Sönümlemeli kanallarda kafes kodlamalı sistemler için birleşik serpiştirme tekniği

Combined interleaving technique for trellis coded systems in feding channels

Tez No: 75424
Yazar: ERSİN ÖZTÜRK
Danışmanlar: DOÇ. DR. ÜMİT AYGÖLÜ
Tez Türü: Yüksek Lisans
Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1998
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı
Bilim Dalı: Elektronik ve Haberleşme Mühendisliği Bilim Dalı
Sayfa Sayısı: 175

Özet

Hareket özgürlüğü sayısal iletişim sistemlerinin gelişimindeki en son eğilimdir. Yer-uydu iletişimi gibi hareketli sistemlerde yüksek kalitede ses ve sayısal veri iletiminin gerçeklenebilirliği için giderek artan bir gereksinim vardır. Bu sistemler hem güç hem de bantgenişliği açısından sınırlıdır ve iletişim ortamı sönümlemeli ve zamanla değişen özelliklere sahiptir. Doppler frekans kayması, bitişik kanallar arasındaki enerji saçılması, vericilerdeki doğrusal olmayan güç kuvvetlendiricileri ile çok yollu sönümleme ve gölgelemenin birleşimi gibi telefon hatlarında olmayan fakat uydu kanallarında olan ek zayıflatma kaynaklan, telefon hatlarında ulaşılan kodlama verimlerine gezgin iletişim sistemlerinde ulaşmayı engellemektedir. Bu tezde, gezgin iletişim sistemlerinde arzu edilen hata başarımını elde etmek için kafes kodlamalı modülasyonu, birleşik serpiştirme teknikleri ile birlikte kullanan bir yöntem önerilmiştir. Kafes kodlamalı modülasyon, sayısal iletişim sisteminin güvenilirliğini, iletim gücünü ve gerekli bantgenişliğini artırmadan güçlendiren birleşik bir kodlama ve modülasyon yapısıdır. Klasik sayısal iletişim sistemlerinde modülasyon ve hata düzeltme kodlaması tasarım sırasında ayrı ayrı düşünülür. Kafes kodlamalı modülasyon, yüksek dereceden modülasyon yapısı kullanımını katlamalı kodlarla birleştirir, alıcıda kod çözme işlemi, kanal çıkışında elde edilen gürültülü işaret vektörlerine dayanarak iki ayrı aşama yerine tek işlemle birleşik olarak yapılır. Sönümlemeli kanallarda istenen iletişim hızlarına ulaşmak için ek tekniklere başvurulması gerekmektedir. Serpiştirme bu tekniklerden birisidir. Bu yöntemde kodlayıcı çıkışındaki simgeler uzun süreli sönümlemenin yaratacağı istatistiksel bağımlılığı yok etmek için serpiştirilirler yani zamanda birbirlerinden uzaklaştırılmış olarak iletilirler. Pratikte serpiştirmenin derinliği sonludur ve maksimum sönümleme süresinin ortalamasına göre seçilir. Bu çalışmada, sönümlemeli ortamda iletişim sisteminin bit hata başarımını iyileştirmek için bit ve koordinat serpiştirmeyi birlikte kullanan bir kodlama ve modülasyon sistemi önerilmiştir. Çalışmada 2/3 oranlı 4 ve 8 durumlu iki farklı katlamalı kod 8-PSK modülasyonu ile birlikte kullanılmıştır. Bu yeni yöntemde, kodlanmış sistemin çeşitliliği 2/3 oranlı katlamalı kodlayıcı çıkışma yerleştirilen 3 bit serpiştirici ve modülatör çıkışma yerleştirilen 2 koordinat serpiştirici yardımıyla artırılmaktadır. Daha önce literatürde yer alan simge serpiştiricili, bit serpiştiricili ve koordinat serpiştiricili 3 ayrı iletişim modeli ile yeni yaklaşım bit hata olasılığı üst sınırlan ve benzetim sonuçlan açısından 2/3 oranlı 4 ve 8 durumlu iki ayrı katlamalı kod ve 8-PSK modülasyonu kullanılarak karşılaştırılmakta ve sonuçlar tablo ve eğriler yardımı ile gösterilmektedir.

