Channel Codes Research For Overlapped Code Division Multiple Access System

The paper researches on the channel codes of overlapped code division multiple access (OVCDMA) system, which includes overlapped code division multiplexing (OVCDM) and turbo product code (TPC).First, this paper simplifies the traditional OVCDM, that means simplifying the second encode matrix to multiplexing matrix, and changing the decode algorithm of second encode matrix from BCJR to symbol soft demap. Also, this paper does some study on the multiplexing matrix searching using EXIT. For the simplified OVCDM, this paper designes the architecture of channel code while the frequency efficiency is 4bps/HZ and 6bps/HZ, and while the channel is AWGN and multi-path fadig. The simulation results show that the performance of simplified OVCDM system is near to the traditional one while the bit error rate is in 10^-5, but there is obvious error floor which needs additional precoding in practical application. This paper also discusses the design of precode and does some simulation for comparison.And then, because of the actual hardware implementation and the bad performance in short frame of simplified OVCDM, and based on one improved soft input soft output TPC decoding by Doctor Fang Jun, this paper studys the TPC decoding in high-order modulation and the soft decoding in multi-path fading for TPC owing the advantages of short frame and hardware implementation. Also this paper analyses the complexity of TPC, turbo code of LTE and simplified OVCDM, compares the performance of TPC and LTE in AWGN by simulation at last.