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استفاده از تبدیل بسته موجک در بهبود عملکرد OFDM به جای روش مرسوم مبتنی بر FFT
Using WPT as a New Method Instead of FFT for Improving the Performance of OFDM Modulation
مقالات پردازش دادههای رقمی
Paper
پژوهشي
Research
<p dir="RTL" style="text-align: justify;"><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;">امروزه در سامانه­های مخابرات دیجیتال نوین، از مالتیپلکس تقسیم فرکانسی متعامد (</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;">) بهعنوان یک روش مدولاسیون مقبول و کارآمد استفاده میشود. در مدولاسیون چندحاملی </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> از تبدیل </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">FFT</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> بهمنظور ایجاد تعامد زیرحامل­ها استفاده می­شود. این تبدیل با استفاده از توابع سینوسی پنجرهشده بهعنوان توابع پایه، بهدلیل سطح بالای گلبرگ­های کناری در طیف فرکانسی، حساسیت بالایی نسبت به تداخل­ بینسمبلی (</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">ISI</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;">) و تداخل بینحاملی (</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">ICI</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;">) دارد. بهمنظور بازیابی سیگنال در گیرنده، با افزودن</span></span></strong> <strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;">پیشوند چرخشی</span></span></strong> <strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;">(</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">CP</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;">) در ابتدای سمبل </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> در فرستنده، می­توان </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">ISI</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> را حذف کرد. همچنین با استفاده از همسان­سازی حوزه فرکانس در گیرنده، بهسادگی سیگنال آغشته به </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">ICI</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> را</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;"></span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> میتوان بازیابی کرد. این افزودن </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">CP</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> میزان قابل توجهی از پهنای باند در دسترس را هدر می­دهد و درنتیجه کارایی پهنای باند را بهطور قابل توجهی کاهش می­دهد. با توجه به ویژگی­های منحصربهفرد موجک­ها نظیر انعطاف­پذیری بالا، سازگاری و محلیبودن آنها، از این تبدیل برای متعامدسازی سامانه چندحاملی در این پژوهش استفاده شده است. درصورت استفاده از موجک، دیگر نیازی به استفاده از تابع پنجره­ی مستطیلی نیست.­ این امر حساسیت سامانه به رانش فرکانسی و نوفه فاز را کم کرده و سامانه را نسبت به اعوجاج کانال چندمسیره و تداخل­های </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">ISI</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> و </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">ICI</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> مقاوم­تر می­سازد. در این مقاله پس از بررسی و مطالعه تبدیل موجک گسسته و تبدیل بسته موجک، دو طرح مدولاسیون </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">FFT-OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> و </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">WPT-OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> برای کانال­های استاندارد شهری </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">TU6</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> و روستایی </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">RA6</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> توسط نرم­افزار </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">MATLAB</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> شبیهسازی شدهاند. این دو کانال استاندارد و معروف توسط استاندارد </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">3GPP</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> معرفی شدهاند که بهعنوان معیار سنجش عملکرد سامانه­های مخابراتی نسلهای </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">3G</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> و </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">4G</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> معرفی شده­اند. کانالهای مورد استفاده در این پژوهش، کانالهای متداولی در سامانههای سیاری مانند سامانههای پخش تلویزیون دیجیتال</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">(DVB) </span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> هستند. نتایج شبیه­سازی بهبود عملکرد سامانه </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">WPT-OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> را نسبت به سامانه </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">FFT-OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> نشان می­دهد. این نتایج نشان</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;"></span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> میدهند که در سامانههای مبتنی بر</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">OFDM</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> مانند </span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">DAB( Digital Audio Broadcast)</span></span></span></strong> <strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">WiMAX (worldwide Interoperability for</span></span></span></strong> <strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;">Microwave Access), DVB( Digital Video Broadcast),</span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> میتوان از این سامانه</span></span></strong><strong><span dir="LTR"><span style="font-family:times new roman,serif;"><span style="font-size:8.0pt;"></span></span></span></strong><strong><span style="font-family:b nazanin;"><span style="font-size:10.0pt;"> استفاده کرد.</span></span></strong><br>
