odfm系統(tǒng)中基于認(rèn)知無線電的頻譜檢測的研究畢業(yè)論文外文翻譯(編輯修改稿)
2024-09-06 09:33 本頁面
【文章內(nèi)容簡介】 the probability is. However, with the increase of SNR, wrong protection even no protection will appear。 to false alarm probability, the larger of the length is, the smaller of the perceived error verdict is, and no protection will also appear if the threshold selection is inappropriate. For effective detection, appropriate parameters need to be selected including spectral detection time and setting the threshold. [5] presented optimized promised criterion, obtained high transmitting efficiency by traversing d, and made total false detecting probability Pe=1+PfPdminimum.III. CYCLOSTATIONARY DETECTIONCyclostationary signal is a random signal whose statistical characteristics cyclically changes over time, including firstorder (mean), second order (correlating function) and higherorder (higherorder cumulants) cyclostationary.For the random signal X(t), when it meets cyclostationary properties, the mean and autocorrelation functions are both cyclical, and the cycle is the same as the signal cycles. The selfcorrelation function of s(t) is defined as Rx=(t+τ2,tτ2), where T is the cycle. If it is shown into Fourier series, Fourier coefficient can be obtained as follows, which is called cyclic autocorrelation function (CAF).RXατ=1/TT2T2Rx(t+τ2,tτ2)exp?(2jπα)dtAccording to WienerKhintchine relations, the Fourier transform of CAF can be obtained and defined as spectrum correlation function (SCF).SXαf=∞∞RXατexp?(2jπft)dtIn project, the discrete form of CAF is obtained using its discrete form as follows.RXατ=1/Tn=0N1x(n+τ2)x*(nτ2)exp?(2jπfαn)Where α=T/n(n=0,1,2,……) is cycle frequency, which is important in cyclostationary detection. The cycle frequency of a cyclostationary signal includes zerocycle frequency and nonzero cycle frequency. Where, the zero cycle frequency corresponds to the smooth part of signal, and only nonzero cycle frequency portrays the cyclostationarity of signal.The carrier of OFDM signal has multiple α loop frequency, and meets the secondorder cyclostationarity properties. Cyclostationary detection of OFDM signals is done using the method of evolutionary optimal detection presented by Dandawate [6]. A delayed matrix is constructed as statistical detecting amount, then correlating function matrix is estimated using matrix, which is appropriate for the situation that there is no priori information of primary user signal in the receiver, and cyclostationary feature of detecting signal is obtained to verdict whether there is protecting signal. However, the method can be optimized in terms of its basic assumptions due to its high plexity, chooses and obtains the most effective time delay parameterτs, then obtains the maximum amplitude RXατ. According to Dandawate theory, detecting probability Pd∝|RXατ| detects all of the cycle frequency and sums the detectable quantity by the sum criteria, where M is the number of random variables, and meets the χ2distribution.Pd=1Q(λi=1Lμ(τs(i))2i=1Lμ(τs(i)))The simulated signal abovementioned is detected circularly, then the coordinate axes are normalized and the results are as follows.Figure 4. Twodimensional autocorrelation function of signalFigure 5. Relationship between delay and autocorrelation functionThe two pictures above are twodimensional picture of autocorrelation function of the signal RXατ, varied from cycle frequency α and delayτ and α = 0 c
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