A method of inserting downlink demodulation Reference Signals into Resource Blocks of slots within Orthogonal Frequency Division Multiplexing (OFDM) frames at an OFDM transmitter forming part of an OFDM system, the transmitter having at least one antenna and the system having at least one transmission layer, includes the steps of: for each antenna, inserting cell-specific Reference Signals at sub-carriers indices evenly spaced by a fixed sub-carrier spacing; and selectively inserting UE-specific Reference Signals into nodes of a rectangular lattice formed from unused Resource Elements in a group of Resource Blocks of two consecutive slots, the nodes being located at the same sub-carrier indices as the cell-specific Reference Signals and further being equally spaced by a fixed number of symbol indices within the group of Resource Blocks.