Fig. 1. H matrix and the corresponding tanner graph of (6,4) LDPC code.

Fig. 2. Schematic diagram of base matrix expansion.

Fig. 3. Performance of constructed LDPC codes with R=22/68, Z=64\96\128\192\256\384. Code length =4325\6528\8704\13,056\17,408\26,612 bits, binary phase-shift keying (BPSK) modulation, max iteration num=50.

Fig. 4. performance of constructed LDPC codes with R=10/52, Z=80\128\160\256\320\384. Code length =4160\6656\8320\13,312\16,640\19,968 bits, binary phase-shift keying (BPSK) modulation, max iteration num=50.

Fig. 5. Experimental setup of CVQKD scheme based on GG02 protocol. CW laser, continuous-wave laser; BS, beam splitter; AM, amplitude modulator; PM, phase modulator; ATT, attenuator.

Fig. 6. (a) FER values of constructed QC-LDPC codes with rates 1/4 and 10/52. (b) FER values of constructed QC-LDPC codes with rates 1/3 and 22/68. Both (a) and (b) are binary phase-shift keying (BPSK) modulation, max iteration num=500, frame num=10,000. (c) Reconciliation efficiency of the QC-LDPC codes for FERs of 0.5, 0.1, 0.01, and 0.001.

Fig. 7. Secret key rate as a function of distance for a CVQKD system with a homodyne detector, excess noise ϵ=0.01, detection efficiency η=0.6, electronic noise Vel=0.01, and the attenuation factor α of the quantum channel set to be 0.2 dB/km. Pinkish-red dashed curve and blue dashed curve show the QC-LDPC codes (length 62,800, rates 1/4 and 1/3) in the DVB-S2 protocol, respectively. Red solid curve shows the QC-LDPC code (length 68,000, rate 22/68) in the 5G protocol.

Table 1. Two Types of BGs Given in the 5G Protocol

Table 2. Sets of LDPC Lifting Size Z