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V.32 modemin hata başarımının bilgisayar benzetimi ile incelenmesi

Performance analysis of V.32 modem

  1. Tez No: 14420
  2. Yazar: CEM KÖSE
  3. Danışmanlar: YRD. DOÇ. DR. ÜMİT AYGÖLÜ
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1991
  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: 108

Özet

İV ÖZET Bu çalışmada ilk olarak» daha yüksek hızda veri iletim olanağı sağlayan kafes kodlu modül asyon tekniğinin gelişimi kısaca anlatılmıştır. Modemler hakkında genel bir bilgi verilmiş ve tipleri hakkında açıklamalar yapılmıştır. CCITT V32 modeminin özellikleri verilmiş ve modül asyon ve demodülasyon işlemleri anlatılmıştır. Daha sonra, kafes kodlu modül asyonun yapısı ve temel kavramları anlatılmıştır. 9600 bps'lik V32 modeminde seçimli olarak kullanılmakta olan 90 'lik dönmeyle değişmeyen geri beslemeli kodlar için kurallar verilmiş, tasarım adımları açıklanmıştır. Alıcıda demodülasyon sırasında oluşan işaret elemanları dönmelerinin giderilmesinde kullanılan farksal kodlama yöntemi an latı lmı ştı r. İdeal olmayan iletim kanalı nedeniyle oluşan simgelerarası girişim anlatılmış ve giderme çaresi olarak uyar 1 amali dengeleyicilere giriş yapılmıştır. Bu çalışmada kullanılan ve dengeleyici çıkışında oluşan karesel ortalama hatayı en aza indirmeye çalışan» iki boyutlu en küçük ortalama karesel hata algoritması açıklanmıştır. Benzetim modelinde ilk olarak kanal etkisi gözönüne alınmadan çeşitli işaret gürültü oranlarına göre vi terbi kod çözme tekniği kullanılarak hata başarım eğrisi çıkarılmıştır. Daha sonra sisteme kanal eklenmiş ve getirdiği bozucu etkiyi giderebilmek için ayrıca bir uyar lamalı dengeleyici alıcı girişine yerleştirilmiştir. Bu durumda karar devresinde elde edilen hatanın iterasyon sayısına göre değişim eğrileri, kanal ve dengeleyici çıkış işaret eğrileri çizdirilmiş ve dengeleyicin sistemin çeşitli işaret gürültü oranları için hata başarım eğrisi çıkarılmıştır.

Özet (Çeviri)

