Rateless Coded OFDM System Design Based On Inter-carrier Interference Self-cancellation

In the modern communication system,mobile communication technology is a rapidly developing and widely used technology.Meanwhile,the number of users has increased dramatically.The Orthogonal Frequency Division Multiplexing?OFDM?technology is the core technology of mobile communication systems because of its good anti-interference ability and high spectrum utilization.As a multicarrier modulation technology,it can provide higher quality services in wireless communication networks.Since the sub-carriers of the OFDM system overlap with each other,the orthogonality between subcarriers is required.However,the frequency offset exits between the oscillators at the transmitter end and the receiver,which will destroy the orthogonality between the subcarriers and cause inter-carrier interference?ICI?.In order to suppress the negative impact brought by ICI,the ICI self-cancellation method is often applied to OFDM systems and has been studied widely because of its low complexity and significant performance.On the other hand,rateless code is a channel coding method with variable code rate,which is generated according to the output nodes degree profile.Its achievable rate can adaptively approach the current channel Shannon limit without channel state information?CSI?at the transmitter.Meanwhile,the robustness of the rateless code is excellent and the rateless code can reduce the feedback overhead.Based on the above discussion,this paper mainly studies the transmission scheme of the rateless coded OFDM system with ICI self-cancellation and the optimization of the degree profile.The main contents and contributions of this paper are as follows:Firstly,we consider a point-to-point rateless coded OFDM system.During each transmission round,the transmitter keeps transmitting to the receiver using Raptor code until the receiver feeds back an ACK.We use ICI self-cancellation precoding to combat frequency offset at the transmitter,wherein the data symbol vectors are multiplied with some precoding matrix before transmission.The pre-encoded signals are transmitted to the receiver,and the Belief Propagation?BP?algorithm is used to recover the original message through soft demodulation.In order to further improve the system robustness,we jointly optimize the precoding matrix and the degree profile of Raptor code with only statistical channel state information?CSI?is assumed at the transmitter.We mainly consider the joint optimization of two set of parameters.One is the coefficient of the matrix and the other is the degree profile of Raptor code.The optimization problem is formulated based on the extrinsic information transfer?EXIT?analysis of the decoding process at the receiver.The advantage of the scheme is that the CSI is not required at the transmitter,which reduces the system overhead.Simulation results verify that the scheme with the optimized precoding matrix and degree profile can achieve the largest tolerable frequency offset.The scheme achieves a BER under10^-4 at a smaller overhead and approximates the average transmission rate of the system.Then we extend the point-to-point system scheme,and study the transmission scheme and optimization of the ICI self-cancellation and degree profile for the rateless coded OFDM system in a distributed antenna system?DAS?.DAS can extend coverage and improve channel capacity.Thus it become a highly competitive wireless communication architecture solution.In this paper,we consider a single-user two-antenna system.The user encodes the original information with Raptor code.The codeword is modulated and multiplied by the ICI self-cancellation matrix,then the signals are sent to the antenna units which cover the user.It should be noted that the received signal of each antenna unit has different frequency offset.Each unit will preprocess the received signal to obtain the baseband signal.Then the quantizer quantizes the baseband signal and transmit it to the central processing unit.The central processing unit jointly decodes according to the signals uploaded by each antenna unit to recover the original information.We jointly optimize the ICI precoding matrix and the degree profile of rateless code with only statistical CSI at the transmitter.Simulation shows that the optimized scheme achieves the largest tolerable frequency offset compared with the system without ICI self-cancellation.And it is within only 8%away from the theoretical limit based on EXIT.