Bilişsel telsiz iletişimde otomatik yineleme istemeli karma aktarım protokolleri
Hybrid relaying protocols with automatic repeat request in cognitive wireless communication
Yazar: MURAT BARAN
Danışman: PROF. DR. HASAN ÜMİT AYGÖLÜ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Savunma Teknolojileri Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Savunma ve Savunma Teknolojileri = Defense and Defense Technologies

Özet:
Telsiz iletişim teknolojilerindeki gelişmeler sonucunda daha karmaşık, daha güvenilir ve daha hızlı telsiz iletişim sistemleri boyutça küçülmüş ve maliyetleri azalmıştır. Bunun yanında telsiz iletişim sistemlerinin başarımlarının tel ile bağlı iletişim sistemlerinin başarımlarına yaklaşması telsiz iletişim sistemlerine olan kullanıcı talebini hızla artırmıştır. Bilindiği gibi telsiz iletişim sistemleri radyo spektrumunu paylaşarak iletişim kurarlar. Bu nedenle radyo spektrumu belli telsiz iletişim tekniklerinde kullanılmak üzere bölgelere ayrılmış, bu radyo spektrum bölgeleri de ihtiyaca göre servis sağlayıcıların kullanımına ayrılmıştır. Ancak kullanıcı sayısındaki hızlı artış, söz konusu radyo spektrum bölgelerinin daha etkin kullanılması gereksinimini ortaya çıkarmıştır. Bu amaçla literatürde, belli bir radyo spektrum aralığının kullanım haklarına sahip birincil kullanıcılarla aynı radyo spektrum aralığını kullanmak isteyen ikincil kullanıcıların söz konusu radyo spektrum aralığını nasıl ortaklaşa paylaşabileceklerini öneren bilişsel radyo yöntemleri önerilmiştir.
Bilişsel radyo yöntemlerinde ikincil kullanıcılar araya serme yönteminde birincil kullanıcının kullanmadığı radyo spektrum boşluklarını kullanarak, üstüne serme yönteminde birincil kullanıcının başarımını artırmak koşuluyla birincil kullanıcıyla işbirliği yaparak ve altına serme yönteminde birincil kullanıcının alıcısında izin vereceği girişim üst sınırını aşmamak koşuluyla iletişim kurabilirler. Bu üç bilişsel radyo yönteminde görüldüğü gibi, ikincil kullanıcıların başarımları birincil kullanıcıların izin verdiği ölçütlere doğrudan bağımlıdır. 
Bu tez çalışmasında altına sermeli bilişsel radyoda ikincil kullanıcıda kullanılmak üzere otomatik yineleme istemeli karma aktarım protokolleri önerilmiştir. Önerilen protokollerde kullanılan karma aktarım (XD) yönteminde röle hem yarı-çift yönlü (HD) hem de tam-çift yönlü (FD) röle olarak çalışmakta olup, böylece ikincil kullanıcının HD röleli sistemlere göre daha yüksek etkin veri hızı değerlerine ulaşabilmesi amaçlanmıştır. Ancak XD röle sistem üzerinde girişime neden olacağı için sistem güvenilirliğini artırmak amacıyla otomatik yineleme isteme yöntemi de sistem yapısına dahil edilmiştir.
XD rölenin sisteme getirisini inceleyebilmek için aynı koşullar altında HD röleli sistem tasarımlarına da tez kapsamında yer verilmiştir. Bu bakımdan önerilen her bir XD röleli sistem tasarımının etkin veri hızı, verimi (normalize etkin veri hızı) ve paket başarımı, aynı koşullar altında tasarlanan HD röleli sistemler ile kuramsal çıkarımlar ve benzetim yoluyla karşılaştırılmıştır. Yapılan karşılaştırmalar sonucunda tasarlanan XD röleli sistemlerin, XD rölenin sistemde girişime neden olması nedeniyle HD röleli sistemlere göre paket başarımlarının bir miktar daha kötü olduğu, buna karşılık HD röleli sistem tasarımlarına göre çok daha yüksek etkin veri hızı ve verim değerlerine ulaşabildiği görülmüştür.

Abstract:
More complex, more reliable and faster wireless communication systems become smaller and cheaper thanks to evolution of wireless communication technology. Besides, customer's demand increases due to wireless communication systems' performance converging to that of wired communication systems. As known, wireless communication systems communicate over a limited radio spectrum which is divided into subbands dedicated to the service providers. However, in current fixed spectrum allocation context in which almost all the spectrum is under lisence, the leased spectrum subbands are seriously under-utilized. The rapid increase in the number of user's demand neccessitates new techniques for efficiently use of the radio spectrum. For this purpose, cognitive radio techniques are proposed in the literature. Cognitive radio techniques organize primary users to share their lisenced spectrum with unlisenced secondary users. Primary users usually do not use their spectrum band effectively in time, frequency or space. Those primary user's absences are called as spectrum holes. Therefore, these spectrum holes inspire unlisenced secondary users to realize their communications over these holes. In this case, secondary users must fulfill some requirements. First, secondary users must have the ability to sense the presence of primary users on the spectrum. Second, secondary users must have the ability to hear primary user's communication. Third but not neccessarily, secondary users must can sense the presence of other secondary users if there are any. Cognitive radio techniques can be classified as interweave, overlay and underlay paradigm.
