Rateless Coded Multi-user Downlink Transmission In Cloud Radio Access Network

The trend of the next generation communication system is heterogeneous network and smaller cellular,which lead to the issues of severe inter-cell interference and cell association.At the same time,the communication system will face the explosive growth of user equipments and their demands for data transmission which bring the urgent requirement for the improvement of the capacity of the radio access network(RAN).Cloud radio access network(C-RAN)is a revolutionary mobile network architecture which can potentially solve the above challenges.In C-RAN,the the remote radio headers(RRH)and baseband processing units(BBU)of all access nodes are separated.The RRHs are closer to users,and all the BBUs are centralized in a virtualized BBU pool.The BBU pool and the RRHs are connected through the fronthaul with limited capacity.Compared with the traditional RAN system,the channel state information(CSI)in C-RAN is complicated and uncertain,which lead to the issue of choosing appropriate channel coding scheme.In this work,we refer to the rateless code for coded transmission in C-RAN.The rateless code has been applied to cooperative relay system and distributed antenna system due to its system adaptivity.The characteristic of rateless code makes it adaptable to the transmission in C-RAN.Based on the above summaries,in this paper we propose a rateless coded transmission scheme and optimization for the multi-user downlink of C-RAN.The main contributions of this work can be summarized as follows:Firstly,we design the rateless coded transmission scheme for multi-user downlink of C-RAN.For the BBU pool,we adopt the specially designed Raptor code for rateless encoding on the message for each user.Then a precoder based on zero forcing(ZF)structure is used to do precoding on the modulated signals,which is then followed by a fixed scalar quantizer to perform signal compression.The compression signals are transmitted through capacity-limited fronthaul to the corresponding RRHs.The RRHs convert the compressed signals into baseband signals and broadcast the signals to each user.For the users,we assume that they are aware of the ZF precoder,then we propose a scheme of compression noise estimation to derive the soft information from the received signals.Finally the belief propagation(BP)algorithm is adopted to recover the messages.Secondly,to further improve the performance of the proposed downlink rateless coded transmission scheme,we study the joint optimization of rateless encoding and the precoding.We mainly consider the joint optimization of two set of parameters.The first are the power coefficients of the basis vectors in the ZF precoder.The second are the output-symbol degree profiles and the average input-symbol degree of the Raptor code used for downlink data of each user.We formulate the optimization problem according to the extrinsic information transfer(EXIT)function analysis on the decoding process at the user to maximize the sum-throughput and min-throughput,which is constrained by guaranteeing a successful decoding.Then,we refer to a algorithm with a two loop structure to solve the optimization problem,which is validated through computer simulations.The results show that the proposed algorithm can effectively improve the performance of the C-RAN system.