Quantum communication networks enable secure transmission of information between remote sites. However, at present, photon losses in the optical fiber limit communication distances to less than 150 kilometers. The quantum repeater idea allows extension of these distances. In practice, it involves the ability to store quantum information for a long time in atomic systems and coherently transfer quantum states between matter and light. Previously known schemes involved atomic Raman transitions in the UV or near-infrared and suffered from severe loss in optical fiber that precluded long-distance quantum communication.

Master of Science

Information, products and services can be requested and purchased via the Interactive Video Data Service (IVDS) system developed by The Center for Wireless Telecommunications, Virginia Tech. This system consists of three components - User control, Repeater unit and a Host program. The user requests a service using his/her television remote (User control). A transceiver (User control) located near the television set responds to user requests by extracting information hidden in the commercial's audio, and transmitting information to the repeater unit. The receiver unit decodes received messages and forwards them in capsules to the Host component. Thus the user requests are received by the host system. The repeater unit is a real-time operating system with its in-built hardware and software functions. Application specific software can be written using the existing software drivers and libraries (kernel) to decode and process messages. The Host program monitors and responds to received user messages. This thesis focuses on the repeater unit hardware setup and discusses the application software implementation developed to receive messages from the transceiver box and to retransmit the messages in a different format over the Internet. The software specifications included no incoming message loss, ability to statically hold 10000 user messages, time-stamp and location-stamp (using a GPS receiver) forwarded messages, scheduling messages for retransmission based on message priority, and retransmission using the point-to-point protocol (PPP) using a dial-up modem connection. In order to achieve better performance the existing software kernel was re-written in some sections. This thesis also discusses some of the system limitations from the repeater unit's perspective. / Master of Science

Syftet med undersökningen är att tillföra kunskap och förståelse för resenärers motiv till att återbesöka en destination upprepade gånger samt förstå hur de utvecklar lojalitet gentemot den platsen. Studien genomfördes utifrån en kvalitativ ansats. Nio respondenter deltog i semistrukturerade intervjuer som berörde deras erfarenheter och känslor kring resvanor. Som teoretisk utgångspunkt har sammanställningar utformats utifrån den tidigare forskningen som grund för analys av empiriskt material. Push- och pull-faktorer presenteras inledningsvis, som därefter följs av en modell om lojalitetsutveckling. Som stöd tilllojalitetsutvecklingsmodellen presenteras en tillitsutvecklingsteori. Slutligen tillämpas lojalitetskategoriseringar som gör det möjligt att placera en respondents lojalitet i två olika kategoriseringar. En analys av det insamlade materialet påvisade att beroende på vilket intresse en individ har kan resemotivationen skilja sig från person till person. Mat och klimat var båda motiverande faktorer för respondenterna att åka tillbaka. Majoriteten av respondenterna ansåg även att kunskapen om destinationen motiverade dem till att fortsätta åka tillbaka då denna gav en känsla av att vara hemma på semestern samt att de kan då koppla av på en gång. Studiens resultat visade även att lokalbefolkningen hade stor påverkan när det gäller respondenternasutveckling av lojalitet gentemot destinationen.

Recent years, Orthogonal Frequency Division Multiplexing (OFDM) system gets more and more attentions for its great benefit to the optical fiber communication system for improving the transmission performance. Not only better performance in long distance transmission but also high bit-rate is attractive feature of the OFDM system. The OFDM technology has been developed for the wireless communication system, and is now used in Asymmetric Digital Subscriber Line (ADSL), ETSI Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), High Definition Television (HDTV), Wireless Local Area Network (WLAN) and so on. The OFDM is extensively used in broadband wired and wireless communication systems, and it solves the problem of intersymbol interference (ISI) effectively. Actually, the OFDM is a kind of Multi-Carrier Modulation (MCM), and it is not a brand-new technology. The history of the OFDM can be traced back to 1966 when Chang of Bell Labs introduced the concept of the OFDM. In this thesis, I will briefly introduce the background of optical fiber communication system, then, explain my motivation and the structure of this thesis. Next, theoretical study has been conducted. For the simulation, I discuss the performance of the OFDM system related to the number of subcarriers and the erbium-doped fiber amplifier (EDFA) repeater output power. It revealed that small number of subcarriers had better transmission performance. In my study case, repeater output power of 0dBm was the optimum condition. In addition, experimental study has been conducted. For the experiment, the waveform required for the OFDM signal generation was calculated offline, and it was realized by the Arbitrary Waveform Generator (AWG). Finally, this thesis is concluded.

