畢業(yè)論文翻譯(無(wú)線局域網(wǎng)接收器的高效自動(dòng)增益控制算法和結(jié)構(gòu))

1、無(wú)線局域網(wǎng)接收器的高效自動(dòng)增益控制算法和結(jié)構(gòu)il-gu lee *, sok-kyu lee下一代無(wú)線局域網(wǎng)研究團(tuán)隊(duì)，電子和電信研究院，柯亭161棟，顧儒城區(qū)，大田305700，大韓民國(guó)2006年2月23日收稿；2006年10月31日收到修訂的稿件；2006年11月1日收錄2007年1月11日可在線使用摘要接收機(jī)的性能前端限制了所給定的通信鏈路的質(zhì)量和范圍?；诿鞔_定義的系統(tǒng)參數(shù)和結(jié)構(gòu)的設(shè)計(jì)可以使整個(gè)系統(tǒng)在性能、成本和市場(chǎng)化方面有巨大的差異。值得強(qiáng)調(diào)的是，我們需要一種改進(jìn)的數(shù)字自動(dòng)增益控制(agc)，應(yīng)用于多輸入多輸出、正交頻分復(fù)用(mimo - ofdm)的無(wú)線局域網(wǎng)(wlans),爭(zhēng)取成為

2、即將到來(lái)的802.11n標(biāo)準(zhǔn)（在2004年8月，heejung yu et al.建議采用無(wú)線etri局域網(wǎng)作為泰科全球網(wǎng)絡(luò)、ieee 802.11文件（文件號(hào)11-04-0923-00-000n）的說(shuō)明; h.yu,t.jeon,s.lee,為下一代無(wú)線局域網(wǎng)設(shè)計(jì)的雙頻mimo-ofdm系統(tǒng)在2005年5月電機(jī)及電子學(xué)工程師聯(lián)合會(huì)通信國(guó)際會(huì)議（icc）上獲得通過(guò)）。在本文中，我們提出一種有效的算法和實(shí)施面向下一代無(wú)線局域網(wǎng)的數(shù)字自動(dòng)增益控制系統(tǒng)。該自動(dòng)增益控制算法有兩個(gè)反饋增益控制以提高收斂速度,且同時(shí)保持agc電路的穩(wěn)定性。另外，在各種滿足不動(dòng)點(diǎn)約束和精度要求的實(shí)驗(yàn)中獲得了實(shí)際應(yīng)用中需要的

3、一套完整參數(shù)。關(guān)鍵詞：自動(dòng)增益控制，無(wú)線局域網(wǎng); mimo-ofdm技術(shù)，接收器結(jié)構(gòu)1、簡(jiǎn)介agc電路應(yīng)用于許多系統(tǒng)，這些系統(tǒng)的輸入信號(hào)電平在非常大的動(dòng)態(tài)范圍內(nèi)變化。在高數(shù)據(jù)傳輸速率的數(shù)字通信系統(tǒng)中，尤其是在突發(fā)包分組交換系統(tǒng)中，如無(wú)線局域網(wǎng)，每個(gè)數(shù)據(jù)包開(kāi)始會(huì)引入了一個(gè)大信號(hào)變化。來(lái)解調(diào)一個(gè)改善信噪比的接收信號(hào)，自動(dòng)增益控制可用于維持基帶信號(hào)的平均功率在我們期望值的附近。 agc對(duì)于應(yīng)用于下一代無(wú)線局域網(wǎng)技術(shù)的mimo-ofdm具有重要意義，它確保實(shí)現(xiàn)接收端的性噪比滿足實(shí)際要求，從而確保實(shí)數(shù)據(jù)傳輸速率。過(guò)去已經(jīng)有一些關(guān)于自動(dòng)增益控制的研究，這些研究提出自動(dòng)增益控制算法和目前實(shí)施所遇到的問(wèn)題。在

