In this dissertation, we studied and addressed the privacy-preserving and authentication techniques for some network applications, where existing internet security solutions cannot address them straightforwardly due to different trust and attack models and possibly constrained resources. For example, in a centralized dynamic spectrum access (DSA) system, the spectrum resource licensees called incumbent users (IUs), have strong operational privacy requirements for the DSA service provider called spectrum access system (SAS), and hence SAS is required to perform spectrum computation without knowing IUs' operational information. This means SAS can at most be considered as a semi-trusted party which is honest but curious, and common anonymization and end-to-end encryption cannot address this issue, and dedicated solutions are required. Another example is that in an intra-vehicle Controller Area Network (CAN), the transmitter can only embed 64 bits of message and its authentication tag into on message frame, which makes it difficult to achieve message authentication in real-time with sufficient cryptographic strength. The focus of this dissertation is to fill the gap of existing solutions with stronger security notion and practicability. On the topic of privacy-preserving DSA systems, we firstly explored existing solutions and proposed a comparative study. We additionally proposed a new metric for evaluation and showed the advantages and disadvantages of existing solutions. We secondly studied the IU location privacy in 3.5GHz band ESC-based DSA system and proposed a novel scheme called PriDSA. PriDSA addresses malicious colluding SAS attack model through leveraging different and relatively lightweight cryptography primitive with novel design, granting stronger security notion and improved efficiency as well. We thirdly studied the operational privacy of both IU and secondary users (SUs) in a general centralized SAS based DSA system and proposed a novel framework called PeDSS. Through our novel design that integrates differential privacy with secure multi-party computation protocol, PeDSS exhibits great communication and computation overhead compared to existing solutions. On the topic of lightweight message authentication in resource-constrained networks, we firstly explored message authentication schemes with high cryptographic strength and low communication-overhead and proposed a novel scheme called CuMAC. CuMAC provides a flexible trade-off between authentication delay and cryptographic strength, through the embodiment of a novel concept that we refer to as accumulation of cryptographic strength. We secondly explored the possibility of achieving both high cryptographic strength and low authentication delay and proposed a variant of CuMAC called CuMAC/S. By employing the novel idea of message speculation, CuMAC/S achieves enables the accumulation of cryptographic strength while incurring minimal delay when the message speculation accuracy is high. / Doctor of Philosophy / The privacy-preserving and message authentication issues of some network applications are distinctive from common internet security due to different attack models and possibly constrained resources, and these security and privacy concerns cannot be addressed by applying existing internet security solutions straightforwardly. For example, in a centralized dynamic spectrum access (DSA) system, the spectrum resource licensees called incumbent users (IUs), have strong operational privacy requirements for the DSA service provider called spectrum access system (SAS), and hence SAS is required to perform spectrum computation without knowing IUs' operational information. This means SAS can at most be considered as a semi-trusted party which is honest but curious, and common anonymization and end-to-end encryption cannot address this issue, and dedicated solutions are required. Another example is that in an intra-vehicle Controller Area Network (CAN), the transmitter can only embed 64 bits of message and its authentication tag into on message frame, which makes it difficult to achieve message authentication in real-time with sufficient cryptographic strength. We addressed the privacy issue of DSA systems by proposing novel schemes incorporating efficient cryptographic primitives and various privacy-preserving techniques, achieving a greatly higher efficiency or stronger privacy-preserving level. We addressed the lightweight authentication issue of resource-constrained networks by employing the novel concept of security accumulation and message speculation, achieving high cryptographic strength, low communication overhead, and probable low latency.