Interweave cognitive radio is the most opportunistic cognitive radio paradigm besides others. Secondary users detect the spectrum holes and communicate over those spectrum holes without causing any harm on primary user's receiver. The newest secondary user in the network assumes all other secondary users as primary user if there are multiple secondary users.
In overlay cognitive radio, secondary user must cooperate with primary user to increase the primary user's performance. Primary user will not share radio spectrum with the secondary user if it can not help improving its performance. Otherwise, secondary user acts as relay to improve primary user's performance by cooperating with it. Secondary user must have the primary user's codework to help. Also, primary user should have the ability to sense secondary user's presence in this paradigm, unlike the other two cognitive radio paradigms.
In underlay cognitive radio, primary user allows secondary user's communication under limited interference in its receiver. Secondary user has to change its transmit power with respect to the primary user's interference limit. As a result, in cognitive radio, secondary user's performance is very dependent on primary user's restrictions.
In this thesis, automatic repeat request (ARQ) based hybrid (XD) relaying protocols are proposed to be used in secondary users of a cognitive radio network. Half duplex (HD) relaying protocols with automatic repeat request are also considered to evaluate XD relaying protocols' gains.
Despite there are different ARQ techniques in the literature, the simplest stop-and-wait ARQ technique is applied in the thesis coverage. According to stop-and-wait ARQ, transmitter sends a packet and stops to wait receiver's response. Receiver sends back acknowledgement (ACK) signal if it successfully receives the transmitter packet. Otherwise, the receiver sends nothing or NACK signal to request second transmission of the same packet.
HD relay receives packets from the transmitter and transmits packet to the receiver in discrete time slots. In HD relayed systems, transmitter trnasmits a packet to relay in the first phase and relay transmits transmitter's packet to the receiver in the second phase. Despite HD relaying, full duplex (FD) relay receives transmitter's new packet and transmits transmitter's previous packet to the receiver simultaneously. For this reason, FD relayed systems can achieve higher throughputs than HD relayed systems. However, FD relayed systems suffer from self-interference at FD relay and interference at the receiver if transmitter and receiver are close enough to each other. To achieve higher throughputs than HD relayed systems and to reduce the disturbance due to the interference in FD relayed systems, hybrid relaying (XD) technique is considered in the proposed protocols. According to that, XD relayed systems work as FD relayed systems if there is no outage for both transmitter-relay and relay-receiver channels. XD relay work as HD relay if outage occurs in transmitter-relay channel or relay-receiver channel.
In this thesis, to achive higher rate then HD relayed systems and more reliablity then FD relayed systems, XD relayed protocols are proposed. XD relayed systems still suffer from interference like FD relayed systems. So, to achieve more reliable systems with XD relay, stop-and-wait ARQ is involved in the proposed XD relayed protocols. Consequently, all proposed XD relayed protocols work with full duplex relaying unless there is no outage between system's nodes. On the other hand, XD relay works as HD relay if outage occurs in transmitter-relay channel or relay-receiver channel. XD relay will stop FD relaying and try to successfully receive the retransmitted new packet if outage occurs only in transmitter-relay channel. Like that, transmitter stops sending new packet and allows XD relay to re-transmit the previous packet to the receiver if outage occurs only in relay-receiver channel. First XD relay re-transmits previous packet to the receiver and then the transmitter re-transmits the new packet to relay if outage occurs for both transmitter-relay and relay-receiver channels simultaneously.
ARQ-based HD relayed protocols are also considered in the thesis to evaluate the proposed ARQ-based XD relayed protocols' performance in terms of throughput, normalized effective data rate and packet success. Theoritical analysis is performed for both types of protocols and supported via computer simulations. 
The thesis is organized as follows. In Section 2, the basic consepts like fading channel models, channel capacity, outage probability, automatic repeat-request technique are revized. Section 3 deals with basic cooperation methods with different relaying protocols are revisited. Cognitive radio technique is presented in Section 4. New ARQ-based XD relayed protocols are developed in Section 5 and compared with their correspanding ARQ-based HD relayed protocols. The new protocols differ in ARQ process applied only at the transmitter, only at the relay and at both transmitter and relay. The non-ARQ protocols are also taken as reference schemes. In Section 6, the best ARQ-based XD and HD relayed protocols are applied at the secondary user of an underlay cognitive radio network. As seen from the performance results of Section5 and 6, XD relayed protocols achieve higher throughput then HD relayed protocols in return of a slight loss in the number of successively transmitted packets.