Methods, techniques, and systems for transmitting a data frame of an electromagnetic (EM) telemetry signal to or from a downhole location can be performed using multi-channel transmissions in which at least two different channels at two different frequencies are used to send a long header and the data frame. At least a portion of the long header is transmitted at a first frequency, and at least a portion of the data frame and any remaining portion of the long header not transmitted at the first frequency are transmitted at one or more additional frequencies different from each other and the first frequency. In one example, only two channels are used: one channel is used to transmit all of the long header, another channel is used to transmit all of the data frame, and either of the channels is used to transmit an identifier identifying the type of the data frame.

A communication device for transmitting orthogonal frequency division multiplexed (OFDM) signals in a wireless communication system. The device includes a plurality of antenna elements that transmit the OFDM signals to a receiver over a plurality of transmission channels in the wireless communication system. The device also generates weight coefficients applied to each of the plurality of subcarrier signals, and controls an amplitude and/or phase of the plurality of subcarrier signals as a function of said weight coefficients.