A method and apparatus for using demodulation reference signal based channel state information feedback in Orthogonal Frequency Division Multiplexing\u2014multiple-input multiple-output systems is disclosed. The offset error is compensated for in normal operation. A diamond electron source having an electrode and a substrate which contains the phosphorous-doped diamond film and emitting electron beams from the phosphorous-doped diamond film when voltage is applied between the electrode and the substrate. The drill is inserted into a preliminary hole drilled beforehand and cuts a distal end of the preliminary hole to form a hole including an expanded distal end having a greater diameter. For a given distance from the detector, the 2D PRF to be applied is formed by scaling the representative 2D PRF using the scale for that distance.