  Wideband extremely large-scale multiple-input-multiple-output (XL-MIMO) is a
promising technique to achieve Tbps data rates in future 6G systems through
beamforming and spatial multiplexing. Due to the extensive bandwidth and the
huge number of antennas for wideband XL-MIMO, a significant near-field beam
split effect will be induced, where beams at different frequencies are focused
on different locations. The near-field beam split effect results in a severe
array gain loss, so existing works mainly focus on compensating for this loss
by utilizing the time delay (TD) beamformer. By contrast, this paper
demonstrates that although the near-field beam split effect degrades the array
gain, it also provides a new possibility to realize fast near-field beam
training. Specifically, we first reveal the mechanism of the near-field
controllable beam split effect. This effect indicates that, by dedicatedly
designing the delay parameters, a TD beamformer is able to control the degree
of the near-field beam split effect, i.e., beams at different frequencies can
flexibly occupy the desired location range. Due to the similarity with the
dispersion of natural light caused by a prism, this effect is also termed as
the near-field rainbow in this paper. Then, taking advantage of the near-field
rainbow effect, a fast wideband beam training scheme is proposed. In our
scheme, the close form of the beamforming vector is elaborately derived to
enable beams at different frequencies to be focused on different desired
locations. By this means, the optimal beamforming vector with the largest array
gain can be rapidly searched out by generating multiple beams focused on
multiple locations simultaneously through only one radio-frequency (RF) chain.
Finally, simulation results demonstrate the proposed scheme is able to realize
near-optimal nearfield beam training with a very low training overhead.
