| The Pulse Code Modulation/Frequency Modulation (PCM/FM) is widely used in most reentry Telemetry System in our country. When the code rate is low (500Kbps or less), the transmitting signal suffering only flat fading and increasing the system power can reduce the bit-error rate (BER). At present, the data transfer rate of PCM/FM reentry telemetry system can reach 2Mbps in our country. Because of the increase of the speed rate, the signal suffers from frequency select fading, which causes the serious intersymbol interference (ISI). Thus, increasing the system power can't reduce the BER. So wireless communication systems frequently make use of channel coding to improve the anti-jamming capability. In recent years, The Turbo codes and LDPC codes which can approach Shannon limit and effectively resist channel fading are of widespread concerned and have been applied to various communications. Turbo codes with pseudo-random properties is constructed by combining convolutional code and random interleaver. After certain iterations between two soft input/soft output (SISO) decoder, Turbo codes realize the iteration decoding. LDPC code is another kind of Shannon Capactiy Achieving linear block code. Compared with the Turbo codes, LDPC codes based on parallel iterative decoding algorithm of sparse matrix have low algorithm complexity of decoding, less computing works and lower error level layer. All of these features make the LDPC codes become a challenge to Turbo codes.This thesis studies mainly about the optimization problem of degree distribution of irregular LDPC Codes on RICE channel and the performance analysis of PCM/FM reentry telemetry system based on Turbo codes and LDPC codes. The conclusions gained are as follows:Firstly, the performance of Turbo and LDPC decoding algorithm on Gaussian channel is analyzed. We compare the decoding performance of regular LDPC codes and irregular LDPC codes. The performance of irregular LDPC codes on which degree distribution is well optimized is better than regular LDPC codes even the code length is shorter. In fact, the gain can achieve 0.6 dB-1.5dB.Secondly, on the basis of the structural characteristics PCM/FM reentry telemetry system, we build the architecture model of the encoded PCM/FM reentry telemetry system based on Turbo/LDPC codes. According to the reentry telemetry channel characteristics, we established a channel simulation model for encoding PCM/FM reentry telemetry system, and apply Turbo Code and LDPC code in PCM/FM reentry telemetry system, build the construction of encoded PCM/FM reentry telemetry system and analyse the system. The results showed that in the telemetry channel, the anti-bit-error performance of encoded PCM/FM reentry telemetry system has been greatly improved. The encoded PCM/FM reentry telemetry system has at least 7dB channel gain compared to the non-encoded reentry telemetry system.Finally, we study the optimization problem of degree distribution of irregular LDPC Codes. A method based on Exit graph for degree distribution optimization of irregular LDPC Codes on Rician channel is proposed. This method modify the computing formula of Variable Node Decoder (VND) curve under a condition of not knowing channel state information (NCSI). A result of degree optimization is also given. The result shows that the method is easy to implement and the LDPC codes also have a good performance after optimization. |
PDF Full Text Request