Recent disasters, including the 9/11 terrorist attacks, Hurricane Katrina, the London subway bombings, and the California wildfires, have all highlighted the limitations of current mobile communication systems for public safety first responders. First, in a point-to-point configuration, legacy radio systems used by first responders from differing agencies are often made by competing manufacturers and may use incompatible waveforms or channels. In addition, first responder radio systems, which may be licensed and programmed to operate in frequency bands allocated within their home jurisdiction, may be neither licensed nor available in forward-deployed disaster response locations, resulting in an operational scarcity of usable frequencies. To address these problems, first responders need smart radio solutions which can bridge these disparate legacy radio systems together, can incorporate new smart radio solutions, or can replace these existing aging radios. These smart radios need to quickly find each other and adhere to spectrum usage and access policies. Second, in an infrastructure configuration, legacy radio systems may not operate at all if the existing communications backbone has been destroyed by the disaster event. A communication system which can provide a new, temporary infrastructure or can extend an existing infrastructure into a shaded region is needed. Smart radio nodes that make up the public safety infrastructure again must be able to find each other, adhere to spectrum usage policies, and provide access to other smart radios and legacy public safety radios within their coverage area. This work addresses these communications problems in the following ways. First, it applies cognitive radio technology to develop a smart radio system capable of rapidly adapting itself so it can communicate with existing legacy radio systems or other smart radios using a variety of standard and customized waveforms. These smart radios can also assemble themselves into an ad-hoc network capable of providing a temporary communications backbone within the disaster area, or a network extension to a shaded communications area. Second, this work analyzes and characterizes a series of rendezvous protocols which enable the smart radios to rapidly find each other within a particular coverage area. Third, this work develops a spectrum sharing protocol that enables the smart radios to adhere to spectral policies by sharing spectrum with other primary users of the band. Fourth, the performance of the smart radio architecture, as well as the performance of the rendezvous and spectrum sharing protocols, is evaluated on a smart radio network testbed, which has been assembled in a laboratory setting. Results are compared, when applicable, to existing radio systems and protocols. Finally, this work concludes by briefly discussing how the smart radio technologies developed in this dissertation could be combined to form a public safety communications architecture, applicable to the FCC's stated intent for the 700 MHz Band. In the future, this work will be extended to applications outside of the public safety community, specifically, to communications problems faced by warfighters in the military. / Ph. D.

The unlicensed wireless spectrum offers exciting opportunities for developing innovative wireless applications. This has been true ever since the 2.4 GHz band and parts of the 5 GHz bands were first opened for unlicensed access worldwide. In recent years, the 5 GHz unlicensed bands have been one of the most coveted for launching new wireless services and applications due to their relatively superior propagation characteristics and the abundance of spectrum therein. However, the appetite for unlicensed spectrum seems to remain unsatiated; the demand for additional unlicensed bands has been never-ending. To meet this demand, regulators in the US and Europe have been considering unlicensed operations in the 5.9 GHz bands and in large parts of the 6 GHz bands. In the last two years alone, the Federal Communications Commission in the US has added more than 1.2 GHz of spectrum in the pool of unlicensed bands. Wi-Fi networks are likely to be the biggest beneficiaries of this spectrum. Such abundance of spectrum would allow massive improvements in the peak throughput and potentially allow a considerable reduction of latency, thereby enabling support for emerging wireless applications such as augmented and virtual reality, and mobile gaming using Wi-Fi over unlicensed bands. However, access to these bands comes with its challenges. Across the globe, a wide range of incumbent wireless technologies operate in the 5 GHz and 6 GHz bands. This includes weather and military radars, and vehicular communication systems in the 5 GHz bands, and fixed-service systems, satellite systems, and television pick-up stations in the 6 GHz bands. Furthermore, due to the development of several cellular-based unlicensed technologies (such