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Sayısal işaret işlemci ile V32 modem

V32 modem with digital signal processor

  1. Tez No: 22037
  2. Yazar: ÖMER DOĞAN
  3. Danışmanlar: DOÇ. DR. MEHMET BÜLENT ÖRENCİK
  4. Tez Türü: Yüksek Lisans
  5. Konular: Bilgisayar Mühendisliği Bilimleri-Bilgisayar ve Kontrol, Computer Engineering and Computer Science and Control
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1992
  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: 119

Özet

SAYISAL İŞARET İŞLEMCİ İLE V32 MODEM ÖZET Bu çalışmada, sayısal işaret işleme algoritmaları kullanılarak CCITT'nin tarif ettiği V32 modemin alıcı ve verici fonksiyonlarının yazılımının gerçeklenmesi amaçlanmıştır. Bu amaçla, sayısal işaret işleme algoritmalarını gerçeklemeye uygun olan bir sayısal işaret işlemci TMS320C30 seçilmiş ve temel özellikleri anlatılmıştır. Modulator katı olarak QAM-32 seçilmiş ve alıcı tarafta da buna uygun olarak Hubert dönüştürücü kullanılmıştır. Alıcı ve gönderici tarafına ilişkin blok şema verilmiş ve alt bölümlerin nasıl gerçekleştirildiği ayrıntılı olarak anlatılmış, ardından programlar sunulmuştur. vı

Özet (Çeviri)

