The Mobility Impact in IEEE 802.11p Infrastructureless Vehicular Networks

Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. Due to high speed of nodes and their frequent disconnections, it is difficult to design a MAC scheme in VANETs that satisfies the quality-of-service requirements in all networking scenarios. In this thesis, we provide a comprehensive evaluation of the mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay, as well as the impact of mobility factors. The study also presents a relation between the mobility factors and the respective medium access behavior. Moreover, a new unfairness problem according to node relative speed is identified for both broadcast and unicast scenarios. To achieve better performance, we propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. Extensive simulation results show the significant impact of mobility on the IEEE 802.11p MAC performance, an identification of a new unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes

Throughput optimization in MPR-capable multi-hop wireless networks

Recent advances in the physical layer have enabled the simultaneous reception of multiple packets by a node in wireless networks. This capability has the potential of improving the performance of multi-hop wireless networks by a logarithmic factor with respect to current technologies. However, to fully exploit multiple packet reception (MPR) capability, new routing and scheduling schemes must be designed. These schemes need to reformulate a historically underlying assumption in wireless networks which states that any concurrent transmission of two or more packets results in a collision and failure of all packet receptions. In this work, we present a generalized model for the throughput optimization problem in MPR-capable multi-hop wireless networks. The formulation incorporates not only the MPR protocol model to quantify interference, but also the multi-access channel. The former is related with the MAC and routing layers, and considers a packet as the unit of transmission. The latter accounts for the achievable capacity of links used by simultaneous packet transmissions. The problem is modeled as a joint routing and scheduling problem. The scheduling subproblem deals with finding the optimal schedulable sets, which are defined as subsets of links that can be scheduled or activated simultaneously. Among other results, we demonstrate that any solution of the scheduling subproblem can be built with |E| + 1 or fewer schedulable sets, where |E| is the number of links of the network. This result contrasts with a conjecture that states that a solution of the scheduling subproblem, in general, is composed of an exponential number of schedulable sets. The model can be applied to a wide range of networks, such as half and full duplex systems, networks with directional and omni-directional antennas with one or multiple transmit antennas per node. Due to the hardness of the problem, we propose several polynomial time schemes based on a combination of linear programming, approximation algorithm and greedy paradigms. We illustrate the use of the proposed schemes to study the impact of several design parameters such as decoding capability and number of transmit antennas on the performance of MPR-capable networks

Improving the Performance of Medium Access Control Protocols for Mobile Adhoc Network with Smart Antennas

Requirements for high quality links and great demand for high throughput in Wireless LAN especially Mobile Ad-hoc Network has motivated new enhancements and work in Wireless communications such as Smart Antenna Systems. Smart (adaptive) Antennas enable spatial reuse, increase throughput and they increase the communication range because of the increase directivity of the antenna array. These enhancements quantified for the physical layer may not be efficiently utilized, unless the Media Access Control (MAC) layer is designed accordingly. This thesis implements the behaviours of two MAC protocols, ANMAC and MMAC protocols in OPNET simulator. This method is known as the Physical-MAC layer simulation model. The entire physical layer is written in MATLAB, and MATLAB is integrated into OPNET to perform the necessary stochastic physical layer simulations. The aim is to investigate the performance improvement in throughput and delay of the selected MAC Protocols when using Smart Antennas in a mobile environment. Analytical methods were used to analyze the average throughput and delay performance of the selected MAC Protocols with Adaptive Antenna Arrays in MANET when using spatial diversity. Comparison study has been done between the MAC protocols when using Switched beam antenna and when using the proposed scheme. It has been concluded that the throughput and delay performance of the selected protocols have been improved by the use of Adaptive Antenna Arrays. The throughput and delay performance of ANMAC-SW and ANMAC-AA protocols was evaluated in details against regular Omni 802.11 stations. Our results promise significantly enhancement over Omni 802.11, with a throughput of 25% for ANMAC-SW and 90% for ANMC-AA. ANMAC-AA outperforms ANMAC-SW protocol by 60%. Simulation experiments indicate that by using the proposed scheme with 4 Adaptive Antenna Array per a node, the average throughput in the network can be improved up to 2 to 2.5 times over that obtained by using Switched beam Antennas. The proposed scheme improves the performances of both ANMAC and MMAC protocols but ANMAC outperforms MMAC by 30%