SUMMARY PERFORMANCE ANALYSIS OF V.32 MODEM Developments in semiconductor technology cause many theoretical digital signal processing techniques be used in practice and as a result of this, digital systems are preferred to analogue systems. Low speed modems desi gned accor di ng to phase and frequency modulation principle at the beginning» are developed according to ways having higher error per for mance. In this study, the structure of a CCITT V32 modem using trellis coded modulation technique is investigated and a communication system consisting this modem is studied for error performance by computer simulation. The trellis coded modulation technique in channels with limited bands had been first introduced by Ungerboeck, which gives better results, has taken its place in today's data communication systems. In this way, coding and modulation processes are considered as a whole. The excess obtained by expanding the signal set to be transmitted, as in con vol uti onal encoders, is used to enlarge the free Euclid distance between signal sequences. In receiver, by using Vi terbi decoder, trellis coded modulation signals are directly processed. State transitions in encoder structure are determined from modulated channel signals and, thus, instead of free Hamming distance, free Euclid distance is becoming the measure for performance.. Ungerboeck, has shown that, for PSK and QAM systems, with trellis structures increasing transmitted signal set element number by double, 3-4 dB coding gains can be obtained.VI Lee-Fang Wei, by designing binary encoders transparent to 90* 180*270° phase ambiguities, matching binary sequences in accordance with channel signals has shown that those ambiguities can be eliminated. By using such coding techniques it is possible to transmit data in higher speeds with the same band width. In recommandations published by CCITT, the error performance of high speed modems are increased in this way. It is true that the telephone network is the most common network today. Therefore, this network is also used for other communication means other than telephone conversations. Most important among them is no doubt, the data communication. But telephone network is not suitable for digital systems to be connected directly. This is because, that this network is suitable for transmitting only the frequency band 300-3400 Hz, cannot transmit direct current and due to the group delay properties the impulse shapes are disturbed. The modems are used to convert digital signals to voiee frequency signals. By using various techniques, the bit speed can be hold faster than line transmission speed. Basis of information packaging is corresponding one modulation step to a sequence of bits, instead of one bit. Information packed like this shall be decoded on the receiving side. Modulation being multilevel causes modulation steps to be reduced and as a result the signal to noise ratio is reduced. It becomes possible to reach data speeds, which are under normal conditions not transmit t able on telephone channels, by keeping speed low on line. Such procedures are used in modems working% at 2400 bps or higher speeds. In CCITT V32 modem, coming 4 bits data sequence is changed into a two-dimension transmission signal by using trellis coding modulation technique. In order to supply the uniform distribution of signal to frequency area before this transformation, the data coming to modem is put through a scrambler. In this modem, where data speed is 9600 bps and transmission speed is 2400 symbol /s, the transmitted signal occupies the whole telephone band and therefore the seperation of channels is supplied by echo cancelling techniques.vıı In part three, the trellis coded modulation technique is investigated and basic notions are given. In technique of trellis coded modulation, information is modulated by an encoder with finite states and the signal received at receiver is turned to its original state by a sequance of decoders. 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 sturueture causes the distances between signals grow significantly even 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 con vol uti onal codes. The difference in trellis ceded modulation is that modulation signals are corresponded to trellis branches instead of binary numbers. Convol uti onal channel coding with expanded signal space is a technique improving error performance of synchronous data transmission without limiting data speed or requiring broad band width. Generally speaking, this coding technique consist of applying m data bit groups to the Input of a finite state machine. This machine expands each input group to m+1 bit groups by keeping some specific bits stored in encoder and being in conformity with predetermined logical combi nati ons. This coding technique, being a special case of convol uti onal codes, is realized by mapping on grouped signal sets. Signal subsets are determined so that the distance between them is largest. This process depends on the principle of dividing signal set into smaller subsets and thus increasing distance between elements in set. Vi terbi algoritm used in decoding binary convol uti onal codes is also used in trellis coded modulation procedure. In this algoritm, 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.VI 1 1 Since the minimum Euclid distance between valid signal elements is increased, coding gain occours. Against this advantage, phase ambiguities during obtaining carrier signal in receiver demodulation cause received signal elements be rotated according to transmitted ones. This causes errors in sequentially received data and system's performance is reduced. As it is used to be, the problem of phase ambi qui ties can be solved by differential coding technique. Here differential coding technique is applied to convol uti onal channel coding technique and by this, obtained cedes are not effected by signal element rotations. If it is supposed that the Gauss noi se is added to the channel, si nee at the receiver decoding operation is done by Vi terbi algoritm, system's error probability is limited by free distance from bottom. In part four, disturbances in signals due to non -ideal transmission channel and eliminating methods are given. In telephone networks, characteristics of line to be connected and the number of exchanges are not known previously. When the trasnsmission speed rises, the changes in average characteristics in used channel may prevent healthy transmission. For this reason in high speeds, in order to prevent inter symbol interference adaptive equalizers shall be used. Adaptive equalizers in basic are digital filters, whose cofficient can be adjusted according to a determined a proximity measure. By using such filters, the opposite of telephone channel characteristic is approached and the total frequency characteristic of transmission media is straightened. Intersymbol interference is reduced to minimum. For this reason various adapted stabilizing methods are developed. Before starting data transmission, by sending a suitable signal sequence the stabilizer coefficients are determined. Since the channel properties are not known previously, by measures in receiver it is possible to proximate the stabilizer coefficients to most suitable values step by step. According to different ways of calculating coefficients, different stabilizing methods are obtained. In method of the least mean square error, the mean square error occured at the output of equalizer is tried to be minimized. In systems quadrature amplitude modulation is used, the signal is divided among twoIX carriers which has QO° phase difference. For this reason in systems using this modulation technique, channel and equalizer are modeled as a complex function. In part five, in order to study the error performance of this this modem a simulation model is constructed. First of all, by using this model, error performance of channels with Gauss noise is examined. In this examination modulation and demodulation operations and channel 's disturbing effect have not been considered. In second part of simulation, effect of transmission channel has been considered and by using an adaptive equalizer this disturbing effect has been tried to be eliminated. After adding channel and adaptive equalizer to the system, for ideal and disturb channel, the change of equalizer's mean square error with iteration number is studied and equalizer's input and output signals has been plotted in two dimensional plane. In the system with channel and equalizer, for ideal and disturbing channel, the change of error performance with signal to noise ratio has been examined.

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