A method of transmitting over a digital communications channel, the method implemented at a transmitter-side, comprising: allocating, to a first reference signal for a channel estimation by a first user equipment, a first group of transmission resources from a first two-dimensional resource plane, which is represented by a first and second orthogonal axes corresponding to a first transmission dimension and a second transmission dimension, respectively for transmission; allocating, to a second reference signal for a channel estimation by a second user equipment, a second group of transmission resources from the first two-dimensional resource plane for transmission; performing a two-dimensional transform on a multiplexing of the first reference signal having the first group of transmission resources and the second reference signal having the second group of transmission resources from the first two-dimensional resource plane to generate a corresponding transformed signal in a second two-dimensional resource plane represented by a third and a fourth orthogonal axes corresponding to a third transmission dimension and a fourth transmission dimension respectively, wherein the first two-dimensional resource plane comprises a delay-Doppler plane and the second two-dimensional resource plane comprises a time-frequency plane; performing one or more operations on the transformed signal to generate a formatted signal according to a transmission format of the digital communications channel; and transmitting the formatted signal over the digital communications channel, wherein transmission resources used by the first reference signal and the second reference signal are orthogonal in the delay-Doppler plane. allocating, to a first reference signal for a channel estimation by a first user equipment, a first group of transmission resources from a first two-dimensional resource plane, which is represented by a first and second orthogonal axes corresponding to a first transmission dimension and a second transmission dimension, respectively for transmission; allocating, to a second reference signal for a channel estimation by a second user equipment, a second group of transmission resources from the first two-dimensional resource plane for transmission; performing a two-dimensional transform on a multiplexing of the first reference signal having the first group of transmission resources and the second reference signal having the second group of transmission resources from the first two-dimensional resource plane to generate a corresponding transformed signal in a second two-dimensional resource plane represented by a third and a fourth orthogonal axes corresponding to a third transmission dimension and a fourth transmission dimension respectively, wherein the first two-dimensional resource plane comprises a delay-Doppler plane and the second two-dimensional resource plane comprises a time-frequency plane; performing one or more operations on the transformed signal to generate a formatted signal according to a transmission format of the digital communications channel; and transmitting the formatted signal over the digital communications channel, wherein transmission resources used by the first reference signal and the second reference signal are orthogonal in the delay-Doppler plane.