as Licensed Assisted Access and New Radio Unlicensed, NR-U), the competition for channel access among unlicensed devices has also been increasing. Thus, coexistence across wireless technologies in the 5 GHz and 6 GHz bands has emerged as an extremely challenging and interesting research problem. In this dissertation, we first take a comprehensive look at the various coexistence scenarios that emerge in the 5 GHz and 6 GHz bands as a consequence of new regulatory decisions. These scenarios include coexistence between Wi-Fi and incumbent users (both in the 5 GHz and 6 GHz bands), coexistence of Wi-Fi and vehicular communication systems, coexistence across different vehicular communication technologies, and coexistence across different unlicensed systems. Since a vast majority of these technologies are fundamentally different from each other and serve diverse use-cases each coexistence problem is unique. Insights derived from an in-depth study of one coexistence problem do not help much when the coexisting technologies change. Thus, we study each scenario separately and in detail. In this process, we highlight the need for the design of novel coexistence mechanisms in several cases and outline potential research directions. Next, we shift our attention to coexistence between Wi-Fi and vehicular communication technologies designed to operate in the 5.9 GHz intelligent transportation systems (ITS) bands. Until the development of Cellular V2X (C-V2X), dedicated short range communications (DSRC) was the only major wireless technology that was designed for communication in high-speed and potentially dense vehicular settings. Since DSRC uses the IEEE 802.11p standard for its physical (PHY) and medium access control (MAC) layers, the manner in which DSRC and Wi-Fi devices try to gain access to the channel is fundamentally similar. Consequently, we show that spectrum sharing between these two technologies in the 5.9 GHz bands can be easily achieved by simple modifications to the Wi-Fi MAC layer. Since the design of C-V2X in 2017, however, the vehicular communication landscape has been fast evolving. Because DSRC systems were not widely deployed, automakers and regulators had an opportunity to look at the two technologies, consider their benefits and drawbacks and take a fresh look at the spectrum sharing scenario. Since Wi-Fi can now potentially share the spectrum with C-V2X at least in certain regions, we take an in-depth look at various Wi-Fi and C-V2X configurations and study whether C-V2X and Wi-Fi can harmoniously coexist with each other. We determine that because C-V2X is built atop cellular LTE, Wi-Fi and C-V2X systems are fundamentally incompatible with each other. If C-V2X and Wi-Fi devices are to share the spectrum, considerable modifications to the Wi-Fi MAC protocol would be required. Another equally interesting scenario arises in the 6 GHz bands, where 5G NR-U and Wi-Fi devices are likely to operate on a secondary shared basis. Since the 6 GHz bands were only recently considered for unlicensed access, these bands are free from Wi-Fi and NR-U devices. As a result, the greenfield 6 GHz bands provide a unique and rare opportunity to freshly evaluate the coexistence between Wi-Fi and cellular-based unlicensed wireless technologies. We study this coexistence problem by developing a stochastic geometry-based analytical model. We see that by disabling the listen before talk based legacy contention mechanism---which has been used by Wi-Fi devices ever since their conception---the performance of both Wi-Fi and NR-U systems can improve. This has important implications in the 6 GHz bands, where such legacy transmissions can indeed be disabled because Wi-Fi devices, for the first time since the design of IEEE 802.11a, can operate in the 6 GHz bands without any backward compatibility issues. In the course of studying the aforementioned coexistence problems, we identified several gaps in the literature on the performance analysis of C-V2X and IEEE 802.11ax---the upcoming Wi-Fi standard. We address three such gaps in this dissertation. First, we study the performance of C-V2X sidelink mode 4, which is the communication mode in C-V2X that allows direct vehicular communications (i.e., without assistance from the cellular infrastructure). Using our in-house standards-compliant network simulator-3 (ns-3) simulator, we perform simulations to evaluate the performance of C-V2X sidelink mode 4 in highway environments. In doing so, we identify that packet re-transmissions, which is a feature introduced in C-V2X to provide frequency and time diversity, thereby improving the system performance, can have the opposite effect if the vehicular density increases. In fact, packet re-transmissions are beneficial for C-V2X system performance only at low vehicular densities. Thus, if vehicles are statically configured to always use/disable re-transmissions, the maximum potential of this feature is not realized. Therefore, we