</p>
<p style="text-align: justify;"><strong>Orthogonal frequency division multiplexing (OFDM) is used </strong><strong>in order to provide immunity against very hostile multipath channels</strong> <strong>in many modern communication systems.</strong><strong>. </strong><strong>The OFDM technique divides the total available frequency bandwidth into several narrow bands. In conventional </strong><strong>OFDM</strong><strong>, FFT algorithm is used to provide orthogonal subcarriers. Intersymbol interference (ISI) and intercarrier interference (ICI) impairements are caused by time domain rectangular windowed sine and cosine basis functions. FFT-OFDM is a very popular multi–carrier modulation (MCM) technique</strong><strong>. I</strong><strong>t has some interesting features such as low complex modulation/demodulation implementation, simple and fast frequency domain channel estimation/ equalization. Also, by transmitting data over different parallel frequencies, FFT-OFDM has spectrum efficiency due to overlapped sub-channels and immunity against fading channels. </strong><strong>Unfortunately, </strong><strong>FFT-OFDM </strong><strong>has serious drawbacks i.e. high sensitivity to ISI and ICI which caused by time domain rectangular windowed sine and cosine basis functions and their high level side lobes in frequency domain. For this purpose,</strong> <strong>cyclic prefixes (CP) are added at the beginning of the OFDM symbols and </strong><strong>this causes bandwidth and power inefficiencies.</strong><br>
<strong>In order to provide more efficient MCM </strong><strong>technique</strong><strong>, besides preserving the advantages of conventional FFT-OFDM, discrete wavelet modulation (DWM) and wavelet packet modulation (WPM) have been introduced in recent years. </strong><strong>Therefore, it is possible to use time domain equalization (TEQ) or overlap frequency domain equalization (overlap FEQ) to reduce the interferences effectively in the absence of CP. Although TEQ techniques are more complicate than FEQ in conventional OFDM, WPT-OFDM has bandwidth and power enhanced efficiencies and this makes it so appropriate for digital communication systems.</strong><br>
<strong>In recent years, several studies have been done </strong><strong>on the</strong><strong> wavelet theory, wavelet and WPM modulation in comparison with FFT-OFDM. </strong><strong>Because of the good performance of WPT, a number of studies are still on the performance of WPT in hostile channels with more details. </strong><strong>Also, there are a number of </strong><strong>studies about various kinds of FEQ and TEQ such as zero force (ZF) and minimum mean square error (MMSE) in the peresence of AWGN and some fading channels. These researches also contain the comparison of FEQ for FFT-OFDM and overlap FEQ for WPT-OFDM. </strong><br>
<strong>Todays, 3GPP standard is spread in different domains like 3G, 4G and LTE-A technologhies. In this paper, a</strong><strong>ll the parameters are chosen according to 3GPP standards. F</strong><strong>or demonstrating the benefits of discrete WPT, two OFDM modulation schemes, i.e. FFT-OFDM and WPT-OFDM with two applied channels i.e. 6-tap rural area (RA6) and 6-tap typical urban (TU6) channels are considered. </strong><strong>The performance of two systems are investigated by the measure of bit error rate (BER) in different SNRs(dB). </strong><strong>Also, Wavelet families i.e. Haar, Daubechies6 , Symlet5 and Coiflet5 are compared with FFT in OFDM system with QPSK, 16-QAM and 64-QAM constellation mappings. In the receiver side, FEQ is used in FFT-OFDM and overlap FEQ is used in WPT-OFDM to equalize multipath fading channels. This is a comprehensive comparison between FFT-OFDM and WPT-OFDM with different constellations, a number of wavelet families, different equalizer with two applied channels in order to implement a real environment. </strong><strong>The simulation results demonstrate performance improvement of the system using WPT-OFDM scheme. In order to evaluate the performance of these two OFDM techniques, the required SNRs for reaching BER =10<sup>-3</sup> are extracted and compared for both systems. It was observed that one can obtain better performance by using Haar wavelets as orthogonal basis function rather than FFT in OFDM modulation. We achieved better performance by using Haar wavelets rather than FFT in OFDM modulation. A</strong><strong>s a result, WPT-OFDM can be applied , with better performance, in different OFDM-based applied technologhies such as </strong><strong>DAB( Digital Audio Broadcast),</strong> <strong>WiMAX( worldwide Interoperability for Microwave Access), DVB( Digital Video Broadcast)</strong><strong>.</strong><br>
</p>
مدولاسیون چندحاملی, تبدیل فوریه سریع, تبدیل بسته موجک, استاندارد 3GPP
Multicarrier modulation, Fast Fourier Transform, Wavelet Packet Transform, 3GPP standard
121
136
http://jsdp.rcisp.ac.ir/browse.php?a_code=A-10-546-1&slc_lang=fa&sid=1
saeed
Ghazi-Maghrebi
سعید
قاضی مغربی
s_ghazi2002@yahoo.com
10031947532846007878
10031947532846007878
No
Islamic Azad University, Yadgar-e-Imam Khomeini (RAH) Shahr-e-Ray
دانشکده مهندسی برق –واحد یادگار امام خمینی (ره)شهرری، دانشگاه آزاد اسلامی
arbayan
khordadpoor
فربیان
خردادپور
farbayan_khordadpoor@yahoo.com
10031947532846007879
10031947532846007879
Yes
Islamic Azad University, Yadgar-e-Imam Khomeini (RAH) Shahr-e-Ray
دانشکده مهندسی برق –واحد یادگار امام خمینی (ره)شهرری، دانشگاه آزاد اسلامی