V32 MODEM WITH DIGITAL SIGNAL PROCESSOR SUMMARY During the last thirty years there has been a rapidly rising need for higher speed data transmission to furnish computer communications. The initial demand was in connection by commercial applications. This need has been met for the most part by utilizing the wide-spread network of voice bandwith channels developed for voice communications. To transmit digital signals over these analog channels, which pass frequencies in the range 300-3400 Hz, it is necessary to modulate a voice-frequency carrier signal and for a data receiver to demodulate this signal. A data tranceiver is consequently known as a MODEM. Such a modem serves to interconnect data terminal equipment and the modulated voice-frequency signals of the communication channel. Developments in semiconductor technology cause many theoretical digital signal processing techniques be used in practice and a result of this, digital systems are preferred to analogue systems. Low speed modems designed according to phase and frequency modulation principle at the beginning, are developed according to ways having higher error performance. A CCITT recommended V32 data modem, described in this work uses QAM-32 with trellis coded modulation scheme to achieve 9600 bps full-duplex communication over voice-grade telephone channels. It also uses echo cancelling technique to seperate receive and transmit signals. In this work, we are concerned with the development of digital signal processing algorithms, utilized to implement a 9600 bps CCITT recommended V32 data modem. In modulation, the input data is scrambled by dividing the data by one of the two generating polinomial as defined in the appropriate CCITT recommendation. The purpose of the scrambler is not data security, since the type of scrambler used is self synchronising, but rather is to insure a suitable level of randomness in the data stream. This randomness prevents long strings of all marks or all spaces, thereby spreading the transmitted energy uniformly over the passband as required by the modem receiver's adaptive processes. VllA self sychronizing scrambler/descrambler shall be included in the modem. Each transmission direction uses a different scrambler. According to the direction of trans mission, the generating polinomial is: -18 -23 Call mode modem generating polinomial = 1 + x + x,or -5 -23 Answer mode modem generating polinomial = 1 + x + x Figure 3.1. is an example of the scrambler used CCITT V32. At the transmitter, the scrambler shall effectively divide the message data sequence by the generating polino mial. The coefficients of the quotients of this division. taken is descending order, from the data sequence which shall appear at the output of the scrambler. At the receiver the received data sequence shall be multiplied by the scrambler generating polinomial to recover the message sequence. The scrambled data stream to be transmitted is divided into groups of four consecutive data bits. As shown in figure 3.2., the first two bits in time Qln and Q2n in each group, where the subscript n designates the sequence number of the group, are first dif f erantially encoded into Yin and Y2n according to table-2. The two dif f erantially encoded bits Yin and Y2n are used as input to a systematic convolutional encoder which generates a redundant bit YOn. This redundant bit and the four information-carrying bits Yln,Y2n,Q3n and Q4n are then mapped into the coordinates of the signal element to be transmitted according to the signal spaces diagram shown in figure 1.1 and as listed in table 2b. 32 points which include complex values are stored in ROM ( Read Only Memory ). Raised-cosine type filter minimizes the distortion caused by the time response of the filter to baud samples. Distortion of this type is often called intersymbol inter ference since the ringing caused by one symbol interferes with other symbols. The term raised-cosine refers to the shape of the filter characteristics, since the slope of the band edge is a cosine raised above the axis with the point of symmetry at the corner frequency. In figure 3.3 the several characteristics of raised-cosine filter are shown. The modulator multiplies the base band signal from the lowpass filter by the carrier signal. The carrier is a viiisinusoidal wave at 1800 Hz. Since the base band signal is complex, the modulator is complex. Figure 3.4. illustrares this type of modulator. The values of sinWct are generated by tables in ROM. The modulator output is sent to AFE ( Analog Front End ) chip. In AFE the modulator output is converted from digital to analog signal by D/A converter. In demodulation, the receiver analog input must be filtered by a bandpass filter to remove unwanted high frequency noise, energy from the transmit band and low frequency power line interference. The telephone network generates echos at points internal to and near end of a telephone connection. Echo cancellation is one means used to combat this echo for data transmission. In practice it is desirable to cancel the echoes in both directions of the connection. For this purpose two adaptive cancellers are necessary. The near-end talker for one of the cancellers is the far-end talker for the other. When it is attempted to transmit data signals through the network, the same echoes are encountered as in speech transmission. When half duplex data transmission is used, echoes present no problem since there is no receiver on the transmitting end to be affected by the echo. In full duplex transmission, where the data signals are transmitted in both direction data signal in one direction can interfere with the data signal flowing in the opposite direction, unless these two data signals are in nonoverlapping frequency bands. One approach to the full duplex data transmission is the use of an echo canceller to isolate the two directions of the transmission. There is a transmitter and receiver on each end of connection, and a hybrid is used to provide a virtual four wire connection between the transmitter on each end and receiver on the opposite end. Host canceller implementations described neglect completely the affect of nonlinear distortion in the echo path or in the echo replica. An exception to this uses an echo n canceller with 2 taps to synthesize an impulse response of n samples. This canceller, called a“memory compensation”or“table look-up”canceller, assigns an independent output to each possible combination of n transmitted bits and, thus is completely general as to the kind of nonlinearity that it can n correct. The price paid for this generality is 2 taps, IXrather than n, and a structure in which, at each sample time, only one tap weight can be updated. The consequences of this are that for large n the required memory becomes very large and the adaptation very slow. Hubert type demodulation is used in systems where digital signal processing is employed. Hubert transformer shifts in effect the Fourier components of a signal with -90* in phase. Figure 4.12. shows the block diagram of this kind of demodulator. In figure 4.12. Sp is the signal received from the line and called“in-phase”, Sq is the Hilbert transformation of this signal and called“quadrature”. Sq = X * sinWct + Y * cosWct Letting 6=Wct. from fig. 4.3 we find that, | Xp Lxqj cos8 -sine sine cose Sp Sq LÛ4J This multiplications space. The sampling at signal can be moved to Hilbert phase shifter. form a rotation in 2-dimensional the baud rate of the demodulated the left and placed just after the Ve have seen the overall demodulation process assuming ideal conditions. There are many blocks in fig. 4.12. Proper sampling tracking the changes i This is called timing r with pandpass filteri the outputting the r the use of DPLL output signal has now been f timed. The remaining intersymbol interfere introduced by the co equalizer attempts to r interference (ISI). Th tunable filter that delay by producing a channel. The adaptive adaptive equalizer. phase is accompl n the envelope of t ecovery.Timing reco ng the square of t esult to DPLL circu as a clock input t iltered, gain contr distortion consi nee, phase error, mmunication channe emove the main eff e adaptive aqualizi attemps to correct response that is th equalizer is a ished by closely he received signal, very loop commences he input signal and it, culminates with o ADC. The received oiled, and properly sts primarly of frequency error 1. The adaptive ects to intersymbol er is effectively a for amplitude and e complement of the decision feed-backAt the equalizer output the received signal points match the transmitted signal points with the addition of relatively small error vectors. The decision logic examines these points to determine the closest ideal point to reach the received point. In this work, Trellis coding is used to modulation. In this modulation technique, by using expanded signal sets, coding and signal mapping processes are applied making the minimum Euclid distance between signal sequences the largest. Using this type of structure causes the distances between signals grow significantly even if the same information speed, band width and signal power is used. The reason for use of Trellis term is that those systems can be described with similar state transition diagrams to Trellis diagrams in binary convolutional codes. Viterbi algorithm used in decoding binary convolutional codes is also used in Trellis coded modulation procedure. In this algorithm, decoding operation is started after a certain sequence length of signals are received. This means, a decision operation is not done as soon as the signal is received. This sequence length is called code decoding depth and determined by designer. It is clear that to choose this parameter as big as possible shall reduce the error during decoding. Finally the decoded data is descramled and sent to Data Terminal Eqiupment. XI

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