This dissertation considers a deep reinforcement learning (DRL) setting under the practical challenges of real-world wireless communication systems. The non-stationary and partially observable wireless environments make the learning and the convergence of the DRL agent challenging. One way to facilitate learning in partially observable environments is to combine recurrent neural network (RNN) and DRL to capture temporal information inherent in the system, which is referred to as deep recurrent Q-network (DRQN). However, training DRQN is known to be challenging requiring a large amount of training data to achieve convergence. In many targeted wireless applications in the 5G and future 6G wireless networks, the available training data is very limited. Therefore, it is important to develop DRL strategies that are capable of capturing the temporal correlation of the dynamic environment that only requires limited training overhead. In this dissertation, we design efficient DRL frameworks by utilizing echo state network (ESN), which is a special type of RNNs where only the output weights are trained. To be specific, we first introduce the deep echo state Q-network (DEQN) by adopting ESN as the kernel of deep Q-networks. Next, we introduce federated ESN-based policy gradient (Fed-EPG) approach that enables multiple agents collaboratively learn a shared policy to achieve the system goal. We designed computationally efficient training algorithms by utilizing the special structure of ESNs, which have the advantage of learning a good policy in a short time with few training data. Theoretical analyses are conducted for DEQN and Fed-EPG approaches to show the convergence properties and to provide a guide to hyperparameter tuning. Furthermore, we evaluate the performance under the dynamic spectrum sharing (DSS) scenario, which is a key enabling technology that aims to utilize the precious spectrum resources more efficiently. Compared to a conventional spectrum management policy that usually grants a fixed spectrum band to a single system for exclusive access, DSS allows the secondary system to dynamically share the spectrum with the primary system. Our work sheds light on the real deployments of DRL techniques in next generation wireless systems. / Doctor of Philosophy / Model-free reinforcement learning (RL) algorithms such as Q-learning are widely used because it can learn the policy directly through interactions with the environment without estimating a model of the environment, which is useful when the underlying system model is complex. Q-learning performs poorly for large-scale models because the training has to updates every element in a large Q-table, which makes training difficult or even impossible. Therefore, deep reinforcement learning (DRL) exploits the powerful deep neural network to approximate the Q-table. Furthermore, a deep recurrent Q-network (DRQN) is introduced to facilitate learning in partially observable environments. However, DRQN training requires a large amount of training data and a long training time to achieve convergence, which is impractical in wireless systems with non-stationary environments and limited training data. Therefore, in this dissertation, we introduce two efficient DRL approaches: deep echo state Q-network (DEQN) and federated ESN-based policy gradient (Fed-EPG) approaches. Theoretical analyses of DEQN and Fed-EPG are conducted to provide the convergence properties and the guideline for designing hyperparameters. We evaluate and demonstrate the performance benefits of the DEQN and Fed-EPG under the dynamic spectrum sharing (DSS) scenario, which is a critical technology to efficiently utilize the precious spectrum resources in 5G and future 6G wireless networks.

In an effort to make more spectrum available, recent initiatives by the FCC let mobile providers offer spot service of their licensed spectrum to secondary users, hence paving the way to dynamic secondary spectrum markets. This dissertation investigates secondary spectrum markets under different regulatory regimes by identifying profitability conditions and possible competitive outcomes in an oligopoly model. We consider pricing in a market where multiple providers compete for secondary demand. First, we analyze the market outcomes when providers adopt a coordinated access policy, where, besides pricing, a provider can elect to apply admission control on secondary users based on the state of its network. We next consider a competition when providers implement an uncoordinated access policy (i.e., no admission control). Through our analysis, we identify profitability conditions and fundamental price thresholds, including break-even and market sharing prices. We prove that regardless of the specific form of the secondary demand function, competition under coordinated access always leads to a price war outcome. In contrast, under uncoordinated access, market sharing becomes a viable market outcome if the intervals of prices for which the providers are willing to share the market overlap. We then turn our attention to how a network provider use carrier (spectrum) aggregation in order to lower its break-even price and gain an edge over its competition. To this end, we determine the optimal (minimum) level of carrier aggregation that a smaller provider needs. Under a quality-driven (QD) regime, we establish an efficient way of numerically calculating the optimal carrier aggregation and derive scaling laws. We extend the results to delay-related metrics and show their applications to profitable pricing in secondary spectrum markets. Finally, we consider the problem of profitability over a spatial topology, where identifying system behavior suffers from the curse of dimensionality. Hence, we propose an approximation model that captures system behavior to the first-order and provide an expression to calculate the break-even price at each network location and provide simulation results for accuracy comparison. All of our results hold for general forms of demand, thus avoid restricting assumptions of customer preferences and the valuation of the spectrum.

