| Modulation,demodulation and channel error-correcting code are all the key technologies in modem communication system owing to the requirements of communication distance,communication capability and transmission quality.As an error-correcting code technology,convolutional code is widely used because of its outstanding error-correcting performance.The Viterbi algorithm is a maximum-likelihood decoding algorithm for convolutional codes.This paper designs a convolutional code decoding module used in a DQPSK modulation-demodulation systerm with VHDL language,adopting the(2,1,7)convolutional codes and corresponding Viterbi algorithm.The Viterbi decoder uses the 16-quantizationlevel of the path metric to instead of the conventional 8-quantizationlevel in order to improve the system gain.Meanwhile,the specific hardware implementation program of flame synchronization is also presented.First of all,the principle of DQPSK modulation-demodulation is introduced,and the feasible whole systerm scheme is proposed while the performance is also analysed.Then,combining the principle and classification of convolutional code decoding,we give the concrete parameters of this Viterbi decoder,present a 4-bit soft decision decoder appropriated for this systerm,which has fully parallel ACS and 42 trace-back depth.Moreover,each module of the Viterbi decoder are designed and implemented with VHDL language in Xilinx FPGA platform by means of modularized solution.Simulation results indicate that the whole system can work normally.In the end,the flame synchronization which is very important for the demodulation systerm is also implemented by referring to the structure of flame. |