propose a simple and effective, distributed re-transmission control mechanism named Channel Congestion Based Re-transmission Control (C2RC), which leverages the locally available channel sensing results to allow vehicles to autonomously decide when to switch on re-transmissions and when to switch them off. Second, we present a detailed analysis of the performance of Multi User Orthogonal Frequency Division Multiple Access (MU OFDMA)---a feature newly introduced in IEEE 802.11ax---in a wide range of deployment scenarios. We consider the performance of 802.11ax networks when the network comprises of only 802.11ax as well as a combination of 802.11ax and legacy stations. The latter is a practical scenario, especially during the initial phases of 802.11ax deployments. Simulation results, obtained from our ns-3 based simulator, give encouraging signs for 802.11ax performance in many real-world scenarios. That being said, there are some scenarios where naive usage of MU OFDMA by an 802.11ax-capable Wi-Fi AP can be detrimental to the overall system performance. Our results indicate that careful consideration of network dynamics is critical in exploiting the best performance, especially in a heterogeneous Wi-Fi network. Finally, we perform a comprehensive simulation study to characterize the performance of Multi Link Aggregation (MLA) in IEEE 802.11be. MLA is a novel feature that is likely to be introduced in next-generation Wi-Fi (i.e., Wi-Fi 7) devices and is aimed at reducing the worst-case latency experienced by Wi-Fi devices in dense traffic environments. We study the impact of different traffic densities on the 90 percentile latency of Wi-Fi packets and identify that the addition of a single link is sufficient to substantially bring down the 90 percentile latency in many practical scenarios. Furthermore, we show that while the addition of subsequent links is beneficial, the largest latency gain in most scenarios is experienced when the second link (i.e., one additional) link is added. Finally, we show that even in extremely dense traffic conditions, if a sufficient number of links are available at the MLA-capable transmitter and receiver, MLA can help Wi-Fi devices to meet the latency requirements of most real-time applications. / Doctor of Philosophy / Wireless networks have become ubiquitous in our lives today. Whether it is cellular connectivity on our mobile phones or access to Wi-Fi hotspots on laptops, tablets, and smartphones, never before has wireless communication been as integral to our lives as it is today. In many wireless communication systems, wireless devices operate by sending signals to and receiving signals from a central entity that connects to the wired Internet infrastructure. In the case of cellular networks, this entity is the cell tower deployed by the operators (such as ATandT, Verizon, etc. in the US), while the Wi-Fi router deployed in homes and offices plays this role in Wi-Fi networks. There is also another class of wireless systems, where wireless devices communicate with each other without requiring to communicate with any central entity. An example of such a distributed communication system---which is fast gaining popularity---is vehicular ommunication networks. End-user devices (e.g. cellphone, laptop, tablet, or a vehicle) can communicate with each other or the central entity only if they are both tuned to the same frequency channel. This channel can lie anywhere within the radio frequency spectrum, but some frequency channels (the collection of channels is referred to as frequency bands) are more favorable—--in terms of how far the signal sent over these channels can reach—--than others. Another dimension to these frequency bands is the licensing mechanism. Not all frequency bands are free to use. In fact, most frequency bands in the US and other parts of the world are licensed by the regional regulatory agencies. The most well-known example of this licensing framework is the cellular network. Cellular operators spend large amounts of money (to the tune of billions of dollars) to gain the privileges of exclusively operating in a given frequency band. No other operator or wireless device is then allowed to operate in this band. Without any external interfering wireless device, cellular operators can guarantee a certain quality of service that is provided to its customers. Thus, the benefits of using licensed frequency bands are obvious but these bands and their associated benefits come at a high price. An alternative to licensed frequency bands are the unlicensed ones. As the name suggests, unlicensed frequency bands are those where any two or more wireless devices can communicate with each other (subject to certain rules) without having to pay any licensing fees. Unsurprisingly, because there is no limit to who or how many devices can communicate over these bands, wireless devices in these bands frequently experience external interference, which