Nonorthogonal Multiple Access With Orthogonal Time–Frequency Space Signal Transmission

Orthogonal time-frequency space (OTFS) is being pursued in recent times as a suitable wireless transmission technology for use in high-mobility scenarios. In this article, we propose nonorthogonal multiple access (NOMA) based OTFS which may be called NOMA-OTFS system and evaluate its performance from a system-level and link-level perspective. The challenge lies in the fact that while OTFS transmission technology is known for its resilience to high-mobility conditions, while NOMA is known to yield high spectral efficiency (SE) in low-mobility scenarios in comparison to orthogonal multiple access (OMA). We present a minimum mean square error (MMSE)-successive interference cancellation based receiver for NOMA-OTFS, for which we derive expression for symbol-wise postprocessing SINR in order to evaluate system sum SE. We develop power allocation schemes to maximize the sum SE in the high-mobility version of NOMA. We further design a realizable codeword-level SIC (CWIC) receiver using low-density parity check (LDPC) codes along with MMSE equalization for evaluating link-level performance of such practical NOMA-OTFS system. The system-level and link-level performance of the proposed NOMA-OTFS system are compared against benchmark OMA-OTFS, OMA-orthogonal frequency division multiplexing (OMA-OFDM) and NOMA-OFDM schemes. From system-level performance evaluation, we observe interestingly that NOMA-OTFS provides higher system sum SE than OMA-OTFS. When compared to NOMA-OFDM, we find that outage SE of NOMA-OTFS is improved at the cost of decrease in mean SE. The link-level results additionally show that the developed CWIC-based NOMA-OTFS receiver performs significantly better than NOMA-OFDM in terms of block error rate, goodput, and throughout.