msk調制解調系統(tǒng)設計的外文翻譯

1、<p><b>　　畢業(yè)論文外文翻譯</b></p><p>　　題 目 MSK調制解調系統(tǒng)的設計 </p><p>　　學生姓名 趙 琪 學號 0813024034 </p><p>　　所在院(系) 物理與電信工程學院

2、 </p><p>　　專業(yè)班級  通信工程通信081 </p><p>　　指導教師 魏 瑞 </p><p>　　2012 年 3 月 15 日</p><p>

3、;<b>　　一：中文譯文</b></p><p>　　調頻寬頻傳輸MX589 GMSK調制移鍵控(msk)0.3 GMSK 1200</p><p><b>　　作者</b></p><p>　　Fred Kostedt, Engineer for MX-COM.</p><p>　　James

4、C. Kemerling, Engineer for MX-COM.</p><p><b>　　引言</b></p><p>　　隨著計算機的普及，數(shù)據(jù)傳輸在當今社會的需求也在不斷的增加，進而出現(xiàn)了傳輸數(shù)據(jù)的無線鏈路。二進制數(shù)據(jù)組成的“一零”和“零一”的過渡，產(chǎn)生了豐富的諧波頻譜內容，這并不適合射頻傳輸。因此，數(shù)字調制領域已得到了蓬勃的發(fā)展。從最近的標準可以看到，如

5、蜂窩數(shù)字分組數(shù)據(jù)（CDPD）和Mobitex*指定高斯濾波最小頻移鍵控（GMSK）的調制方法就是數(shù)字調制領域的先進技術。</p><p>　　GMSK是一種簡單而有效的數(shù)字調制的無線數(shù)據(jù)傳輸辦法。為了使我們更好的理解GMSK數(shù)字調制的方法，我們會分析MSK和GMSK的基本理論知識，以及如何運用GMSK數(shù)字調制方法來實現(xiàn)CDPD和Mobitex系統(tǒng)。</p><p>　　GMSK調制解調器降

6、低了系統(tǒng)的復雜性，從而降低系統(tǒng)的成本。但是，也有一些重要的實施細則需要加以考慮。本文將涵蓋其中的一些細節(jié)，把重點放在“典型”調頻收音機拓撲接口的單芯片中的基帶調制解調器的中頻/射頻部分。</p><p><b>　　背景</b></p><p>　　如果我們看一下傅里葉級數(shù)展開的一個數(shù)據(jù)信號，我們可以看到諧波延伸到無窮遠。當這些諧波被總結，他們給它的數(shù)據(jù)信號急劇轉變。

7、因此，一個過濾了的NRZ數(shù)據(jù)流用來調制射頻載波將產(chǎn)生相當大的RF頻譜的帶寬。當然，有嚴格的FCC的法規(guī)頻譜和這種制度，這種使用情況通常被認為是不切實際的。但是，如果我們在開始就移除高次諧波的傅立葉級數(shù)（即讓數(shù)據(jù)信號通過一個低通濾器），其過程中的數(shù)據(jù)將逐步急劇減少。這表明，premodulation過濾是一種在無線數(shù)據(jù)傳輸過程中減少被占領的頻譜的很有效的方法。除了緊湊的頻譜，無線數(shù)據(jù)調制方案必須在有噪音的情況下能獲得良好的誤碼率（BER）

8、性能。其性能也應該是線性的獨立的功率放大設備從而允許使用C類功率放大器。</p><p>　　為了滿足上述標準，學術領域提出的“數(shù)據(jù)傳輸”就是是滿載的調制策略。大部分的關于數(shù)據(jù)位或特殊階段的相位，頻率或振幅的術語。一些較顯著的技術列于表1。</p><p>　　調制技術 縮寫</p><p>　　頻移鍵控

9、 FSK</p><p>　　多層次的頻移鍵控 MFSK</p><p>　　連續(xù)相位頻移鍵控 CPFSK</p><p>　　最小頻移鍵控 MSK</p><p>　　高斯最小頻移鍵控

10、 GMSK</p><p>　　馴服調頻 TFM</p><p>　　相移鍵控 PSK</p><p>　　正交相移鍵控 QPSK</p><p>　　差分正交相移鍵控

11、 DQPSK</p><p>　　差分正交相移鍵控 DQPSK</p><p>　　正交調幅 QAM</p><p><b>　　表1 ：調制格式</b></p><p>　　表1中所列出來的每個調制格式適合特定的應用。一般情況下，計劃依賴于