4、文獻(xiàn)3,4中，作者提出了一種實(shí)現(xiàn)簡(jiǎn)單的數(shù)字自動(dòng)增益控制結(jié)構(gòu)，其定位于ieee802.11a標(biāo)準(zhǔn)。文獻(xiàn)3的作者提出一種簡(jiǎn)單的多站式自動(dòng)增益控制方案。在文獻(xiàn)4中，作者提出了一種基于雙自相關(guān)的同步自動(dòng)增益控制接口的方案。在這些論文中，對(duì)理論問(wèn)題進(jìn)行了分析，并且提供了沒(méi)有詳細(xì)考慮實(shí)施過(guò)程中的限制條件而得到的仿真結(jié)果。在本文中，提出了新的agc結(jié)構(gòu)，它包括一個(gè)大增益更新循環(huán)和一個(gè)小增益更新循環(huán)，用來(lái)提高收斂速度，并同時(shí)保持agc電路的穩(wěn)定。此外，它還可以動(dòng)態(tài)控制mimo- ofdm系統(tǒng)接收到大變化能量信號(hào)的增益，該信號(hào)能量變化由隨時(shí)間頻率偏移的多徑衰落引起的。本文的其余部分安排如下：在第二節(jié)中，給出下一

5、代無(wú)線局域網(wǎng)的框架模型；在第三節(jié)中，描述了接收器的整體結(jié)構(gòu)。后面詳細(xì)地描述每個(gè)子塊，并給出各自部分結(jié)構(gòu)：自動(dòng)增益控制在第4節(jié)；數(shù)字放大器在第5節(jié)載以及波監(jiān)聽(tīng)在第6節(jié)；在第7節(jié)中，展現(xiàn)該設(shè)計(jì)的性能；最后，在第8節(jié)我們得出結(jié)論。2、框架模型下一代wlan是一種基于分組的高通量mimo - ofdm系統(tǒng)，該系統(tǒng)工作在 5 ghz頻帶。圖1和2顯示了下一代無(wú)線局域網(wǎng)的數(shù)據(jù)包結(jié)構(gòu) ，該結(jié)構(gòu)在文獻(xiàn)1,2中有詳細(xì)描述。每個(gè)數(shù)據(jù)包包含一個(gè)檢測(cè)頭， 預(yù)測(cè)信道和同步信道。報(bào)頭可以被雙方識(shí)別以用來(lái)通信鏈接。傳統(tǒng)的ofdm報(bào)頭由10個(gè)相同的短正交頻分復(fù)用(ofdm)輔助符號(hào)（ti，i = 1，210；每個(gè)符號(hào)包含16

6、個(gè)樣品）和2個(gè)相同的長(zhǎng)正交頻分復(fù)用(ofdm) 輔助符號(hào)（ti，i = 1，2；每個(gè)符號(hào)包含64個(gè)像ieee802.11a的樣品）。在mimo- ofdm模式，長(zhǎng)正交頻分復(fù)用(ofdm)符號(hào)，（ti，i = 1，2），該符號(hào)在信號(hào)之后傳輸，提供信道測(cè)量能力。短的輔助符號(hào)是用于信號(hào)檢測(cè)，自動(dòng)增益控制，多樣性，粗采樣和頻率同步。為了確保接收到的信號(hào)增益控制及時(shí)和提供穩(wěn)定增益的可靠傳輸，接收器設(shè)計(jì)人員可以使用短報(bào)頭來(lái)調(diào)整接收到的信號(hào)強(qiáng)度達(dá)到到最佳水平，該調(diào)整通過(guò)在接收信號(hào)路徑上可動(dòng)態(tài)調(diào)節(jié)的各種信號(hào)處理元件來(lái)實(shí)現(xiàn)。圖1 傳統(tǒng)的ofdm數(shù)據(jù)包結(jié)構(gòu)模型圖2 mimo-ofdm數(shù)據(jù)包結(jié)構(gòu)模型長(zhǎng)輔助符號(hào)被設(shè)計(jì)