With the continued rise in wildfires in California, and around the world, technological advancements are needed to improve the safety and effectiveness of wildland firefighters. One area that provides an opportunity for such development is the deployment of temporary communications networks. Currently, radio repeaters are set up on mountain tops in the response area; such repeaters do not provide flexibility once installed, still have blind spots, and require the time of valuable assets like helicopters to install. This thesis will establish the feasibility of airborne radio repeaters for wildland firefighting. In order to successfully demonstrate the feasibility of such an airborne system, the resulting system should be rapidly deployable, improve communications range and reliability, and be compatible with existing regulations and guidelines. The design process for the repeater payload is described, as well as important troubleshooting steps. The resulting product is then compared to the initial requirements through testing and observation. Although audio filtering provided by off-the-shelf handheld radios prevented the repeater from functioning as intended, the proposed 2m/70cm dual-band digital communications relay was capable of being carried by the Altavian Nova and was able to successfully demonstrate the feasibility of such a system. As such it will be an important contribution to communications needed for fighting future wildfires.

When using a wireless repeater in a network, an overlap zone between the repeater and the primary access point that holds the internet connection is created. The theoretical throughput for connections through the repeater is reduced by half since the repeater can’t send and receive data at the same time when using only one channel which was the case in this thesis. To solve the problem with the overlap zone, an algorithm that runs in the form of a bash script on two openWRT-based access points where one acts as a repeater and the other one is a gateway with an internet connection was developed. The script uses the signal strength value (RSSI) to decide whether a wireless device is located within the overlap zone by comparing the measured value against a set threshold where the gateway still delivers higher throughput than the repeater. Wireless devices in the overlap zone are blocked on the repeater and will then connect to the gateway instead. Wireless devices that are located outside the overlap zone will connect to the access point that yields the strongest signal strength. The throughput to the gateway was measured at several locations with different signal strengths to both the gateway and the repeater. Wireless devices within the overlap zone that were connected to the repeater before the script was run gained a higher throughput after the script was run and made it connect directly to the gateway instead. / Vid användning av en trådlös repeater i ett nätverk skapas en överlappningszon mellan repeatern och den primära anslutningspunkten som har uppkopplingen mot internet. Den möjliga datagenomströmningshastigheten för uppkopplingar via repeatern halveras eftersom repeatern inte kan sända och ta emot samtidigt på samma kanal. För detta examensarbete fanns inte möjligheten att använda fler kanaler. För att lösa problemet med överlappningszonen utvecklades en algoritm som körs i form av ett bash-skript på två openWRT-baserade anslutningspunkter där den ena agerar som repeater och den andra är en gateway med internetuppkoppling. Skriptet använder värdet på signalstyrka (RSSI) för att avgöra om en trådlös enhet befinner sig i överlappningszonen genom att kontrollera uppmätt värde mot ett tröskelvärde där gatewayen fortfarande ger högre datagenomströmningshastighet än repeatern. Trådlösa enheter i överlappningszonen blockeras på repeatern och kopplar då upp till gatewayen istället. Utanför överlappningszonen kopplas enheter upp till den anslutningspunkt som har starkast signalstyrka. Datagenomströmningen till gatewayen mättes på flera platser med olika signalstyrkor till både gatewayen och repeatern. Trådlösa enheter i överlappningszonen som var uppkopplade till repeatern innan skriptet kördes fick en högre datagenomströmningshastighet efter att skriptet kört och styrt om anslutningen direkt till gatewayen istället.

This thesis presents a new approach to extend and reduce the transmission range in wireless systems. Conditioning is defined as purposeful electromagnetic interference that affects a wireless signal as it propagates through the air. This interference can be used constructively to enhance a signal and increase its energy, or destructively to reduce energy. The constraints and limitations of the technology are described as a system model, and a flow chart is used to describe the circuit process. Remaining theoretical in nature, practical circuit implementations are foregone in the interest of elementary simulations depicting the interactions of modulated signals as they experience phase mismatch. Amplitude modulation and frequency modulation are explored with using both positive and negative conditioning, and conclusions to whether one is more suitable than the other are made.

