Browsing by Subject "Wireless communication systems."

Now showing 1 - 20 of 32

Open Access
Analysis and mitigation of interference in multi-radio multi-channel wireless mesh networks
(2013) Uluçınar, Alper Rifat
Wireless mesh networking, which is basically forming a backbone network of mesh routers using wireless links, is becoming increasingly popular for a broad range of applications from last-mile broadband access to disaster networking or P2P communications, because of its easy deployment, self-forming, self-configuration, and self-healing properties. The multi-hop nature of wireless mesh networks (WMNs) aggravates inter-flow interference and causes intra-flow interference and severely limits the network capacity. One technique to mitigate interference and increase network capacity is to equip the mesh routers with multiple radios and use multiple channels. The radios of a mesh router can then simultaneously send or receive packets on different wireless channels. However, careful and intelligent radio resource planning, including flow-radio and channel assignment, is necessary to efficiently make use of multiple radios and channels. This first requires analyzing and modeling the nature of co-channel and adjacent channel interference in a WMN. Through real-world experiments and observations made in an indoor multihop multi-radio 802.11b/g mesh networking testbed we established, BilMesh, we first analyze and model the nature of co-channel and adjacent channel interference. We conduct extensive experiments on this testbed to understand the effects of using multi-radio, multi-channel relay nodes in terms of network and application layer performance metrics. We also report our results on using overlapping in addition to orthogonal channels for the radios of the mesh routers. We then turn our attention to modeling and quantifying adjacent channel interference. Extending BilMesh with IEEE 802.15.4 nodes, we propose computational methods to quantify interference between channels of a wireless communication standard and between channels of two different standards (such as Wi-Fi and ZigBee). Majority of the studies in the literature on channel assignment consider only orthogonal channels for the radios of a multi-radio WMN. Having developed quantitative models of interference, next we propose two optimization models, which use overlapping channels, for the joint flow-radio and channel assignment problems in WMNs. Then we propose efficient centralized and distributed heuristic algorithms for coupling flows and assigning channels to the radios of a WMN. The proposed centralized and distributed schemes make use of overlapping channels to increase spectrum utilization. Using solid interference and capacity metrics, we evaluate the performances of the proposed schemes via extensive simulation experiments, and we observe that our schemes can achieve substantial improvement over single-channel and random flow-radio and channel assignment schemes.
Open Access
Antenna analysis
(2009) Tunç, Celal Alp
Multiple-input-multiple-output (MIMO) wireless communication systems have been attracting huge interest, since a boost in the data rate was shown to be possible, using multiple antennas both at the transmitter and receiver. It is obvious that the electromagnetic effects of the multiple antennas have to be included in the wireless channel for an accurate system design, though they are often neglected by the early studies. In this thesis, the MIMO channel is investigated from an electromagnetics point of view. A full-wave channel model based on the method of moments solution of the electric field integral equation is developed and used in order to evaluate the MIMO channel matrix accurately. The model is called the channel model with electric fields (MEF) and it calculates the exact fields via the radiation integrals, and hence, it is rigorous except the random scatterer environment. The accuracy of the model is further verified by the measurement results. Thus, it is concluded that MEF achieves the accuracy over other approaches which are incapable of analyzing antenna effects in detail. Making use of the presented technique, MIMO performance of printed dipole arrays is analyzed. Effects of the electrical properties of printed dipoles on the MIMO capacity are explored in terms of the relative permittivity and thickness of the dielectric material. Appropriate dielectric slab configurations yielding high capacity printed dipole arrays are presented. The numerical efficiency of the technique (particularly for freestanding and printed dipoles) allows analyzing MIMO performance of arrays with large number of antennas, and high performance array design in conjunction with well-known optimization tools. Thus, MEF is combined with particle swarm optimization (PSO) to design MIMO arrays of dipole elements for superior capacity. Freestanding and printed dipole arrays are analyzed and optimized, and the adaptive performance of printed dipole arrays in the MIMO channel is investigated. Furthermore, capacity achieving input covariance matrices for different types of arrays are obtained numerically using PSO in conjunction with MEF. It is observed that, moderate capacity improvement is possible for small antenna spacing values where the correlation is relatively high, mainly utilizing nearly full or full covariance matrices. Otherwise, the selection of the diagonal covariance is almost the optimal solution.MIMO performance of printed rectangular patch arrays is analyzed using a modified version of MEF. Various array configurations are designed, manufactured, and their MIMO performance is measured in an indoor environment. The channel properties, such as the power delay profile, mean excess delay and delay spread, are obtained via measurements and compared with MEF results. Very good agreement is achieved.
