1.premise
The grid defined in this way consists of 𝑁 points along the delay period, with spacing Δ𝜏 and 𝑀 points along the Doppler period, with spacing 𝛥𝜐, resulting with a total of 𝑁𝑀 grid points inside the fundamental rectangular domain
2.description
where the first factor is localized along delay (time) and the second factor is localized along Doppler (frequency).
3.characteristic
In other words, the local structure of the OTFS carrier waveform resembles that of TDM while the global structure resembles that of FDM.