Design Of De-mapper And Viterbi Decoder In COFDM System

COFDM is a popular modulation technique for high rate wireless communication over frequency-selective fading channels. Based on DVB-T receiver chip design, this paper mainly presents an implementation of Demapper module and Viterbi Decoder module for COFDM system.Viterbi algorithm is well known as a maximum-likelihood decoder that receives information and gets any possible path on the trellis diagram, and then finds an optimal path with minimum path metric. BMGU, PMUU and SMU are the three main components of Viterbi decoder. The complexity of Viterbi decoder increases exponentially with the constraint length. In order to trade off between the speed, area and power dissipation of the chip, some modifications have been made to the standard Viterbi algorithm, and a Viterbi decoder is developed with the following features:1. 4-bit-soft-input.2. Up to 32 Mbps decoding speed with a throughput rate of 2 bit per clock.3. 32 CSA units.4. Carry-lookahead Adder for CSA unit.5. Improved adaptive Viterbi algorithm for parallel state hierarchy.6. Pre-lookahead Traceback approach with a configurable traceback depth.A simplified 4-bit-soft-output demapping algorithm for QPSK, 16-QAM and 64-QAM constellation is designed. Taking the CSI into consideration, the Demapper module is suitable for both flat-fading channel and frequency-selective fading channel. Two CSI calculation algorithms can be configured according to the variation speed of the channel.A float-C model and a fixed-C model have also been made to evaluate and verify the performance of the algorithm. The whole design has been simulated on VCS, tested on NiosⅡFPGA development board, synthesized on Design Compiler and static-timing-analyzed on PrimeTime.