A Signal Modulation Method of Direct Sequence Spread Spectrum Based on Modulated m-sequence
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摘要: 为提高直接序列扩频通信系统的频谱利用率,该文提出在m序列周期等于信息比特位宽整数倍情况下的直接序列扩频信号——调制m序列的信号调制与解调方法(m-DSSS)。首先结合调制m序列相关性数值仿真结果验证m-DSSS信号捕获可行性,然后根据信息比特的排列组合情况,设计一种基于多通道的m-DSSS信号调制和解调方法,并对m-DSSS信号通过加性高斯白噪声信道后的抗干扰能力建立数学模型。最后在相同频谱利用率条件下,与码移键控(CSK)调制方式进行了仿真对比试验。仿真结果表明,m-DSSS信号不仅比CSK信号具有更低的副峰,且当载波采用BPSK调制时,m-DSSS还可通过判断极性来辅助信号捕获,在误码率优于1e-3条件下,m-DSSS信号比CSK信号有2 dB以上优势,验证了m-DSSS调制用于直扩系统的可行性。Abstract: To raise the spectrum utilization, a modulation and demodulation method called modulated Direct Sequence Spread Spectrum (m-DSSS) is proposed, which assign the period of m-sequence to be equal to the integer multiple of data bit. Firstly, the feasibility of m-DSSS signal acquisition is verified by numerical simulation results of correlation. Then, according to the permutation of information bits, a multi-channel signal modulation and demodulation method for m-DSSS is designed, and the mathematical model reacting the anti-interference ability of passing through the additive Gaussian white noise channel is established. Finally, the simulation and contrast test are carried out with Codes Shift Keying (CSK) modulation under the same spectrum utilization. The simulation results show that m-DSSS signal not only has lower side peak than CSK signal but also can assist signal acquisition by judging polarity when BPSK modulation is adopted. When bit error rate is better than 1e-3, m-DSSS signal has more than 2 dB advantage than CSK signal, which verifies the feasibility of m-DSSS used in DSSS system.
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表 1 10级调制m序列与信息比特对应关系
信息比特组合 第1组码片与原
m序列对应码片
的相位关系第2组码片与原
m序列对应码片
的相位关系第3组码片与原
m序列对应码片
的相位关系000 反相 反相 反相 001 反相 反相 同相 010 反相 同相 反相 011 反相 同相 同相 100 同相 反相 反相 101 同相 反相 同相 110 同相 同相 反相 111 同相 同相 同相 表 2 8通道条件下信号捕获仿真试验参数
参数类型 用户1 用户2 m序列生成多项式 x10+x3+1 x10+x4+x3+x1+1 m-DSSS每个周期调制比特数 3 3 CSK循环移位次数 8 8 CSK每次循环移位码片数 128(7次)、127(1次) 128(7次)、127(1次) 载波调制方式 BPSK BPSK 载波频率(Hz) 2500 2500 信号采样频率(Hz) 100000 100000 信号延迟(扩频码片数) 200 300 信噪比(dB) –15 –15 信号数据长度(扩频码片数) 8184 8184 相关运算步进长度(扩频码片数) 1 1 -
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