Anten seçimli fiziksel katman ağ kodlama
Physical layer network coding with antenna selection
Yazar: MUSTAFA YAĞIMLI
Danışman: PROF. DR. İBRAHİM ALTUNBAŞ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı / Telekomünikasyon Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Son yıllarda telsiz iletişim sistemlerinin sönümlemeli kanallardaki hata başarımlarını iyileştirmek ve kanal sığalarını arttırmak amacıyla literatürde önerilen en önemli çeşitleme tekniklerinden birisi, alıcıda ve/veya vericide birden fazla anten kullanılarak sağlanan anten çeşitlemesidir. Bununla birlikte, anten çeşitleme tekniğinin özellikle güç ve boyut sınırlı gezgin birimlerde uygulanması pratikte zorluklara neden olmaktadır. Bu tür kısıtlardan dolayı, literatürde iletişimin gezgin birimler arasında işbirliğine dayandığı işbirlikli çeşitleme tekniği önerilmiştir. Bu yolla, kaynak, işaretini hedef alıcıya doğrudan ve/veya dolaylı olarak röle üzerinden yarı dupleks olarak iki zaman aralığında iletebilmektedir. İşbirlikli çeşitleme ile birlikte röleli telsiz iletişim sistemleri önem kazanmıştır. Elde edilen hata başarımını daha da iyileştirebilmek için, literatürde, röleli telsiz iletişim sistemleri ile çok antenli yapıların birlikte kullanıldığı pek çok yöntem önerilmiştir. Bu alanda yapılan çalışmaların arasından, daha düşük güç tüketimi ve devre karmaşıklığı sağlayan verici ve/veya alıcı anten seçimi tekniği öne çıkmaktadır. Anten seçimi tekniği, sağladığı hata başarımı ve anten çeşitlemesi kazancı bakımından özellikle gezgin birimler için çok uygundur. Bununla beraber, röleli telsiz iletişim sistemlerinin yarı dupleks olarak çalışması, bant verimliliklerini tam duplekse göre yarıya düşürmektedir. Bant verimliliği sorununa çözüm olarak literatürde ağ kodlama önerilmiştir. Klasik ağ kodlama, çift yönlü röleli kanallardaki iki terminalin iletişimi için gerekli dört zaman aralığını üç zaman aralığına indirmektedir. Klasik ağ kodlama tekniğine dayalı telsiz iletişim sistemlerinin kanal kapasitesini ve bant verimliliğini daha fazla arttırmak için literatürde fiziksel katman ağ kodlama önerilmiştir. Klasik ağ kodlamalı sistemlerin aksine, fiziksel katman ağ kodlama tekniğini kullanan sistem, çift yönlü röleli kanallardaki bilgi aktarımını iki zaman aralığında gerçekleştirebilmektedir. Bu sayede, fiziksel katman ağ kodlama tekniği veri hızında önemli ölçüde iyileşme sağlamaktadır. Literatürde, çok antenli fiziksel katman ağ kodlamalı sistemleri, hız ve hata başarımı açısından inceleyen çalışmalar bulunmaktadır. Bu çalışmalarda, çok antenli fiziksel katman ağ kodlama tekniği kullanarak iletim hızlarının arttırılmasına karşın, anten çeşitlemesinin ancak sınırlı sayıda teknik ile arttırılabileceği gösterilmiştir.Bu tez çalışmasında, literatürde ilk defa anten seçimli fiziksel katman ağ kodlama tekniği ve bu tekniğe dayalı üç farklı sistem önerilmiştir. Önerilen sistemler, tek antenli iki terminal ile çok antenli bir röleden oluşmakta ve rölede kuvvetlendir ve aktar tekniği kullanılmaktadır. Önerilen sistemlerin çıkışında sağlanan anlık işaret gürültü oranı değerlerine ilişkin hata olasılığı ifadeleri, frekans seçici olmayan ve yavaş sönümlemeli Rayleigh kanallarda M-seviyeli faz kaydırmalı anahtarlama modülasyonu için moment üretme işlevi tekniği kullanılarak çıkarılmıştır. Bunun yanı sıra, türetilen hata olasılığı sınır ifadeleri için asimptotik çeşitleme derecesi analizi yapılmıştır. Bilgisayar benzetimleri yapılarak analizlerin doğruluğu gösterilmiştir. Elde edilen teorik sonuçlara ve benzetim sonuçlarına göre, önerilen sistemlerin çeşitleme derecesinin röledeki toplam anten sayısına eşit olduğu ve bu sayede sistemlerin hata başarımlarının klasik fiziksel katman ağ kodlamalı sisteme göre önemli ölçüde iyileştiği gösterilmiştir. Ayrıca önerilen sistemlere ilişkin hata başarımları kendi aralarında karşılaştırılmış ve içlerinden en iyi başarımı, iki zaman aralığı kullanan ve iki anten seçimi yapan sistemin sağladığı gösterilmiştir.

