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Sayısal yayıncılıkta yeni bir modülasyon tekniği OFDM/COFDM

Başlık çevirisi mevcut değil.

  1. Tez No: 55975
  2. Yazar: SELAMİ ÇELEBİOĞLU
  3. Danışmanlar: DOÇ.DR. MELİH PAZARCI
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Sayısal işaret işleme, Yayıncılık, Digital signal processing, Publishing
  7. Yıl: 1996
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: Belirtilmemiş.

Özet

ÖZET Günümüzde sayısal sistemler, her sektörde olduğu gibi yayıncılık sektöründe de, analog sistemlere üstünlükleri nedeniyle, tercih edilmektedir. Son yıllarda sayısal işaret işleme ve geniş çaplı tümdevre tasarım teknolojilerinde sağlanan gelişme, sayısal sistemlerin daha ucuz ve verimli kullanılmasına yol açmıştır. Bu tezde sayısal yayıncılıkta, son yıllarda adından sıkça bahsettiren dikey frekans çoğullamalı modülasyon OFDM (Orthogonal Frequency Division Multiplexing) ve OFDM 'e ileri yönde hata düzeltme (FEC: Forward Error Correction) uygulanarak elde edilen COFDM (Coded-OFDM) teknikleri anlatılmıştır. Ayrıca bu tekniklerin sayısal ses yayını (DAB.Digital Audio Broadcasting) ve sayısal video yayını (DVB.Digital Video Broadcasting) üzerine yapılmış uygulamalarına yer verilmiştir. İlk bölümde OFDM/COFDM 'in tercih nedenleri ve tarihçesi anlatılmıştır. İkinci bölümde ise çok taşıyıcılı paralel iletişim sistemlerinin genel özellikleri verilip, OFDM/COFDM 'in matematiksel temelleri anlatılmıştır. Ayrık Fourier dönüşümü (DFT: Discrete Fourier Transform) ve hızlı Fourier dönüşümü (FFT:Fast Fourier Transform) teknikleri kullanılarak OFDM işaretinin nasıl oluşturulup modüle ve demodüle edildiği anlatılmıştır. Ayrıca COFDM sistemlerinde kullanılan hata düzeltme kodlarının karşılaştırmaları verilmiştir. Üçüncü bölümde ise OFDM/COFDM 'in DAB ve DVB sistemleri üzerinde kullanımı incelenmiştir. DAB sistemlerinde OFDM 'in farklı bir kullanımı tanıtılmıştır. COFDM sistemlerinin DVB uygulamaları ve bu uygulamalardan bazılarına ilişkin bilgiler DVB başlığı altında incelenmiştir. Dördüncü bölümde ise OFDM/COFDM Modülasyon tekniklerinin üstünlüklerine sebep olan faktörler ile sistemin günümüz için dezavantaj olan yönleri belirtilmiştir. IX

Özet (Çeviri)