12、兩個級別（如QAM調制，QPSK調制），需要有更好的信號信噪比（SNR）的超過兩個級別的與計劃類似的BER性能。此外，在無線環(huán)境里，多層次的計劃，通常需要更大的功率放大器，其線性超過兩個級別的計劃。事實上，GMSK使用兩個級別連續(xù)相位調制（CPM）的格式廣受歡迎和采納。另一點，其主張是，允許使用C級功率放大器（相對非線性）和數(shù)據(jù)傳輸速率接近頻道帶寬（取決于濾波器的帶寬和信道間隔）。</p><p><b&g

13、t;　　GMSK的理論基礎</b></p><p>　　在詳細討論GMSK之前，我們需要回顧MSK，從而導出GMSK。MSK是一個連續(xù)相位調制的調制方案，即調制載波在任何階段都沒有相位不連續(xù)性而且頻率的變化發(fā)生在載波的零通道。MSK的獨特之處是由于一個合乎邏輯的頻率0和1之間的關系：邏輯0的頻率和邏輯1的頻率的不同之處就是總是相當于一半的數(shù)據(jù)傳輸速率。換言之，調制指數(shù)為0.5的MSK，并定義為<

14、/p><p>　　m = _f x T</p><p><b>　　where,</b></p><p>　　f = |flogic 1 – flogic 0|</p><p>　　T = 1/bit rate</p><p>　　例如，1200比特每秒的MSK數(shù)據(jù)基帶信號可以由1200赫茲的邏輯頻率

15、1和1800赫茲邏輯頻率0組成（見圖1）。</p><p>　　圖1 ：1200波特率的MSK數(shù)據(jù)信號;a）NRZ數(shù)據(jù)，b）的MSK信號</p><p>　　基級的MSK，如圖1所示，在無線數(shù)據(jù)傳輸系統(tǒng)的數(shù)據(jù)傳輸速率與relatvely相比有低帶寬的頻道，這是一個強有力的手段。MX-COM加載裝置如MX429和MX469是單芯片解決方案的基帶MSK系統(tǒng)，將調制解調電路在一個芯片上。<

16、/p><p>　　另一種方法實現(xiàn)的MSK調制，可實現(xiàn)直接輸入NRZ數(shù)據(jù)為一個頻率調制器且其調制指數(shù)為0.5（見圖2）。這種做法實質上是相當于基級的MSK。但是，直接的辦法是壓控振蕩器的射頻/中頻部分，即在基帶的MSK的電壓頻率轉換發(fā)生在基級。</p><p>　　圖2 ：直接的MSK調制</p><p>　　MSK最根本的問題的是，其頻譜并非緊湊的，即不足以實現(xiàn)數(shù)據(jù)傳輸

17、速率接近射頻頻道的帶寬。一種頻譜的策略的MSK擴展揭示旁瓣遠遠高于數(shù)據(jù)速率（見圖4）。無線數(shù)據(jù)傳輸系統(tǒng)，需要更有效地利用射頻頻道帶寬，這是必要的，以減少MSK上旁瓣的能量。早些時候，我們提出一個簡單的手段來減少這一能量，即數(shù)據(jù)流提交給調制器之前，先經(jīng)過一低通濾波器（前調制過濾）。預調制低通濾波器必須有一個狹窄的帶寬即急劇截止頻率和很少的超脈沖響應。這就是高斯濾波器的特點，它有一個個脈沖響應的特點是古典高斯分布（鐘形曲線），如圖3所示，注

18、意沒有通過的或響鈴的。</p><p>　　圖3 ：高斯濾波器脈沖響應BT=0.3和BT=0.5</p><p>　　圖3描述了脈沖響應的高斯濾波器為BT=0.3和BT=0.5與濾波器的-3dB帶寬及數(shù)據(jù)傳輸速率有關即：</p><p>　　因此，對數(shù)據(jù)傳輸速率為9.6kbps和BT為0.3的高斯過濾器-3dB的截止頻率是2880Hz。</p><

