Diversity and PAPR Enhancement in OTFS using Indexing

In this paper, we investigate indexing of delay-Doppler (DD) bins in orthogonal time frequency space (OTFS) modulation and propose indexing designs that enhance the diversity order of OTFS. It has been shown in the literature that the asymptotic diversity order of conventional OTFS (without indexing) is just one. In this paper, we show that indexing in the DD domain in OTFS can be used to enhance the asymptotic diversity order to two. Towards this, we consider 2-dimension indexing in OTFS, where indexing is done along both delay as well as Doppler axes. We achieve the enhanced diversity order of two by proposing indexing designs that provably eliminate all the rank one difference matrices. Our simulation results also validate this analytically proven diversity order of two in indexed OTFS. In addition, indexing is shown to offer improved PAPR (peak-to-average power ratio) performance of OTFS.