A method of transmitting signals includes transmitting a signal from a plurality of antennas over a plurality of subcarriers. The signal is on-off keyed and comprises a plurality of on periods and a plurality of off periods. Each on period comprises, on each subcarrier, a frequency domain symbol associated with the subcarrier. The frequency domain symbol is phase shifted from each antenna by a respective factor of a set of factors associated with the subcarrier. The set of factors is different for at least two of the subcarriers.