Conceção e desempenho de retransmissões sem fios cooperativas

Doutoramento em Engenharia Eletrotécnica/TelecomunicaçõesIn recent years, a new paradigm for communication called cooperative communications has been proposed for which initial information theoretic studies have shown the potential for improvements in capacity over traditional multi-hop wireless networks. Extensive research has been done to mitigate the impact of fading in wireless networks, being mostly focused on Multiple-Input Multiple-Output (MIMO) systems. Recently, cooperative relaying techniques have been investigated to increase the performance of wireless systems by using diversity created by different single antenna devices, aiming to reach the same level of performance of MIMO systems with low cost devices. Cooperative communication is a promising method to achieve high spectrum efficiency and improve transmission capacity for wireless networks. Cooperative communications is the general idea of pooling the resources of distributed nodes to improve the overall performance of a wireless network. In cooperative networks the nodes cooperate to help each other. A cooperative node offering help is acting like a middle man or proxy and can convey messages from source to destination. Cooperative communication involves exploiting the broadcast nature of the wireless medium to form virtual antenna arrays out of independent singleantenna network nodes for transmission. This research aims at contributing to the field of cooperative wireless networks. The focus of this research is on the relay-based Medium Access Control (MAC) protocol. Specifically, I provide a framework for cooperative relaying called RelaySpot which comprises on opportunistic relay selection, cooperative relay scheduling and relay switching. RelaySpot-based solutions are expected to minimize signaling exchange, remove estimation of channel conditions, and improve the utilization of spatial diversity, minimizing outage and increasing reliability.Nos últimos anos foi proposto um novo paradigma de comunicação, chamado de comunicação cooperativa, para o qual estudos iniciais de teoria da informação demonstraram ter potencial para melhorias na capacidade em redes sem fios tradicionais multi-hop. Uma extensa pesquisa tem sido realizada para mitigar o impacto da atenuação em redes sem fios, tendo-se debruçado principalmente em sistemas Multiple-Input Multiple-Output (MIMO). Recentemente têm sido investigadas técnicas de retransmissão cooperativas para aumentar o desempenho de sistemas sem fios, usando a diversidade criada por diferentes antenas individuais com o objetivo de atingir o mesmo nível de desempenho dos sistemas MIMO com dispositivos de baixo custo. A comunicação cooperativa é um método promissor para atingir uma elevada eficiência na ocupação espectral e melhorar a capacidade de transmissão em redes sem fios. A comunicação cooperativa tem por ideia base a junção de recursos de nós distribuídos para melhorar o desempenho global de uma rede sem fios. Em redes cooperativas os nós cooperam para ajudarem-se mutuamente. Um nó cooperativo que ofereça ajuda estará agindo como um intermediário ou mediador, podendo transmitir mensagens da origem para o destino. A comunicação cooperativa explora a natureza da transmissão em difusão das comunicações sem fios para formar antenas múltiplas virtuais com vários nós de rede independentes e com antenas únicas. Esta investigação visou contribuir para a área científica das redes sem fios cooperativas. O foco da pesquisa foi nos protocolos de controlo de acesso ao meio (MAC) com retransmissão cooperativa. Especificamente, proponho uma arquitetura para enquadrar a retransmissão cooperativa, chamada RelaySpot (ponto de retransmissão), que explora a seleção oportunista de retransmissores, o escalonamento de retransmissores cooperativos e a comutação entre retransmissores. As comunicações baseadas na RelaySpot deverão ter uma troca de sinalização reduzida, não usam estimativas das condições do canal e melhoram o aproveitamento da diversidade espacial, minimizando a interrupção e aumentando a fiabilidade

Improving the Performance of Medium Access Control Protocols for Mobile Adhoc Network with Smart Antennas