19、;p>　　從圖3中，注意圖中，分散在BT=03的3位期和BT=0.5的2位期上的現(xiàn)象。引起的這種現(xiàn)象稱為碼間干擾（ISI）。BT=0.3的鄰近符號或比特率間的相互干擾比BT=0.5的鄰近符號或比特率間的相互干擾更加厲害。當GMSK的參數(shù)BT為1時就相當于的MSK。換言之，MSK并不是有意引起碼間干擾的。更大的碼間干擾使頻譜更加緊湊，讓解調更加困難。因此，從MSK發(fā)展到高斯調制濾波的MSK即考慮到頻譜密度的緊湊特點。圖4顯示了正?；?/p>

20、的譜密度的MSK和GMSK。請注意GMSK上減少的旁瓣能源。所以，這意味著GMSK與MSK相比通道間隔在鄰近頻道干擾處應當更加嚴格。</p><p>　　圖4：MSK和GMSK的功率譜密度</p><p><b>　　性能測量</b></p><p>　　GMSK調制器的性能通常是量化的測量信號的信噪比（SNR）和誤碼率（BER）。與信噪比的有

21、關的Eb/N0，其公式為：</p><p><b>　　Where</b></p><p>　　S=signal power</p><p>　　R=data rate in bits per second</p><p>　　=noise power spectral density(watts/Hz)</p&g

22、t;<p>　　=energy per bit</p><p><b>　　最新標準</b></p><p>　　GMSK已經(jīng)通過了許多的無線數(shù)據(jù)通信協(xié)議。其中具體的兩個GMSK調制系統(tǒng)分別是蜂窩數(shù)字分組數(shù)據(jù)（CDPD）和Mobitex。</p><p>　　CDPD使用閑置蜂窩傳輸語音頻道數(shù)據(jù)傳輸?shù)姆忾]空氣時間蜂窩系統(tǒng)，發(fā)送數(shù)據(jù)

23、速率在19.2kbps且使用參數(shù)BT為0.5。由于這種高數(shù)據(jù)速率，因而促進了30kHz信道間隔的蜂窩網(wǎng)絡形成和GMSK的頻譜保全。語音優(yōu)先于數(shù)據(jù)而且可以將數(shù)據(jù)傳輸中斷，迫使CDPD系統(tǒng)尋求新的閑置蜂窩通道。這可能被證明是對一個以19.2kbps的數(shù)據(jù)速率在一個高度擁擠且時間有限的地區(qū)執(zhí)行命令時，對其吞吐量形成的一個障礙。</p><p>　　CDPD也將被加入到現(xiàn)有的蜂窩基礎設施中，因此，它將會提供廣泛的覆蓋范圍

24、。覆蓋范圍大和易用性強似乎是CDPD系統(tǒng)的最大的優(yōu)點。在比預期慢部署CDPD的人有焦慮，也許是有點緊張了它的潛力。</p><p>　　與其競爭的專用的數(shù)據(jù)系統(tǒng)，如Mobitex并非是無足輕重的。雖然Mobitex與比CDPD（8kbps）相比具有較低的數(shù)據(jù)速率，它也并不與蜂窩語音傳輸分享其信道。但有幾個奧妙之處如這將是使最終用戶難以選擇最適合其需求的多少實際吞吐量潛力的系統(tǒng)。Mobitex的選擇8kbps的數(shù)據(jù)

25、速率且BT參數(shù)設置為0.3，這樣使其比CDPD承受了更嚴格的頻道間隔（12.5kHz），但由BT=0.3帶來的更大的符號間干擾限制了系統(tǒng)的耐噪聲性也讓信號產(chǎn)生了失真。狹窄的頻帶也限制了Mobitex單元間的頻偏程度。</p><p>　　CDPD和Mobitex的應用采用的對packetting數(shù)據(jù)的前向糾錯技術。圖5顯示了典型的數(shù)據(jù)包結構，這兩個系統(tǒng)進行比較。前向糾錯（FEC）有助于提高系統(tǒng)在信道條件不好時候的

26、吞吐量。</p><p>　　圖5：CDPD和Mobitex的典型的數(shù)據(jù)包結構</p><p><b>　　實施的考慮</b></p><p>　　設計一個GMSK調制器/解調器似乎是一個簡單的任務。大多數(shù)教科書本調制器作為一個“簡單”高斯濾波器級聯(lián)的壓控振蕩器。然而，在實踐中一般并不那么簡單。許多章節(jié)中一個典型的廣播電臺，如合成器，IF濾波器