Process variations and circuit aging continue to be main challenges to the power-efficiency of VLSI circuits, as considerable power budget must be allocated at design time to mitigate timing variations. Modern designs incorporate adaptive techniques for variation compensation to reduce the extra power consumption. The efficiency of existing adaptive approaches, however, is often significantly attenuated by the fine-grained nature of variations in nanometer technology such as random dopant fluctuation, litho-variation, and different rates of transistor degradation due to non-uniform activity factors. This dissertation addresses the limitations from existing adaptation techniques, and proposes new adaptive approaches to effectively compensate the fine-grained variations. Adaptive supply voltage (ASV) is one of the effective adaptation approaches for power-performance tuning. ASV has advantages on controlling dynamic and leakage power, while voltage generation and delivery overheads from conventional ASV systems make their application to mitigate fine-grained variations demanding. This dissertation presents a dual-level ASV system which provides ASV at both coarse-grained and fine-grained level, and has limited power routing overhead. Significant power reduction from our dual-ASV system demonstrates its superiority over existing approaches. Another novel technique on supply voltage adaptation for variation resilience in VLSI interconnects is proposed. A programmable boostable repeater design boosts switching speed by raising its internal voltage rail transiently and autonomously, and achieves fine-grained voltage adaptation without stand-alone voltage regulators or additional power grid. Since interconnect is a widely recognized bottleneck to chip performance and tremendous repeaters are employed on chip designs, boostable repeater has plenty of chances to improve system robustness. A low cost scheme for delay variation detection is essential to compose an efficient adaptation system. This dissertation presents an area-efficient built-in delay testing scheme which exploits BIST SCAN architecture and dynamic clock skew control. Using this built-in delay testing scheme, a fine-grained adaptation system composed of the proposed boostable repeater design and adaptive clock skew control is proposed, and demonstrated to mitigate process variation and aging induced timing degradations in a power as well as area efficient manner.

In the field of wireless multiple-input multiple-output (MIMO) communications, remarkable capacity enhancements may be achieved in certain environments relative to single-antenna systems. In a non-line of sight (NLOS) environment with rich multipath, the capacity potential is typically very good, but in a line of sight (LOS) environment with a high Rician K-factor, the capacity improvement may be severely limited or almost disappear. The objective of the research described in this dissertation has been to develop a more thorough understanding of the capacity limitations of MIMO in a LOS environment and explore methods to improve that capacity. It is known that for a LOS link with a given range, an optimal antenna configuration, which usually involves large antenna spacings, can be computed to maximize the capacity. A method is here proposed for achieving near-maximum MIMO capacity in LOS environments with suboptimal array configurations. Suboptimal arrays may include small antenna spacings and/or arrays rotated off normal. The method employs single-antenna full-duplex, amplify-and-forward relays, otherwise known as "wireless repeaters." We have designated this concept repeater-assisted capacity enhancement (RACE) for MIMO. Potential applications include tower-mounted or building-top cellular backhaul and high-speed wireless bridge links (explored in Chapter 5) and ground-to-air sensor network backhaul links and base-to-mobile links in a cellular configuration (explored in Chapter 7). We have analyzed this concept in simulation for point-to-point and point-to-multipoint links and have found the following critical parameters for system design and deployment: orientation, antenna spacing, and antenna patterns of the transmit (TX)/receive (RX) MIMO arrays; and position, noise figure, TX/RX isolation, and antenna patterns associated with the repeater(s). Simulation results for an n[subscript R] xn[subscript T] MIMO link demonstrate nearly a factor of n=min{n[subscript R], n[subscript T] } improvement in capacity relative to a single-input single-output (SISO) link using n-1 optimally placed wireless repeaters supporting the link. Other portions of analysis presented include the development of a determinant-based metric for capacity (D) and an exploration of upper and lower bounds of capacity as a function of D. The position of repeaters is analyzed theoretically and a metric introduced based on D intended to quickly and intuitively determine optimal positions for repeaters assisting a given MIMO link based on TX/RX node steering vectors.