  The bloom of the fifth generation (5G) communication and beyond serves as a
catalyst for physical layer key generation techniques. In 5G communications
systems, many challenges in traditional physical layer key generation schemes,
such as co-located eavesdroppers, the high bit disagreement ratio, and high
temporal correlation, could be overcome. This paper lists the key-enabler
techniques in 5G wireless networks, which offer opportunities to address
existing issues in physical layer key generation. We survey the existing key
generation methods and introduce possible solutions for the existing issues.
The new solutions include applying the high signal directionality in
beamforming to resist co-located eavesdroppers, utilizing the sparsity of
millimeter wave (mmWave) channel to achieve a low bit disagreement ratio under
low signal-to-noise-ratio (SNR), and exploiting hybrid precoding to reduce the
temporal correlation among measured samples. Finally, the future trends of
physical layer key generation in 5G and beyond communications are discussed.
