Performance Comparison of Equalization Schemes for OTFS over Time-Varying Multipath Channels

In this paper, simple linear equalization schemes in time-frequency (TF) and delay-Doppler (dD) domains for orthogonal time frequency space (OTFS) are investigated, and their demapping is concretely derived to adhere to channel coding by taking effective desired channel, effective residual interference, and effective noise, capturing demodulation and equalization, into account. The BLER performance shows that OTFS with TF-domain minimum mean squared error (MMSE) (TF-MMSE) and dD-domain MMSE (dD-MMSE) can achieve higher diversity order than that of CP-OFDM with TF-ZF in varied types of channel: i) high delay spread and low mobility in non-line-of-sight, ii) medium delay spread and high mobility in line-of-sight (LoS), and iii) low delay spread and very high mobility in LoS. It is observed from numerical link level simulations that OTFS with TF-MMSE and dD-MMSE can achieve superior BLER performance compared to CP-OFDM with TF-ZF in all the channel scenarios, but there is a cross-over point where it is started that OTFS becomes better than CP-OFDM in terms of BLER performance in the first channel type. Also, matched filter bound with iterative demapping and decoding for OTFS is drawn to provide lower bounds in BLER performance and to compare diversity order with those of TF-MMSE and dD-MMSE for OTFS.