Theoretical Research Of MIMO Diversity System Based On LDPC Codes

Low-density parity-check(LDPC)codes have powerful error-correcting ability and are easy to implement,which have been adopted as the channel coding scheme of long codes for next generation mobile communications.Multiple input multiple output(MIMO)can improve the validity and reliability of the systems significantly when the timefrequency resources are limited.In practice,diversity techniques are always applied to reduce the effects caused by channel fading.Combining the LDPC codes with the MIMO technology can not only employ diversity gains but also encoding gains.This thesis mainly studied the implementation scheme and the performance of the proposed MIMO diversity systems based on LDPC codes.Firstly,this thesis analyzed and calculated the Shannon limits of different modulation schemes under Gaussian channel,frequency non-selective fast fading channel and frequency non-selective slow fading channel.The solution of the Shannon limits can be divided into two steps: Firstly obtain the calculation formula of channel capacity;Then calculate the Shannon limits with numerical integration and linear interpolation.Therefore,the key is how to calculate the channel capacity.For Gaussian channel,this thesis gave a general expression of the channel capacity for different modulation schemes.For frequency non-selective fast fading channel,this thesis assume the the perfect interweaving is possible and channel state information is known at receiver.Hence the capacity of channel is the ergodic capacity.For frequency non-selective slow fading channel,referencing to the concept of outage probability,this thesis proposed a mathematical model of frequency non-selective slow fading channel.Besides,the channel capacity was derived.Then,this thesis investigated the MIMO diversity system based on LDPC codes under time selective fading channel.The code rate of LDPC code used in this thesis is1/2,which means the length of the information bits is equal to the length of the check bits.In other words,there is a unique correspondence between information bits and check bits.This thesis put forward a soft decoding algorithm based on signal processing and message passing.This algorithm modified the LLR of the information bits and the check bits making the best of channel information,which improve the error rate performance of the system.Finally,aiming at frequency selective fading channel,this thesis studied the performance of the MIMO system based on LDPC codes.Compared with time selective fading channel,there are many distinguish multi-path in frequency selective fading channel.For the receiver,there are extra frequency gains.Employing LDPC codes into MIMO diversity systems to improve the reliability of communication systems under frequency selective fading channel.In addition,this thesis amended the soft decoding algorithm based on signal processing and message passing,to make it suitable for frequency selective fading channel.