The Design of New ARQ Techniques in the Turbo Code

[[abstract]]Since the turbo code introduced by Berrou et al.[1], it attracts much attention because it provides very good coding gain in additive white Gaussian noise (AWGN) channel. In [1], the BER performance can be obtained to 10/sup -5/ at an Eb/No of just 0.7 decibels; however, large block sizes of 65,536 bits are required. In our simulations, when the block sizes are 8,192 bits and Eb/No is under 2 decibels, the BER would be worse than 10/sup -2/, no matter what iteration times are. In this environment, the transmitted packet would be in error because the BER is too high, therefore the Automatic Repeat reQuest (ARQ) technique is needed. However, if the channel is still very noisy, the retransmitted data will need to be retransmitted again and again because it is almost impossible that the packet is error free after decoding when the BER is worse than 10/sup -2/. In order to reduce retransmission times, we will propose two ARQ techniques base on the turbo code in this paper which architectures are shown in Fig. 1 and Fig. 2. Although the authors proposed similar technique in [2], our techniques are much simpler.In our architecture of encoder, the data was packed with the cyclic redundancy code (CRC) before feeding into the turbo code encoder. The receiver will request retransmission if the CRC check failed in the hard decision output of the turbo code decoder. In order to reduce the retransmission times, we use all the transmitted signals to decide the output even the signals are decoded failed in its decoding time. Two architectures of decoder are described as follows:In the first method (Fig.1) that we offered, the received data are decoded using the Soft Output Viterbi Algorithm (SOVA) decoding method introduced in [3]. However, in this method, we combined the soft-output of the retransmitted signal with previous decoded signal which was decoded failed and fed to the hard decision. If the CRC check failed, the transmitter will be requested retransmitted until the CRC check successfully. The difference between the first and second methods is that the second method combined the retransmitted data and the previous transmitted signal before feeding to the decoder such as Fig.2 shown and the soft-output of the decoder is directly sending to the hard decision.The simulation results are shown in the Fig.3. In our simulation, the decoder iterates 10 times for each frame.From these simulation results we find that the second method is better than the first method and using the second method the average retransmission times are much fewer. This can make the turbo code more efficient on data transmission in poor transmission environment.[[sponsorship]]國際無線電科學聯合會中華民國委員會; 中央大學; 中華電信研究所[[conferencetype]]國內[[conferencedate]]19991220~19991221[[booktype]]紙本[[iscallforpapers]]Y[[conferencelocation]]桃園縣, 臺