The rising popularity of wireless services resulting in spectrum shortage has motivated dynamic spectrum sharing to facilitate e cient usage of the underutilized spectrum. Wideband spectrum sensing is a critical functionality to enable dynamic spectrum access by enhancing the opportunities of exploring spectral holes, but entails a major implemen- tation challenge in compact commodity radios that have limited energy and computation capabilities. The sampling rates speci ed by the Shannon-Nyquist theorem impose great challenges both on the acquisition hardware and the subsequent storage and digital sig- nal processors. Sub-Nyquist sampling was thus motivated to sample wideband signals at rates far lower than the Nyquist rate, while still retaining the essential information in the underlying signals. This thesis proposes several algorithms for invoking sub-Nyquist sampling in wideband spectrum sensing. Speci cally, a sub-Nyquist wideband spectrum sensing algorithm is proposed that achieves wideband sensing independent of signal sparsity without sampling at full bandwidth by using the low-speed analog-to-digital converters based on sparse Fast Fourier Transform. To lower signal spectrum sparsity while maintaining the channel state information, the received signal is pre-processed through a proposed permutation and ltering algorithm. Additionally, a low-complexity sub-Nyquist wideband spectrum sensing scheme is proposed that locates occupied channels blindly by recovering the sig- nal support, based on the jointly sparse nature of multiband signals. Exploiting the common signal support shared among multiple secondary users, an e cient coopera- tive spectrum sensing scheme is developed, in which the energy consumption on signal acquisition, processing, and transmission is reduced with the detection performance guar- antee. To further reduce the computation complexity of wideband spectrum sensing, a hybrid framework of sub-Nyquist wideband spectrum sensing with geolocation database is explored. Prior channel information from geolocation database is utilized in the sens- ing process to reduce the processing requirements on the sensor nodes. The models of the proposed algorithms are derived and veri ed by numerical analyses and tested on both real-world and simulated TV white space signals.

Abstract The commercial long-term evolution (LTE) networks of today offer fast and regionally wide access to the Internet and to the commercial applications and services at a reasonable price. At the same time, public safety (PS) users are still communicating with old-fashioned, second-generation voice and data services. Recently, the commercial LTE networks have been standardized to offer capabilities to mission-critical users. However, the commercial networks do not yet fully support the coverage requirements of the PS users. Moreover, the commercial infrastructure might be out of order in critical scenarios where PS actors are needed. Thus, the PS users require, for example, rapidly deployed LTE networks to support their own communication. This thesis studies the PS use of commercial operators' LTE networks and rapidly deployed closed LTE networks. The key tasks are to find out how to connect users seamlessly together between the different networks as well as finding out how the frequency planning is implemented. This thesis provides practical design solutions to guarantee network interoperability by connecting the networks as well as radio spectrum utilization solutions by licensed shared access (LSA). While the concept of LSA has been well developed, it has not been thoroughly investigated from the point of view of the PS actors, who have special requirements and should benefit from the concept. Herein, the alternatives for spectrum sharing between PS and commercial systems are discussed. Moreover, the thesis develops a specific LSA spectrum sharing system for the PS actors deploying their own network in scenarios where the commercial networks are insufficient. The solution is a robust LSA-based spectrum sharing mechanism. Note that PS actors also need to be able to utilize the spectrum when the LSA system is not available and when the commercial system has failed. Thus, this thesis proceeds on developing sensing methods for complementing LSA, where the sensing methods guarantee spectrum information for a rapidly deployed PS network. It is shown how PS actors can utilize available spectrum with a secondary spectrum licence. This is a good alternative to reserving the spectrum completely. The work assembles missing pieces of existing methods to ensure the functionality of the commercial and of the supporting rapidly deployed networks, both in terms of spectrum usage and application services. / Tiivistelmä Kaupalliset long-term evolution (LTE) -verkot tarjoavat nopean, edullisen ja alueellisesti kattavan pääsyn Internettiin sekä laajaan valikoimaan sovelluksia. Samaan aikaan turvallisuustoimijat (public safety (PS) -toimijat) käyttävät vanhanaikaisia äänen sekä vaatimattoman datayhteyden tarjoavia verkkoja. LTE-verkot ovat kuitenkin äskettäin standardoitu tarjoamaan valmiudet myös toimintokriittiseen kommunikointiin. Toisaalta, kaupalliset LTE-verkot eivät vielä tarjoa esimerkiksi tarvittavaa alueellista kattavuutta PS-käyttäjille. Lisäksi, kaupalliset verkot saattavat olla epäkunnossa kriittisissä tilanteissa. Tämän vuoksi PS-toimijat tarvitsevat omia nopeasti pystytettäviä LTE-verkkoja tukemaan nykyaikaista viestintäänsä. Opinnäytetyössä tutkitaan näiden nopeasti pystytettävien LTE-verkkojen käyttöä kaupallisten LTE-verkkojen kanssa. Keskeiset tehtävät ovat eri verkkojen PS-toimijoiden saumaton yhdistäminen sekä verkkojen taajuusjaon toteuttaminen. Tämä opinnäytetyö tarjoaa käytännön ratkaisuja verkkojen yhteentoimivuuden takaamiseksi ja radiotaajuuksien jakoratkaisuja lisensoidun jaetun käyttöoikeuden licensed shared access (LSA) -metodin avulla. Vaikka LSA:n käsite on jo pitkälle kehitetty, sitä ei ole tutkittu perusteellisesti PS-toimijoiden näkökulmasta ottaen huomioon heidän erityisvaatimuksensa. Tässä työssä syvennytään näiltä osin LSA järjestelmään yhtenä vaihtoehtona taajuuksien saamiseksi nopeasti pystytettäville verkoille. Lisäksi työssä kehitetään robusti LSA-pohjainen taajuuksien jakamisjärjestelmä nopeasti pystytettäville verkoille tilanteissa, joissa kaupalliset verkot ovat riittämättömät. Huomaa, että PS-toimijoiden on pystyttävä hyödyntämään taajuuksia myös silloin, kun LSA-järjestelmän kaikki osat eivät ole käytettävissä ja kun kaupallinen LTE järjestelmä on alhaalla. Tätä varten opinnäytetyössä kehitetään LSA:ta täydentävä havainnointimenetelmä, jolla taataan taajuustiedot vapaista taajuuksista nopeasti pystytettäville verkoille, sekä näytetään, miten PS-toimijat voivat hyödyntää LSA:ta toissijaisen taajuuslisenssin avulla. Tämä on hyvä vaihtoehto radiospektrin varaamiseksi kokonaan. Työ kokoaa puuttuvia osia olemassa oleviin menetelmiin, jotta voidaan varmistaa kaupallisten verkkojen toimivuus PS-käyttäjille yhdessä niitä tukevien nopeasti pystytettävien verkkojen kanssa taajuuksien käytön ja sovelluspalvelujen osalta.