7、用來(lái)信道預(yù)測(cè)和精細(xì)頻移校正。該信號(hào)包括奇偶校驗(yàn)，長(zhǎng)度和速率等。有一個(gè)短的保護(hù)區(qū)間（gi）和一個(gè)長(zhǎng)的保護(hù)區(qū)間（gi2）組成32或64個(gè)數(shù)據(jù)樣本，分別用來(lái)作為傳統(tǒng)ofdm的長(zhǎng)輔助符號(hào)和長(zhǎng)mimo - ofdm的輔助符號(hào)。在ofdm數(shù)據(jù)域中，每個(gè)波段有四個(gè)子載波作為試點(diǎn)被插入到的位置-21，-7，7，和21。，子載波總數(shù)分別為52和104。3、接收器結(jié)構(gòu)接收器的整體框圖如圖3所示。從三個(gè)天線接收到的3個(gè)信號(hào) 輸入數(shù)字放大器用來(lái)調(diào)整即將到來(lái)信號(hào)的能力達(dá)到目標(biāo)值。數(shù)字前端操作只適用于來(lái)自3個(gè)可用路徑出來(lái)的兩個(gè)接收信號(hào)，達(dá)到降低實(shí)施復(fù)雜度的目的。輸入信號(hào)能量測(cè)量和增益更新是在agc模塊中計(jì)算的。數(shù)字放大器

8、的輸出被監(jiān)測(cè)用來(lái)檢測(cè)信號(hào)是否太大或者太小大而超出載波的敏感范圍。接收到的信號(hào)被引向到一個(gè)+10和-10m赫茲頻率變化的頻道混合器。ofdm輸入符號(hào)緩沖到fft輸入緩沖區(qū)，并且在輸入fft前對(duì)載波頻率偏移（cfo）進(jìn)行糾正。利用相導(dǎo)頻跟蹤塊對(duì)頻率和相位的誤差進(jìn)行估計(jì)和糾正。通過(guò)短報(bào)頭和長(zhǎng)報(bào)頭的自相關(guān)的結(jié)果來(lái)實(shí)現(xiàn)cfo估計(jì)，幀同步和帶檢測(cè)。同步程序完成后，cfo補(bǔ)償數(shù)據(jù)包由128點(diǎn)、基-23 dif fft塊轉(zhuǎn)化為頻域。該fft輸出是位反轉(zhuǎn)順序，該序列通過(guò)使用迫零（zf）方法輸入到mimo檢測(cè)中2。jishu do de xnho bi dngxing do yg tngdo hnh q wi 1

9、0 h 10 zho h de pnl fshng binhu. ofdm fho de shr hunchng fft shr do hunchng q, zib pnl pin y (shux ciw gun) sh zi fft shr jizhng. de pnl h xingwi wch gj, bng lyng zi xing do pn gnzng kui de jizhng. shux ciw gun gj, zhng tngb h di jinc sh yu yg dunq h chngq de xyn z xinggun de jigu. tngb hu chngx wnc

10、hng hu, shux ciw gun bchngdictionary - view detailed dictionary圖3 帶3天線的雙頻段mimo-ofdm接收機(jī)前端結(jié)構(gòu)4、自動(dòng)增益控制接收信號(hào)幅度的自動(dòng)調(diào)整使得adc的動(dòng)態(tài)范圍得以充分利用。agc實(shí)際工作的狀態(tài)轉(zhuǎn)移圖如圖4所示。當(dāng)agc自動(dòng)增益控制塊的啟動(dòng)信號(hào)（agc_en）為低電平時(shí)，該自動(dòng)增益控制塊的狀態(tài)從任意狀態(tài)變成空閑狀態(tài)。第一個(gè)狀態(tài)是功率測(cè)量狀態(tài)（msr），它判定在放大器增益調(diào)整之前信號(hào)峰值是否在adc的動(dòng)態(tài)范圍之內(nèi)。如圖5所示，信號(hào)功率的衡量是根據(jù)每個(gè)天線積累的實(shí)部絕對(duì)值（同相分量）和虛部（正交分量）的能量來(lái)測(cè)量信號(hào)的信