manifests to the end-user in terms of interruption of service. The best example of a wireless technology that uses unlicensed bands is Wi-Fi. One of the greatest advantages of Wi-Fi networks is that anyone can purchase a Wi-Fi router and deploy it within their homes or offices—--flexibility not afforded by licensed bands. However, this very flexibility and ease-of-use can sometimes contribute negatively to Wi-Fi performance. Arguably, we have all faced scenarios where the performance of Wi-Fi is poor. This is most likely to happen in scenarios where there are hundreds (or even thousands) of neighboring Wi-Fi devices, such as at stadiums, railway stations, concerts, etc. Based on our discussions above, it is clear as to why Wi-Fi performance suffers in such scenarios. Thus, although unlicensed bands are lucrative in terms of low-cost, and ease of use, there is no guarantee on how good a voice/video call or a video streaming session conducted over Wi-Fi will be. The above problem is well-known and well-researched. Regulators, researchers, and service providers actively seek solutions to offer better performance over unlicensed bands. An obvious solution is to make more unlicensed bands available; if all neighboring Wi-Fi users communicate with their respective routers on different channels, everyone could communicate interference-free. The problem, however, is that frequency bands are limited. Even more limited are those bands that support wireless communications over larger distances. Another solution is to improve the wireless technology—if a Wi-Fi device can more efficiently utilize the channel, its performance is likely to improve. This fact has driven the constant evolution of all wireless technologies. However, there are fundamental limits to how much a frequency channel can be exploited. Therefore, in recent years, stakeholders have turned to spectrum sharing. Even though a wireless network may possess an exclusive license to operate on a given frequency band, its users do not use the band everywhere and at all times. Then why not allow unlicensed wireless devices to operate in this band at such places and times? This is precisely the premise of spectrum sharing. In this dissertation, we look at the problem of coexistence between wireless technologies in the 5 GHz and 6 GHz bands. These two bands are extremely lucrative in terms of their relatively favorable propagation characteristics (i.e., their communication range) and the abundance of spectrum therein. Consequently, these bands have garnered considerable attention in recent years with the objective of opening these bands up for unlicensed services. However, the 5 GHz and 6 GHz bands are home to several licensed systems, and the performance of these systems cannot be compromised if unlicensed operations are allowed. Significant activity has taken place since 2013 concerning new technologies being developed, new spectrum sharing scenarios being proposed, and new rules being adopted in these two bands. We begin the dissertation by taking a comprehensive look at these issues, describing the various coexistence scenarios, surveying the existing literature, describing the major challenges, and providing directions for potential research. Next, we look at three coexistence problems in detail: (i) coexistence of dedicated short range communications (DSRC) and Wi-Fi, (ii) coexistence of cellular V2X (C-V2X) and Wi-Fi, and (iii) coexistence of 5G New Radio Unlicensed (5G NR-U) and Wi-Fi. The former two scenarios involve the coexistence of Wi-Fi with a vehicular communication technology (DSRC or C-V2X). These scenarios arose due to considerations in the US and Europe to allow Wi-Fi operations (on an unlicensed secondary basis) in the spectrum that was originally reserved for vehicular communications. Our work shows that because DSRC and Wi-Fi are built on top of fundamentally similar protocols, they are, to an extent, compatible with each other, and coexistence between these two technologies can be achieved by relatively simple modifications to the Wi-Fi protocol. However, C-V2X, owing to its inheritance from the cellular LTE, is not compatible with Wi-Fi. Consequently, significant research is required if the two technologies are to share the spectrum. On the other hand, in the coexistence of 5G NR-U and Wi-Fi, we focus on the operations of these two technologies in the 6 GHz bands. NR-U is a technology that is built atop the 5G cellular system, but is designed to operate in the unlicensed bands (in contrast to traditional cellular systems which only operate in licensed bands). Although these two technologies can coexist in the 5 GHz and 6 GHz bands, we restrict our attention in this dissertation to the 6 GHz bands. This is because the 6 GHz bands are unique in that the entire range of the 6 GHz bands were opened up for unlicensed access all at once recently, and no Wi-Fi or NR-U devices currently operate in these bands. As a