Performance Analysis On Physical-layer Network Coding With Hierarchical Modulation In Asymmetric Channels

In wireless communication networks,the channel state information is complicated and changing.In an asymmetric two-way relay channel(TWRC),the current physical-layer network coding(PNC)systems use different modulation schemes in the two channels in dealing with their different channel states to control the data transmission process.Such schemes can not fully make use of the potential cabability of the communication system and meet the diverse demands of some digital services in which not all bits require the same level of reliability.The hierarchical modulation(HM)can offer different degrees of protection to the transmitted bits according to their relative importance.According to the different channel conditions,it can provide different data rates to users.Under this consideration,this thesis focuses on the performance analysis and design of the physical-layer network coding with the hierarchical modulation in asymmetric channels,including 2/4PSK-PNC,4/16QAM-PNC,joint LDPC coding and the optimal power allocation scheme,such that the better transmission performance can be attained in the asymmetric two way relay channels.The main contributions of this thesis can be summarized as follows:The 2/4PSK-PNC scheme is proposed for the asymmetric TWRC where two signals from the sources are simultaneously received by the relay in one time slot and then broadcast back to the sources in another time slot.The 2/4PSK-PNC demodulation and mapping rules at the relay node is presented,the expressions for calculating the end-to-end bit error rate(BER)are derived under the Rayleigh fading and Additive White Gaussian Noise(AWGN)channels as well.The proposed 2/4PSK-PNC system can not only make full use of the better channel to achieve a high signal transmission rate,but also consider the signal reliability in the worse channel.· In a PNC system employing a higher order modulation of 16QAM,the bitwise exclusive-or mapping of the superimposed signals at the relay may encounter the ambiguity problem.In this thesis,the reasons of such ambiguity are analyzed and explained mathematically,and a joint hierarchical 4/16QAM and Physical-layer Network Coding(4/16QAM-PNC)scheme is proposed to solve this problem.Accordingly,the BER expressions of the 4/16QAM-PNC system are derived for AWGN channels.Simulation results show that the proposed 4/16QAM-PNC system can get over the ambiguity problem at the relay.Compared with the traditional 16QAM-PNC scheme,better communication performance is achieved in the TWRC.For the asymmetric TWRC,to improve the transmission quality of the poor link,two schemes using the Low Density Parity Check(LDPC)channel coding are studied in the 2/4PSK-PNC and 4/16QAM-PNC systems.The decoding algorithms based on the Log Likelihood Ratios Belief Propagation(LLR-BP)are presented.In 2/4PSK-PNC system,based on the BER expression,the optimal power allocation scheme is proposed to obtain the lowest error probability.The simulation results are presented which verify the theoretical analysis results.