Uzay Kaydırmalı Anahtarlama Modülasyonu İçin Yüksek Başarımlı İşbirlikli İletişim

Abstract

Son yıllarda ses ve veri iletiminde artan kullanım oranlarını karşılamak amacıyla çok-giriş çok-çıkışlı telsiz iletişim sistemleri ortaya atılmıştır. Bu çalışmada, çok-giriş çok-çıkışlı sistemlerin sahip olduğu karmaşıklık ve maliyet gibi olumsuzlukları en aza indirirken başarımdan ödün vermeksizin yüksek veri hızları sağlayan uzaysal modülasyon ve onun özel bir biçimi olan uzay kaydırmalı anahtarlama modülasyonu ele alınmaktadır. Uzaysal modülasyonda bilgi genlik/faz modülasyonlarının yanında birden fazla anten arasından belirlenen etkin anten indisleri de kullanılarak hedefe iletilmekte iken uzay-kaydırmalı anahtarlama modülasyonunda bilgi iletimi için yalnızca etkin anten indisinden yararlanılmaktadır. Bu çalışmada, literatürde uzay-kaydırmalı anahtarlama modülasyonunun Rayleigh sönümlemeli kanallardaki bit hata başarımı için literatürde verilene kıyasla bilgisayar benzetim sonuçlarıyla daha uyumlu bir birleşim üst sınırı verilmiştir. Artan modülasyon düzeyi ve alıcı anten sayısı ile birlikte uzay-kaydırmalı anahtarlama modülasyonunun hata başarımının, yalnız genlik/faz modülasyonunun kullanıldığı durumdaki hata başarımını geçtiği bilgisayar benzetimleriyle gösterilmiştir. Uzay-kaydırmalı anahtarlama modülasyonu, her işaretleşme aralığında belli sayıda verici antenden yalnız birinin etkin hale getirilmesine dayandığından klasik biçimiyle verici çeşitlemesi sağlamamaktadır. Tez çalışmasının ikinci aşamasında, uzay-kaydırmalı anahtarlama modülasyonunu işbirlikli iletişim ile bütünleştiren, literatürde verilen yapılar ele alınmış, işaretlerin bir röle üzerinden kestirilip tekrar hedefe iletildiği çöz-ve-aktar tekniği uzay kaydırmalı anahtarlama modülasyonuna uyarlanarak kuramsal hata başarım analizi yapılmıştır. Basitlik, hızlı çalışma ve düşük karmaşıklık gibi avantajlarına karşın çöz-ve-aktar röleli uzay kaydırmalı anahtarlama modülasyonunun başarımının büyük ölçüde rölenin hata başarımına bağlı olduğu ve çeşitleme kazancı sağlamadığı gösterilerek hedefte karar verilirken rölenin olumsuz etkilerini en aza indiren yüksek başarımlı çöz-ve-aktar röleli uzay-kaydırmalı anahtarlama modülasyonlu yeni bir işbirlikli sistem önerilmiştir. Önerilen sistemin hata başarımının klasik çöz-ve-aktar röleli sistemin başarımına olan üstünlüğü, sağladığı çeşitleme kazancı, birleşim üst sınırına dayalı kuramsal analizlerle ve bilgisayar benzetim sonuçlarıyla ortaya konmuştur. Kuramsal hata analizi önce iki verici anten için gerçekleştirilmiş daha sonra genel anten sayısı için genelleştirilmiştir. Son olarak, tezde ele alınan işbirliksiz ve işbirlikli uzay-kaydırmalı anahtarlama modülasyonlu sistemlerin başarımları bir arada verilerek karşılaştırmalar yapılmıştır.