result, we can learn from the mistakes made in the 5 GHz bands, where a vast majority of today's Wi-Fi networks operate. Our work shows that, indeed, we can take decisive steps---such as disabling certain Wi-Fi functions---in the 6 GHz bands, which can facilitate better coexistence in the 6 GHz bands. Finally, in the course of identifying and tackling the various coexistence scenarios in the 5 GHz and 6 GHz bands, we identify some open issues in the performance of new wireless technologies designed to operate in these bands. Specifically, we highlight the need to better understand and characterize the performance of Multi User Orthogonal Frequency Division Multiple Access (MU OFDMA), a feature common in cellular networks but newly introduced to Wi-Fi, in the upcoming Wi-Fi 6 generation of devices. We propose and evaluate an analytical model for the same. We also characterize the performance of Multi Link Aggregation---which a novel feature likely to be introduced in future Wi-Fi 7 devices---that is aimed at reducing the worst-case delay experienced by Wi-Fi devices in dense traffic conditions. Additionally, we identify an issue in the performance of the distributed operational mode of C-V2X. We show that packet re-transmissions, which is a feature aimed at improving the performance of C-V2X, can have a counter-productive effect and degrade the C-V2X performance in certain environments. We address this issue by proposing a simple, yet effective, re-transmission control mechanism.

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The radio frequency spectrum is not being efficiently utilized partly due to the current policy of allocating the frequency bands to specific services and users. In opportunistic spectrum access (OSA), the ``white spaces'' that are not occupied by primary users (a.k.a. incumbent users) can be opportunistically utilized by secondary users. To achieve this, we need to solve two problems: (i) primary-secondary incumbent protection, i.e., prevention of harmful interference from secondary users to primary users; (ii) secondary-secondary network coexistence, i.e., mitigation of mutual interference among secondary users. The first problem has been addressed by spectrum sensing techniques in cognitive radio (CR) networks and geolocation database services in database-driven spectrum sharing. The second problem is the main focus of this dissertation. To obtain a clear picture of coexistence issues, we propose a taxonomy of heterogeneous coexistence mechanisms for shared spectrum access. Based on the taxonomy, we choose to focus on four typical coexistence scenarios in this dissertation. Firstly, we study sensing-based OSA, when secondary users are capable of employing the channel aggregation technique. However, channel aggregation is not always beneficial due to dynamic spectrum availability and limited radio capability. We propose a channel usage model to analyze the impact of both primary and secondary user behaviors on the efficiency of channel aggregation. Our simulation results show that user demands in both the frequency and time domains should be carefully chosen to minimize expected cumulative delay. Secondly, we study the coexistence of homogeneous CR networks, termed as self-coexistence, when co-channel networks do not rely on inter-network coordination. We propose an uplink soft frequency reuse technique to enable globally power-efficient and locally fair spectrum sharing. We frame the self-coexistence problem as a non-cooperative game, and design a local heuristic algorithm that achieves the Nash equilibrium in a distributed manner. Our simulation results show that the proposed technique is mostly near-optimal and improves self-coexistence in spectrum utilization, power consumption, and intra-cell fairness. Thirdly, we study the coexistence of heterogeneous CR networks, when co-channel networks use different air interface standards. We propose a credit-token-based spectrum etiquette framework that enables spectrum sharing via inter-network coordination. Specifically, we propose a game-auction coexistence framework, and prove that the framework is stable. Our simulation results show that the proposed framework always converges to a near-optimal distributed solution and improves coexistence fairness and spectrum utilization. Fourthly, we study database-driven OSA, when secondary users are mobile. The use of geolocation databases is inadequate in supporting location-aided spectrum sharing if the users are mobile. We propose a probabilistic coexistence framework that supports mobile users by locally adapting their location uncertainty levels in order to find an appropriate trade-off between interference mitigation effectiveness and location update cost. Our simulation results show that the proposed framework can determine and adapt the database query intervals of mobile users to achieve near-optimal interference mitigation with minimal location updates. / Ph. D.

