A method in a wireless transmitter encoding and transmitting data over a communication channel and having a plurality of spatial streams differentiable at a receiver, comprising: identifying a plurality of symbol periods within which symbols representing some of the encoded data are to be transmitted, wherein the symbols comprise constellation values for a first plurality of frequency subcarriers; identifying pilot tone values for the plurality of symbol periods, wherein the pilot tones are carried in a second plurality of frequency subcarriers and the pilot tone values vary over the second plurality of frequency subcarriers for at least one symbol period and spatial stream, and wherein the identified pilot tone values are such that the receiver, when receiving at least some of the pilot tones having the identified pilot tone values, is provided with signals enabling the receiver to characterize the communication channel to obtain transmit stream diversity benefits, and wherein at least two nonzero pilot tone values are present over two spatial streams in a given symbol period and more than one nonzero pilot tone is present in a symbol period and wherein the at least two nonzero pilot tone values are selected such that they are orthogonal in the time dimension and in the frequency dimension; wherein orthogonality in the time dimension is achieved using a Walsh-Hadamard mapping matrix and orthogonality in the frequency dimension is achieved using the same Walsh-Hadamard mapping matrix shifted by some columns; and transmitting the symbols and the pilot tones over the plurality of spatial streams for the plurality of symbol periods, the pilot tones providing the receiver with information about the effects of the communication channel or impairments on the spatial streams transmitted. identifying a plurality of symbol periods within which symbols representing some of the encoded data are to be transmitted, wherein the symbols comprise constellation values for a first plurality of frequency subcarriers; identifying pilot tone values for the plurality of symbol periods, wherein the pilot tones are carried in a second plurality of frequency subcarriers and the pilot tone values vary over the second plurality of frequency subcarriers for at least one symbol period and spatial stream, and wherein the identified pilot tone values are such that the receiver, when receiving at least some of the pilot tones having the identified pilot tone values, is provided with signals enabling the receiver to characterize the communication channel to obtain transmit stream diversity benefits, and wherein at least two nonzero pilot tone values are present over two spatial streams in a given symbol period and more than one nonzero pilot tone is present in a symbol period and wherein the at least two nonzero pilot tone values are selected such that they are orthogonal in the time dimension and in the frequency dimension; wherein orthogonality in the time dimension is achieved using a Walsh-Hadamard mapping matrix and orthogonality in the frequency dimension is achieved using the same Walsh-Hadamard mapping matrix shifted by some columns; and transmitting the symbols and the pilot tones over the plurality of spatial streams for the plurality of symbol periods, the pilot tones providing the receiver with information about the effects of the communication channel or impairments on the spatial streams transmitted.