mobilecommunications-ch2調(diào)制技術(shù)-資料下載頁
2025-05-11 22:58本頁面
【正文】 的 自相關(guān) 特性，有處處為零的 互相關(guān) 特性； ?有盡可能大的復(fù)雜度 ?PN序列的 長度應(yīng)能滿足擴(kuò)展帶寬的需要 ?PN序列應(yīng)具有近似白噪聲的頻譜性質(zhì)，任何時(shí)間上不同的兩段噪聲都不完全相同 擴(kuò)頻地址碼的特性 常用 PN序列： M序列、 Gold序列、 Walsh正交碼 偽隨機(jī)（ PN）序列 50 CDMA多址碼 ? CDMA系統(tǒng) ：用戶、信道和基站均依靠 擴(kuò)頻多址碼 識(shí)別 ?基站的識(shí)別 不同相移的 PN序列（短 PN碼） ?信道的識(shí)別 正交的 Walsh函數(shù) ?用戶的識(shí)別 周期足夠長的 PN序列（長 PN碼） ?公共掩碼：移動(dòng)臺(tái)電子序列號(hào)（ ESN） 專用掩碼：移動(dòng)臺(tái)識(shí)別號(hào)（ MIN） 偽隨機(jī)（ PN）序列 51 CDMA 擴(kuò)頻技術(shù) 52 CDMA 擴(kuò)頻技術(shù) 53 CDMA 擴(kuò)頻技術(shù) 54 CDMA 擴(kuò)頻技術(shù) 55 擴(kuò)頻系統(tǒng)的優(yōu)點(diǎn) ?抗干擾能力強(qiáng)，可抗白噪聲、人為干擾、多徑干擾等 ?保密性好。若不知 PN序列生成方法就難以獲取原始信息 ?隱蔽性好。擴(kuò)頻信號(hào)近似白噪聲，難以檢測，寬帶擴(kuò)頻信號(hào)功率譜密度很低，難以監(jiān)聽 ?易于實(shí)現(xiàn)大容量多址通信 CDMA 擴(kuò)頻技術(shù) 56 Modulation Techniques for Mobile Radio Modulation is the process of encoding information from a message source in a manner suitable for transmission. moudulation signal— baseband signal moudulated signal— bandpass signal The basic form of the three modulation methods used for transmitting digital signals are: Amplitude shift keying Frequency shift keying Phase shift keying 57 ?Spreading spectrum increases the modulation rate (chip rate) in order to spread the energy of the information signal on a wide frequency band without modifying the data rate. ?To be able to perform the despreading operation, the receiver must not only know the sequence used to spread the data signal, but the spreading sequence of the received signal and the locally generated spreading sequence must be synchronized. Spreading spectrum Technology 58 ?All users occupy the same frequency at the same time. The receiver will ‘hear’ all the transmitters’ signals mixed together but using the correct code sequence can decipher the required transmission channel. ?Spreading sequence are actually unique streams of 1 and 1 which pose the codes associated to a user. Therefore, users are discriminated thanks to the spreading codes. Spreading spectrum Technology 59 ?In the receiver, the posite signal is correlated with a replica of the code used to spread the data. Thus, low crosscorrelation between the desired and interfering users is important to suppress the multiple access interference. Good autocorrelation properties are required to reliably separate the multipath ponents. Spreading spectrum Technology
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