11、號(hào)功率。用0.8最小二乘方法（32個(gè)在40 mhz采樣的樣本）來(lái)測(cè)量輸入信號(hào)的功率。所選擇的轉(zhuǎn)換為，通過(guò)采用信號(hào)能量的對(duì)數(shù)值可以減少信號(hào)能量值的范圍。如果在信號(hào)功率測(cè)量期間，測(cè)量得到的信號(hào)能力不在adc（adc飽和度）的動(dòng)態(tài)范圍內(nèi)，能量測(cè)量將停止，同時(shí)agc啟用大的增益更新?tīng)顟B(tài)（update_l），用大的增益控制值來(lái)粗調(diào)增益以加快增益調(diào)整。通過(guò)寄存器的設(shè)定值（agc_gainl）來(lái)迅速降低放大器的增益，以達(dá)到加速收斂的目的。增益調(diào)整的尺度固定為3分貝。在信號(hào)功率測(cè)量期間，增益更新是由于在adc飽和度觀察時(shí)候adc已經(jīng)飽和如果在信號(hào)功率測(cè)量期間，adc沒(méi)有飽和度，則測(cè)量的能量和參考能量進(jìn)行比較，

12、計(jì)算出一個(gè)誤差信號(hào)。誤差信號(hào)幅度和參考值的比較是在一個(gè)小增益更新?tīng)顟B(tài)（update_s）中進(jìn)行的。通過(guò)測(cè)量能量的對(duì)數(shù)值和目標(biāo)能量值的比較，可以選擇一個(gè)最大的信噪比，和最小的飽和效應(yīng)的信號(hào)。如果測(cè)得的能量（agc_pwr_log）比目標(biāo)能量（agc_vref）小，由于這是一個(gè)可編程的寄存器，我們可以調(diào)整增益，在增益改變信號(hào)固定下來(lái)之后，通過(guò)寄存器的值使信號(hào)能量等于目標(biāo)能量 如果測(cè)得的能量（agc_pwr_log）比目標(biāo)能量（agc_vref）大，則增益減小，甚至設(shè)置agc_gains，這是寄存器編程。這種額外的增益抑制在接收到的信號(hào)是很大時(shí)可以加快增益調(diào)整和防止飽和情況發(fā)生。一旦放大器的增益更新

13、，自動(dòng)增益控制模塊在等待狀態(tài)內(nèi)等待寄存器編程一段時(shí)間（agc_delay）。該寄存器的值應(yīng)足夠大，使得信號(hào)能很好的在增益變化可控范圍之內(nèi)。wait_cont和wait_last狀態(tài)分別是持續(xù)增益控制進(jìn)程和最后增益控制的等待狀態(tài)。在等待狀態(tài)結(jié)束后，信號(hào)的能量測(cè)量和增益的更新如此反復(fù)進(jìn)行，直到測(cè)量的功能量比目標(biāo)能量小，此時(shí)增益進(jìn)行相應(yīng)的更新。最初的增益由可編程寄存器（agc_ginit）來(lái)設(shè)定。圖4 agc狀態(tài)轉(zhuǎn)移圖圖5 agc方框圖5、數(shù)字放大器數(shù)字放大器是通過(guò)放大或衰減來(lái)調(diào)整信號(hào)能量的大小，它根據(jù)現(xiàn)有的增益狀態(tài)調(diào)整可編程寄存器使得信號(hào)能量達(dá)到目標(biāo)能量。數(shù)字放大器包括一個(gè)增益狀態(tài)單元，它可以存儲(chǔ)

14、對(duì)接收的數(shù)據(jù)包處理所選定的增益狀態(tài)該增益狀態(tài)單元從最高增益狀態(tài)開(kāi)始，以確保最低的功率信號(hào)可以被檢測(cè)和處理。兩個(gè)接收信號(hào)的路徑上都應(yīng)用到了相同的增益調(diào)整量。利用像圖6所示的粗增益步驟，簡(jiǎn)化了數(shù)字放大器的實(shí)施。更新的增益量劃分為6分貝和3分貝兩個(gè)步驟， 6分貝增益更新步驟首先應(yīng)用到輸入信號(hào)，然后再用3分貝步驟進(jìn)行增益調(diào)整。當(dāng)增益控制值和agc參考值相同時(shí)，將不存在增益調(diào)整。圖6 數(shù)字放大器的方框圖6、監(jiān)控adc飽和的載波監(jiān)聽(tīng)不管adc是否飽和，輸入信號(hào)的存在都可以通過(guò)監(jiān)測(cè)被檢測(cè)到。為了提高adc飽和度檢測(cè)的可靠性，使用16個(gè)在40 mhz采樣的連續(xù)樣品?？紤]一個(gè)從天線0接收到的信號(hào)的實(shí)部部分。如果