Özet (Çeviri)

There is a growing need for reliable transmission of high quality voice and digital data in mobile communication systems. In recent years, the number of communication systems that treat voice or image signals as digital data is rapidly increasing. 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. Freedom of mobility is the latest trend in the evolution of digital communication systems. The objective of such a system is to remove the barriers which limit the mobility of the users at both ends of communication link. These systems are both power and bandwidth limited. In a power limited environment, the desired system performance should be achieved with the smallest possible power. One solution is the use of error-correcting codes, which increase the power efficiency by adding extra bits to the transmitted symbol sequence. This procedure requires the modulator to operate at a higher data rate and hence requires a larger bandwidth. İh a bandwidth limited environment, increased efficiency in frequency utilization can be obtained by choosing higher-order modulation schemes, but a larger signal power would be needed to maintain the same signal separation and hence the same error probability. When both limitations are imposed simultaneously, as in the mobile satellite application, it is most often not possible to achieve the desired throughput with either technique acting alone. In the past, coding and modulation were treated as separate operations with regard to overall system design. In particular, most earlier works on coded digital communication systems independently optimised conventional coding with maximised minimum Hamming distance and conventional modulation with maximally separated signals. Modulators and demodulators convert an analog waveform channel into a discrete channel, whereas encoders and decoders correct errors that occur on the discrete channel. In conventional multilevel (amplitude and/or phase) modulation systems, during each modulation interval the modulator maps n binary bits into one of M=2n possible transmit signals and the demodulator recovers the n bits by making an independent M-ary nearest neighbour decision on each signal received. With a code of rate (k/n) n-k redundant check symbols are appended to every k information symbols. Since the decoder receives only discrete code symbols, Hamming distance (the number of symbols in which two code sequences or blocks differ,) is the appropriate measure of distance for decoding and hence for code design.A minimum Hamming distance dmm11 guarantees that the decoder can correct at least (dj^, -l)/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 band-limited. The latter necessarily leads to the use of non-binary modulation. However when modulation and error correction coding are performed in the classical independent manner, disappointing results are obtained. About twenty years ago using random coding bound arguments, it was shown that considerable performance improvement could be obtained by treating coding and modulation as a single entity. TCM is a combined coding and modulation scheme for improving the reliability of a digital transmission system without increasing the transmitted power or the required bandwidth. TCM has been widely recognised as a powerful error control technique suitable for applications in mobile communications. 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. TCM schemes employs redundant non-binary modulation in combination with a finite-state encoder which governs the selection of modulation signals to generate coded signal sequences. The difference is that in TCM schemes, the trellis branches are labelled with redundant non-binary modulation signals rather than with binary code signals. The independent“hard”signal decisions cause an irreversible loss of information İn the receiver. The remedy for this problem is soft- decision decoding. In that procedure, the decoder operates directly on unquantised“soft”output samples of the channel. In the receiver the noisy signals are decoded by a soft-decision maximum likelihood sequence decoder. Shannon's information theory predicted the existence of coded modulation schemes with these characteristics more than four decades ago. The development of effective TCM techniques and today's signal processing technology now allow these gains to be obtained in practice. The most common application of such trellis coded modulation techniques is in the new generation of modems being developed for the telephone channel. In a land-satellite communication system, several reasons most of which relate to the additional source of degradation present on the mobile satellite channel but absent on the telephone channel, would preclude transmission at the high data rates achievable on the telephone channel. First, Doppler frequency shifts due to mobile vehicle motion can be a serious source of performance degradation if not compensated for. Second, the fact that the 5 kHz mobile channel is actually a slot in a frequency- divisions multiple access (FDMA) system, brings on the problem of interference due to energy spill over from adjacent channels. Third the satellite channel is inherently a non-linear one primarily due to HPA in the transmitter. The most serious source of impairment that does not exist on the telephone channel is the combination of multipath fading and shadowing. Fading, which