In one example aspect, a method of transmitting signals is provided. The method comprises transmitting a first multi-carrier on-off keyed signal comprising a plurality of on periods and a plurality of off periods. Transmitting the first signal in each on period comprises, for each subcarrier of a plurality of subcarriers, transmitting, from each antenna of a plurality of antennas, a frequency domain symbol associated with that subcarrier phase shifted by a respective factor of a set of factors associated with that subcarrier, wherein the set of factors associated with that subcarrier is different from the set of factors associated with at least one other subcarrier of the plurality of subcarriers.