If the degree distribution is chosen carefully, the irregular low-density parity-check (LDPC) codes can outperform the regular ones. An image transmission system is proposed by combining regular and irregular LDPC cod...If the degree distribution is chosen carefully, the irregular low-density parity-check (LDPC) codes can outperform the regular ones. An image transmission system is proposed by combining regular and irregular LDPC codes with 16QAM/64QAM modulation to improve both efficiency and reliability. Simulaton results show that LDPC codes are good coding schemes over fading channel in image communication with lower system complexity. More over, irregular codes can obtain a code gain of about 0.7 dB compared with regular ones when BER is 10 -4. So the irregular LDPC codes are more suitable for image transmission than the regular codes.展开更多
Direct-sequence spread spectrum (DSSS) communication possesses low probability of detection and has been widely used in confidential communications. However, pseudo-noise (PN) sequences, used as spreading code in ...Direct-sequence spread spectrum (DSSS) communication possesses low probability of detection and has been widely used in confidential communications. However, pseudo-noise (PN) sequences, used as spreading code in conventional DSSS communications, possess peri- odic character and binary value. In hostile environments, these distinct characters may lead to some important parameters of signals being estimated accurately, and then lead to the leakage of transmitted information. To solve the problem, we propose the chaotic phase modulation (CPM) sequence alternating the PN sequences. CPM sequence has complex values and constant envelope, and also possesses large quantity and good correlation characteristics. Moreover, it has more hidden features than conventional sequences by modulating its phases using chaotic sequence. To improve the data rate, we apply it into the technique of multichannel communica-tion. Simulation results show this scheme's superior bit error ratio (BER) performance, which demonstrates its feasibility in underwater acoustic communications.展开更多
This paper analyzes the performance of both in-band and outband narrowband interference suppression in orthogonal code modulation spread spectrum system, and compares with conventional spread spectrum system at the sa...This paper analyzes the performance of both in-band and outband narrowband interference suppression in orthogonal code modulation spread spectrum system, and compares with conventional spread spectrum system at the same information transmission rate and spread spectrum bandwidth. Simulation results indicate that the antijamming (narrowband interference) performance obtained from orthogonal code modulation spread spectrum system is much better than that from conventional spread spectrum system.展开更多
文摘If the degree distribution is chosen carefully, the irregular low-density parity-check (LDPC) codes can outperform the regular ones. An image transmission system is proposed by combining regular and irregular LDPC codes with 16QAM/64QAM modulation to improve both efficiency and reliability. Simulaton results show that LDPC codes are good coding schemes over fading channel in image communication with lower system complexity. More over, irregular codes can obtain a code gain of about 0.7 dB compared with regular ones when BER is 10 -4. So the irregular LDPC codes are more suitable for image transmission than the regular codes.
文摘Direct-sequence spread spectrum (DSSS) communication possesses low probability of detection and has been widely used in confidential communications. However, pseudo-noise (PN) sequences, used as spreading code in conventional DSSS communications, possess peri- odic character and binary value. In hostile environments, these distinct characters may lead to some important parameters of signals being estimated accurately, and then lead to the leakage of transmitted information. To solve the problem, we propose the chaotic phase modulation (CPM) sequence alternating the PN sequences. CPM sequence has complex values and constant envelope, and also possesses large quantity and good correlation characteristics. Moreover, it has more hidden features than conventional sequences by modulating its phases using chaotic sequence. To improve the data rate, we apply it into the technique of multichannel communica-tion. Simulation results show this scheme's superior bit error ratio (BER) performance, which demonstrates its feasibility in underwater acoustic communications.
文摘This paper analyzes the performance of both in-band and outband narrowband interference suppression in orthogonal code modulation spread spectrum system, and compares with conventional spread spectrum system at the same information transmission rate and spread spectrum bandwidth. Simulation results indicate that the antijamming (narrowband interference) performance obtained from orthogonal code modulation spread spectrum system is much better than that from conventional spread spectrum system.