Open Access
Application and improvement of SAGE algorithm for channel parameter estimation
(2008) Bodur, Harun
In recent years, Multiple Input Multiple Output (MIMO) systems have gained importance due to the improvements on the performance of radio systems. Channel parameter estimation is an important factor in the design and optimization of MIMO systems. In this thesis, channel parameters such as delay, angles (azimuth and elevation) of arrival (AoA) and departure (AoD), Doppler frequency and polarization are estimated from measurement data using Space Alternating Generalized Expectation-Maximization (SAGE) algorithm. One of the focuses of this thesis is to reduce the computational complexity of the algorithm by using Particle Swarm Optimization (PSO) technique. Additionally, a new initialization procedure is proposed to get better estimates and to improve the processing time of the algorithm. Moreover, performance of the SAGE algorithm and improvements on the algorithm are tested via extensive measurement data. It is found that SAGE algorithm is a powerful tool for channel estimation and it can further be improved by the aforementioned propositions.
Open Access
Atmospheric turbulence modeling and aperture analysis for optimizing receiver design and system performance on free space optical communication links
(2012) Meriç, Haşim
Strong turbulence measurements that are taken using real time optical wireless experimental setups are valuable when studying the effects of turbulence regimes on a propagating optical beam. In any kind of Free Space Optical (FSO) system, knowing the strength of the turbulence thus the refractive index structure constant (C 2 n ), is beneficial for having an optimum bandwidth of communication. Even if the FSO Link is placed very well-high-above the ground just to have weak enough turbulence effects, there can be severe atmospheric conditions that can change the turbulence regime. Having a successful theory that will cover all regimes will give us the chance of directly processing the image in existing or using an additional hardware thus deciding on the optimum bandwidth of the communication line at firsthand.In literature, simulation of beam propagation through turbulent media has always been a tricky subject when it comes to moderate-to-strong turbulent regimes. Creating a well controlled turbulent environment is beneficial as a fast and a practical approach when it comes to testing the optical wireless communication systems in diverse atmospheric conditions. For all of these purposes, strong turbulence data have been collected using an outdoor optical wireless setup placed about 85 centimeters above the ground with an acceptable declination and a path length of about 250 meters inducing strong turbulence to the propagating beam. Variety of turbulence strength estimation methods as well as frame image analysis techniques are then been applied to the experimental data in order to study the effects of different parameters on the result. Such strong turbulence data is compared with existing weak and intermediate turbulence data. The Aperture Averaging (AA) Factor for different turbulence regimes as well as the inner and outer scales of atmospheric turbulence are also investigated. A new method for calculating the Aperture Averaging Factor is demonstrated deducing spatial features at the receiver plane. Controlled turbulent media is created using multiple phase screens each having supervised random variations in its frequency and power while the propagated beam is calculated using Fresnel diffraction method. The effect of the turbulent media is added to the propagated beam using the modified Von Karman spectrum. Created scintillation screens are tested and compared with the experimental data which are gathered in different turbulence regimes within various atmospheric conditions. We believe that the general drawback of the beam propagation simulation is the difference in terms of spatial distribution and sequential phase textures. To overcome these two challenges we calculate the Aperture Averaging Factors to create more realistic results. In this manner, it is possible to create more viable turbulent like scintillations thus the relationship between the turbulence strength and the simulated turbulence parameters are distinctly available. Our simulation gives us an elusive insight on the real atmospheric turbulent media. It improves our understanding on parameters that are involved in real time intensity fluctuations that occur in every optical wireless communication system.
