Noncoherent Orthogonal Time Frequency Space Modulation

Abstract

The recently-developed orthogonal time frequency space (OTFS) modulation is capable of transforming the timevarying fading of the time-frequency (TF) domain into the time-invariant fading representations of the delay-Doppler (DD) domain. The OTFS system using orthogonal frequency-division multiplexing (OFDM) as inner core naturally requires the subcarrier spacing (SCS) ∆f to be larger than the maximum Doppler frequency ϑmax, i.e. ∆f > ϑmax, when perfect channel state information (CSI) knowledge is assumed. However, for the first time in literature, we explicitly demonstrate that the practical OFDM-based OTFS systems have to double their SCS in order to facilitate CSI estimation, requiring ∆f ′ = 2∆f > 2ϑmax.In order to mitigate this loss, we propose a novel noncoherent OTFS system, which is capable of operating at ∆f > ϑmax. The major challenge in this context is the mitigation of the DDdomain interference without CSI. Against this background, we draw an analogy between the input-output model of OTFS and that of V-BLAST, where V-BLAST's blind inter-antenna interference mitigation technique is invoked. Moreover, we propose to partition the DD-domain modulated symbols into groups, where space-time block coding is invoked in order to eliminate the DDdomain interference within each group. Our simulation results demonstrate that the proposed noncoherent OTFS is capable of substantially outperforming its coherent counterparts relying on CSI estimation.