A method of increasing the coherence bandwidth on shortwave channel using the narrow beam forming

Shortwave communication works with frequency-selective fading channel, of which coherence bandwidth is an important parameter describing the "flat" channel bandwidth to be used by single carrier system. As has been known, increasing the coherence bandwidth can facilitate larger bandwidth system with faster data rate transmission and simplified system with reduced receiver complexity. This paper proposes a method of using narrow beam to concentrate spatially the transmitted signal power on the main path and suppress the undesired multipath signals resulting in delay spread. For realizing the functions motioned above, the adaptive technique is taken to set up and maintain the narrow beam link for the point-to-point communication. The theoretical study shows that the coherence bandwidth can be effectively increased by this method. The calculations are performed for different number of antennas over the mid-range channel and the results confirm this approach.