Ldpc Codes Sess System

LDPC codes are good codes which had been invented by Gallager.Self-encoded spread spectrum(SESS) system was first brought forward by LimNguyen, the spread spectrum sequence of which is fully built by the source data.In this paper, We summarized the researches of the LDPC codes and SESSsystem. We introduced some code structures, encoding methods of the LDPC codesand the belief propagation algorithm and the density evolution. We introduced thetechnique of the basic SESS system, analyzed its performance in AWGN channel andcharacteristics such as the self-interference, the multi-access and the multi-path. Theperformance of the single user SESS system is the same as the conventional directspread spectrum system when Eb/No is higher than 4dB. The error propagationcoursed by self-interference worsens the system performances while the Eb/No is low.In this paper, We brought forward an approach which can enhance theperformance of the SESS system. We constructed the SESS-LDPC system bycombining the LDPC codes and the SESS system and utilized the strong errorcorrecting abilities of the LDPC codes to improve the performance of the system.We constructed two types of LDPC codes and proved the excellentperformances of the (3,6) Mackay codes and the eIRA code through simulation. Wecompared the encoding complexity and the code structure of the two types of codes.We simulated the SESS-LDPC system and proved that the LDPC codes canimprove the performance of the SESS system, and we also pointed out that thereexisted a threshold which was not effected by the performance of error correctingcodes. We pointed out that the correlation of the spread spectrum sequences decidedthe multi-access performance of the system. We adopted orthogonal Hadamard codeas the spread spectrum sequence and proved that the threshold decreased and themulti-access performance was improved.