DirectSequenceSpreadFrequencyTechniques直接序列扩频技术.docx

Experiment onDirect Sequence Spread Frequency TechniquesStudent name:Tianhao DengBUPT numberlass number:2009215109Abstract: This experiment is designed to learn the fundamentals of the directsequence spread spectrum and code division multiple address techniques.Experiment PurposeGrasp the principle of direct sequence spread spectrum(DSSS)Grasp the method of DSSS modulation and demodulation.Contents of the experimentIn this experiment, the following items need to be done.(1)Write a program to generate the m sequence with p(x) = 1+x5+x7+x8+x9 +x13+x15.(2)Draw the autocorrelation of the generated m-sequence.(3) Generate a signal;(4) Write a program to encode the generated signal by an A-law PCM encoder(please refer to books to find how an A-law PCM encoder works);(5) Modulate the encoded signal by QAM with center frequency fc=256Hz;(6) Demodulate the modulated signal;(7) Decode the demodulated signal, and compare the decoded signal with the generated signal.Principle of the experiment(1)The performance of DSSSA direct sequence spread spectrum modulator and demodulator is shown inFig. (1), where a pseudorandom sequence generator generates the spreading sequence. Fig 1.Direct sequence spread spectrum system(2)Generator a m-sequenceAn m-sequence generator is usually adopted as the pseudorandom sequence generator, which is a linear feedback shift registers (LFSR) output. A polynomialis used to describe the connections of the linear feedback shift registers. For example, the polynomial is corresponding to the LFSR of the Fig. (2), wheredenotes a connection.Fig 2.Linear feedback shift register(3)Autocorrelation of the generated m-sequenceThe periodic autocorrelation of a m-sequence is In MATLAB, the autocorrelation can be obtained the function [ACF, Lags, Bounds] = autocorr(Series) where series is the input sequence and ACF is the autocorrelation of the sequence. The code is as follow :autocorr(mSequence);(4)A-law PCM encoderA-law compression is a logarithmic compr