27、，功率amplfier等已遠非理想的行為。獨特的是，這些合成器給GMSK調制提出了一個獨特的問題。由0或1組成的數(shù)據(jù)模式使頻譜響應擴展到附近的直流。大多數(shù)頻率合成器將不會呼應此低頻信號（一個典型的綜合有效的高通濾波器特性）。對合成器而言有兩種最常見的調制方式將大大有助于在不理想的情況下的實施，即“兩點調制”和“正交調制”。</p><p><b>　　兩點調制</b></p>

28、<p>　　兩點調制（見圖5）用分裂高斯過濾信號規(guī)避這一合成問題；其中一部分是針對振蕩器調制輸入的，其他部分是用來調節(jié)TCXO。TCXO不是頻率控制反饋環(huán)。因此，TCXO可被低頻部分的信號調制，其輸出有效地總結了頻率合成器里的信號調制壓控振蕩器。綜合信號的頻譜響應延伸到直流信號。</p><p>　　圖6 ：兩點調制的無線電框圖</p><p><b>　　I和Q的調制

29、</b></p><p>　　正交（I和Q）調制還可以有效地消除合成器的缺點。在I和Q調制中，高斯過濾數(shù)據(jù)信號分離成同相（I）和正交相位（Q）的組成部分。已調射頻信號是由混合的I和Q元件的頻率最多的射頻載波組成。合成器的作用現(xiàn)在已經(jīng)減少到僅僅改變載波頻率的選擇信道上了。達到正交調制的最佳性能的關鍵在于準確建立I和Q的組件。</p><p>　　圖7：I和Q電臺框圖</p&

30、gt;<p>　　基帶I和Q信號可以被用來創(chuàng)建全通相移網(wǎng)絡。為了所有頻率波段的利益這個網(wǎng)絡必須保持I和Q信號間90度的關系。</p><p><b>　　解調</b></p><p>　　解調GMSK信號需要多注意維護一個純正波形一樣調制的信號。選擇高斯預調制濾波器主要有三個原因：</p><p>　　1) 窄帶和急劇截止。<

31、;/p><p>　　2) 較低的超脈沖響應。</p><p>　　3) 保存的過濾器輸出脈沖地區(qū)。</p><p>　　第一個條件使GMSK調制的頻譜有效率，它也提高了其在解調時的抗噪聲性能時。第二個條件使GMSK低相位失真。這是一個重大的關切點，在接收器的信號解調到基帶時候，必須注意設計中的IF過濾，以保護這一特點。第三個條件，確保協(xié)調一致的信號。當然這是一個相當嚴格

32、的而且不是物理高斯濾波器容易實現(xiàn)的，一個的相位響應可以保持線形，因此能充分的被的相干解調。</p><p>　　在大多數(shù)系統(tǒng)上的上述目標的限制還包括：</p><p><b>　?。?） 數(shù)據(jù)速率</b></p><p>　　（2） 發(fā)送濾波器的帶寬（BT）的</p><p><b>　?。?） 頻道間隔<

33、;/b></p><p>　?。?） 允許相鄰信道的干擾</p><p>　?。?） 尖峰載波偏差</p><p>　?。?） Tx和Rx載波頻率精確度</p><p>　　（7） 調制器和解調器的線性度</p><p>　?。?） 接收中頻濾波器的頻率和相位特性。</p><p>　　這

34、些制約因素是所有部件的平衡，必須能夠提供可靠的GMSK系統(tǒng)。數(shù)據(jù)傳輸速率，TX帶寬參數(shù)BT，峰值載波偏差，和在接收器和發(fā)送器之間的載波頻率的準確性都是IF濾波器的寬度所必須的。IF濾波器應具有足夠的寬度，以適應上述參數(shù)中的最大的變化，使接收到的信號將不會進入到濾波器的周圍。IF濾波器的周圍在較高頻率組成的部分接收的數(shù)據(jù)可以引入過量的群時延（相位失真）。通順的IF濾波器應該有很少或根本沒有群時延，更多群時延的產(chǎn)生，在接收端就會產(chǎn)生錯誤的數(shù)