Requirements for high quality links and great demand for high throughput in Wireless LAN especially Mobile Ad-hoc Network has motivated new enhancements and work in Wireless communications such as Smart Antenna Systems. Smart (adaptive) Antennas enable spatial reuse, increase throughput and they increase the communication range because of the increase directivity of the antenna array. These enhancements quantified for the physical layer may not be efficiently utilized, unless the Media Access Control (MAC) layer is designed accordingly. This thesis implements the behaviours of two MAC protocols, ANMAC and MMAC protocols in OPNET simulator. This method is known as the Physical-MAC layer simulation model. The entire physical layer is written in MATLAB, and MATLAB is integrated into OPNET to perform the necessary stochastic physical layer simulations. The aim is to investigate the performance improvement in throughput and delay of the selected MAC Protocols when using Smart Antennas in a mobile environment. Analytical methods were used to analyze the average throughput and delay performance of the selected MAC Protocols with Adaptive Antenna Arrays in MANET when using spatial diversity. Comparison study has been done between the MAC protocols when using Switched beam antenna and when using the proposed scheme. It has been concluded that the throughput and delay performance of the selected protocols have been improved by the use of Adaptive Antenna Arrays. The throughput and delay performance of ANMAC-SW and ANMAC-AA protocols was evaluated in details against regular Omni 802.11 stations. Our results promise significantly enhancement over Omni 802.11, with a throughput of 25% for ANMAC-SW and 90% for ANMC-AA. ANMAC-AA outperforms ANMAC-SW protocol by 60%. Simulation experiments indicate that by using the proposed scheme with 4 Adaptive Antenna Array per a node, the average throughput in the network can be improved up to 2 to 2.5 times over that obtained by using Switched beam Antennas. The proposed scheme improves the performances of both ANMAC and MMAC protocols but ANMAC outperforms MMAC by 30%

Energy aware routing protocols in ad hoc wireless networks

In Mobile Ad hoc Network, communication at mobile nodes can be achieved by using multi-hop wireless links. The architecture of such a network is based, not on a centralized base station but on each node acting as a router to forward data packets to other nodes in the network. The aim of each protocol, in an ad hoc network, is to find valid routes between two communicating nodes. These protocols must be able to handle high mobility of the nodes which often cause changes in the network topology. Every ad hoc network protocol uses some form of a routing algorithm to transmit between nodes based on a mechanism that forwards packets from one node to another in the network. These algorithms have their own way of finding a new route or modifying an existing one when there are changes in the network. The novel area of this research is a proposed routing algorithm which improves routing and limits redundant packet forwarding, especially in dense networks. It reduces the routing messages and consequently power consumption, which increases the average remaining power and the lifetime of the network. The first aim of this research was to evaluate various routing algorithms in terms of power. The next step was to modify an existing ad hoc routing protocol in order to improve the power consumption. This resulted in the implementation of a dynamic probabilistic algorithm in the route request mechanism of an ad hoc On-Demand Distance Vector protocol which led to a 3.0% improvement in energy consumption. A further extension of the approach using Bayesian theory led to 3.3% improvement in terms of energy consumption as a consequence of a reduction in MAC Load for all network sizes, up to 100 nodes.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Probabilistic route discovery for Wireless Mobile Ad Hoc Networks (MANETs)

