Learning Based Adaptive Rateless Coded Transmission Technology And VOFDM System

Modern wireless communications face the challenges caused by noise and interferences, which can be modeled as large-scale fading and small-scale fading such as multi-path transmis-sion, Doppler effect and frequency spread. With these problems, not only the reliability and quality of communications are threatened, but also the probability of outage is increased. To improve such situation, a variety of error correcting codes are put into implement, among which concatenated codes and Turbo codes are widely used. However, these classic fixed-rate codes seem helpless when it comes to variable channels, since they are not able to adapt to the channel variations and give rise to overheads. Thus, more and more attentions are paid to Rateless codes, which is essentially a kind of forward incremental redundant codes with the ability of adjusting its transmission rate adaptively. This paper studies the design of rateless coded transmission system, and relative modulation problems. The contents are listed as follows:We first propose a Q-learning based method to utilize rateless rate adaption for rateless cod-ed transmission system. Ideally, a rateless transmitter just sends the signals infinitely until an acknowledgement (ACK) frame is received. Correspondingly the receiver initials the decoding process once it receives one signal and add the signals received afterward into the process until the decoding process succeeds. Then it sends an ACK frame back. However, in practice, the transmission is always implemented in frames each containing some number of symbols, and the receiver initials decoding process after some number of frames are received. Thus the over-head of decoding delay and signaling is decreased. The proposed algorithm is designed for the transmitter so that it is able to learn the current channel state according to history and feedback, which indicates how many frames should be sent in sequence. The Q-learning based algorithm is designed through redefining cost function and reward function and a probability based decision strategy. Simulations show that throughput of the transmission is increased, and both the ratio of retransmission and cost of delay are decreased.We also propose an iterative receiver structure which essentially realizes the joint equaliza-tion and decoding based on concatenated VOFDM demodulator and rateless decoder. A novel algorithm named iterative equalization and decoding (IED) algorithm for the proposed iterative receiver structure is also developed. Treated as the generalization of OFDM and Single-Carrier Frequency Domain Equalization (SC-FDE), VOFDM operates in unit of vectors instead of sepa-rated symbols. It not only increases the diversity order, but also decreases peak-to-average power ratio (PAPR) and sensitivity to carrier frequency offset (CFO) when compared to conventional OFDM. IED algorithms are implemented through leveraging the continuously-updated extrinsic information between the decoder and V-OFDM equalizer. The process continues until all the check equations are satisfied or the maximum number of iteration is reached. We also apply a few iterative demodulation methods to the iterative receiver, among which soft interference cancellation is proved to offer a good trade-off between BER performance and calculation com-plexity.At last, we propose a modulation method combining VOFDM and OFDM with index mod-ulation (OFDM-IM). OFDM-IM is a novel modulation method based on conventional OFDM proposed in recent years, of which the basic idea is inspired by spatial modulation. Instead of utilizing every subcarrier to carry constellations, only some of the subcarriers are chosen. These chosen sub-carriers are called active sub-carriers and correspondingly the rest are called inac-tive subcarriers. The uncertainty of sub-carriers provide more information to transmit. Hence, the transmission system utilizing OFDM-IM has higher throughput. However, simulations show that even though OFDM-IM obtain more diversity gain at high signal to noise ratio (SNR) region, it can not compete with OFDM at lower SNR region. To make the modulation more practical and improve the overall performance of rateless coded transmission system, we apply index modula-tion to VOFDM. Simulations see the improvement of both throughput and BER performance at low SNR region.