for mobile satellite channels can be assumed to be modelled by a Rician distribution, not only introduces an error floor into the system but also makes the problem of carrier recovery more difficult. Finally even if the above sources of degradation were absent, the power limitation imposed by the mobile satellite channel would preclude transmission at the high data rates achievable on the telephone 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 innovative aspect of TCM is the concept that convolutional encoding and modulation should not be treated as separate entities, but rather, as a unique operation. As a conclusion, the received signal, instead of being fist demodulated and then decoded, is processed by a receiver that combines demodulation and decoding in a single step. The consequence of this is that the parameter governing the performance of the transmission system is not the free hamming distance of the convolutional code, but rather, over the additive white Gaussian noise channel, the free Euclidean distance between transmitted signal sequences. Thus the optimization of the TCM design will be based on Euclidean distances rather than on Hamming distances, so that the choice of the code and of the signal constellation will not be performed separately. 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 maximise directly the“free distance”(minimum Euclidean distance) between coded signal sequences. This allowed the construction of modulations codes whose free distance significantly exceed the minimum distance between uncoded modulation signals, at the same information rate, bandwidth, and signal power. 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 depending on the value of the Rician parameter K, the ratio of direct plus specular power to diffuse power. In particular for small values of K (the channel tends toward Rayleigh), the primary design criteria for high signal to noise ratio become the length of the shortest error event path and the product of branch distances along that path with a secondary considerations. Thus at low values of K, the longer is the shortest error event path and the larger is the product of the branch distances along that path the better the code will perform even though dfo* does not achieve its optimum value over the AWGN. As K increases the significance of these primary and secondary considerations shift relative to one another until K reaches infinity (AWGN) in which case optimum performance is once again achieved by a trellis code designed to maximise dfree. A mobile communication channel suffers from noise and fading due to multipath propagation. Fading causes burst errors in transmitted data. Error correcting codes are used to improve error performance in a mobile data communication. However a burst error causes decoding errors. In a radio communication system operating over a multipath environment, such as a mobile radio or indoor wireless system, the communication channel is often modelled as a Rayleigh fading channel. On slow Rayleigh fading channels, most error correcting schemes fail to provide significant coding gain. It is well known that the probability of error may be made to decrease exponentially with average energy to noise ratio with optimal diversity, even though the channel is subject to fading. The order of diversity for any coded system with a symbol interleaver is the minimum number of distinct symbols between two code words. Thus diversity can only be increased by preventing parallel transitions and increasing the constraints length of the code. If the fades are long enough, without interleaving coding deteriorates the error performance. This typically occurs when slow mobiles in adispersive environment. For example, coding for pedestrians communicating through land mobile channels should not be used without a technique alleviating the effects of the long fades. These techniques aim that uncorrelating the received energy of each consecutive code symbol. Spread spectrum and interleaving are two of these techniques. Interleaving is used to make burst errors into random errors which can be corrected by error correcting codes. The performance of error correcting codes is improved for a fading channel by utilising channel state information (CSI) in the decoding algorithm. For slowly fading channel coded modulation/interleaving showed good performance improvement. Interleaving destroys the fading correlation making the coded scheme acts as a kind of diversity technique. The order of diversity for these coded schemes is defined by the shortest error event path through the trellis of the code, higher diversity is obtained at the expense of more complexity. Since the minimum squared Euclidean distance is secondary factor in fading channels, an optimum scheme for AWGN channels may not be optimum for fading channels. Zehavi proposed an alternative approach for increasing the diversity of a coded system with as small as possible reduction in the free Euclidean distance of the code. The proposed system is based on a convolutional encoder followed by three bit interleavers. This approach yields a coded system with high diversity, which is proportional to de^ of the code. The results suggest that on a Rayleigh channel, the standard trellis codes may not be the correct approach for improving the reliability of the communication channel. It is shown that