Radio frequency spectrum is a scarce natural resource that is utilized for many services including surveillance, navigation, communication, and broadcasting. Recent years have seen tremendous growth in use of spectrum especially by commercial cellular operators. As a result, cellular operators are experiencing a shortage of radio spectrum to meet bandwidth demands of users. Spectrum sharing is a promising approach to solve the problem of spectrum congestion as it allows cellular operators access to more spectrum in order to satisfy the ever growing bandwidth demands of commercial users. The US spectrum regulatory bodies are working on an initiative to share 150 MHz of spectrum, held by federal agencies, in the 3.5 GHz band with commercial wireless operators. This band is primarily used by radar systems that are critical to national defense. Field tests have shown that spectrum sharing between radars and communication systems require large separation distance in order to protect them from harmful interference. Thus, novel methods are required to ensure spectrum sharing between the two systems without the need of large protection distances. In order to efficiently share spectrum between radars and communication systems at the same time and in the same geographical area, a novel method is proposed that transforms radar signal in such a way that it does not interfere with communication systems. This is accomplished by projecting the radar signal onto null space of the wireless channel between radar and communication system. In order to understand the effects of the proposed sharing mechanism -- in urban, sub-urban, and littoral areas -- new channel models, specifically, two- and three-dimensional channel models are designed that capture azimuth and elevation angles of communication systems and helps in placing accurate nulls. In addition, interference coming from communication systems into radar receivers is analyzed and radar performance is accessed. Using this information, resource allocation schemes are designed for communication systems that take advantage of the carrier aggregation feature of the LTE-Advanced systems. This further helps in dynamic sharing of spectrum between radars and communication systems. The proposed signal projection approach not only meets radar objectives but also meets spectrum sharing objectives. However, there is a trade-off as signal projection results in some performance degradation for radars. Performance metrics such as probability of target detection, Cramer Rao bound and maximum likelihood estimate of target's angle of arrival, and beampattern of radar are studied for performance degradation. The results show minimal degradation in radar performance and reduction in exclusion zones, thus, showing the efficacy of the proposed approach. / Ph. D.

In the push for spectrum sharing and open spectrum access, the 3.5 GHz frequency band is under consideration for small cells and general Wireless Local Area Networks (WLAN) in the United States. The same band is beginning to see deployment in China, Japan, and South Korea, for the 4G Long Term Evolution (LTE) cellular standard to increase coverage and capacity in urban areas through small cell deployment. However, since the adoption of this band is new, there is a distinct shortage of propagation data and accurate channel modeling at 3.5 GHz in indoor environments. These models are necessary for cellular coverage planning and evaluating the performance and feasibility of wireless systems. This report presents the results of a fixed wireless channel measurement campaign at 3.5 GHz. Measurements were taken in environments typical of indoor wireless deployment: traditional urban indoor office, hallway, classroom, computer laboratory, and atrium areas, as well as within a hospital. Primarily Non Line of Sight (NLOS) experiments were carried out in areas with a controllable amount of partitions separating the transmitter and receiver in order to document material-based attenuation values. Indoor-to-outdoor measurements were carried out, focusing on attenuation due to common exterior building materials such as concrete, brick, wood, and reinforced glass. Documented metrics include large scale path loss, log-normal shadowing, and channel power delay profiles combined with delay spread characteristics for multipath analysis. The statistical multi-antenna diversity gain was evaluated to gauge the benefit of using multi-antenna systems in an indoor environment, which has much greater spatial diversity than an outdoor environment. Measurements were compared to indoor path loss models used for WLAN planning in the low GHz range to investigate the applicability of extending these models to 3.5 GHz. / Master of Science

The radio spectrum that can be used for wireless communications is a finite but extremely valuable resource. During the past two decades, with the proliferation of new wireless applications, the use of the radio spectrum has intensified to the point that improved spectrum sharing policies and new mechanisms are needed to enhance its utilization efficiency. This dissertation studies spectrum sharing and coexistence on both licensed and unlicensed bands for wireless networks. For licensed bands, we study two coexistence paradigms: transparent coexistence (a.k.a., underlay) and policy-based network cooperation (a.k.a., overlay). These two paradigms can offer significant improvement in spectrum utilization and throughput performance than the interweave paradigm. For unlicensed band, we study coexistence of Wi-Fi and LTE, the two most poplar wireless networks. / Ph. D.

