  Discrete-modulation continuous-variable quantum key distribution has the
potential for large-scale deployment in the secure quantum communication
networks due to low implementation complexity and compatibility with the
current telecom systems. The security proof for four coherent states
phase-shift keying (4-PSK) protocol has recently been established by applying
numerical methods. However, the achievable key rate is relatively low compared
with the optimal Gaussian modulation scheme. To enhance the key rate of
discrete-modulation protocol, we first show that 8-PSK increases the key rate
by about 60\% in comparison to 4-PSK, whereas the key rate has no significant
improvement from 8-PSK to 12-PSK. We then expand the 12-PSK to two-ring
constellation structure with four states in the inner ring and eight states in
the outer ring, which significantly improves the key rate to be 2.4 times of
that of 4-PSK. The key rate of the two-ring constellation structure can reach
70\% of the key rate achieved by Gaussian modulation in long distance
transmissions, making this protocol an attractive alternative for high-rate and
low-cost application in secure quantum communication networks.