Open Access
Channel assignment and routing for multi-radio wireless mesh networks
(2008) Özdemiray, Ahmet Murat
Wireless Mesh Network is a promising technology since it extends the range of wireless coverage by multi-hop transmission between routers. However, in multihop networks the total throughput decreases with increasing number of nodes and hops. To increase the total throughput, some mesh routers are equipped with multiple radios to use the available bandwidth of multiple non-overlapping channels. However, channel assignment should be done carefully to effectively use this available bandwidth. Moreover, the optimal channel assignment algorithm is NP-hard. In this thesis, we propose a joint channel assignment and routing solution to effectively use the available bandwidth for multi-radio wireless mesh networks with given network topology and traffic profile. Initially, we predict the final routes of the flows and estimate the loads on the links using these path predictions and given traffic profile. Then three different heuristics determine the assignment order of the links. Then the least busy channel among the available channels is assigned to the link. Finally, our routing algorithm routes the flows such that the selected path is the least busy path among the alternatives. We evaluated our channel assignment and routing algorithm using ns-2 simulator which supports multiple channels and multiple radios per node and we compared our results with single channel WMNs, and different algorithms for multi-radio multi-channel WMNs. The results show that our joint algorithm successfully achieves up to 5 times more throughput than single channel WMN with using just 2 radios and 3 channels. Our algorithms also out-performs other compared channel assignment algorithms for multi-radio multi-channel WMNs.
Open Access
Channel assignment and routing for multi-radio wireless mesh networks
(2008) Gülten, Sıtkı
In this study, we analyze the channel assignment and routing problem for multi-radio wireless mesh networks. We assume that each router has more than one radio, the system operates in a time-slotted mode, and channel assignments are static. In particular, within a time slot the channel assignments for radio connections have to obey the interference constraint. The union of all the connections established throughout the time horizon should result in a strongly connected network where each node can communicate with every other node within the given maximum hop-distance or the diameter value. The objective is to minimize the number of time slots used while respecting the interference and diameter restriction. An integer linear program is proposed as an exact methodology to solve the problem for small scale networks. For larger network sizes, three type of heuristic approaches are developed. In order to evaluate the quality of the heuristic solutions, the lower bound of the model is strengthened through the use of valid inequalities and lagrangian relaxation. The subgradient algorithm is used in lagrangian relaxation models to find optimal solutions or lower bounds. The heuristics are tested on a large set of varying network topology instances. The computational experiments illustrate that improvement heuristic based on local search is the most suitable approximation technique.
Open Access
Continuous flow routing (CFR) : a wireless ad hoc network routing protocol for supporting multimedia flows
(2006) Kara, Ahmet
Multimedia flows require special routing layer support in ad hoc networks due their unique characteristics and certain requirements on packet delay, jitter, loss rate and bandwidth. In this thesis we propose a wireless ad hoc routing protocol (called CFR) with route discovery and maintenance mechanisms, that is specifically designed for better supporting multimedia flows in wireless ad hoc networks. Since multimedia flows are long durational, it is important to route them through stable routes in order to minimize route failures and disturbance on the flows. We propose to improve the stability by considering the energy drain rates and estimated remaining lifetimes of nodes while selecting the best routes. Additionally we provide a maintenance scheme that acts pro-actively and re-routes the traffic if a node starts getting very low energy. For this we define two thresholds on remaining lifetime of a node. After the first threshold is reached, the node re-routes some of its flows so that the traffic load on the node is reduced. This helps to an even distribution of traffic to nodes of an ad hoc network. After the second threshold is reached, the node has very few energy and therefore redirects all its traffic to some other routes so that the flows are not disturbed by the route failure when the node depletes all of its energy and can not route anymore. We implemented the protocol as part of an ns2-based simulator and proved that it works correctly. Additionally, we compared the protocol against some other similar protocols. The results show that CFR can indeed help supporting multimedia flows better.
Open Access
Distributed channel aware link scheduling for CSMA based wireless networks with time-varying channels and delay sensitive applications
(2011) Erkan, Bahadır
In wireless networks, interference between neighboring links is an important issue. The link scheduling algorithm controls the interference between neighboring links such that no adjacent links can be concurrently active. Distributed throughput optimum algorithms for the link scheduling problem have been proposed in the literature. However, the maximum packet delays of these distributed throughput optimum algorithms can become arbitrarily large, which significantly degrades the performances of delay sensitive applications such as “Skype”. In this thesis, we propose two distributed link scheduling algorithms: a full opportunistic algorithm and a delay based adaptive algorithm. The proposed algorithms, while maintaining throughput optimality, increase the average delay performance of the previously proposed throughput optimum scheduling algorithms by 20% under the fading radio channel. We propose a new metric “Effective Goodput”, which measures the rate of packets that are successfully received before their respective playout times for delay sensitive applications. The delay based distributed adaptive scheduling algorithm proposed in the thesis increases the “Effective Goodput” by nearly 100% compared with the throughput optimum scheduling algorithms proposed in the literature.