15、adc輸出樣本的數(shù)值絕對(duì)值大于某個(gè)閾值（cs_th_sat，500），且大于或等于一個(gè)可編程的寄存器數(shù)值（cs_th_cnt_sat，4），我們就標(biāo)記adc已經(jīng)達(dá)到飽和。 4個(gè)信號(hào)的組成部分（兩個(gè)天線接收到信號(hào)的實(shí)部和虛部）中任意一個(gè)都可以標(biāo)記adc已經(jīng)達(dá)到飽和。載波監(jiān)聽(tīng)的框圖如圖7所示。圖7 載波監(jiān)聽(tīng)的方框圖7、性能評(píng)估我們運(yùn)用了50 ns的均方根時(shí)延擴(kuò)展信道模型。同時(shí)，該模型包括射頻損傷和一個(gè)具有10 db補(bǔ)償?shù)睦展β史糯笃骱鸵粋€(gè)具有零極點(diǎn)相位噪聲的模型。發(fā)射機(jī)和接收機(jī)振蕩器的頻率不穩(wěn)定會(huì)引起殘留頻率誤差的存在。由于模數(shù)轉(zhuǎn)換器（adc）的使用，使得所有仿真事件都有時(shí)間/頻率偏移。數(shù)據(jù)包大

16、小固定為1 k字節(jié)。仿真模型的傳輸速率固定為36 mbps（兆比特每秒）和54 mbps（兆比特每秒），使用qpsk，16 -qam和64- qam的調(diào)制方案，這些方案都采用mimo雙波段技術(shù)。因此，實(shí)際的數(shù)據(jù)傳輸速率分別為72 mbps（兆比特每秒）的，144 mbps（兆比特每秒）和216 mbps（兆比特每秒）。使用不同的調(diào)制方案仿真得到的包差錯(cuò)率繪制在圖8上。fl和fx分別指仿真結(jié)果中的浮點(diǎn)類型和定點(diǎn)類型。雖然大多數(shù)調(diào)制方案的定點(diǎn)與浮點(diǎn)有相似的性能，然而在使用16 -qam和64- qam的調(diào)制方案時(shí)，由量化誤差引起的性能上的差距很明顯，在10的包差錯(cuò)率時(shí)，兩種方案損失的信噪比分別為0

17、.3和0.7分貝。我們?cè)趫D8表明：推薦的算法及其實(shí)施在多徑衰落的50ns均方根時(shí)延擴(kuò)展和40 ppm的時(shí)間/頻率偏移中具有很好的效果。就已知的約束而言，該算法在28分貝信噪比大約有10包差錯(cuò)率。考慮到實(shí)施的復(fù)雜性和性能之間的折中，我們提出的算法和它的實(shí)施對(duì)于mimo- ofdm系統(tǒng)接收器而言，是一個(gè)很好的折中解決方案。圖8 調(diào)制方案在包差錯(cuò)率之間的比較在圖9中，在16-qam和r= 3/ 4條件下，目標(biāo)信號(hào)在仿真過(guò)程中的幅度調(diào)整變化。agc_vref是在3分貝條件下，在agc完成之后的目標(biāo)信號(hào)幅度。例如，agc_vref= 13對(duì)應(yīng)著目標(biāo)信號(hào)的幅度為2 （13/2）。我們發(fā)現(xiàn)，通過(guò)對(duì)信號(hào)頻帶的

18、使用和雙頻段的使用進(jìn)行不同的設(shè)置，接收機(jī)的性能可以得到提高，如表1所示。如表2所示，agc_gainl和agc_gains寄存器值的設(shè)置需要考慮包差錯(cuò)率的結(jié)果，和輸入信號(hào)的能量范圍和更新的次數(shù)，因?yàn)閍gc需要在短報(bào)頭結(jié)束之前很好的完成。agc_init是初始增益設(shè)置。如果信號(hào)需要被放大，初始增益將增加到最大值。如果信號(hào)很大，需要加以抑制，初始增益將減少到0。當(dāng)agc_ginit是20，增益控制為3分貝時(shí)，最大的信號(hào)抑制可達(dá)到20*3 = 60分貝，最大的信號(hào)放大可以達(dá)到（32-20）*3 = 36分貝。初始增益應(yīng)該設(shè)置得足夠大以達(dá)到具有抑制信號(hào)的能力。在agc_ginit= 20這個(gè)例子中，有