SUMMARY OFDM/COFDM MODULATION TECHNIQUES IN DIGITAL BROADCASTING The orthogonal frequency division multiplexing OFDM is a form of multicarrier modulation technique that was introduced three decades ago. Recent advances in digital signal processing (DSP) and very large scale integrated circuit technologies have paved the way for the massive implementation of OFDM techniques in the consumer electronics field. Digital audio broadcasting (DAB), designed for broadcasting multiple radio programs from terrestrial or satellite transmitters to mobile receivers in Europe, can be given as an successful implementation of OFDM technique. The concept of using parallel data transmission and frequency division multiplexing was published in mid 60s. The idea was to use parallel data and frequency division multiplexing (FDM) with overlapping subchannels to avoid the use of high speed equalization and to combat impulsive noise, and multipath distortion as well as to fully use the available bandwidth. If we try look at the basic principles of OFDM, we should first understand the concept of parallel transmission and multiple carriers; In a conventional serial data system, the symbols are transmitted sequentially, with the frequency spectrum of each data symbol allowed to occupy the available bandwidth. But a parallel system is one in which several sequential streams of data transmitted simultaneously, so that at any moment many data elements are being transmitted. In such a system, the spectrum of an individual data element normally occupies only a small part of the available bandwidth. This parallel approach has the advantage of spreading out a frequency selective fade over many symbols. This effectively randomizes burst errors caused by fading or impulse interference, so that, instead of several adjacent symbols symbols being completely destroyed, many symbols are only slightly distorted. By the help of this effect we can reconstruct majority of transmitted data without forward error correction(FEC). Because of dividing entire channel bandwidth into many narrow subbands, the frequency response over each individual channel is relatively flat. Since each subchannel covers only a small fraction of the original bandwidth, equalization is simpler than in a serial system. In a standard parallel data system, the total signal frequency band is divided into N non-overlapping frequency subchannels. Each subchannel is modulated with a separate symbol and and then, the N subchannels are frequency division multiplexed. By carefully selecting carrier spacing, such as letting the carrier spacing equal to the reciprocal of the useful symbol period, intersymbol interferenceamong the channels can be eliminated. OFDM can be simplified as a form of multicarher modulation where its carrier spacing is such that each subcarrier is orthogonal to the other subcarriers. Fast Fourier transform FFT and DFT techniques are discussed for modulation. Consider a data sequence (d0,d,, d2 dN _,), where each d“ is a complex number d”= a“+jbM. If a discrete Fourier transform (DFT) is performed the result vector D = (D0, D,, DN_X) of N complex numbers, N-l N-l Dm= ^dne-^2mmlN)= £^ry2<'m,m=0,1,2 N-1 (s.1) n=0 n=Q where f ”=n/(NAt), tm=mAt, At is an arbitrarily chosen symbol duration of the serial data sequence d“. The real part of the vector D has components, N-l ym=H(fii”cos2nfntm+bnsm2nfntm), m=0,1,.... N-1 (s.2) n=0 If these components are applied to a low-pass filter at time intervals At, a signal is obtained that closely approximates the frequency division multiplexed signal. Y(t) = Z(a“ cos2nf”t +b“ smlTvfJ ), 0< t <NAt (s.3) n=0 If the carriers are separated by a multiple of 1/T, where T is the modulation period and are characterized by the fact that spectrum of different overlaps. A constellation is associated to each carrier and during one symbol period, the transmitted signal of the k th carrier is given by sk=akco${27tfki)±bkim{27tfkt), (s 4) sk=rkcos(27tfkt + <pk), where ak, bk are the coordinates of the current symbol for the k th carrier rk, <pk, are the polar coordinates of the current symbol for the k th carrier XIand fk is the frequency of the k th carrier. If we consider now an infinite transmission time, the signal for the k th carrier becomes, +00 s* = Z rj,k cos(2t^ (r - jT) + <pJJc )n{t - jT), (s.5) /=-co where, n{t) = 1, 0 < t < T, n(t) = 0, otherwise (s.6) In a multicarrier modulation, the transmitted signal is a summation of N modulated carriers:./.=-» +=0 N-\ ?*(>)= E Hrjkcos{2nfk{t~jT) + <pk) * *(t-jT) (s.7) If we consider that each carrier conveys symbol taken in a two- dimensional constellation with 2a points and modulated during T seconds, the bit-rate D can be shown as, £> = ^bits/s (s.8) The total bandwidth occupied by the N carriers is given by: ”N-\ 3 N + 5, ns BT =-+*j = -. (s.9) The spectral efficiency tj is, *=iH^blt/8'Hz- (s10) If we look at the OFDM system design parameters, we see three basic parameters, Xll1. Useful Symbol Duration:The useful symbol duration T affects the carrier spacing and coding latency. To maintain the data throughput, a longer useful symbol duration results in an increase of the number of carriers and the size of FFT (assuming that the signal constellation is fixed). 2. Number Of Carriers: The number of subcarriers can be determined based on the channel bandwidth, data throughput and useful symbol duration.The number of carriers corresponds to the number of complex points being processes in FFT. 3. Modulation scheme: The modulation scheme used in an OFDM system can be selected based on the requirement of power or spectrum efficiency. In general, the selection of modulation scheme applying to each sub channel depends solely on the compromise between the data rate requirement and transmission robustness. An advantage of OFDM is that different modulation schemes can be used on different subchannels. In this Thesis QAM is selected for modulation scheme. By using time and frequency diversity OFDM provides a means to transmit data in a frequency selective channel. However, it can not suppress fading itself. Depending on their position in the frequency domain, individual subchannels can be affected by fading. This requires the use of channel coding to further protect the transmitted data. Mostly used code for channel coding is the Trellis code. A coded OFDM system is called a COFDM system. OFDM is used in DAB system which is designed to transmit data audio in the FM band. In this system a signal is divided over a large number of 2 or 4-PSK modulated orthogonal subcarriers. The subcarriers of 6 different programs are multiplexed in one beam to reduce the effects of frequency selectivity of the transmission channel. A new system based on OFDM is mentioned, in which the carriers of each programs are transmitted in one FM-channel with a bandwidth of 200 KHz instead of multiplexed with the carriers of other programs. As a conclusion of DAB application, the traditional OFDM system used in DAB needs a minimum bandwidth of 1.5 MHz, so it can't be implemented in the present FM-System. But with the new technique it is possible to transmit single program in a single channel. As an application of COFDM system, DVB is mentioned. The 4 projects in Europe for terrestrial TV broadcasting all depend on COFDM. COFDM causes problems, because of its sensitivity to sub channel interference, in NTSC systems. Inspite of this advantages in North America COFDM is inspected as an alternative for HDTV broadcasting. Advantages that COFDM offers in dense networks are briefly given. The following COFDM features are mentioned, 1. Corresponding to time diversity, COFDM feature is coding plus interleaving 2. Corresponding to frequency diversity, COFDM feature is coding plus frequency interleaving 3. Corresponding to space diversity, COFDM feature is coding plus guard interval. xinAs a result, OFDM/COFDM modulation techniques offer an efficient and secure way for digital broadcasting. Some problems still occur, such as frequency offset correction in OFDM. But it is obvious that these techniques will be widely used in the near future for both DAB and DVB. XIV

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