Open Access
Estimation of receiver sampling clock timing impurity impact on channel orthogonality in OFDM based communication systems
(2009) Tanyeri, H. Onur
The growing need for high-speed wireless communication systems has led communication engineers to design and implement communication systems at higher frequencies where more bandwidth is available, use digital modulation schemes with more complex constellations and place carriers closer together with little guard-band in the pursuit of designing communication systems closer to the channel capacity. These new designs have placed tighter constraints on the performance of oscillators and timing devices of transceivers. In this work, the effects of timing clock jitter on the receiver Analog-to-Digital Converter (ADC) of Orthogonal Frequency Division Multiplexing (OFDM) based communication systems are examined and Inter-Carrier Interference (ICI) effects are quantified in order to prevent unnecessary over designs in OFDM ADC circuitry. In this respect, a simulation tool that synthesizes jitter processes with defined spectral characteristics is prepared. The generated jitter processes are utilized in an OFDM simulation tool that quantifies the ICI levels caused by receiver ADC sampling jitter. Using these two tools, ICI levels of certain OFDM systems are examined and guidelines for OFDM ADC circuitry design are proposed.
Open Access
High power high efficiency microwave power amplifier design using Class-E topology
(2010) Coşkun, Akif Alperen
Power consumption is a major problem in wireless technology. Since power ampli- fier is one of the most power consuming element, efficiency of the power amplifier should be optimized. Switching amplifiers typically offer very high efficiency. As the frequency increases, the efficiency drops. Class-E is a switching amplifier which is suitable for microwave frequencies. It attains a very high efficiency with proper voltage and current characteristics. Different techniques to obtain these characteristics are possible. In this work, we implement a 38 dBm power amplifier at 2 GHz. Different methods are examined to obtain a high efficiency behavior. We obtained a power added efficiency of 60%.
Open Access
Implementation of a coded-reference ultra-wideband system
(2011) Gürlevik, Osman
Coded-reference ultra-wideband (CR UWB) systems provide orthogonalization of the reference and data signals in the code domain to facilitate communications without the need for complex channel estimation and have significant advantages over the previous techniques in terms of performance and/or implementation complexity. This thesis presents a UWB testbed as a general experimental platform to explore pulse-based UWB communications and discusses design and implementation issues. A testbed is built as a flexible solution for hardware implementation of a CR UWB system.
Open Access
Joint link
(2009) Kaybal, Fazlı
In this thesis, we study the joint scheduling and routing problem in spatial reuse Time Division Multiple Access (STDMA) based multi-channel/multiradio/multi-rate wireless mesh networks (WMNs). The main objective of the joint scheduling and routing problem addressed in thesis is to reduce the number of required TDMA time slots to deliver all packets to their destinations. Since the optimum solution to the problem is NP-hard, we propose a greedy iterative solution methodology. The problem is formulated as an integer linear program (ILP) under the physical interference model. We consider two versions of the problem in order to investigate the factors affecting the capacity of WMNs. In the first one, we perform scheduling and routing when the number of channels and number of radios are varied for multi-rate WMNs where nodes are equipped with omni-directional antennas. This analysis is done for both single-class (best-effort traffic) and two-class (best-effort and delay sensitive classes) traffic models. We then extend this analysis by adding the power control scheme which allows transmitters to change the transmitting powers slot-by-slot. Finally, joint scheduling and routing problem is extended for WMNs where nodes are equipped with multiple sectored antennas. We show that the network performance is improved with more radio resources, e.g., using multiple orthogonal channels, multiple radios per node, transmit power control scheme, and directional antennas in terms of delay and total dissipated energy. The network throughput when using 3 channels and 3 radios is increased by up to 67.2% compared to single channel WMNs and the total dissipated energy is reduced by up to 45.5% with transmit power control scheme. Finally, when directional antennas with 6 sectors are used at both transmitters and receivers, the network throughput increases by up to 72.6% compared to omni-directional antenna case.