19、60分貝的空間去抑制信號(hào)。自動(dòng)增益控制寄存器仿真參數(shù)的設(shè)置如表1所示。圖9 agc參考值 (agc_vref)的調(diào)整表1 可編程寄存器的設(shè)置agc registersvalues agc_delay32agc_gainl12agc_gains8agc_ginit20agc_vref13 (雙頻帶使用)/10 (單頻帶使用)表2 agc循環(huán)次數(shù)agc_gainlagc_gains# of update_l# of update_sper (%)012020918.410172816.111464813.364460812.7128858912.71812472124.6接收器中agc的快速收斂電

20、路減少了將接收信號(hào)調(diào)整到adc工作范圍內(nèi)的時(shí)間。本文提出的的數(shù)字agc電路包括一個(gè)大增益更新循環(huán)和一個(gè)小增益更新循環(huán)，用來(lái)加快收斂速度，并且同時(shí)維持控制輸入信號(hào)電平穩(wěn)定。圖10為1000包，在27分貝、64 - qam和r= 3 /4 條件下的仿真結(jié)果。大增益更新循環(huán)可以很快的將接收信號(hào)調(diào)整到期望的范圍。小增益更新循環(huán)慢慢撫平接收信號(hào)，以避免ad轉(zhuǎn)換器達(dá)到飽和并且加輸入信號(hào)電平的收斂快速度。數(shù)字放大器輸入信號(hào)電平與時(shí)間關(guān)系數(shù)字放大器輸出信號(hào)電平與時(shí)間關(guān)系圖10 數(shù)字放大器輸入/輸出信號(hào)電平8、結(jié)論在本論文中，設(shè)計(jì)的自動(dòng)增益控制電路用來(lái)調(diào)整接收信號(hào)的強(qiáng)度，通過(guò)接收路徑上可以處理各種信號(hào)的大動(dòng)態(tài)范

21、圍元件來(lái)使接收信號(hào)達(dá)到一個(gè)恒定的最佳能量水平附近。該自動(dòng)增益控制電路包括一個(gè)大增益更新循環(huán)和一個(gè)小增益更新循環(huán)，用來(lái)加快收斂速度，并且同時(shí)保持自動(dòng)增益控制電路的穩(wěn)定。此外，它可以用來(lái)動(dòng)態(tài)控制由多徑衰落、時(shí)間和頻率偏移引起大變化范圍接收信號(hào)的增益， 以確保及時(shí)地對(duì)接收信號(hào)進(jìn)行增益控制，提供穩(wěn)定增益進(jìn)而得到可靠的傳輸。參考文獻(xiàn)1 heejung yu et al., ieee 802.11 wireless lans etri proposal specication for ieee 802.11 tgn, ieee 802.11 document, doc. no. 1104092300000

22、n, august, 2004.待添加的隱藏文字內(nèi)容12 h. yu, t. jeon, s. lee, design of dualband mimo-ofdm system for next generation wireless lan, in: ieee international conference on communications (icc), may, 2005.3 v.p.g. jimenez, m.j.f.g. garcia, f.j.g. serrano, a.g. armada, design and implementation of synchronization

23、 and agc for ofdmbased wlan receivers, ieee trans. consum. electron. 50 (4) (2004) 10161025.4 a. fort, w. eberle, synchronization and agc proposal forieee 802.11a burst ofdm systems, globecom 3 (12) (2003) 13351338.ecient automatic gain control algorithm and architecture for wireless lan receiversil

24、gu lee *, sokkyu leenext generation wireless lan research team, etri, 161 gajeongdong, yuseonggu, daejeon 305700, republic of koreareceived 23 february 2006; received in revised form 31 october 2006; accepted 1 november 2006available online 11 january 2007abstractthe performance of a receiver fronte

