On The Design Of Uplink Rateless Coded Transmission Strategy In Cloud Radio Access Network

In order to cope with the rapidly increasing number of users and the amount of data in the current communication network,improve the quality of wireless network service and reduce the cost of network deployment,Cloud Radio Access Network(C-RAN)is expected to become a transformative access network architecture in 5G communication networks.The typical feature of C-RAN is that the remote radio head(RRH)of each access node in the network is completely separated from the baseband unit(BBU),and the RRH is closer to the user,while each BBU converges backward into a virtual BBU pool.The work done by the traditional base station is implemented in RRHs and BBU pool respectively.Compared with the traditional cellular network architecture,the network state and the channel state in C-RAN are more complex.It is difficult for the transmitter to know the accurate channel state information in advance before transmitting the information.This puts forward higher requirements for choos-ing appropriate channel coding schemes in C-RAN.Rateless coding is an efficient error control coding technique which is adaptive to current network channel state and transmission characteristics.The adaptive characteristics of rateless codes make it suitable for the flexible transmission mechanism in C-RAN.Based on this,this paper mainly studies the design and optimization of uplink access transmission scheme based on rateless coding in C-RAN.The main contents and contributions of the thesis are as follows:First of all,we design a rateless coded uplink transmission scheme for single-user scenarios in C-RAN.The main ideas of this scheme are as follows: First,the LDPC code is used as a pre-code to encode the information to be sent to the Raptor code.Then,the code word modulation is continuously sent to each RRH covering the user,and each RRH preprocessing unit preprocesses the received signal to obtain baseband signals.Finally,the BBU pool uses a belief propagation decoding algorithm based on Tanner graph to decompress and decode all the signals uploaded by RRH.When the BBU pool correctly recovers the original information of the user,it feeds back the ACK signal to the user through RRH.Secondly,we design a output node degree profile optimization algorithm for the rateless coded transmission scheme.Because there is no universally optimal degree profile for Raptor codes in AWGN channel,it is necessary to design the optimal degree profile for different SNR to approach Shannon capacity of the channel.Firstly,we analyze the Extrinsic Mutual Information Transfer(EXIT)function for the iterative decoding at BBU pool based on Gaussian approximation.Then,under the constraint of error-free decoding at the BBU pool,we use the maximization of the system throughput as the objective function and obtain the optimal output node degree profile by solving the linear programming(LP)problem.Finally,the numerical simulation results show that the performance of the degree profile obtained by the proposed optimization algorithm is close to the upper bound of the system.Finally,based on MATLAB GUI platform,we carry out a visualization implementation of the algorithm of rateless code degree profile optimization presented in this thesis.The visualization platform makes it more intuitive and easy to use the optimization algorithm for the output degree profile of the rateless code under the actual system conditions.On this GUI platform,we only need to input the relevant parameters,such as the number of users,the number of RRHs,the channel coefficient of each link,the transmit power and so on.The resulted optimal degree profile and coefficient can also be visualized.