Mobile wireless ad hoc networks (MANETs) have become of increasing interest in view of their promise to extend connectivity beyond traditional fixed infrastructure networks. In MANETs, the task of routing is distributed among network nodes which act as both end points and routers in a wireless multi-hop network environment. To discover a route to a specific destination node, existing on-demand routing protocols employ a broadcast scheme referred to as simple flooding whereby a route request packet (RREQ) originating from a source node is blindly disseminated to the rest of the network nodes. This can lead to excessive redundant retransmissions, causing high channel contention and packet collisions in the network, a phenomenon called a broadcast storm. To reduce the deleterious impact of flooding RREQ packets, a number of route discovery algorithms have been suggested over the past few years based on, for example, location, zoning or clustering. Most such approaches however involve considerably increased complexity requiring additional hardware or the maintenance of complex state information. This research argues that such requirements can be largely alleviated without sacrificing performance gains through the use of probabilistic broadcast methods, where an intermediate node rebroadcasts RREQ packets based on some suitable forwarding probability rather than in the traditional deterministic manner. Although several probabilistic broadcast algorithms have been suggested for MANETs in the past, most of these have focused on “pure” broadcast scenarios with relatively little investigation of the performance impact on specific applications such as route discovery. As a consequence, there has been so far very little study of the performance of probabilistic route discovery applied to the well-established MANET routing protocols. In an effort to fill this gap, the first part of this thesis evaluates the performance of the routing protocols Ad hoc On demand Distance Vector (AODV) and Dynamic Source Routing (DSR) augmented with probabilistic route discovery, taking into account parameters such as network density, traffic density and nodal mobility. The results reveal encouraging benefits in overall routing control overhead but also show that network operating conditions have a critical impact on the optimality of the forwarding probabilities. In most existing probabilistic broadcast algorithms, including the one used here for preliminary investigations, each forwarding node is allowed to rebroadcast a received packet with a fixed forwarding probability regardless of its relative location with respect to the locations of the source and destination pairs. However, in a route discovery operation, if the location of the destination node is known, the dissemination of the RREQ packets can be directed towards this location. Motivated by this, the second part of the research proposes a probabilistic route discovery approach that aims to reduce further the routing overhead by limiting the dissemination of the RREQ packets towards the anticipated location of the destination. This approach combines elements of the fixed probabilistic and flooding-based route discovery approaches. The results indicate that in a relatively dense network, these combined effects can reduce the routing overhead very significantly when compared with that of the fixed probabilistic route discovery. Typically in a MANET there are regions of varying node density. Under such conditions, fixed probabilistic route discovery can suffer from a degree of inflexibility, since every node is assigned the same forwarding probability regardless of local conditions. Ideally, the forwarding probability should be high for a node located in a sparse region of the network while relatively lower for a node located in a denser region of the network. As a result, it can be helpful to identify and categorise mobile nodes in the various regions of the network and appropriately adjust their forwarding probabilities. To this end the research examines probabilistic route discovery methods that dynamically adjust the forwarding probability at a node, based on local node density, which is estimated using number of neighbours as a parameter. Results from this study return significantly superior performance measures compared with fixed probabilistic variants. Although the probabilistic route discovery methods suggested above can significantly reduce the routing control overhead without degrading the overall network throughput, there remains the problem of how to select efficiently forwarding probabilities that will optimize the performance of a broadcast under any given conditions. In an attempt to address this issue, the final part of this thesis proposes and evaluates the feasibility of a node estimating its own forwarding probability dynamically based on locally collected information. The technique examined involves each node piggybacking a list of its 1-hop neighbours in its transmitted RREQ packets. Based on this list, relay nodes can determine the number of neighbours that have been already covered by a broadcast and thus compute the forwarding probabilities most suited to individual circumstances

Medium access control protocol for visible light communication in vehicular communication networks

