Coordinated transmission without power variations

The two-channel problem is analyzed for the case in which equal power must be allocated to each transmitter. The performance improvement available using coordinated transmission is described. The scheme provides optimum theoretical performance when the SNR varies between channels and/or the interference is correlated, without varying the transmitted power for each channel. It is shown that a simplified scheme using only rotations in the transmitter and receiver achieves most of the improvement of the general scheme. Variations employing fixed rotations in the transmitter are shown to yield significant performance improvement over conventional schemes with very little cost in complexity. The high speed digital subscriber line using pulse amplitude modulation is chosen as an example