Research On Application Of Fountain Codes In Mobile Communication Systems

With the rapid development of mobile communication technology, the reliability requirements of channel coding in mobile communication systems are increasing, and the related research of new channel coding technology is very popular. Fountain codes is a new type of channel coding. Different from the traditional fixed rate coding, fountain codes can generate infinite number of encoded bits and the transmitter does not need to know the channel state information. Therefore, fountain codes can adapt to various channel conditions very well. In the paper, we take the Vo LTE scene and the eMTC scene as examples and complete the research on application of fountain codes in mobile communication systems with simulation.First, we propose the LT coding scheme with limited feedback. In unicast scenarios, if the decoder can’t find a coded symbol whose degree is one, the receiver would choose the most useful source symbol and send the sequence number of the source symbol to the transmitter. By sending the most useful source symbol again, the decoder can continue to decode so that the speed of convergence is faster and the number of encoded symbols to recover all the source symbols reduces effectively, when comparing to traditional LT scheme. In broadcast scenarios, each receiver chooses several the most useful source symbols and the transmitter retransmit the source symbol which is required by the most receivers. The simulation results show that the LT codes with limited feedback can use very few feedback bits to reduce the number of encoded symbols for successful recovery and also eliminate the error floor, in both unicast and broadcast scenarios.Second, we propose the physical random network coding scheme based on LT codes to replace the existing Hybrid Automatic Retransmission reQuest scheme used in voice over LTE. We put forward the random network coding scheme, the improved HARQ scheme and the physical random network coding scheme, and compare their performances to the performance of HARQ scheme. According the simulation results, the random network coding scheme which uses LT codes to replace HARQ scheme directly can’t obtain better performance than HARQ scheme. The improved HARQ scheme which combines the idea of LT codes and HARQ scheme can’t satisfy performance improvement in all signal-to-noise ratio. The physical random network coding scheme which applies LT codes to the physical layer directly uses LT codes to fight against the noise and obtains better performance than the existing HARQ scheme.Finally, we propose rateless scheme based on LT codes to replace the repetition scheme using in eMTC scene. In rateless scheme, the transmitter continues encoding and creates encoding bits which can be loaded in a RB in each time slot before sending the RB to the receiver. The receiver keeps on receiving encoding bits and attempts to decode. If the receiver succeed to decoding, the receiver would send an ACK signal to the transmitter. Otherwise, the receiver would wait to receive encoding bits in the next time slot. We complete simulations under two circumstances, aiming at whether the TTI number for a single packet transmission is fixed or not. The simulation results show that the rateless scheme has better performance than the repetition scheme and the rateless scheme is more flexible in actual use.