35、據(jù)速率導致誤碼率（BER）性能的降低。從經(jīng)驗和規(guī)則出發(fā)，支配的群延遲只有不到10％的時間是可以承受的。你高興的知道要獲得這樣的性能需要認識其他的一些影響誤碼率的因素：帶寬參數(shù)BT，信號強度，衰落等。還可以采取一些相均衡的措施是有助于減少群延遲的，但如果在中頻濾波器的設計步驟中得到控制，這些群延遲是可以避免的。</p><p>　　CDPD和Mobitex的標準是針對前面提到的兩個由MXCOM打造的設備制造的：MX

36、589和MX909。這兩個設備被設計成系統(tǒng)的傳送和接受基礎的接口。MX589是一種能以數(shù)據(jù)傳輸速率4kbps-40kbps速率傳輸以及參數(shù)BT為0.5或0.3的多用途的設備。該器件是在一個CMOS進程中實施的，可以在低電源電壓（3.0-5.0伏特）和小電流（1.5毫安@3.0V）中運作。數(shù)字數(shù)據(jù)接口是一個用于發(fā)送和接收的同步串行位。</p><p>　　MX909，也是一個在CMOS進程中實施的，是專門用于Mob

37、itex型系統(tǒng)的，通信的參數(shù)為0.3。其數(shù)據(jù)傳輸速率可達4kbps-19.2kbps，但應設置為8kbps實現(xiàn)Mobitex的兼容性。它可在電源電壓為4.5-5.5伏特和通常的3.0毫安情況下運行。數(shù)據(jù)接口是一個平行微處理器的I/O兼容總線，而且MX909包含的的所有電路需要執(zhí)行前向糾錯編碼和解碼的Mobitex格式。</p><p>　　基帶GMSK信號的解調使用的是高斯型低通濾波器和時鐘提取和參考電平補償電路

38、與數(shù)據(jù)提取電路。信號的傳輸部分首先經(jīng)過高斯低通濾波器相似。信號零交叉參考和時鐘頻率被提取然后使用峰值檢測電路和鎖相環(huán)去調諧。這一配合努力幫助改善了信號瓣上的噪音。一旦“鎖定”那電路中被提取出來的信號將被準確的解調出來。</p><p>　　峰值檢測電路可以調整直流信號1.5位的變化。這種“夾緊”模式被使用是在載波第一次被接收器檢測的時候。該PLL具有寬帶收購模式，至少可以在不到8點零交叉參考數(shù)時鎖定到一個信號。使

39、用這兩種方式，讓各種設備在接受端感知到載波之后以最快的速度啟動數(shù)據(jù)的解碼。兩個鎖相環(huán)及尖峰跟蹤電路的模式平常不會工作，一旦初期收購方式獲得了鎖他們將給予更好的抗噪性。GMSK基帶信號的性質要求在直溜系統(tǒng)附近需要有良好的響應就像提到的調制部分一樣。更隨機數(shù)據(jù)格局沒有大的直流分量，對任何highpass交流電耦合可能需要用于面對的基帶信號的GMSK調制器或解調器具有較不敏感的特性。MX589的誤碼率（BER）性能如圖8。此圖顯示不同設備的h

40、ighpass在誤碼率上的特性描繪。</p><p>　　Figure 8: BER performance of the MX589</p><p>　　這個數(shù)字數(shù)據(jù)從一個以8kbps和帶寬參數(shù)為0.3以及噪聲帶寬等于1比特率的靜態(tài)系統(tǒng)中得到。隨著噪聲帶寬等于比特率，并假設噪聲頻譜在基帶時候是平坦的，X軸在本質是Eb/N0。作為一種替代辦法，以完整的DSP實現(xiàn)，這兩種設備都提供符合成本效

41、益的解決方案和空間保守的調制解調要求的CDPD和MobitexGMSK系統(tǒng)。</p><p><b>　　總結</b></p><p>　　GMSK提供了一種簡單、高效的頻譜調制方法的無線數(shù)據(jù)傳輸系統(tǒng)（如CDPD和Mobitex）。MX-Com的MX589和MX909的基于GMSK調制解調器提供了單片機解決方案，并協(xié)助執(zhí)行了以GMSK系統(tǒng)為使用標準的調頻收音機拓撲。&