Open Access
Joint routing, gateway selection, scheduling and power management optimization in wireless mesh networks
(2011) Uzunlar, Onur
The third generation (3G) wireless communications technology delivers user traffic in a single step to the wired network via base station; therefore it requires all base stations to be connected to the wired network. On the other hand, in the fourth generation (4G) communication systems, it is planned to have the base stations set up so that they can deliver each other’s traffic to a small number of base stations equipped with wired connections. In order to improve system resiliency against failures, a mesh structure is preferred. The most important issue in Wireless Mesh Networks (WMN) is that the signals that are simultaneously transmitted on the same frequency channel can interfere with each other to become incomprehensible at the receiver end. It is possible to operate the links at different times or at different frequencies, but this also lowers capacity usage. In this thesis, we tackle the planning problems of WMN, using 802.16 (Wi-MAX) protocol, such as deploying a given number of gateway nodes along with operational problems such as routing, management of power used by nodes and scheduling while maximizing the minimum service level provided. The WMN under consideration has identical routers with fixed locations and the demand of each router is known. In order to be able to apply our results to real systems, we work with optimization models based on realistic assumptions such as physical interference and single path routing. We propose heuristic methods to obtain optimal or near optimal solutions in reasonable time. The models are applied to some cities in Istanbul and Ankara provinces.
Open Access
A link-state based on-demand routing protocol supporting real-time traffic for wireless mobile ad hoc networks
(2007) Görbil, Gökçe
Wireless ad hoc networks have gained a lot of popularity since their introduction and as many wireless network interface cards provide support for ad hoc networking, such networks have also seen real-life deployment for non-specialized purposes. Wireless mobile ad hoc networks (MANETs) are currently the most common type of ad hoc networks, and such networks are especially esteemed for their mobility support and ease of deployment due to their ad hoc nature. As most common network applications, such as the Web, FTP, email, and instant messaging, are data-centric and do not operate under strict time constraints, MANETs have been deployed to enable such non-real-time applications in the past. However, with the increasing use of real-time applications over ad hoc networks, such as teleconferencing, VoIP, and security and tracking applications where timeliness is of importance, real-time traffic support in multi-hop wireless mobile ad hoc networks has become an issue. We propose an event-driven, link-state based, on-demand routing protocol to enable real-time traffic support in such multi-hop wireless mobile ad hoc networks. Our protocol, which is named Elessar, is based on link-state topology dissemination, but instead of the more common periodic link-state messaging scheme, we employ event-driven link-state messages in Elessar, where topology changes are the events of interest. Through such an approach, we aim to lower the overhead of our protocol, especially for low-mobility cases, which is currently the most commonly encountered case with ad hoc networks deployed with machines directly interacting with humans, such as PDAs and laptops. Due to its link-state nature, our protocol is able to support non-real-time traffic without any further action. In order to support real-time traffic, however, we employ a direct cost dissemination mechanism, which only operates on-demand when there are one or more real-time flows in the network. We aim to provide soft quality-of-service (QoS) guarantees to real-time flows through intelligent path selection, without any resource reservation. We also aim to provide such QoS guarantees throughout the lifetime of a real-time flow, even in the face of node failures and mobility, by dynamic path adaptation during the lifetime of the flow. Elessar is able to support real-time and non-real-time traffic concurrently, as well as various different types of concurrent real-time traffic, such as delay- and loss-sensitive traffic. Our protocol, therefore, does not aim to support a single type of real-time traffic, but rather a plethora of different types of real-time traffic. Elessar is completely distributed, dynamic and adaptive, and does not require the underlying MAC protocol to be QoS-aware. We analyse our design choices and the performance of our protocol through realistic simulation experiments conducted on the OMNeT++ discrete event simulation platform, using the INET framework. We have used the IEEE 802.11b MAC protocol during our simulations and have employed the random waypoint mobility model to simulate mobility. Our experimental results show that Elessar is able to efficiently provide real-time traffic support for different types of traf- fic flows, even in the face of mobility. Our protocol operates best for smallto-medium-sized networks where mobility rates are low-to-medium. Once the mobility rate exceeds a certain threshold, intelligent path selection cannot cope satisfactorily with the high dynamism of the environment and the overhead of Elessar exceeds acceptable levels due to its event-driven link-state nature.
