  Beamforming technology is widely used in millimeter wave systems to combat
path losses, and beamformers are usually selected from a predefined codebook.
Unfortunately, traditional codebook design neglects the beam squint effect, and
this will cause severe performance degradation when the bandwidth is large. In
this letter, we consider that a codebook with fixed size is adopted in the
wideband beamforming system. First, based on the rectangular beams with
conventional beam coverage, we analyze how beam squint affects system
performance and derive the expression of average spectrum efficiency. Next, we
formulate optimization problem to design the optimal codebook. Simulation
results demonstrate that the proposed codebook spreads beam coverage to cope
with beam squint and significantly slows down the performance degradation.