Recent achievements in the automotive industry related to lighting apparatuses include the use of LED or laser technology to illuminate the vehicle environment. This advancement resulted in greater energy efficiency and increased safety with selective illumination segments. A secondary effect was creating a new field for researchers in which they can utilize LED fast modulation using the Pulse Width Modulation (PWM) signal. Using LED to encode and transmit data is a relatively new and innovative concept. On the other field, there have been advancements in vehicular communication using radio frequency at 2.4 or 5GHz. This research focuses mainly on a field in which visible light augments or replaces radio frequency communication between vehicles. This research also investigates the effect of asymmetry on network performance using Visible Light Communication (VLC) in vehicular networks. Different types of asymmetry were defined and tested in real-world simulation experiments. Research results showed that asymmetry has a negative influence on network performance, though that effect is not significant. The main focus of the research is to develop a lightweight and new Media Access Control (MAC) protocol for VLC in vehicular networks. To develop a MAC protocol for VLC, special software was developed on top of the existing Network Simulation Environment (NSE). A new VLC MAC protocol for Vehicle to Vehicle (V2V) was benchmarked using a defined set of metrics. The benchmark was conducted as a set of designed simulation experiments against the referent IEEE 802.11b MAC protocol. Both protocols used a newly defined VLC-equipped vehicle model. Each simulation experiment depicted a specific network and traffic situation. The total number of scenarios was eleven. The last set of simulations was conducted in realworld scenarios on the virtual streets of Suffolk, VA, USA. Using defined metrics, the test showed that the new VLC MAC protocol for V2V is better than the referent protocol.Nedavna dostignuća u automobilskoj industriji koja se tiču opreme za osvjetljivanje uključuju korištenje LED ili laserskih rasvjetnih tijela za osvjetljivanje okoline. Ovime se postižu uštede u potrošnji energije kao i povećana sigurnost u prometu. LED rasvjeta je uniformnija od običnih žarulja tako da osvjetljenje bude ravnomjernije i preciznije. Obzirom da su LED selektivne moguće je odabrati segment ceste koji se želi osvijetliti. Upravo ta fleksibilnost LED otvara novi prostor za istraživače gdje mogu koristiti PWM signal za modulaciju podataka. PWM je poseban signal koji ima varijabilnu širinu pulsa na izlazu. Istraživači i znanstvenici mogu koristiti LED za kodiranje i prijenos podataka između automobila. Prednosti korištenja komunikacije u vidljivom dijelu elektro-magnetskog spektra (eng.VLC) je u činjenici da taj segment nije zaštićen licencama te je otvoren za slobodno korištenje. Osim toga, vidljivo, neintenzivno svjetlo nema biološki negativnih posljedica. Kod korištenja PWM signala za modulaciju, postojeći izlaz svjetla i njegova funkcija (osvjetljivanja ceste) nisu narušeni. Ljudsko oko ne može detektirati oscilacije tako visoke frekvencije (oko 5 kHz) S druge strane, komponente koje mogu primiti poslani signal su foto diode ili kamere. Kamere su već prisutne na modernom vozilu u obliku prednje kamere ili stražnje kamere za pomoć pri parkiranju. U svakom slučaju, tehnologija je već prisutna na modernom vozilu. Na drugom području, znanstvenici rade na komunikaciji između vozila koristeći radio valove niže frekvencije 2.4 ili 5 GHz. Komunikacija između automobila je predmet standardizacije i mnoge zemlje već propisuju pravila za obaveznu ugradnju opreme za takav oblik komunikacije. Prednost takvog koncepta je razmjena podatka; od onih za zabavu pa do kritičnih i sigurnosnih podataka npr. informacija o nadolazećem mjestu gdje se dogodila prometna nesreća. Ovo istraživanje se fokusira na proširenje ili zamjenu radio komunikacije sa komunikacijom koristeći vidljivi dio spektra (npr. LED i kamere). Jedan od glavnih nedostataka takvog koncepta je ne postojanje adekvatnog i specijaliziranog protokola za kontrolu pristupa mediju (eng. MAC). Drugi problem je nepoznati efekt asimetrije u VLC komunikaciji na performanse mrežne komunikacija. Ovo istraživanje je prepoznalo i klasificiralo različite tipove asimetrije. Svaki tip je testiran u sklopu simulacijskog eksperimenta u stvarnim scenarijima. Pokazalo se je da asimetrija negativno utječe na mrežne performanse, međutim taj efekt nije značajan jer uzrokuje manje od 0.5 % neuspješno poslanih poruka. Glavni fokus istraživanja je razvoj novog i pojednostavljenog MAC protokola za VLC komunikaciju između automobila. Kako bi se razvio novi MAC protokol nad VLC tehnologijom u prometnim mrežama, bilo je nužno napraviti i novu razvojnu okolinu koja se bazira na postojećim mrežnim simulatorima. Novi VLCMAC protokol za komunikaciju između automobila je testiran koristeći definirani set metrika. Testovi su napravljeni u obliku simulacijskih eksperimenata u kojima su uspored¯ivane performanse novog i referentnog protokola. Referentni protokol, u ovom istraživanju je IEEE 802.11b MAC protokol. U sklopu ovog rada definiran je i model vozila opremljen VLC tehnologijom. U simulacijskim eksperimentima je korišten isti model vozila za oba protokola. Za potrebe istraživanja je definirano jedanaest simulacijskih eksperimenata, svaki od njih opisuje specifične situacije u mrežnim komunikacijama kao i u prometu. Završni simulacijski scenariji uključuju okolinu iz stvarnosti, mreža ulica grada Suffolka, SAD. Osim stvarnih ulica, vozila su se kretala i razmjenjivala podatke koristeći mrežnu komunikaciju na kompletnom ISO/OSI mrežnom stogu sa zamijenjenim MAC podslojem. Razvojna okolina uključuje preciznu provjeru fizičkih karakteristika na razini putanje zrake svjetlosti. Ova preciznost je bila nužna kako bi simulacije bile što vjerodostojnije stvarnim sustavima. Obzirom da se radi o mnogo kalkulacija, obično računalo nije dostatno za izvođenje simulacijskih eksperimenata; zbog toga su se eksperimenti izvodili na klasteru računala Sveučilišta u Zagrebu. Koristeći definirane metrike, istraživanje je pokazalo kako je novi VLC MAC protokol za komunikaciju između automobila bolji od referentnog protokola.