this approach yields a better coding gain over a Rayleigh channel as compared to the previous approach. Gagnon and Haccoun proposed a technique consisting of the permutation of each coordinate of the transmitted signals. This permutation may be viewed as an interleaving at the level of individual coordinates. An interleaving of coordinates is applied after the mapper. With this interleaving of coordinates, the constellation of the signal being transmitted may be modified. For example, the interleaving of coordinates for an 8-PSK signal set results in an approximate 16-QAM constellation on the channel. In this thesis, a coded system which combines a regular convolutional code, bit interleaving, coordinate interleaving, and an appropriate decoding technique is proposed. The proposed system deals with a method for increasing the diversity of the coded system with as small as possible reduction in the free Euclidean distance. The diversity of a coded system can be increased by using a regular convolutional code followed by three bit interleavers and a 8-PSK signal mapper followed by two coordinate interleavers. Bit interleaving is applied before the mapper and coordinate interleaving is applied after the mapper. A group of 3 bits at the output of the bit interleavers is mapped into the 8-PSK signal set according to the Gray mapping. The real and imaginer parts of the mapping signal are interleaved individually and new signals occur. Resulting interleaved signal points are digitally pulse shaped, and transmitted over the channel. At the receiver, the faded noisy version of the transmitted channel signal is passed through a coordinate deinterleavers, three metric computation units, and metric deinterleavers. Finally the decision on the transmitted sequence is taken with the aid of the Viterbi algorithm.The proposed system is evaluated with an analysis model. Ideal interleavers and deinterleavers are assumed, so that the combined interleavers and mapping can be viewed as three statistically independent communication modulators and channels. The outputs of the encoder are transmitted by three random modulators. At the receiver, the faded noisy version of the transmitted channel symbol is passed through three coordinate deinterleavers. Received signal components are deinterleaved and the original mapped signals are reconstructed. Then deinterleaved signals are passed through three metric computation units. An optimal decoder calls for a complicated metric which takes into account the apiori probabilities of transmission of all possible channel symbols associated with the output bit c'a. In selecting a decoding metric, a trade-off exists between simplicity of implementation, robustness of the system, and error performance. The decoder input unit computes the branch metric corresponding to all possible values of c. Finally these metrics are fed to the decoder which employs the Viterbi algorithm to find the binary data sequences C with the highest cumulative sum of metrics. Note that most of the coding gain was obtained through the diversity of the code, generated by the bit and coordinate interleavers. This gain is larger than the loss associated with the random nature of the modulation and the suboptimal decoding. The upper bound on the bit error rate of this approach is compared to the other interleaving techniques. Simulation programs were run to compare the four interleaving systems (symbol interleaving, coordinate interleaving, bit interleaving and bit-coordinate interleaving) with 4 and 8 state trellis codes. The simulation results verified that the proposed system outperforms its previously given counterparts. The results indicate that the best convolution^ code in conjunction with the proposed system (8-PSK modulation, bit interleavers, coordinate interleavers, and an appropriate soft decision metric) is a more attractive means of improving the reliability over a fading channel than the other systems. It will be assumed that the effect of the fading on the phase of the received signals is fully compensated for either by tracking it with some form of phase locked loop or with pilot tone calibration techniques. Thus these results will only reflect the degradation due to effect of the fading on the amplitude of the received signal. Also only the case is considered where interleaving/deinterleaving is employed to further combat the fading. This allows for considerable simplification of the analysis and is of great practical interest. The thesis is organised as follows. In section 1, previous works in the literature about the subject and contributions of the thesis are introduced. In section 2, fading environment and trellis coded modulation are explained and a baseline coding scheme for fading channels is considered. The baseline system uses a trellis code followed by a symbol interleaver. In section 3, individual interleaving techniques such as bit interleaving and coordinate interleaving given in the literature are overviewed. Finally in section 4, the new method which employs bit and coordinate interleavers together in order to increase the diversity of a coded system is explained. Upper bounds and simulations results for the bit error rate of the 2/3 rate coded 8- PSK system with 4 and 8 states and the comparisons with the previously given interleaved systems are presented.

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