Recent advances in spectrum access technologies, such as software-defined radios, have made dynamic spectrum sharing (DSS) a viable option for addressing the spectrum shortage problem. However, these advances have also contributed to the increased possibility of "rogue" transmitter radios which may cause significant interference to other radios in DSS. One approach for countering such threats is to employ a transmitter authentication scheme at the physical (PHY) layer. In PHY-layer authentication, an authentication signal is generated by the transmitter, and embedded into the message signal. This enables a regulatory enforcement entity to extract the authentication signal from the received signal, uniquely identify a transmitter, and collect verifiable evidence of a rogue transmission that can be used later during an adjudication process. There are two primary technical challenges in devising a transmitter authentication scheme for DSS: (1) how to generate and verify the authentication signal such that the required security and privacy criteria are met; and (2) how to embed and extract the authentication signal without negatively impacting the performance of the transmitters and the receivers in DSS. With regard to dealing with the first challenge, the authentication schemes in the prior art, which provide privacy-preserving authentication, have limited practical value for use in large networks due to the high computational complexity of their revocation check procedures. In this dissertation, the novel approaches which significantly improve scalability of the transmitter authentication with respect to revocation, are proposed. With regard to dealing with the second challenge, in the existing PHY-layer authentication techniques, the authentication signal is embedded into the message signal in such a way that the authentication signal appears as noise to the message signal and vice versa. Hence, existing schemes are constrained by a fundamental tradeoff between the message signal's signal to interference and noise ratio (SINR) and the authentication signal's SINR. In this dissertation, the novel approaches which are not constrained by the aforementioned tradeoff between message and authentication signals, are proposed. / Ph. D.

For the past 70 years, the Federal Communications Commission (FCC) has been the licensing authority for wireless spectrum. Traditionally, spectrum was commercially licensed to primary users with defined uses. With the growth of personal communication systems in the 1990''s, unallocated spectrum has become a scarce commodity. However, since most primary users are active only at certain times and places, much of the allocated spectrum remains underutilized. Substantial holes exist in the spatio-temporal spectrum that could be opportunistically used by unlicensed secondary users. As a result, the FCC is considering allowing secondary users to opportunistically use frequencies that are not being used by primary users. If multiple secondary users are present in the same geographical area, the concept of Dynamic Spectrum Sharing (DSS) allows these users to share the opportunistic spectrum. If several secondary users want to use a limited set of frequency resources, they will very likely interfere with each other. Sensing is a distributed technique where each transmitter/receiver pair senses (both passively and actively) the available channels and uses the channel that provides the best performance. While sensing alone allows sharing of the spectrum, it is not the optimal method in terms of maximizing the capacity in such a shared system. If we allow the secondary users to collaborate and share information, optimal capacity might be reached. However, collaboration adds another level of complexity to the transceivers of the secondary users, since they must now be able to communicate (Note that in general, the secondary users may have completely different communication protocols, e.g., Wi-Fi and Bluetooth). Additionally, optimizing the capacity of the available spectrum could have other negative side effects such as impacting the fairness of sharing the resources. Our primary goal is to explore the benefit of this cost-benefit tradeoff by determining the capacity increase obtainable from collaboration. As a secondary goal, we also wish to determine how this increase in capacity affects fairness. To summarize, the goal of this work is to answer the question: Fundamentally, what is the benefit of collaboration in Dynamic Spectrum Sharing? / Master of Science