Sharif Digital Repository

In this thesis, we review the properties of the two dimensional quantum electrodynamics, called Schwinger model and then we study the noncommutative version of the Schwinger model. The most important properties of the Schwinger model in commutative space are the production of dynamical mass for photon like the Higss mechanism and also the bosonization of the model by integrating out the fermionic degrees of freedom and describing it through the free bosonic action. The first section of this thesis includes two parts. At the first, we investigate the corrections to the dynamical photon mass in the noncommutative Schwinger model by using the photon’s vacuum polarization diagrams at one loop... 

In recent years, UWB multiple access communication systems have received a great attention due to its many advantages. The most important advantages of a UWB system can be summarized as its high data rates, low power consumption, fading immunity and high precision routing. One of the important problems in a UWB system implementation is in its receiver part in which in order to use a coherent RAKE receiver, the channel coefficients have to be estimated. Since UWB channel is a dense multipath fading channel with a very high number of resolvable paths, the channel coefficient estimation based on using training sequence is very complicated. To resolve this problem, transmit reference (TR)... 

With recent successes in laboratory tests, and in atmosphere and space demonstrations, there is no doubt that the free-space optical (FSO) communications is ready for operational deployment. Large bandwidth, unlicensed spectrum, excellent security, and quick and inexpensive setup are among its most attractive features. Despite their attractive features, FSO communications suffer from several challenges in practical deployment; the major of them is fading or scintillation and path loss. To overcome such limitations, Channel Coding along with MIMO and Cooperative techniques have been proposed. Although the promising effects of cooperative transmissions in RF communications have greatly been... 

The compute-and-forward (CMF) method has shown a great promise as an innovative approach to exploit interference towards achieving higher network throughput. Here, instead of recovering single messages, the relays attempt to reliably decode (compute) and pass an integer linear combination of the transmitted messages, referred to as an equation, to the destination. By receiving sufficient equations,the destination can solve the linear equation system to recover the desired messages.Two main challenges of the CMF methods are the computational complexity and the rank failure problem at the destination. In this thesis, to decrease the implementation as well as the computational complexity of the... 

Infrared wireless local area networks are a suitable choice for a fast and cheap wireless network, providing rates over 10 Mbps with access to unregulated bandwidth. Among multiple-access techniques for infrared wireless networks, CDMA1 has attracted the most interest, due to its simple structure and power ecency. However, spreading the spectrum in CDMA prohibits exploiting the low-rate strong channel codes. The other limiting factor in infrared networks is the channel dispersion. In this thesis, these two issues are considered and an internally coded scheme for time-hopping CDMA, suitable for wireless infrared communication is proposed and with accurate modelling of infrared dispersive... 

In this thesis, we consider the problem of spectrum sensing in cognitive radio networks. First, the collaborative energy detectors based spectrum sensing are investigated in the case of known noise variance for two models of primary user (PU) signal, i.e. random and unknown deterministic signals. Since the derived optimum collaborative energy detector requires the signal-to-noise ratio (SNR) of secondary users (SU) and it has complex structure, the generalized likelihood ratio (GLR) detector is proposed for both models of PU signal which leads to the same decision rules for both models. Simulation results show that the performance of the proposed GLR detector is near to that of optimal... 

Multiple access technologies are one of the key features of different generations of wireless communication. From the first generation to the fourth generation, orthogonal multiple access (OMA) schemes have been utilized to serve multiple users in different orthogonal resource blocks in time, frequency or code domain. However, the new application scenarios for the fifth generation impose the requirements of higher system throughput, massive connectivity, and low latency, which can not be fulfilled with OMA schemes. Therefore, non-orthogonal multiple access (NOMA) schemes have attracted increasing attention in both industry and academia. In NOMA, multiple users may occupy the same resource... 

Future mobile networks are expected to not only bring more wireless connectivity and higher data rates, but also to support several high-quality services including low-latency, ultra-reliable, and energy-efficient communications. To support these needs, intelligent reflecting surfaces (IRSs) have recently attracted a lot of attention due to their potential for reconfiguring the wireless channel and are considered as a promising solution for future mobile generations. On the other hand, the exponential growth of network traffic, and its non-uniform distribution result in network congestion during peak-traffic times and network underutilization during off-peak times. Caching or content... 