42、lt;/p><p>　　雖然的MX-COM組件集成的基帶調制解調器滿足了大多數(shù)的調制信號處理的需要，但一些關鍵的系統(tǒng)設計方面確不容忽視。例如，調制器的配置必須有一個平坦光譜響應下降到直流。此外，該接收器的相位響應，必須是線形的橫貫那些應當在注意的IF濾波器的數(shù)據(jù)所占的帶寬。按照這些建議再加上單芯片基帶調制解調器所保證性能優(yōu)良的誤碼率，將會是低能耗和成本低的系統(tǒng)。</p><p><b&g

43、t;　　二：外文資料原文</b></p><p>　　Transmission Wideband FM Modulator msk 1200 MX589 GMSK 0.3 GMSK</p><p>　　Introduction</p><p>　　The proliferation of computers in today's society

44、has increased the demand for transmission of data over wireless links. Binary data, composed of sharp "one to zero" and "zero to one" transitions, results in a spectrum rich in harmonic content that i

45、s not well suited to RF transmission. Hence, the field of digital modulation has been flourishing. Recent standards such as Cellular Digital Packet Data (CDPD) and Mobitex*specify Gaussian filtered Minimum Shift Keying (

46、GMSK) for their modulation met</p><p>　　GMSK is a simple yet effective approach to digital modulation for wireless data transmission. To provide a good understanding of GMSK, we will review the basics of MSK

47、 and GMSK, as well as how GMSK is implemented in CDPD and Mobitex systems.</p><p>　　GMSK modems reduce system complexity, and in turn lower system cost. There are, however, some important implementation deta

48、ils to be considered. This paper will cover some of these details, focusing on interfacing a single chip baseband modem to the IF/RF section of a "typical" FM radio topology.</p><p>　　Background<

49、;/p><p>　　If we look at a Fourier series expansion of a data signal we see harmonics extending to infinity. When these harmonics are summed, they give the data signal its sharp transitions. Hence, an unfiltered

50、 NRZ data stream used to modulate an RF carrier will produce an RF spectrum of considerable bandwidth. Of course, the FCC has strict regulations about spectrum usage and such a system is generally considered impractical.

51、 But if we start to remove the high frequency harmonics from the Fourier series </p><p>　　The academic field of “Data Transmission” is loaded with modulation strategies that attempt to meet the above criteri

52、a. Most involve translation of data bits or patterns into a particular combination of phase,frequency or amplitude. Some of the more notable techniques are listed in Table 1.</p><p>　　MODULATION TECHNIQUE

53、 COMMON ACRONYM</p><p>　　Frequency Shift Keying FSK</p><p>　　Multi-level Frequency Shift Keying MFSK</p><p>　　Continuous Phase Frequ

54、ency Shift Keying CPFSK</p><p>　　Minimum Shift Keying MSK</p><p>　　Gaussian Minimum Shift Keying GMSK</p><p>　　Tamed Frequency

55、Modulation TFM</p><p>　　Phase Shift Keying PSK</p><p>　　Quadrature Phase Shift Keying QPSK</p><p>　　Differenti

56、al Quadrature Phase Shift Keying DQPSK</p><p>　　Pi/4 Differential Quadrature Phase Shift Keying Pi/4 DQPSK</p><p>　　Quadrature Amplitude Modulation QAM</p>

57、<p>　　Table 1: Modulation Formats</p><p>　　Each of the modulation formats listed in Table 1 is suited to specific applications. In general, schemes that rely on more than two levels (e.g. QAM, QPSK) req

58、uire better signal to noise ratios (SNR) than two-level schemes for similar BER performance. Additionally, in a wireless environment, multi-level schemes generally require greater power amplifier linearity than two-level

59、 schemes. The fact that GMSK uses a two-level continuous phase modulation (CPM) format has contributed to its popularity.</p><p>　　GMSK Basics</p><p>　　Prior to discussing GMSK in detail we need

60、 to review MSK, from which GMSK is derived. MSK is a continuous phase modulation scheme where the modulated carrier contains no phase discontinuities and frequency changes occur at the carrier zero crossings. MSK is uniq

61、ue due to the relationship between the frequency of a logical zero and one: the difference between the frequency of a logical zero and a logical one is always equal to half the data rate. In other words, the modulation i

62、ndex is 0.5 for MS</p><p>　　m = _f x T</p><p><b>　　where,</b></p><p>　　f = |flogic 1 – flogic 0|</p><p>　　T = 1/bit rate</p><p>　　For example,

