Zamanla değişen kanalların kestirimi ve uyarlamalı kodlama
The Estimation of time varying channels and adaptive coding
- Tez No: 66775
- Danışmanlar: DOÇ. DR. ÜMİT AYGÖLÜ
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 130
Özet
ÖZET Hareket özgürlüğü, sayısal iletişim sistemlerinin gelişiminde en son eğilimdir. Böyle bir sistemin amacı, iletişim hattının her iki ucunda, kullanıcının hareket yeteneğini sınırlayan engellerin kaldırılmasıdır. Bu tür hareketli sistemlerde, iletişim hattı sönümlemeli ve zamanla değişen karakteristiklere sahiptir. Buna rağmen, bildik yöntemler, başarımı istenen bir düzeyde tutabilmek için, ses ya da veri iletiminde kanalın en kötü durumuna göre seçilen bir hata düzeltme yeteneğini dikkate alır. Bu yaklaşım hareketli radyo kanallarının zamanla değişen yapılarına yanıt vermede yeterli değildir. Oysa, etkin hata kontrolü, gerçek kanal koşullarına göre, en uygun kodu seçen uyarlamalı bir kontrol sisteminin kurulmasıyla gerçekleştirilebilir. Burada, öncelikli problem herhangi bir anda kanalın içinde bulunduğu durumu“doğru kestirebilmek”; ikincil problem ise, kanalın bu durumunda en iyi başarımı sağlayabilecek kodu seçerek devreye sokabilmektir. Bu tezde, kanal durumunu kestirebilmek için bit - hata olasılığını dikkate alarak literatürdekilerden farklı bir kestirim yöntemi önerilmiştir. Kanalın sönümlemeli yapısı ile etkin savaşım amacıyla, çoklu kafes kodlamalı modülasyon tercih edilmiştir. Ayrıca, literatürde yer alan,“kanalın farklı durumları için farklı kodlama oranlarını kullanma”yaklaşımı terkedilmiş; verim aynı kodlama oranı sayesinde sürekli“sabit”tutulurken, kanalın farklı durumları için seçilen kodların etkinlikleri farklı“işaret eşleştirme yöntemleri”kullanılarak sağlanmıştır.
Özet (Çeviri)
SUMMARY THE ESTIMATION OF TIME VARYING CHANNELS AND ADAPTIVE CODING In recent years, the number of communication or audio - video systems that treat voice or image signals as digital data is rapidly increasing. The material that computers process or store is digital data as well. One of the important characteristics of digital signals is that they are more reliable in a noisy environment than analog signals. Since the detector for digital data may only decide whether each symbol is a 0 or a 1, digital symbols can often be detected perfectly, provided that the noise is weak. However, when the noise is not weak, the detector may make an erroneous decision, that is, it may decide that a symbol is a 1 although it was originally a 0. But if the data are coded, that is, some appropriate check (redundant) symbols are annexed to the data symbols, the decoder can correct or detect certain errors. Thus, when a signal is represented as digital data, we can make the signal detection more reliably by adding check symbol to the data symbols. This technique is called error-control coding. When choosing an appropriate error-control code for a digital communication or recording system, a number of factors must be considered. The first of these is the type of errors that occur in the system. Errors are classified as one of the three types, namely, random errors, burst errors, and byte errors. For each type of error, there exist codes that can effectively detect and/or correct errors of that type. The other factors to be considered are the required error-correction or error- detection capability, the allowable number of redundant symbols, and the size, complexity, and speed of the decoder. Naturally, a trade off among these factors must be made. VIIn addition to the selection of the code, we must choose between two error- control strategies : (1) Correct any errors at the decoder according to a given rule ( FEC - forward error correction ) (2) Request a retransmission of erroneous data block ( ARQ - automatic repeat request ) Freedom of mobility is the latest trend in the evolution of digital communication systems. This trend has portrayed itself in the implementation of digital cellular systems and recent proposals for a universal digital portable communication system [1] - [4]. The objective of such a system is to remove the barriers which limit the mobility of the users at both ends of a communication link. The radio spectrum is a scarce resource. Therefore, one of the most important objectives in the design of a digital portable radio system or a digital cellular system is the efficient usage of the available spectrum to accommodate the ever-increasing demand. This must be done with no sacrifice in power or transmission quality. Power consumption must be minimized in order to limit the size of the portable units and the ensure safety from electromagnetic radiation hazard. To minimize the required power without any sacrifice in bit - error - rate ( BER ) performance or in the required bandwidth, error control techniques can be applied. Trellis - coded modulation ( TCM ) [5], [6], which is based on combining the functions of coding and modulation, has been widely recognized as a powerful error control technique suitable for applications in mobile communications [7], [8]. In classical digital communication systems, the function of modulation and error-correction coding are separated. In conventional multilevel (amplitude and/or phase) modulation systems, during each modulation interval the modulator maps m binary bits into one of M = 2m possible transmit signals, and the demodulator recovers the m bits by making an independent M-ary nearest- neighbour decision on each signal received. Conventional encoders and decoders for error correction operate on binary, or more generally Q - ary, code symbols transmitted over a discrete channel. With a code of rate k/n < 1, n-k redundant check symbols are appended to every k information symbols. Since the decoder receives only discrete code Vllsymbols, Hamming distance (the number of symbols in which two code sequences or blocks differ, regardless of how these symbols differ) is the appropriate measure of distance for decoding and hence for code design. A minimum Hamming distance cC, guarantees that the decoder can correct at least [ ( d^n - 1 ) / 2 ] code-symbol errors. If low signal -to- noise ratios or non-stationary signal disturbance limit the performance of the modulation system, the ability to correct errors can justify the rate loss caused by sending redundant check symbols. Generally, there exist two possibilities to compensate for the rate loss : (1) increasing the modulation rate if the channel permits bandwidth expansion, or (2) enlarging the signal set of the modulation system if the channel is bandlimited. The latter necessarily leads to the use of nonbinary modulation (m > 2). However, when modulation and error-correction coding are performed in the classical independent manner, disappointing results are obtained. TCM has become attractive as a combined