Cognitive radio faces specific challenges due to its special spectrum access scheme. Providing required QoS and preventing irrecoverable interference on the primary users is among the crucial requirements. Beam forming is one of the interference avoidance schemes utilized in MIMO networks. Most of the studies on this scheme assume that each symbol is transmitted via vectors in order to grant desired beam forming and specific combinations in the receiver. Since transmitting multiple symbols at every transmission can provide higher data rates, it seems necessary to exploit beam forming through matrixes instead of vectors. In this thesis, by neglecting the interference of the secondary user,... 

In recent years, coded cooperation has been considered as one of the best methods to cope with fading and at the same time to achieve channel coding gain. On the other hand, the idea of network coding has been introduced for increasing the efficiency of cooperative networks. In addition, it has been shown that only by jointly designing of network coding and channel coding, the capacity will be achievable. Also, to adjust the rate to the channel capacity without requiring the channel state information at the transmitter, Rateless coding has been proposed. In this thesis above techniques are combined to gain their advantages and to improve system performance. To this end, first we consider a... 

Powerful spectrum sensing and access strategies enable cognitive radios to find transmission opportunity in spectral resources dedicated exclusively to the incumbent/primary users (PUs). One of the key effecting factors on cognitive radio network (CRN) throughput is the spectrum sensing sequence used by each secondary user (SU). In this thesis, modeling, performance evaluation, and throughput maximization of a CRN are investigated. More specifically, firstly we evaluate the performance of sequential method for handovers regarding a SU’s average throughput and consumed energy, and an optimization problem is formulated to maximize the throughput through selecting optimal channel sensing time.... 

Opportunistic spectrum access in cognitive radio networks has introduced multiple challenges in the realization of this technology. One of these key challenges is spectrum mobility, in which a secondary user (SU) must be able to vacate channels upon the appearance of a primary user (PU) and reestablish a communication link to avoid service termination, known as spectrum handoff. For a connection-based modeling, multiple interruptions can occur, which lead to multiple handoffs; hence, in a proactive approach, a sequence of target channels, rather than a single target channel must be determined. The first parts of this thesisis dedicated to the analysis of a time-slotted cognitive radio, in... 

In this thesis, we investigate the previously introduced Code-Shifted Reference (CSR) time hopping (TH) ultra-wideband (UWB) communication systems as a potential transmitted scheme reference technique. In this system, the orthogonality between the reference and data bearing signals is acquired in code domain. Also, we consider a simple scheme to clean to obtain a clean template to be used in the correlator receiver for the CSR –UWB, this method was used in UWB system before. In this method, which we call as averaged (Ave.) CSR-UWB, the template for each symbol is obtained by taking the average over all the frames of the symbol interval. It is shown that Ave-CSR system achieves higher... 

In Massive multiple input multiple output (MIMO) systems, due to utilizing a large antenna array in each base station, each user’s channel is orthogonal to one another. These systems have some valuable advantages such as the reduction of energy needed for transmission and significant improvement in spectral efficiency, which makes them be suitable for the next generation of wireless communication systems. In this thesis a massive MIMO working is considered on TDD mode, for wich the channel estimation is done in the base station using the pilot sequences recieved from users. At the beginning of each coherence block, users send their pilot sequences and the rest of the times is allocated to... 

Cognitive Radio (CR), as a new concept to efficiently utilize a licensed spectrum, and this has received intensive attension in recent years. By applying the cognitive technique, an unlicensed user (the secondary user (SU)) is allowed to transmit at the Primary User’s (PU) spectrum band under some conditions. Different approaches have been introduced to be utilized by the SU in order to be able to efficiently transmit at the PR’s spectrum without any sustainsial destructive effects on PR transmission and reception, which one of them is using multiple antenna in the cognitive transmitter. Hereby, CR is able to increase its throughput without substansial harmfull effect on PR’ transmission by...