A Research On The Long PN Code Blind Acquisition In Non-cooperative Communication

Due to the obvious performance of PN (pseudo-noise) sequence, the spread spectrum signalhas high anti-interference, strong confidentiality and low interception probability, which improvethe security and reliability of communication system but also bring severe difficulties to thenon-cooperative communication. Therefore sequence blind acquisition plays an important role innon-cooperative spread spectrum communication receiver. At present, sequence bind acquisitionalgorithms can be classified into two categories: the sequential generator mechanism estimationacquisition algorithms and the sequential estimation acquisition algorithms, when thenon-cooperative receiver has no PN sequences knowledge of the transmitter. But unfortunatelyas to long PN sequence bind acquisition in the environment of non-cooperative communication,the sequential estimation algorithms undoubtedly cannot be used for the limited receivedsequences. Nowadays generator mechanism estimation algorithms still face with three mainproblems summarized as following. First, the error-resilient strength of nonce algorithms are nottoo strong to deal with the higher bit error rate of received sequences, which makes the successprobability of blind acquisition decrease sharply. Second, these methods are not ultimatelysuitable to real-time receiving requirement because of heavy computations, which slow down theacquisition speed. Third, the improvement of acquisition speed leads to a vast decrease ofcapture success rate, which makes the performance of fast acquisition methods quite poor.According to these problems, this dissertation would research on the real-time acquisition oflong PN sequence in non-cooperative communication.Two novel soft generator polynomial estimation methods are proposed from the perspectiveof sequences relationship, both of which can solve the high-order polynomial estimationproblems efficiently. And then, a novel polynomial estimation algorithm using forward andbackward joint Triple Correlation Function is proposed to improve error tolerance and reducecomputation complexity further based on the obvious auto-correlation property of PN sequence.To solve the problems of long code fast acquisition, a novel fast sequential acquisition methodbased on frequency subset is proposed at last. The chief content of this article is as follows.1. A novel algorithm of soft fast generator polynomial estimation based on weighting WalshHadamard Transform (SWWHT-PE). is proposed The mathematical tool of WHT is used toestimate polynomial and three ameliorations are taken out to deal with the problems of directWHT estimation, such as code soft decision, weighting Walsh Hadamard matrix, and coderelationship change. Firstly, the hard decision WHT is ameliorated to soft decision WHT basedon sequence soft information, which would improve its performance of error tolerance. Then, tocompensate the estimation performance loss of multi-taps polynomial, the rows of WalshHadamard matrix are weighted by different coefficients which break its element balance ofGF(2). Lastly, the estimated multi-taps polynomials are changed to few-taps polynomials by thesequence relationship choose. The simulation results show that SWWHT-PE is approximatelyirrelevant with the tapped number and the order of estimated polynomial, and its error toleranceincreases1.5dB than WHT. 2. A novel algorithm of generator polynomial estimation based on reliability accumulation(RA-PE) is proposed. The reliability of received sequence is measured by its soft information,and the state of low reliability sequence is revered. Then, the reliability set of polynomialsestimated by received sequences are estimated by the sequences reliability, which areaccumulated to estimate the most exact polynomial, by which the error tolerance of RA-PE isimproved. The simulation results show that RA-PE is entirely irrelevant with the tapped numberof generator polynomial, and has a good performance to estimate17-order polynomials when thesequence BER is18%. Larger the accumulation is, better the performance of error tolerance is.3. A novel algorithm of the generator polynomial estimation based on forward andbackward joint Triple Correlation Function (FBTCF-PE) is proposed, where TCF peaks are usedto estimate polynomial. According to the equivalent relationship of TCF peak coordinates andsequence phase differences, the computation complexity is decreased by transforming the2-dimensional TCF peak search to1-dimensional sequence acquisition. At the same time, theforward-backward jointed TCF peaks which are2times more than TCF peaks in the some caseare obtained by cyclic correlation operation to improve the estimation precision. The simulationresults show that FBTCF-PE is irrelevant with the tapped number of generator polynomial, andhas a good estimation performance for17-order polynomials of m sequence even when thesequence BER is37%in contrast to its low computation complexity.4. A novel fast sequential acquisition method based on frequency subset is proposed.According to the sequential time-frequency relationship that a bit of frequency informationinclude entire sequences in time domain, the acquisition model based on frequency subset isestablished, where long period sequences in time domain are transformed to frequency subset.Then, two frequency subset schemes are designed: frequency intersecting and frequencysampling (FSSA). The frequency intersecting method uses intersecting frequency subset toestimate while the frequency sampling method uses sampling frequency subset. The key is thatphase weighting factors imported by frequency sampling suppress the correlation backgroundnoises. The simulation results show that the average capture success rate of FSSA is probably50%larger than XFAST. Lastly, a fast sequential acquisition method based on time andfrequency combined domain (TFSA) is proposed, where both the local average sequences andreceived average sequences are used to do frequency sampling acquisition. It can effectivelysolve the problem that bit improvement of acquisition speed leads to vast decrease of capturesuccess rate. The simulation results show that the acquisition speed of TFSA is15times fasterthan time domain fast acquisition methods when the sequence BER is37%and the capture rangeis221. TFSA is quite suitable to long PN sequence acquisition in low SNR environment.