25、nd limits the quality and range of the given communication link. an appropriate design based on welldened system parameters and architecture can make a huge dierence in the performance, cost and marketability of the entire system. in particular, there is a need for improved digital automatic gain co

26、ntrol (agc) for use in multiinput multioutput orthogonal frequency division multiplexing (mimoofdm) systems with application to wireless local area networks (wlans), targeted for the upcoming 802.11n standard heejung yu et al., ieee 802.11 wireless lans etri proposal specication for ieee 802.11 tgn,

27、 ieee 802.11 document, doc. no. 1104092300000n, august, 2004; h. yu, t. jeon, s. lee, design of dualband mimoofdm system for next generation wireless lan, in: ieee international conference on communications (icc), may, 2005. in this paper, we propose an ecient algorithm and implementation of the dig

28、ital agc for next generation wlans. the proposed agc algorithm has two feedback loops for gain control to improve convergence speed, and at the same time maintains the stability of the agc circuit. also, a complete set of parameters for practical implementation is obtained by various experiments wit

29、h xed point constraints and accuracy requirements.keywords: agc; wlan; mimoofdm; receiver architecture1. introductionagc circuits are employed in many systems where the level of an incoming signal can vary over a wide dynamic range. in high data rate digital communication systems, and especially in

30、burst packet switched systems such as wlans, the start of each packet introduces a large signal variation. to demodulate a received signal with an improved signaltonoise ratio, agc can be used to hold the average power of the baseband signal close to a desired level. agc implementation of highthroug

31、hput mimoofdm applications to nextgeneration wlans is important to ensuring achievable operating snr at the receiver and, consequently, achievable data rates.there have been several research contributions that provide automatic gain control algorithms and present implementation issues. in 3,4, the a

32、uthors present the implementation of a simple digital automatic gain control architecture targeting the ieee 802.11a standard. the authors of 3 propose a simple multistop agc scheme. in 4,an agc interface with a synchronization scheme based on double autocorrelation is proposed. in those papers, the

33、 theoretical problem is analyzed and simulation results are provided without considering implementation constraints in detail.in this paper, the proposed architecture includes a large gain update loop and a small gain update loop to improve convergence speed and at the same time maintain the stabili

34、ty of the agc circuit. moreover, it can be used to dynamically control the gain of the received signal for mimoofdm systems with large variations in received signal power caused by multipath fading with time and frequency oset.the remainder of this paper is organized as follows. in section 2, the fr

35、ame model is given for next generation wireless lans, and the overall receiver architecture is presented in section 3. a detailed description for each subblock is then provided in their respective sections: automatic gain control in section 4; carrier sensing block in section 5; and digital amplier

36、in section 6. in section 7, the performance of the proposed design is shown. finally, we conclude in section 8.2. frame modelthe next generation wlan is a packetbased highthroughput mimoofdm system in the 5 ghz band. figs. 1 and 2 show the packet structure of next generation wlan as specied by 1,2.

37、each packet contains a header for detection, channel estimation and synchronization. this preamble is known at both sides of the communication link. the legacy ofdm packet preamble consists of 10 identical short ofdm training symbols ti, i =1,2, .,10, each of which contains 16 samples; and two ident

38、ical long ofdm symbols ti, i =1,2, each of which contains 64 samples as in the ieee 802.11a. for mimoofdm mode, two long ofdm symbols ti, i = 3,4, are transmitted after the signal eld for providing channel measurement capability. the short training symbols are intended for signal detection, automati

39、c gain control, diversity, coarse acquisition, and frequency synchronization purposes. in order to ensure timely gain control for the received signal and provide reliable transmission with stable gain, a receiver designer can use the short preamble to adjust the strength of the received signal to an

40、 optimum level within the dynamic range of various signal processing components in the received signal path.fig. 1. the packet structure of the legacy ofdm mode.fig. 2. the packet structure of the mimo-ofdm mode.the long training symbols are designed to be used for channel estimation and ne frequenc

41、y oset correction. the signal eld includes information for parity, length and rate, etc. there is a short guard interval (gi) and a long guard interval (gi2) that consist of 32 or 64 data samples for the long legacyofdm training symbol and the long mimoofdm training symbol, respectively. in the ofdm

