Design Of Joint Differential Demodulation And Decoding System Based On Polar Codes

Orthogonal Frequency Division Multiplexing(OFDM)is a multi-carrier modulation technology which converts Frequency-selective fading into flat fading.OFDM systems usually need channel estimation to obtain channel state information during coherence detection,which leads to high implementation complexity.In the dynamic and complex multipath channel environment,the channel state information is difficult to obtain,and the channel estimation often requires more complex estimation algorithms or insertion of more pilots to ensure the accuracy of the channel estimation results,which pays a great price,especially in the case of multiple antennas system,which becomes a difficult task to complete.In this case,incoherent detection without channel estimation becomes a better solution.As an important implementation method,differential modulation has lower complexity than coherent detection,but its performance is also worse.Therefore,how to compensate the difference loss has been widely concerned.This paper conducts further research on the basis of differential detection based on iterative OFDM system.The specific research content includes:1.The principle of differential coding in OFDM system is introduced in detail,and the influence of inter-symbol interference and inter-channel interference on time domain difference and frequency domain difference system is derived by mathematical model analysis.It is concluded that frequency domain difference is better than time domain difference.It is more suitable for multipath channels whose channel characteristics change rapidly over time.And carried out simulation verification.Then,based on the iterative system of bit-interleaved,coding and modulation,this paper studies the soft demodulation algorithm and the soft-in and soft-out BCJR algorithm,and deeply explores the essence of the outer information transfer between the inner code and the outer code in the iterative system.2.For the DPSK:OFDM modulation with constant amplitude,a joint differential demodulation and decoding scheme based on Polar codes is proposed.In this iterative scheme,an iterative demodulation method based on signal-to-noise ratio extraction is proposed.This method uses the interleaved iterative information to extract the relative signal-to-noise ratio information between each subcarrier,which is used for the demodulator to update the soft output information,that is,the demodulator is also added to the iteration to avoid the first soft solution.The performance bottleneck caused by the large adjustment error also realizes the full use of soft information.Simulation results show that this scheme can reduce the BER performance gap between coherent OFDM systems and can be used as a low-complexity alternative.The simulation is performed by changing the number of iterations.The simulation results show that the appropriate number of iterations for this scheme is 5 times.3.For DAPSK-OFDM modulation,a joint differential demodulation and decoding scheme based on Polar codes is also proposed.In this iterative scheme,an amplitude and phase correction method based on hard decision feedback is proposed,and the differential decoder and demodulator are combined into one module.This differential soft demodulation module calculates the bit metric value through the received symbols at the preceding and following moments,and then outputs soft information.The proposed correction method needs to add a first-level differential encoder in the feedback link to feed back the information in symbol form,introduce an iterative accumulation factor,and perform amplitude and phase correction on the received symbol by extracting the amplitude and phase information of the feedback symbol.The next round of new soft information output.The simulation results show that the performance of this scheme is significantly better than the traditional differential detection without iteration,and the performance of the coherent OFDM system is not far behind.Due to the hard decision feedback,iterative negative gain will be caused in the low signal-to-noise ratio area.The simulation is performed by changing the number of iterations.The simulation results show that the appropriate number of iterations for this scheme is 3 times.4.Finally,this paper extends the differential joint design idea based on the single-antenna OFDM system to the dual-antenna MIMO-OFDM for design,and proposes an iterative scheme of differential space-frequency coding for the dual-antenna OFDM system.At the transmitting end,based on the idea of differential space-time coding based on group codes and using the unitary matrix characteristics,a finite state transition set is established,and DFBC coding with a code rate of 1 is performed.The receiving end uses a single-antenna OFDM system to jointly design an iterative scheme for DPSK modulation.Complete dual antenna expansion.The simulation results show that this scheme can achieve a differential compensation effect similar to that of a single antenna,which verifies the feasibility of the dual-antenna design scheme.