread significantly degrades its performance in high-mobility scenarios. Recent studies have shown that Orthogonal Time Frequency Space (OTFS) modulation has emerged as a promising alternative, offering superior resilience against time-varying channels by exploiting the delay-Doppler domain representation. In this paper, we present a rigorous mathematical derivation and comparison of OFDM and OTFS by analyzing their respective input-output relationships in wireless communication systems. These mathematical analyses form the foundation for the construction of an experimental comparison model and the design of suitable equalization and detection methods. First, we employ two conventional equalizers, Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE), to evaluate the performance of OFDM and OTFS under identical channel conditions. Experimental results demonstrate that OTFS significantly outperforms OFDM by an order of magnitude in dynamic environments. However, when fractional Doppler shifts exist in OTFS channels, both equalizers suffer a 10-fold performance degradation. To further enhance OTFS detection accuracy, we introduce a Message Passing (MP) detector, which outperforms both ZF and MMSE, particularly in the presence of inter-Doppler interference (IDI) caused by fractional Doppler shifts. Using the sparsity of the delay-Doppler domain, MP detection iteratively refines symbol estimation, achieving superior bit error rate (BER) performance. Within the MP framework, we observe that the algorithm’s parameters, the damping factor, the maximum iterations, and the interference truncation parameter critically influence its effectiveness, computational complexity, and runtime efficiency. Our analysis reveals that the optimal interference truncation parameter increases with signal length. Besides, the damping factor and maximal iterations exhibit strong interdependence, requiring co-optimization through iterative adjustment. Finally, inspired by diversity-combining techniques in Multiple-Input Multiple-Output (MIMO) systems, we applied Equal-Gain Combining (EGC) to OTFS, significantly reducing BER and enhancing system performance in low-SNR regimes. Our findings provide critical insights into the optimization of OTFS-based communication systems for practical high mobility scenarios.