  In this paper we introduce a new 2D modulation technique called OTFS
(Orthogonal Time Frequency & Space) that transforms information carried in the
Delay-Doppler coordinate system to the familiar time-frequency domain utilized
by traditional modulation schemes such as OFDM, CDMA and TDMA. OTFS converts
the fading, time-varying wireless channel into a non-fading, time-independent
interaction revealing the underlying geometry of the wireless channel. In this
new formulation, all QAM symbols experience the same channel and all
Delay-Doppler diversity branches of the channel are coherently combined.
Reference signal multiplexing is done in the time-independent Delay-Doppler
domain, achieving high density pilot packing, which is a crucial requirement
for Massive MIMO. Regardless of the Doppler scenario, OTFS enables approaching
channel capacity through linear scaling of throughput with MIMO order, thus
realizing the full promise of Massive MIMO throughput gains even in challenging
5G deployment settings.
