In a wireless transmitter having a plurality of spatial streams differentiable at a receiver, wherein data is encoded and transmitted over a communication channel with pilot tones to provide the receiver with information about the effects of the communication channel and/or transmitter impairments and/or receiver impairments on the spatial streams transmitted, the benefits can be provided by identifying a plurality of symbol periods within which symbols representing some of the encoded data are to be transmitted, identifying pilot tone values for the plurality of symbol periods, 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 and/or transmitter impairments and/or receiver impairments to obtain transmit stream diversity gain, and transmitting the symbols and the pilot tones over the plurality of spatial streams for the plurality of symbol periods. In some embodiments, the pilot tones are orthogonal over space and time or over space and time and frequency, while in others they are semi-orthogonal. Pilot tone values might be of a constant amplitude and phase shifted, or might also have amplitude variations wherein at least two pilot tones are nonzero for spatial streams for a given symbol period.