42、 data eld, four subcarriers are inserted as pilots into positions 21, 7, 7, and 21 for each band. the total number of subcarriers is 52 and 104 in single and dual band mode, respectively.3. receiver architecturethe overall receiver block diagram is shown in fig. 3. the three received signals from 3

43、antennas are fed into digital ampliers to adjust the power of the incoming signals to the target value. the digital front end operations are applied to only the two received signals out of the 3 available paths to reduce implementation complexity. the power of the input signal is measured and gain u

44、pdate is calculated in the agc block. the digital amplier output is monitored to detect if the signal is large or not for the carrier sensing purpose. the dc oset andi/q imbalance that come from rf components and adc are compensated in each signal path. the received signals are directed to a channel

45、 mixer for +10 and 10 mhz frequency shifting. the input ofdm symbol is buered into the fft input buffer, and the carrier frequency oset (cfo) is corrected at the input of the fft. the frequency and phase errors are estimated and corrected by using the pilot tones in the phase tracking block. the cfo

46、 estimation, frame synchronization and band detection are performed by an autocorrelation result of short and long preambles. after the synchronization process is done, the cfo compensated packets are transformed to the frequency domain by a 128point radix23 dif fft block. the output of fft is the d

47、ata in the bitreversed order, which is fed into the mimo detector 2, which uses the zeroforcing (zf) method.fig. 3. the front-end architecture of dual-band mimo-ofdm receiver with 3 antennas.4. automatic gain controlthe amplitude of the received signal is adjusted so that the dynamic range of the ad

48、c can be fully utilized. the state transition diagram implemented physically for the agc is shown in fig. 4. the agc block state is changed to the idle state from whatever state the agc is in when the agc block enable (agc_en) is deactivated. the rst state is a power measurement state (msr), which d

49、etermines whether the peak signal is within the dynamic range of adc before adjusting the amplier gain. as shown in fig. 5, the signal power is measured by accumulating the absolute real (inphase) and imaginary (quadraturephase) components of each antenna. the power of the input signal is measured f

50、or 0.8 ls (32 samples at 40 mhz sampling). out of the two estimated signal powers, the larger one is selected for gain update. the chosen signal power is converted to a logscale value. it is possible to reduce the range of values by taking log scale for the signal power. if the measured power is out

51、 of the dynamic range of adc (adc saturation) during this power measurement period, the power measurement is stopped and agc makes a coarse adjustment with the large gain update state (update_l) to speed up gain adjustment with the large gain control value. the amplier gain is reduced right away by

52、the amount of the register programmed value (agc_gainl)in order to speed up convergence. the gain step is xed to 3 db. the gain update due to adc saturation is conducted only when adc saturation is observed during the signal power measurement period. if adc is not saturated during the power measurem

53、ent period, the measured power is compared to a reference power to calculate an error signal, and the magnitude of the error signal is compared to a reference value in a small gain update state (update_s). by comparing the measured logscale power with the target power that can be selected tomaximize

54、 the signal to noise ratio and minimize saturation eects. if the measured power (agc_pwr_log)is smaller than the target power (agc_vref), which is a programmable register, then the gain is adjusted so that the signal power is equal to the target power given by the register value after the signal is

55、settled down for the gain change. if the measured signal power is larger than the target power, then the gain is reduced even more given the agc_gains, which is register programmable. this additional gain suppression will speed up the gain adjustment and prevent saturation when the received signal i

56、s large. once the amplier gain is updated, the agc block waits for a register programmed time period (agc_delay) in wait state. this register value should be suciently large so that the signal is well settled down to the gain change. the wait_cont and wait_last state are the wait states of the conti

57、nued gain control process and the last gain control, respectively. afterthe waiting period, the signal power measurement and gain update is repeated until themeasured power is smaller than the target power and gain is updated accordingly. the initial gain is given by the programmable register (agc_g

58、init).fig. 4. state digram of agc block.fig. 5. block diagram of agc.5. digital amplierthe digital amplier is used to scale the incoming signal power either by amplifying or attenuating and adjusts it to the target power specied in a programmable register according to the current gain state. the digital amplier includes a gain state unit that stores the selected g