63、a 1200 bit per second baseband MSK data signal could be composed of 1200 Hz and 1800 Hz frequencies for a logical one and zero respectively (see Figure 1).</p><p>　　Figure 1: 1200 baud MSK data signal; a) NR

64、Z data, b) MSK signal.</p><p>　　Baseband MSK, as shown in Figure 1, is a robust means of transmitting data in wireless systems where the data rate is relatvely low compared to the channel BW. MX-COM devices

65、such as the MX429 and MX469 are single chip solutions for baseband MSK systems, incorporating modulation and demodulation circuitry on a single chip.</p><p>　　An alternative method for generating MSK modulat

66、ion can be realized by directly injecting NRZ data into a frequency modulator with its modulation index set for 0.5 (see Figure 2). This approach is essentially equivalent to baseband MSK. However, in the direct approach

67、 the VCO is part of the RF/IF section, whereas in baseband MSK the voltage to frequency conversion takes place at baseband.</p><p>　　Figure 2: Direct MSK modulation</p><p>　　The fundamental prob

68、lem with MSK is that the spectrum is not compact enough to realize data rates approaching the RF channel BW. A plot of the spectrum for MSK reveals sidelobes extending well above the data rate (see Figure 4). For wireles

69、s data transmission systems which require more efficient use of the RF channel BW, it is necessary to reduce the energy of the MSK upper sidelobes. Earlier we stated that a straightforward means of reducing this energy i

70、s lowpass filtering the data stream prio</p><p>　　Figure 3: Gausssian filter impluse response for BT = 0.3 and BT = 0.5</p><p>　　Figure 3 depicts the impulse response of a Gaussian filter for BT

71、 = 0.3 and 0.5. BT is related to the filter’s -3dB BW and data rate by</p><p>　　Hence, for a data rate of 9.6 kbps and a BT of 0.3, the filter’s -3dB cutoff frequency is 2880Hz.</p><p>　　Still r

72、eferring to Figure 3, notice that a bit is spread over approximately 3 bit periods for BT=0.3 and two bit periods for BT=0.5. This gives rise to a phenomena called inter-symbol interference (ISI). For BT=0.3 adjacent sym

73、bols or bits will interfere with each other more than for BT=0.5. GMSK with BT=_ is equivalent to MSK. In other words, MSK does not intentionally introduce ISI. Greater ISI allows the spectrum to be more compact, making

74、demodulation more difficult. Hence, spectral compactn</p><p>　　Figure 4: Spectral density for MSK and GMSK</p><p>　　Performance Measurements</p><p>　　The performance of a GMSK modem

75、 is generally quantified by measurement of the signal-to-noise ratio (SNR) versus BER. SNR is related to Eb/N0 by</p><p><b>　　Where</b></p><p>　　S=signal power</p><p>　　

76、R=data rate in bits per second</p><p>　　=noise power spectral density(watts/Hz)</p><p>　　=energy per bit</p><p>　　Recent Standards</p><p>　　GMSK has been adopted by man

77、y wireless data communication protocols. Two of the systems specifying GMSK modulation are Cellular Digital Packet Data (CDPD) and Mobitex.</p><p>　　CDPD uses the dead air time on cellular systems by sending

78、 data packets on idle cellular voice channels.Data is transmitted at 19.2kbps using a BT of 0.5. This high data rate is facilitated by the 30kHz channel spacing of the cellular network and the spectral conservation of GM

79、SK. Voice has priority over data and will interrupt data transmission, forcing the CDPD system to seek a new idle cellular channel. This could prove to be an obstacle to the throughput promised by its 19.2kbps data rate

80、wh</p><p>　　CDPD is being added to the existing cellular infrastructure and therefore promises to offer widespread coverage. The coverage and ease of adaptation appear to be the greatest strengths of the CDP

81、D system. The slower-than-expected deployment of CDPD has many people anxious and perhaps a bit nervous about its potential.</p><p>　　Competition from dedicated data systems such as Mobitex is not insignific

82、ant. While Mobitex has a lower data rate than CDPD (8kbps), it is not sharing its channels with cellular voice transmissions. Several subtleties such as this will make it more difficult for end users to select the system

83、 best suited to their needs by obscuring the actual throughput potential of the systems. Mobitex’s choice of 8kbps and a BT of 0.3 afford it a much tighter channel spacing (12.5kHz) than CDPD, but the greate</p>&