Open Access
A peer-to-peer file sharing system for wireless ad-hoc networks
(2004) Sözer , Hasan
In recent years, we have witnessed an increasing popularity of peer-to-peer (P2P) networks. Especially, file sharing applications aroused considerable interest of the Internet users and currently there exist several peer-to-peer file sharing systems that are functional on the Internet. In the mean time, recent developments in mobile devices and wireless communication technologies enabled personal digital assistants (PDA) to form ad-hoc networks in an easy and automated way. However, file sharing in wireless ad-hoc networks imposes many challenges that make conventional peer-to-peer systems operating on wire-line networks (i.e. Internet) inapplicable for this case. Information and workload distribution as well as routing are major problems for members of a wireless ad-hoc network, which are only aware of peers that are within their communication range. In this thesis, we propose a system that solves peer-to-peer file-sharing problem for wireless ad-hoc networks. Our system works according to principles of peer-to-peer systems, without requiring a central server, and distributes information regarding the location of shared files among members of the network. By means of constructing a distributed hash table (DHT) and forming a tree shaped overlay network based on the topology of the network itself, the system is able to answer location queries, and also discover and maintain routing information that is used to transfer files from a source-peer to another peer.
Open Access
Performance analysis of diversity techniques for OFDM and base station cooperation
(2010) Üzeler, Hande
The main goal of the next generation wireless communication systems is to provide high data rate services. In order to deal with performance-limiting challenges that include frequency selective fading channels, power and bandwidth constraints, multiple input multiple output (MIMO) and orthogonal frequency division multiplexing (OFDM) techniques have been proposed as effective techniques to combat fading and to provide high rate reliable transmission. In this thesis we first give an overview of WiMAX as an example of an OFDM system and study the performance of the WiMAX physical layer under different MIMO techniques. We also analyze space-frequency coding and propose a threaded algebraic space-time (TAST) based code. Secondly, since the mobile bandwidth is an expensive and scarce resource, it seems likely that a high frequency reuse will be employed in the future cellular networks to increase spectral efficiency. This means that base stations (BSs) will operate in the same frequency band and therefore cause cochannel interference (CCI) to the users at other cells. CCI is an important performance degrading factor. Therefore our second aim is to investigate BS cooperation techniques to mitigate CCI. We assume that channel state information (CSI) is available at the cooperating BSs and analyze the performance gains due to cooperation when used in conjunction with Alamouti space-time coding.
Open Access
Performance analysis of generic discrete-time queues and applications to telecommunication networks
(2001) Kavak, Egemen Değer
Open Access
Performance analysis of the carrier-sense multiple access protocol for future generation wireless networks
(2013) Köseoğlu, Mehmet
Variants of the carrier-sense multiple access (CSMA) protocol has been employed in many communications protocols such as the IEEE 802.11 and Ethernet standards. CSMA based medium access control (MAC) mechanisms have been recently proposed for other communications scenarios such as sensor networks and acoustical underwater networks. Despite its widespread use, the performance of the CSMA protocol is not well-studied from the perspective of these newly encountered networking scenarios. We here investigate the performance of the CSMA protocol from the point of three different aspects: throughput in networks with large propagation delay, short-term fairness for delay sensitive applications in large networks and energy efficiency-throughput trade-off in networks with battery operated devices. Firstly, we investigate the performance of the CSMA protocol for channels with large propagation delay. Such channels are recently encountered in underwater acoustic networks and in terrestrial wireless networks covering larger areas. However, a mathematical model of CSMA performance in such networks is not known. We propose a semi-Markov model for a 2-node CSMA channel and then extend this model for arbitrary number of users. Using this model, we obtain the optimum symmetric probing rate that achieves the maximum network throughput as a function of the average propagation delay, ¯d, and the number of nodes sharing the channel, N. The proposed model predicts that the total capacity decreases with ¯d −1 as N goes to infinity when all nodes probe the channel at the optimum rate. The optimum probing rate for each node decreases with 1/N and the total optimum probing rate decreases faster than ¯d −1 as N goes to infinity. Secondly, we investigate whether the short-term fairness of a large CSMA network degrades with the network size and density. Our