雙工無線語音數(shù)據(jù)傳輸系統(tǒng)外文翻譯-預(yù)覽頁
【正文】 ion in order to extract the message from the sinusoidal carrier. Since the signal demodulation is performed in the presence of additive noise and possibly other signal distortion, the demodulated message signal is generally degraded to some extent by the presence of these distortions in the received signal. As we shall see, the fidelity of the additive noise, the type and strength of any other additive interference, and the type of any nonadditive interference. Besides performing the primary function of signal demodulation, the receiver also performs a number of peripheral functions, including signal filtering and noise suppression. Digital Communication System An electrical munication system in rather broad terms based on the implicit assumption that message signal is a continuous timevarying waveform. We refer to such continuoustime signal waveforms as analog sources. Analog signal can be transmitted directly via modulation over the munication channel and demodulated accordingly at the receiver. We call such a i munication system an analog munication system. Alternatively, an analog source output may be converted into a digital form and the message can be transmitted via digital modulation as a digital signal at the receiver. There are some potential advantage to transmitting an analog signal by means of digital modulation. The most important reason is that signal fidelity is better controlled though digital transmission than analog transmission. In particular, digital transmission allows us to regenerate the digital signal in longdistance transmission, thus eliminating effects of noise at each regeneration point. In contrast, the noise added in analog transmission is amplified along with the signal when amplifiers are used periodically to boost the signal level in longdistance transmission. Another reason for choosing digital transmission over analog is that the analog message signal may be highly redundant. With digital processing, redundancy may be removed prior to modulation, thus conserving channel bandwidth. Yet a third reason may be that digital munication systems are often cheaper to implement. In some applications, the information to be transmitted is inherently digital。 ., the output of a source is not deterministic. Otherwise, there would be no need to transmit the message. A transducer is usually required to convert the output of a source into an electrical signal that is suitable for transmission. For example, a microphone serves as the transducer that converts an acoustic speech signal. At the destination, a similar transducer is required to convert the electrical signals that are received into a form that is suitable for the user。 而電子郵件和傳真則使得文字信息的遠(yuǎn)距離快速傳遞成為現(xiàn)實(shí)。 1894 年，噢利弗 洛奇（ Oliver Lodge）發(fā)明了一種稱為金屬屑檢測器的靈敏器件，用于檢測無線信號(hào)，他在英國的牛津使用這種靈敏器件演示了在 150 碼的距離上進(jìn)行無線通信；馬克尼（ Marconi）的名字則與無線電報(bào)密切相關(guān)，他于1895 年演示了在大約 2 英里距離上進(jìn)行無線信號(hào)傳 輸， 1897 年馬可波羅又申請了無線電報(bào)系統(tǒng)的專利，并建立了無線電報(bào)和信號(hào)公司。三極管的發(fā)明使得無線電廣播在 20 世紀(jì)初期成為現(xiàn)實(shí)， 1920年在匹茲堡的 KDKA 無線電臺(tái)將調(diào)幅（ AM）廣播信號(hào)送進(jìn)了空中，從那時(shí)起，調(diào)幅無線電廣播在英國及世界各地迅 速發(fā)展。 第一個(gè)電視系統(tǒng)由 于 1929 年在美國建立并進(jìn)行演示試驗(yàn)，倫敦的英國廣播公司（ BBC）于 1936 年開始商業(yè)電視廣播，五年后聯(lián)邦通信委員會(huì)（ FCC）批準(zhǔn)了美國 的電視廣播。產(chǎn)生信息的任何一個(gè)信源，都有一個(gè)基本的特征，即它的輸出是以概率參量描述的，也就是說一個(gè)信源的輸出是不確定的，否則信息傳輸就失去了意義。 通信系統(tǒng)的核心由三個(gè)部分構(gòu)成，即發(fā)信機(jī)，信道和接收機(jī)。例如，在無線電和電視廣播中，聯(lián)邦通信委員會(huì)（ FCC）指定的各個(gè)發(fā)射臺(tái)的頻率范圍，因此發(fā)信機(jī)必須將信息信號(hào)轉(zhuǎn)換到合適的頻率范圍來發(fā)送，以便與分配給此發(fā)信機(jī)的頻率相匹配。通常，調(diào)制需要使用信息信號(hào)來系統(tǒng)地改變正弦載波的振幅，頻率或相位分量。調(diào)制類型的選擇主要基于幾個(gè)因素，如所分配的帶寬，信號(hào)在信道傳輸中遇到的噪聲及干擾的類型，在發(fā)送前進(jìn)行信號(hào)放大所能采用的電子器件等等。 通信信道是一種物理介質(zhì)，用于將來自發(fā)信機(jī)的信號(hào)發(fā)送到接收機(jī)。來自信道上其他 用戶的干擾則是另一種形式的加性噪聲，這種干擾在無線和有線通信系統(tǒng)中都會(huì)經(jīng)常出現(xiàn)。 雙工無線語音數(shù)據(jù)傳輸系統(tǒng)的設(shè)計(jì) 在設(shè)計(jì)一個(gè)通信系統(tǒng)時(shí)，系統(tǒng)設(shè)計(jì)者應(yīng)從統(tǒng)計(jì)分析的角度，對在物理信道上的信號(hào)失真建立數(shù)學(xué)模型。 接收機(jī)。 除了完成信號(hào)解調(diào)這一主要功能，接收機(jī)還完成其他一系列的外圍功能，包括信號(hào)濾波和噪聲抑制等。 除此之外，模擬信源的輸 出還可以轉(zhuǎn)換成數(shù)字形式，消息可以通過數(shù)字調(diào)制后發(fā)送，并在接收端解調(diào)成數(shù)字信號(hào)。對于模擬信號(hào)選擇數(shù)字傳輸方式的另一個(gè)原因是，模擬信號(hào)有可能高度冗余，通過數(shù)字處理，可以在調(diào)制前消除冗余度，從 而壓縮帶寬。 在數(shù)字通信系統(tǒng)中，發(fā)信機(jī)和接收機(jī)完成的功能還應(yīng)包括：發(fā)射端消息信號(hào)的離散化，以及接收端消息信號(hào)的合成或內(nèi)插。實(shí)際上，我們希望使用盡可能少的二進(jìn)制數(shù)來表示信源輸出（消息）。信道編碼的目的是受控方式在二進(jìn)制信息序列中引入一些冗余度，以便接收機(jī)用于提高接收數(shù)據(jù)的可靠性，改善接收信號(hào)的保真度。發(fā)送信息中沒有冗余度時(shí)，解調(diào)器必須判定在任意給定的時(shí)間間隔內(nèi)，發(fā)送的是M 個(gè)波形中的哪一個(gè)
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