Abstract:
Researchers in the communication society have mainly interested in performance improvement of wireless communication systems due to the fact that multipath fading has destructive effect on the performance of wireless communication systems. Diversity is one of the efficient techniques to combat the effects of multipath fading. The basic idea of diversity is that obtaining independently faded multiple replicas of transmitted signals at the receiver. Hence, the severity of the fading channel can be reduced, and this results in the increase of the error performance of the system. Several kinds of diversity techniques can be realized in different domains such as time, frequency, spatial, polarization and their combinations.In recent years, one of the most important diversity techniques proposed in the literature is the antenna diversity which is also known as spatial diversity. The antenna diversity is obtained by using more than one antenna at the transmitter and/or receiver. By the use of multiple antennas, multiple input multiple output systems have attracted great interest, and it is shown that multiple input multiple ouput transmission increases the system capacity and improves the error performance of the system significantly. The antenna diversity techniques mostly used at the receiver are maximum ratio combining and selection combining. Also, space time block and trellis coding techniques are the ones applied at the transmitter. In addition, the antenna selection technique, which can be used at the transmitter and/or the receiver side, has been proposed to obtain the multi antenna advantage with lower hardware complexity. Transmitting and /or receiving signals through a selected subset antenna provide full diversity transmission.In general, wireless devices have small size and limited power. Application of the antenna diversity especially on these devices causes practical difficulties. Due to these kind of restrictions, cooperative diversity, which is based on cooperation between mobile and/or fixed units, has been proposed in the literature. In the cooperative diversity technique, the source transmits its signal to the destination directly and/or through the relay. The relay either decodes or simply amplifies the received source signal and forwards it. Thus, destination receives two independently faded signals and doubles the diversity order of the system compared to the non cooperating scheme.Wireless relay communication systems have gained importance with the cooperative diversity. Also, relayed transmission provides extended coverage area without additional power transmission. In the literature, there are a remarkable number of studies that multi-antenna structures and wireless relay communication systems are used together. Among these studies, the antenna selection technique is the most popular one since it provides lower power consumption and circuit complexity. Therefore, the antenna selection technique is very suitable especially for mobile units.On the other hand, wireless relay communication systems operate in half-duplex mode that relay and terminals can not transmit and receive at the same time. Hence, four time slots are required for mutually exchange information between terminals. As a result, bandwidth efficiency of wireless relay systems are reduced by half compared to the full-duplex mode. Network coding is proposed in the literature as a solution to this problem. Classical network coding is a technique that each node of the network is allowed to perform coding instead of simply relaying incoming data. Each node performs "exclusive or" operation on the received information bits and generates its ouput. Employing network coding in wireless relay networks reduces the number of time slots to three. Besides, it was shown that network coding does not only provide higher bandwidth efficiency, but also other benefits such as saving energy, enhancing robustness, balancing network traffic, improving security. Using network coding reduces the total required messages and transmission power in the network as well. As a result, lifetime of energy-limited networks becomes prolonged. Network coding can use the rest of the links and distribute the traffic in a wider range of network. Moreover, without complex encryption algorithms, security of network can be strengthened by network coding.Because of the above advantages, the network coding has recently attracted much research interest. In order to further increase the bandwidth efficiency and channel capacity, physical layer network coding is proposed in the literature. Physical layer network coding uses the property of electromagnetic waveforms to perform coding. This enhances the suitability of the physical layer network coding system for wireless medium nature. Especially, physical layer network coding technique embrace the interference to improve the throughput performance of the wireless networks. In contrast to the classical network coding systems, the system based on physical layer network coding can exchange information over two-way relay channels in only two time slots. In the first time slot, terminals send their own signals, and the relay receives the superposed signal from two terminals. In the second time slot, the relay broadcasts the superposed signal to the terminals, and each terminal recover the other terminal?s information by subtracting its own signal from the received signal. In this way, physical layer network coding technique provides a substantial improvement in data rate.In the literature, physical layer network coding systems with multiple antennas have been investigated in terms of rate and error performance. It was shown that high data rates can be achieved by the use of physical layer network coding technique with multiple antennas. However, there are only few techniques providing antenna diversity gain. In one of the first papers, space time block coding and network coding techniques were used together and it was shown that the system based on these techniques achieve the full diversity order. Then, max-min antenna selection technique was applied to the system based on the network coding with multi-antenna relay, and shown that the system also provides full diversity gain. Lastly, maximum ratio transmitting technique is used with physical layer network coding technique, providing full diversity antenna gain.In this thesis, three different physical layer network coding systems with antenna selection have been proposed for the first time in the literature. The proposed systems consist of two single-antenna terminals and a multi-antenna relay that uses amplify and forward technique. The first proposed system accomplishes the two-way communication between terminals in three time slots. In the first and second time slots, terminals send their messages to relay respectively, and relay selects the antenna which has highest instantaneous signal to noise ratio. In the third time slot, relay combines the received signals and amplifies and forwards the signal over two different selected antennas with half transmission power.The second proposed system operates within two time slots achieving bidirectional communication. In the first time slot, both terminals transmit their information signals, and relay receives the superposed signal via only one antenna by performing the max-min antenna selection criteria on its antennas. In the second time slot, relay amplifies and forwards the signal through its selected antenna with full transmission power.The third proposed system uses two time slots as in the second proposed system. Unlike the second system, the third system selects two antennas at the relay. Antennas are selected seperately according to the highest path gain. The relay makes transmission using these two antennas with half power in the second time slot.All the proposed systems are analyzed according to their error performance. Towards this goal, error probability expressions of the instantaneous signal to noise ratio value obtained by the output of the proposed systems over frequency non-selective and slowly Rayleigh fading channels for M-ary phase shift keying modulation are derived by using the moment generating function method. Besides, asymptotic diversity orders are derived from the upper and lower bound expressions of error probability. Theoretical results are confirmed by computer simulations. According to obtained theoretical and simulation results, the diversity order of the systems is equal to the number of total antennas at the relay, and thus the proposed systems provide significantly better error performance with respect to the one-antenna relay systems. Also, error performances of the proposed systems are compared with each other. Consequently, the third system which uses two time slots for communication and two antennas selected at the relay gives the best error performace.For future studies, all the proposed systems can be examined under different channel models such as Nakagami-m fading and shadowing. The new systems with multiple antennas and/or multiple relays can be also considered. The optimization of the instantaneous power allocation at the terminals and relay can be investigated. All the above mentioned cases can be analyzed according to imperfect channel estimation and correlated antennas conditions.