In this article, a novel method of performing interference avoidance in next-generation single-carrier ultrawideband block transmission systems is presented. This method relies on the application of a time-domain window to each data block prior to transmission, where the window is optimized such that the required notch depth is achieved for each transmission. Furthermore, a modification of this technique is also detailed, whereby the complexity can be reduced from O(N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) to O(N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) where N is the number of symbols per block. It is shown that the proposed techniques overcome many of the drawbacks of current methods such as transmit power control, frequency notching, and active interference cancellation.