In recent years, multi-input multi-output (MIMO) wireless telecommunication systems has been proposed to meet the increasing usage ratios for voice and data transmission. Multi-input multi-output systems can be used in different ways in order to achieve multiplexing, diversity and antenna gains. Beside their advantages, multi-input multi-output systems also have disadvantages such as inter-channel interference, inter-antenna synchronization and multiple radio frequency chains. Inter-channel interference: Multiple antenna channels of one transmitter transmitting at the same time interfere to each other’s. Inter-antenna synchronization: Especially for space-time based systems, transmitting all symbols at the same time is required for the best performance. Multiple radio frequency chains: In order to make multiple antenna work, every antenna needs its hardware. In this study, spatial modulation, which achieves high data rates without sacrificing the performance while minimizing disadvantages of multi-input multi-output (MIMO) systems like complexity and cost, is discussed firstly. Data is transmitted to the destination by using active antenna indices chosen from a few antennas in addition to amplitude/phase modulations in spatial modulation. Spatial modulation transmitter has a combined modulator which performs amplitude/phase and antenna indice modulations. One part of transmitted bit block determines the amplitude/phase modulated symbol and the other assigns the active antenna. In order to decide the transmitted bit block, receiver estimates which symbol is sent from which transmitter antenna. Sub-optimum and optimum detection rules, which are given in the literature, are explained for spatial modulation receiver. In sub-optimum receiver, active antenna indice is detected firstly using maximal ratio combining (MRC). Secondly, amplitude/phase modulated symbol is estimated with demodulation function. In optimum receiver, all possible antenna indices and amplitude/phase modulated symbols are tested at the same time using maximum likelihood (ML) rule. For spatial modulation, its own advantages and disadvantages are listed finally. After that, space shift keying modulation, which is special and simpler version of the spatial modulation because of disabling amplitude/phase modulation and using only active antenna indices, is studied in detail. Using maximum likelihood detection rule given in the literature, obtaining pairwise error probability for space shift keying modulation is explained. In this study, a new union bound is given for the bit error probability of space shift keying modulation in Rayleigh fading channels, which is more consistent with the simulation results compared to that given in the literature. It is shown that with increasing modulation level and receiver antenna number, the error performance of space shift keying modulation exceeds that of the system where amplitude/phase modulation is used. Cooperative communication, which emerged as an alternative of multi-input multi-output system, is investigated shortly in this study. Being a kind of virtual multi-input multi-output system, cooperative communication has different methods determined according to transmitting form of relay. These methods are classified in three topics: amplify-and-forward relaying, detect-and-forward relaying and coded cooperation. Cooperative communication protocols, which assigns the working behavior of the relay and recevier, are also explained. They are fixed relaying, selective relaying and incremental relaying. Then, high performance demodulation technique in the literature for decode-and-forward relaying using classical amplitude/phase modulations is explained in detail. For decode-and-forward relaying, maximum diversity gain can be achieved in amplitude/phase modulations by maximal ratio combining (MRC). Nevertheless, maximal ratio combining can’t obtain exact diversity gain. So, a new technic called cooperative-maximal ratio combining (C-MRC) is developed [20]. C-MRC doesn’t use only relay-destination parameters when estimating the signal coming from source. Taking into account source-relay-destination transmitting line as a single way, an adaptive weight coefficient for this way is calculated. This coefficient is used to weighting the relay’s effect when deciding in the destination. Thanks to the weighting, full diversity gain can be achieved. Space shift keying modulation with its classical form doesn’t provide transmit diversity. It is because; only one antenna of a few antennas is active during every signaling interval. In the second part of thesis study, systems integrating space shift keying modulation with cooperative communication in the literature are discussed. In this context, amplify-and-forward relayed space shift keying modulation is described briefly. Its performance is given with theoretical and simulation curves. It is assumed that one amplify-and-forward relay with one receiving and transmitting antenna amplifies the signal coming from source with a specific gain value and forward it to destination. Destination estimates the signal utilizing source-destination and relay-destination lines both. Exact diversity gain is achieved in the destination by the amplify-and-forward relayed space shift keying modulation. After the amplify-and-forward relayed space shift keying modulation, decode-and-forward relayed cooperative communication methods proposed to space shift keying modulation in the literature are analyzed and it is explained that these methods work under some assumptions. So, classical detect-and-forward relaying method working under any assumptions and being complexity-free is adapted to space shift keying modulation in this study. In classical detect-and-forward relaying, relay just estimate the signal coming from source and send it to the destination. Theoretical error performance analysis of detect-and-forward relayed space shift keying modulation is derived considering the relay. Probability of making erroneous decision in the destination in case of relay makes mistake is explained. It is shown that when relay sends erroneous signal, bit error probability in the destination gets constant with high signal-to-noise ratio and aproaches to one with increasing transmitting antenna number. The performance of detect-and-forward relayed space shift keying modulation and effect of relay to the performance are showed with the theoretical and simulation results. Despite of its advantages such as simplicity, fast working, and low-complexity, it is showed that the performance of decode-and-forward relayed space shift keying modulation is highly dependent on the error performance of relay and does not provide diversity gain. So, a new cooperative scheme called high performance decode-and-forward relayed space shift keying modulation which minimizes the negative effects of relay when deciding in the destination is proposed. In the high performance decode-and-forward relayed space shift keying modulation, relay’s effect on deciding in the destination is weighted adaptively by modifiying detection rule with a weight coefficient. It is calculated by using the Euclidian distance between channel fading coefficents affecting the performance of space shift keying modulation directly. Firstly, weights of source-relay and relay-destination lines are calculated instantaneously, then weight of source-relay-destination line considered as a single transmission line like in C-MRC is obtained by the smallest of two weights. The superiority of the proposed system’s error performance over classical decode-and-forward relayed system’s and its diversity gain are introduced with theoretical analysis based on the union bounding technique and computer simulation results. To derive the bit error probability of the high performance decode-and-forward relayed space shift keying modulation system, condition of the signal coming from the relay and two possible weight coefficients are considered. Theoretical error analysis is firstly performed for two transmitter antennas and then it is generalized for all number of transmitter antennas. Finally, comparisons are made between performances of noncooperative and cooperative space shift keying modulation systems which is discussed in the thesis. Simulation results are given for different numbers of transmitting antennas. The performance of the high performance decode-and-forward relayed space shift keying modulation is superior than decode-and-forward relayed space shift keying modulation and very close to the amplify-and-forward relayed space shift keying modulation with its diversity gain.