coding and modulation technique for digital transmission over bandlimited channels. Its main attraction comes from the fact that it allows the achievement of significant coding gains over conventional uncoded multilevel modulation without compromising bandwidth efficiency. In the receiver, the noisy signals are decoded by a soft-decision maximum- likelihood sequence decoder. The independent“hard”signal decisions cause an irreversible loss of information in the receiver. The remedy for this problem is soft-decision decoding. In that procedure, the decoder operates directly on unquantized“soft”output samples of the channel. Signal waveforms representing information sequences are most impervious to noise-induced detection errors if they are very different from each other. Mathematically, this translates into the requirement that signal sequences should have large distance in Euclidean signal space. The essential new concept of TCM that led to the afore-mentioned gains was to use signal-set expansion to provide redundancy for coding, and to design coding and signal-mapping functions jointly so as to maximize directly the“free distance”(minimum Euclidean distance) between coded signal sequences. Pragmatic TCM performs nicely in AWGN environment. But it is not nearly as good in the fading environment. The major reason for this degradation in viiiperformance is the presence of parallel transitions in their state diagram which gives rise to single signal error events. As the channel for either a particular or vehicular unit, which we are interested in, can be characterized by time - varying multipath fading; it is needed to solve that problem. Although appropriate criterion for optimum TCM design on the AWGN channel is the maximization of the free Euclidean distance; when TCM is used on a Rician fading channel with interleaving / deinterleaving, the design of the code for optimum performance is guided by other factors: The length of the shortest error event path and the product of branch distances along that path. When these properties are used as a motivation for good code design, multiple trellis coded modulation (MTCM) [8], [9], wherein more than one channel symbol is assigned to each trellis branch, is a natural choice in this situation. Errors statistics on most real channels, for instance multipath radio channels (mobile terrestrial and satellite, HF, UHF or VHF), telephone links and magnetic recording systems, are time varying. Traditional methods suggest the use of fixed rate codes for the transmission of speech or data on a time - varying channel. Fixed rate codes, however, fail to explore the time varying nature of the channel. Therefore, in order to keep the performance at a desirable level, they are designed for average or worst channel conditions. Efficient error control on time - varying channels can be performed by implementing an adaptive control system where the optimum code is selected according to the actual channel conditions [11] - [13]. The availability of a wide range of code rates without changing the basic codec structure motivate the use of punctured convolutional codes [14] - [17] with the soft - decision Viterbi decoding algorithm in adaptive manner [18]. Alternatively, Alamouti and Kallel proposed“adaptive TCM”[19]. According to this scheme, during good channel conditions, more information is sent using high rate pragmatic trellis codes. As channel conditions become worse, lower rate trellis codes and repetition codes are applied. Though statistical characteristics of real channels can significantly vary with time, propagation experiments for various types of channels [20] - [22] indicate that the basic system parameters remain constant over short time intervals. Error statistics for most real channels can thus be obtained by using a quasi - stationary model. In this model, it is assumed that the IXchannel is“stationary”over certain short time interval. A time - varying channel is thus represented by M stationary-state model for representing real channels is the finite - state Markov chain model [23]. In order to implement the adaptive coding scheme it is necessary to use a return channel. There are two different methods to determine the current channel state. The first one, which is proposed by Vucetic [18], is based on counting the number of erroneous blocks. That is, the channel state is chosen according to the channel bit error rate. In the second method, which is proposed Alamouti and Kallel [19], the current state is determined, computing immediate signal- to-noise ratio (SNR). In this thesis, we have proposed an adaptive trellis-coded scheme based on FEC to explore the time-varying nature of Rayleigh fading channels. The scheme is extremely effective in combating fading and is pragmatic in terms of its design and implementation. Assuming that the channel has two states; GOOD and BAD, we used Gilbert's channel model. While there is only AWGN in the GOOD state, transmitted symbols are disturbed by AWGN and Rayleigh fading in the BAD state. We employed two different 8PSK multiple TCM codes, one of them being optimum on AWGN and the other in fading environment, having the same code rate, 2/3. So in our work, we proposed constant throughput for the nonstationary channel. The difference of the effectiveness between the two codes comes from their different signal mapping. The scheme responds to the actual channel by selecting the optimum code for corresponding state. In order to estimate the channel state, it is needed to use a return channel. The rate of transmission in the feedback channel effectively reduces the overall throughtput of the system. However, we assumed that, the degradation in throughtput performance due to the transmission in the feedback channel required for adaptation purposes is insignificant. In the thesis, we ignored the effect of the transmission in feedback channel.The thesis is organized as follows. In Section I, the time-varying channels are considered. The main items related with the adaptive coding and previous works are introduced. In Section II, error - control coding is explained. In Section III, time-varying channel models given in the literature are overviewed. Section IV concerns with the explanations on TCM and MTCM. The necessity of MTCM and also, Viterbi decoding algorithm are explained there. In Section V, the codes employed are explained. System model and channel state estimation procedures are discussed. And the presentation of the adaptive trellis-coded scheme which have been proposed is in this section. Finally, In Section VI, the performance of the proposed scheme is analysed by computer simulations and performance comparisons are made between the usage of fixed rate codes and the adaptive scheme. XI
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