Soft Decision Feedback Equalizer Design And FPGA Implementation Of A GSM Monitoring System

Now,with the rapid development of mobile communication technology,GSM military and civil products such as GSM monitoring system derived from special areas such as signal reconnaissance and communication countermeasure,it still plays an irreplaceable role in the field of communication security.In practical communication systems,the wireless channel is not ideal,the severe inter-symbol interference(ISI)caused by the multipath effects makes it impossible for GSM monitoring system to recover the original symbol sequence directly from the captured signal.Therefore,it is necessary to equalize the signal to eliminate ISI,enables the monitoring system to perform voice listening quickly and accurately.Based on the parallel processing of FPGA and the characteristics of GSM signal,in order to solve the problem of fewer number of signal paths monitored simultaneously by traditional GSM monitoring system based on DSP,this paper presents a design and implementation scheme of soft decision equalizer based on FPGA which is suitable for monitoring system.Firstly,the synchronized signal is derotated to remove the complex number to simplify subsequent operations,then the channel parameters are obtained by channel estimation and the maximum signal-to-noise ratio signal is obtained by matched filtering.Finally,the data is sent to the MLSE equalization processor with soft decision function to get the decision sequence and soft value,and then sent to the subsequent module to complete the decoding and other operations.Soft decision information can effectively reduce the bit error rate of decoding and improve the overall performance of the monitoring system.At the same time,in the structure design,channel estimation and matched filtering modules are optimized,and the whole module is time-division multiplexed,which saves hardware logic resources,reduces delay and improves the working frequency of the whole system.By utilizing the parallel processing ability of the FPGA,the multi-channel design of the system enables a single development board to achieve the simultaneous equalization of up to 32 channels of signals,that is,to simultaneously monitor 32 channels of speech signals,and simultaneously process more signals than the traditional DSP-based monitoring system.This paper uses ModelSim to timing simulate each sub-module,and uses XC7K325 T card to carry out on-chip logic analysis,which verifies that the functions of each module meet the design requirements.Finally,the whole soft-decision equalizer is burned into the board card and connected to the GSM monitoring system for practical testing.By dialing the operator's customer service hotline with several test mobile phones,the monitoring system can successfully monitor the voice calls of multiple mobile phones,which verifies the correctness and feasibility of the design of the softdecision equalizer.