Research On Modem In High Speed Facsimile Communication System

Nowadays facsimile operation keeps high speed of growth in our country, but research on fax technology hardly gets any progress. Modem technology is monopolized by foreigners.To develop our fax industry and achieve the indigenization of fax technology, the first thing we need to do is achieve some breakthrough for modem technology, which is the key technology in facsimile. Consider above of these actualities, this paper's research task is to develop modem software based on PC. Summarily this paper completes the work as follows:(1) Relative ITU-T modem recommendations are researched, and V.17 modem symtem is designed independently.(2) Key modules in modem are further researched, such as QAM, synchronization technology, AGC, trellis coded modulation, Viterbi Algorithm, adaptive equilazer etc.(3) Gardner Algorithm for symbol synchronization is investigated and the disadvantage is found out while algorithm used for QAM signals. A kind of modified algorithm is adopted based on the analysis. The simulation indicates the validity of this improvement.(4) Some exist carrier recovery algorithm are compared, from which an appropriate algorithm is selected out and implemented, and the theoretic demonstratation is presented meanwhile. The experiment result proves the validity of the algorithm.(5) Considering the properties of TCM and Viterbi Algorithm, two kinds of equalizer are investated. Complex LMS Algorithm is researched and the TCM adative equalizer with pre-decision is designed. A new method with Euclidian distance associated with energy distribution characteristic for decision is presented. Simulation result indicates this method has better performance than traditional method when carrier recovery error exists.(6) The V.17 modem software is realized with C++ programming language based on PC platform and the reliability is tested meanwhile. The realization scheme of modem software based on sound card using multi-thread technology is discussed.(7) Tests for key modules are completed, and a local closed-loop test system is designed for the simulation of V.17 modem system.Results of simulation experiment demonstrate that the modem software can achieve satisfied performance, which makes the facsimile system support 14400b/s data rate. The anticipated target is accomplished.