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.

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

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.

abstract: Cognitive radio (CR) and device-to-device (D2D) systems are two promising dynamic spectrum access schemes in wireless communication systems to provide improved quality-of-service, and efficient spectrum utilization. This dissertation shows that both CR and D2D systems benefit from properly designed cooperation scheme. In underlay CR systems, where secondary users (SUs) transmit simultaneously with primary users (PUs), reliable communication is by all means guaranteed for PUs, which likely deteriorates SUs’ performance. To overcome this issue, cooperation exclusively among SUs is achieved through multi-user diversity (MUD), where each SU is subject to an instantaneous interference constraint at the primary receiver. Therefore, the active number of SUs satisfying this constraint is random. Under different user distributions with the same mean number of SUs, the stochastic ordering of SU performance metrics including bit error rate (BER), outage probability, and ergodic capacity are made possible even without observing closed form expressions. Furthermore, a cooperation is assumed between primary and secondary networks, where those SUs exceeding the interference constraint facilitate PU’s transmission by relaying its signal. A fundamental performance trade-off between primary and secondary networks is observed, and it is illustrated that the proposed scheme outperforms non-cooperative underlay CR systems in the sense of system overall BER and sum achievable rate. Similar to conventional cellular networks, CR systems suffer from an overloaded receiver having to manage signals from a large number of users. To address this issue, D2D communications has been proposed, where direct transmission links are established between users in close proximity to offload the system traffic. Several new cooperative spectrum access policies are proposed allowing coexistence of multiple D2D pairs in order to improve the spectral efficiency. Despite the additional interference, it is shown that both the cellular user’s (CU) and the individual D2D user's achievable rates can be improved simultaneously when the number of D2D pairs is below a certain threshold, resulting in a significant multiplexing gain in the sense of D2D sum rate. This threshold is quantified for different policies using second order approximations for the average achievable rates for both the CU and the individual D2D user. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017