84、lt;p>　　Both CDPD and Mobitex employ forward error correction in their packetting of data. Figure 5 shows the typical packet structures of these two systems for comparison. Forward error correction (FEC) helps improve

85、the systems' throughput when less than ideal channel conditions exist.</p><p>　　Figure 5: Typical packet structures for CDPD and Mobitex</p><p>　　Implementation Considerations</p><

86、;p>　　The design of a GMSK modulator/demodulator appears to be a straightforward task. Most textbooks present the modulator as a “simple” Gaussian filter cascaded with a VCO. However, in practice it is generally not th

87、at simple. Many of the sections in a typical radio such as the synthesizer, IF filter, power amplfier, etc. have far from ideal behavior. In particular, the synthesizer presents a unique problem for GMSK modulation. Data

88、 patterns consisting of several consectutive ones or zeros have a sp</p><p>　　Two point modulation</p><p>　　Two point modulation (see Figure 5) circumvents this synthesizer problem by splitting

89、the Gaussian filtered signal; one portion is directed to the VCO modulation input, the other portion is used to modulate the TCXO.The TCXO is not in the frequency control feedback loop. Hence, the TCXO can be modulated b

90、y the low frequency portion of the signal, and its output is effectively summed with the signal modulating the VCO in the synthesizer. The composite signal has a spectral response extending down</p><p>　　Fig

91、ure 6: Two point modulation radio block diagram</p><p>　　I and Q modulation</p><p>　　Quadrature (I and Q) modulation can also be effective in eliminating synthesizer shortcomings. In I and Q mod

92、ulation, the Gaussian filtered data signal is separated into in-phase (I) and quadrature phase (Q) components. The modulated RF signal is created by mixing the I and Q components up to the frequency of the RF carrier, wh

93、ere they are summed together. The role of the synthesizer has now been reduced to merely changing carrier frequency for channel selection. The key to optimum performance w</p><p>　　Figure 7: I and Q radio bl

94、ock diagram</p><p>　　Baseband I and Q signals can be created by using an all-pass phase shifting network. This network must maintain a 90 degree phase relationship between the I and Q signals for all frequen

95、cies in the band of interest.</p><p>　　Demodulation</p><p>　　Demodulation of the GMSK signal requires as much attention to the preservation of an unadulterated wave form as does modulation of th

96、e signal. The choice of a Gaussian shaped pre-modulation filter was made for three main reasons[1]:</p><p>　　1) narrow bandwidth and sharp cutoff</p><p>　　2) lower overshoot impulse response<

97、/p><p>　　3) preservation of the filter output pulse area.</p><p>　　The first condition gives GMSK modulation its spectral efficiency. It also improves its noise immunity when demodulating. The seco

98、nd condition affords GMSK low phase distortion. This is a major concern when the receiver is demodulating the signal down to baseband, and care must be taken in the design of the IF filtering to protect this characterist

99、ic. The third condition ensures the coherence of the signal. While this is quite strict and not realizable with a physical Gaussian filter, the phase </p><p>　　In most systems the constraints on the above go

100、als also include</p><p>　　- Data Rate</p><p>　　- Tx filter bandwidth (BT)</p><p>　　- Channel Spacing</p><p>　　- Allowable adjacent channel interference</p><p

101、>　　- Peak carrier deviation</p><p>　　- Tx and Rx carrier frequency accuracy</p><p>　　- Modulator and Demodulator linearity</p><p>　　- Rx IF filter frequency and phase characteris

102、tics.</p><p>　　These constraints are all part of the balance that must be struck to provide a robust GMSK system. The data rate, Tx BT, peak carrier deviation, and carrier frequency accuracy between receiver

103、 and transmitter all contribute to the necessary width of the IF filter. The IF filter should have sufficient width to accommodate the maximum variations in the above parameters so that the received signal will not run i

104、nto the skirts of the filter.The skirts of the IF filter can introduce excessive amounts</p><p>　　The CDPD and Mobitex standards mentioned earlier are addressed by two devices manufactured by MXCOM:the MX589

105、 and the MX909. Both devices are designed to interface to the transmitter and receiver of a system at baseband. The MX589 is a versatile device capable of operating at data rates from 4kbps-40kbps and at BT’s of 0.5 or 0