results suggest that (a) the throughput region that can be achieved within the acceptable limits of shortterm fairness reduces as the number of contending neighboring nodes increases for random regular conflict graphs, (b) short-term fair capacity weakly depends on the network size for a random regular conflict graph but a stronger dependence is observed for a grid topology. We also present related results from the statistical physics literature on long-range correlations in large systems and point out the relation between these results and short-term fairness of CSMA systems. Thirdly, we investigate the energy efficiency of a CSMA network proposing a model for the energy consumption of a node as a function of its throughput. We show that operating the CSMA network at a very high or at a very low throughput is energy inefficient because of increasing carrier-sensing and sleeping costs, respectively. Achieving a balance between these two opposite operating regimes, we derive the energy-optimum carrier-sensing rate and the energy-optimum throughput which maximize the number of transmitted bits for a given energy budget. For the single-hop case, we show that the energy-optimum total throughput increases as the number of nodes sharing the channel increases. For the multi-hop case, we show that the energy-optimum throughput decreases as the degree of the conflict graph of the network increases. For both cases, the energy-optimum throughput reduces as the power required for carrier-sensing increases. The energy-optimum throughput is also shown to be substantially lower than the maximum throughput and the gap increases as the degree of the conflict graph increases for multi-hop networks.
Open Access
A Performance comparision of polar codes with convolutional turbo codes
(2009) Özgür, Üstün
Polar codes introduced recently by Arıkan are the first low-complexity codes achieving symmetric capacity for arbitrary binary-input discrete memoryless channels (B-DMCs). Although being theoretically significant, their practical significance is an issue that has not yet been fully explored. Previous studies have compared polar codes with Reed-Muller codes, where it was found that polar codes can outperform them. In this thesis, to investigate how polar codes perform against state-of-the-art forward error correction (FEC) codes used in practice, we implement a IEEE 802.16 based link-level Worldwide Interoperability for Microwave Access (WiMAX) simulator which incorporates several WiMAX FEC options, and polar codes. IEEE 802.16 standards family define standards for current and next generation broadband wireless access, which will make high data rate multimedia applications in mobile environments a reality. Next generation broadband access standard, pursued by the IEEE 802.16 Task Group m is a work in progress, and requires even more sophisticated error correction schemes so that higher throughput, better QOS, higher mobilities, wider ranges and lower latencies are supported. We perform performance comparison simulations with the convolutional turbo codes (CTC) configurations defined in IEEE 802.16e to see how much of a performance gap exists between polar codes and CTCs. The main findings of the thesis are that, although the polar codes achieve capacity for specific conditions, as expected, for the code lengths and channel conditions we have simulated, the performance of them cannot compete with that of the CTCs with equivalent rates and lengths. It remains a task to see whether polar codes can achieve similar performances with CTCs when used as component codes in other configurations and aid in the advancement of new communication technologies.
Open Access
Performance of static and adpative subchannel allocation schemes for fractional frequency reuse in WiMAX networks
(2011) Kar, Görkem
We study the downlink performance of WiMAX under fractional frequency reuse (FFR) model. Conventional cellular planning methods can be used for broadband wireless access systems that operate in point-to-multipoint (PMP) configuration based on OFDMA/OFDM such as WiMAX. As an alternative planning method, FFR has been recently proposed for OFDMA/OFDM based cellular systems. FFR divides the cell into two regions: the inner and outer cell. Mobile Stations (MS) inside the inner cell can use the entire frequency band (achieving full frequency reuse), while MSs in the outer ring use a fraction of the band (having fractional frequency reuse). Transmissions in the inner and outer cells occur during different time periods so that users at the cell edge experience less interference. In this thesis, we investigate the effect of dynamically changing the number of subcarriers allocated to inner and outer cells. We use two metrics: total cell throughput and Jain’s fairness index for the distribution of cell throughput among MSs. As the ratio of subcarriers allocated to inner cell increases, the total cell throughput increases while the fairness index decreases. We use the product of cell throughput and fairness index in order to study the trade-off between the two metrics. We show that by dynamically adjusting the ratio of subcarriers allocated to the inner cell based on the user distribution, the throughput