【正文】 this noise represents the interference generated by other base stations that are using the same frequency band.In the receiver section, the ining signal is first filtered by the Receive Filter block, which implements a filter matched to the transmit filter. The filters in this demo are designed to maximize the signal power within the desired frequency band.Then the filtered signal is sent to the IS95A Rev Ch Detector library block, which contains the reverse channel rake receiver. The rake receiver consists of three rake fingers that are set to different delays to handle up to three multipaths. Each active rake finger performs the despreading of the input data with the short PN sequence, followed by despreading with the long code. This is followed by the correlation with the entire set of 64 Walsh codes. The energies in the I and Q ponents are added, and the results from the fingers are added together. This is processed by the Walsh demodulator, which generates decisions in groups of 6 bits, the size used for modulation. The Walsh demodulator outputs both soft decisions and bipolarvalued hard decisions. both are gated by the data burst randomizer signal.The Error Rate Calculation block pares the transmitted bits and the received decisions, and produces the raw (that is, without channel coding) bit error rate. This rate is displayed in the model window during the simulation.Visible Results of the DemoThe demo shows the raw bit error rate (BER) for the reverse channel. The BER depends on the channel conditions and the number of rake receiver fingers active. As the signaltonoise ratio or the channel conditions are changed, the effect of these on the raw BER can be seen by running the model for these different conditions.Changing Demo ParametersThis section suggests some ways that you can alter the parameters in the demo after you understand the model.The simulation duration in this demo is set to 2 seconds. Because the frame duration in IS95A is second, this model processes 100 frames. This gives a sufficient time for the BER measurements to converge at reasonable SNR settings. To run the simulation for a longer or shorter time, select Parameters from the Simulation menu and change the Stop time to times the number of frames you want to simulate.To change the data rate, doubleclick on the Data Source icon. Then doubleclick on the Mobile Station Transmitter Data Rate icon and change the Data Rate parameter. Choices are Full, Half, Quarter, and OneEighth rates.To change the long code mask, doubleclick on the Spreading and Modulation icon. Then doubleclick on the IS95A Rev Ch Walsh Modulation and Spreading icon, and enter a new value for the Long code mask parameter. This parameter can be any nonnegative integer less than 2421. The block uses the binary representation of this number to generate the code. The value for the Long code mask parameter is also required in the IS95A Rev Ch Detector library block. Experiment wit。 on the receiver side, that sequence is decoded by simply integrating for each channel: for channel k, the information bit is retrieved from the sign of the integral:Reverse Link In the reverse link of IS95, Walsh codes are not used in that manner but simply encode bits in a 64ary encoding scheme: that s each 6bit sequence is mapped to a 26 = 64 bit Walsh code.In further evolutions like IS2000 and IS856, the reverse link multiplexes several channels similarly to the forward link, and Orthogonal properties of the Walsh codes provide the multiplexing scheme:Short CodeShort codes are bit sequences with very specific autocorrelation functions. IS95 uses a 215 1 = 32767 chip short code. For a first example let us use here a 22 1 = 3 bit long short code: 100.Permutated, that short code has the following property: if +1 is counted for every bit of the permutated sequence that is identical to the bit of the original sequence, and 1 is counted when that bit is different, every permutated sequence totals 1, whereas the original sequence obviously totals its length .If we note S,S(n) the above putation, we have for a short code sequence:This is a remarkable property of autocorrelation between bit sequences that is used in CDMA. As another example, verify the following two short code sequences (of length 23 1 = 7) verify the same property as the table above: s1 = 0011101 and s2 = 1110010. These two sequences are used for Gold codes in other CDMA standards. And another short code (of length 25 1 = 31) is s3 = 0000101011101100011111001101001.IS95 uses a much longer such sequence (215 1 = 32768 chips) which provides many possible sequences orthogonal to one another, used to differentiate between sectors on the forward link.Long CodeLong code is a sequence of bits used for its pseudoorthogonal properties。參考文獻(xiàn)[1] 　樊昌信等. 通信原理(第四版) [M] .北京:國(guó)防工業(yè)出版社，2000. 162 164 ，301 303.[2] 　郭梯云,等. 移動(dòng)通信[M] . 西安:西安電子科技大學(xué)出版社,2001. 293 298.[3] 　鄧華,等. MATLAB 通信仿真及應(yīng)用實(shí)例詳解[M] . 北京:人民郵電出版社,2003. 207 211 ,224 228 ,314 318.[4] 　胡鋼,等. 基于MATLAB 的遠(yuǎn)程數(shù)據(jù)傳輸系統(tǒng)的仿真研究[J ] .計(jì)算機(jī)仿真. 2001 , (5) :73 75.[5] 　徐素妍,曹坤梅. 基于MATLAB 語(yǔ)言的現(xiàn)代通信仿真分析[J ] .計(jì)算機(jī)應(yīng)用. 2001 , (8) :52 53.[6] 　邱剛. 基于完全互補(bǔ)碼的CDMA 通信系統(tǒng)仿真研究[D] .西安: 西南交通大學(xué), 2002.[7] 　Haykin S. Communication Systems (4t h Edition) [M] . Bei2jing : Publishing House of Elect ronics Indust ry , 2004.[8] 　Smit h P J , Shafi M , Gao H. 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